Appendix N Geotechnical Assessment Report · 2017-03-21 · McArthur River Mine Overburden Management Project Draft Environmental Impact Statement N Appendix N Geotechnical Assessment

McArthur River Mine

Overburden Management Project

Draft Environmental Impact Statement

NAppendix N

Geotechnical Assessment Report

Pando

MRM

McArthur River Mine

Preliminary Geotechnical Assessment

NOEF, IPD and final pit limit stability

McArthur River Mine OMP EIS

February 2017 (Version 4)

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Pando (Australia) Pty Ltd ABN 94 149 551 735

Contents 1 Introduction ............................................................................................................................... 1

1.1 Background......................................................................................................................... 1

1.2 Scope and objectives .......................................................................................................... 3

1.3 Historic and other studies .................................................................................................. 4

2 NOEF Assessment ....................................................................................................................... 5

2.1 Data sources ....................................................................................................................... 5

2.1.1 Overall ........................................................................................................................ 5

2.1.2 NOEF materials testing ............................................................................................... 5

2.2 Hydrology ......................................................................................................................... 12

2.2.1 Foundation hydrogeology ........................................................................................ 13

2.3 Design process .................................................................................................................. 14

2.4 NOEF Design and construction methodology .................................................................. 15

2.4.1 Design ....................................................................................................................... 15

2.4.2 Construction methodology ...................................................................................... 19

2.4.3 Stage sequencing ...................................................................................................... 20

2.4.4 Surface drainage and the Cover System .................................................................. 21

2.4.5 Foundation preparation ........................................................................................... 23

3 NOEF Stability analyses ............................................................................................................ 24

3.1 Standards and guidelines ................................................................................................. 24

3.2 Shear strength parameters .............................................................................................. 25

3.2.1 Cover system materials ............................................................................................ 25

3.2.2 Natural alluvium ....................................................................................................... 25

3.2.3 CCL, advection and basal clay layers ........................................................................ 26

3.2.4 Residual shear strength ............................................................................................ 27

3.2.5 Bedrock ..................................................................................................................... 27

3.2.6 Overburden rock ...................................................................................................... 27

3.2.7 Summary of adopted parameters ............................................................................ 29

3.3 Pore pressure characterisation ........................................................................................ 30

3.3.1 Conceptual pore pressure model ............................................................................. 30

3.4 Design Sections ................................................................................................................ 31

3.4.1 Model framework ..................................................................................................... 32

3.4.2 Precursor conditions ................................................................................................ 39

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3.4.3 Adopted parameters ................................................................................................ 39

3.4.4 Pore pressure model setup ...................................................................................... 40

3.4.5 Discussion of pore pressure response...................................................................... 41

3.5 Seismic loading ................................................................................................................. 42

3.6 Analysis methods ............................................................................................................. 42

3.7 Stability model scenarios ................................................................................................. 43

4 NOEF Analysis results ............................................................................................................... 44

4.1 General ............................................................................................................................. 44

4.2 Results .............................................................................................................................. 44

4.2.1 Pore pressure ........................................................................................................... 45

4.2.2 Overall slope stability ............................................................................................... 46

4.2.3 Cover system stability .............................................................................................. 46

4.2.4 CCL permeability sensitivity ..................................................................................... 46

4.2.5 Seismic loading ......................................................................................................... 46

4.2.6 Residual shear strength ............................................................................................ 47

5 NOEF findings and considerations ........................................................................................... 48

5.1 Key findings ...................................................................................................................... 48

5.2 Required work program in preparation for implementation ........................................... 48

5.2.1 Sampling and testing ................................................................................................ 49

5.2.2 Design and analysis .................................................................................................. 50

5.3 Monitoring network ......................................................................................................... 51

5.3.1 Pore pressure monitoring ........................................................................................ 51

5.3.2 Settlement ................................................................................................................ 52

5.3.3 Infiltration rates ....................................................................................................... 53

5.4 Independent external review ........................................................................................... 53

6 In-Pit Dump .............................................................................................................................. 53

6.1 Background....................................................................................................................... 53

6.2 Description of design and construction ........................................................................... 54

6.3 Design analysis ................................................................................................................. 55

6.3.1 Stability model setup ................................................................................................ 57

6.4 Discussion of results ......................................................................................................... 60

6.5 Ground control management plan (GCMP) considerations............................................. 61

6.5.1 Monitoring ................................................................................................................ 62

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6.5.2 Geotechnical management ...................................................................................... 62

7 Long-term stability – Open Pit ................................................................................................. 62

7.1 Background....................................................................................................................... 62

7.2 Current geotechnical design ............................................................................................ 64

7.3 In-pit tailings disposal ....................................................................................................... 67

7.4 Ongoing design program and recommendations ............................................................ 68

8 References ................................................................................................................................ 69

9 Limitations ................................................................................................................................ 70

List of Tables

Table 1 Overburden types permitted in NOEF zones......................................................................... 8

Table 2 Shale overburden historic strength parameters (URS 2008)................................................ 9

Table 3 Geotechnical specification for CCL ..................................................................................... 10

Table 4 Indicative geotechnical testing frequency for CCL construction ........................................ 11

Table 5 Erosion Classification (after O’Kane 20161) ........................................................................ 22

Table 6 Adopted growth media by slope aspect (after O’Kane 20161) ........................................... 23

Table 7 Design Criteria .................................................................................................................... 24

Table 8 UQ recommended strength parameters ............................................................................ 28

Table 9 Summary of adopted parameters for stability analyses .................................................... 29

Table 10 Pore pressure model boundary setup approach .............................................................. 41

Table 11 Pore pressure modelling, lower boundary condition. ....................................................... 41

Table 12 Base case summary of results (FOS) .................................................................................. 44

Table 13 Base case summary of results for critical seismic coefficient (Ky for FOS=1) .................. 45

Table 14 Sensitivity cases - liner conductivity– summary of results (FOS) ..................................... 45

Table 15 Indicative foundation sampling schedule .......................................................................... 49

Table 16 Base case stability model parameters .............................................................................. 58

Table 17 Summary of IPD stability assessment results ................................................................... 61

Table 18 Design slope configuration parameters (modified after PSM, 2011) ............................... 66

List of Figures

Figure 1 Overall site layout (courtesy of Metserve/MRM) ............................................................... 2

Figure 2 Stages of the current NOEF ................................................................................................. 4

Figure 3 Location of geotechnical and hydrological sampling locations around the planned NOEF 7

Figure 4 An example of MS-NAF “Shale” rock ................................................................................... 9

Figure 5 An example of MRM “breccia” rock. ................................................................................. 10

Figure 6 1% AEP McArthur River flood extents (after WRM 2017) ................................................. 12

Figure 7 Hydraulic conductivity ranges by unit (after KCB 2016) .................................................... 14

Figure 8 Simplified scientific discipline design process (modified after MRM, 2015) .................... 15

Figure 9 Generalised stability section geometry with key features ................................................. 18

Figure 10 NOEF design with indicative stage boundaries (2016 aerial photo background) ........... 20

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Figure 11 Drainage plan (after O’Kane 20161) ................................................................................ 21

Figure 12 Foundation drainage system layout (KCB 2016) ............................................................. 24

Figure 13 Summary of triaxial test results from various foundation and clay resource programs. 26

Figure 14 UNC and UQ data points with recommended shear strength envelopes. ...................... 29

Figure 15 Conceptual model of CCL and cover system of the slope during the wet season .......... 31

Figure 16 Location of 2D stability analysis sections ........................................................................ 32

Figure 17 Section A-A (5600N) ....................................................................................................... 34

Figure 18 Section B-B (4400N)......................................................................................................... 35

Figure 19 Section C-C North (6800E) ............................................................................................... 36

Figure 20 Section C-C South (6800E) ................................................................................................ 37

Figure 21 Section D-D (oblique NW sector) .................................................................................... 38

Figure 22 Water content retention curves (after O’Kane 20163) .................................................... 39

Figure 23 Permeability functions (after O’Kane 20163) .................................................................. 40

Figure 24 Maximum Design Event chart for yield coefficient vs deviatoric displacement. ............ 47

Figure 25 Satellite monitoring with SqueeSARTM monitoring system (http://tre-

altamira.com/mining/#pit-monitoring) ........................................................................................... 52

Figure 26 North IPD at completion of the final design .................................................................... 55

Figure 27 Conceptual IPD location with stability sections. ............................................................. 56

Figure 28 Section A – Oblique section through North IPD with pit wall foundation ...................... 56

Figure 29 Section B – Oblique section through North IPD with pit wall foundation ...................... 57

Figure 30 Northern Slope- Western Fold Zone Domain (Domain 1, after PSM, 2011) ................... 58

Figure 31 Section A - Slide model setup .......................................................................................... 59

Figure 32 Section B – Slide model setup .......................................................................................... 59

Figure 33 Section B – Example of stability analysis results, filtered for FOS<2 ............................... 60

Figure 34 Section A – Example of stability analysis results, filtered for FOS≤2.0 ........................... 61

Figure 35 Mine design stage progressions ...................................................................................... 63

Figure 36 Design structural domains (PSM 2011) ........................................................................... 65

Figure 37 Conceptual model for each design domain - ore zone shown in red (after PSM 2011) . 66

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1 Introduction

1.1 Background

The McArthur River Mine (MRM) is currently operated as an open cut zinc and lead mine located

approximately 700 kilometres south-east of Darwin and 90 kilometres south-west of the MRM’s

Bing Bong concentrate port facility in the Gulf Region of the Northern Territory.

The mine was initially operated as an underground operation from 1995 to 2006 processing up to

1.8M dry metric tonnes per annum (DMT/a) of ore. Phase 2, which commenced in 2005 with the

transition to an open cut operation, increased output to the rate of 2.5M DMT/a open cut.

In 2013, with an anticipated shortfall in the worldwide future production of zinc, MRM expanded

the operation from 2.5M DMT/a to 5.5M DMT/a. This “Phase 3” expansion, with the increase in

production rate, lowered the mining cost and increased the volume of ore that could be

economically extracted. This increase also increased the volume of overburden that needed to be

extracted and stored.

MRM are preparing an Environmental Impact Statement (EIS) as part of an approval process due

to a change in waste classification. Under a new proposal, overburden material from the open cut

operation is no longer planned to be stored in external East Overburden Emplacement Facility

(OEF) and South OEF destinations. The revised mine plan places the majority of overburden in a

modified North Overburden Emplacement Facility (NOEF), with the remainder in smaller EOEF

and SOEF dumps and an in-pit dump. To limit the extent of potential impact, the proposed NOEF

design includes a height increase from approximately 80m to a maximum of 140m above natural

ground level, with a similar footprint to that presented in Phase 3.

The site layout is graphically shown in Figure 1.

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Figure 1 Overall site layout (courtesy of Metserve/MRM)

The overarching objective of the NOEF is to provide a safe and secure facility for the storage of

potentially chemically reactive overburden. To achieve this objective, it must be both physically

and chemically stable through both the operational period (approximately 15 years) and post-

closure (1,000 years plus). The cover system of the NOEF is a key element in limiting oxidation and

water ingress, and comprises a 0.5m thick compacted clay liner (CCL) within approximately 1.6m

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of the external surface of the dump. Therefore, the geotechnical stability of the NOEF is important

in attaining both physical and chemical stability objectives.

The open cut has been planned to extend the current excavation from approximately 160m deep

to 420m deep over 20 years. Overburden mined from the open cut operations is planned to be

exclusively placed in an in-pit dump (IPD) in the last six years of the mining operation. Following

the cessation of mining, the tailings will be reprocessed, with the spent tailings deposited in the

mined-out void over a ten year period. Additionally, non-benign materials reclaimed from the

EOEF, SOEF and general rehabilitation of operational areas will be placed in the open cut void.

After partial backfilling, the open cut void remaining would be approximately 175m deep, and

then filled with water over approximately five years to become a mine pit lake. The lake is

planned to be gradually opened up to the external environment over time as water quality

enables, with regular inflows of McArthur River floodwaters. This permanent lake would not be

legally accessible to the public, nor a resource for use by industry or landholders. Whilst the lake

will be open to the external environment, barrages at the entrances to the lake will repel most

large fauna. Thus, the shallow lake riparian habitat is not expected to be a major habitat in the

region for species of national significance. Note however, that should mine pit lake water quality

not meet the targets, then the mine levee wall surrounding the lake would not be breached (or

would be re-instated), and would be required to remain intact.

1.2 Scope and objectives

This assessment aims to address the following criteria for the NOEF:

Assessment of overall stability;

Assessment of proposed construction methods and their impact on stability, and

identification of areas where opportunity may exist for improvement;

Identification of limiting areas requiring further assessment; and

Recommend monitoring for validating the performance of the facility against the model.

The objectives of the open cut assessment with respect to geotechnical stability are to:

Describe the geotechnical aspects of the mine slope design;

Describe the geotechnical requirements to maintain a physically stable workplace during

the operations period, including around the IPD; and

Requirements to demonstrate a stable landform post-closure with limited risk of

catastrophic failure that may impede its function as a mine pit lake or jeopardise the

integrity of the mine levee wall.

The geotechnical related objectives of the IPD assessment are:

Appraise the viability of the concept to maintain a physically stable workplace for operations occurring on top of, and below the toe of, the IPD during the operations phase;

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Identify geotechnical sensitivities with the design; and

Describe the likely design, operational management and monitoring considerations.

1.3 Historic and other studies

For the NOEF, several site investigative studies have been undertaken to characterise the

foundation, the overburden and engineered materials.

The studies associated with the current Central West (CW) stage of the NOEF (i.e. the dump stage

being developed between 2016 and mid-2018, shown in Figure 2) have focussed on geotechnical

and hydraulic characterisation of the unconsolidated and poorly consolidated alluvial sediments

that overly the basement with the expressed aim of:

identifying areas of low strength that may compromise the OEF foundation;

investigation of the foundation for any preferential flow paths; and

locate sources of suitable clayey materials for use in compacted clay layers (CCLs).

Figure 2 Stages of the current NOEF

With the plan to increase the NOEF height from approximately 80 metres to up to 140 metres

above ground level (mAGL), further test work was undertaken to characterise the shear strength

properties of the various material types forming the internal core and near-surface structures of

the NOEF.

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In addition to comprehensive shear strength testing, detailed hydraulic assessment of the facility

has been completed to understand the hydraulic response of the NOEF cover system, facility core

and the underlying groundwater system.

The aforementioned studies are referenced throughout the text and detailed in the references

section of this document.

Geotechnical investigations and stability assessments have also been completed on the open cut.

The last major review was conducted as part of the Phase 3 EIS, with the work completed in 2012

(Soliman, 2012 and PSM, 2011). Note that the final open cut limits and geometry have not

materially changed from the Phase 3 work to the OMP EIS.

2 NOEF Assessment

2.1 Data sources

2.1.1 Overall

Data sources and types used in this evaluation include:

CW NOEF design, construction and CCL testing (MRM 2015);

University of Queensland (UQ) and University of Newcastle (UNC) testing (UQ 2016);

Dispersion testing and mineral geochemical classification (MRM 2015);

Klohn Crippen Berger (KCB) Groundwater Impact Assessment (KCB 2017);

O’Kane Consulting - Cover System and Landform Design (O’Kane1-4);

Geological mapping (MRM corporate dataset)

KCB site investigation for clay and alluvial materials (KCB 2016).

2.1.2 NOEF materials testing

a Alluvial foundation

Subsurface data has been collected for a range of purposes, including investigation and

construction of the CW (NOEF) stage and exploration for clayey alluvium for use in CCL

manufacture. Most recent studies include:

2004 Test Pits (Golders 2004) – 21 test pits investigating the general vicinity of the OEF.

2008 Test Pits (URS 2008) – 10 test pits investigating the OEF foundation.

2011 Test Pits (Hatch 2011) – 33 test pits investigating clay borrow sources around the OEF area.

2012 – 2016 MRM QA/QC Testing Register

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2013 Boreholes (Golders 2013) – 23 drill holes & geophysics conducted in the East and South East stages (refer to Figure 3) part of clay borrow investigations.

2014 Test Pits (Cardno/KCB 2015) – 63 test pits conducted throughout the CW footprint.

2014 Test Pits (WPROD, Red earth Engineering (REE) 2014) – 39 test pits conducted within the footprint of the future WPROD.

2014/2015 (KCB 2015) – borehole investigation for sampling and testing of potential clay construction materials.

2015 WPROD north test pits (REE 2015) – Test pits in the northern end of WPROD, CWD and EPROD.

2015 KCB/MRM (KCB 2015) in-fill sampling program in the NOEF clay borrow pits.

Figure 3 provides an overview of the location of sampling points from investigative phases

conducted over the last 8 years. The figure does not include the close spaced test pit locations,

such as those for QA/QC or CCL sourcing and dam construction, which also provide useful data

but limited to the specific purpose. The white outline defines the limits of the NOEF.

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Figure 3 Location of geotechnical and hydrological sampling locations around the planned NOEF

Detailed site specific geotechnical investigations are completed as part of the MRM detailed

planning process completed for all OEFs. These investigations will include the future NOEF

footprint. A description of the planned investigation and monitoring program is outlined in

Section 5.

b Overburden rock characterisation

The overburden rock is categorised using a geochemical classification system (MRM/KCB 2014)

based on the reactivity of the material when exposed to air and water. As the design of the

facility is fundamentally defined by geochemical considerations, the internal structures and layout

are broadly described with geochemical designations. The current classification subsets the

overburden rock into five categories (refer to Chapter 6: Waste characterisation):

Low Salinity Non-Acid Forming rock (High Capacity) [LS-NAF(HC)];

Metalliferous Saline Non-Acid Forming rock (High Capacity) [MS-NAF(HC)];

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Metalliferous Saline Non-Acid Forming rock (Low Capacity) [MS-NAF(LC)];

Potentially Acid Forming rock (High Capacity) [PAF(HC)]; and

Potentially Acid Forming rock (Reactive) [PAF(RE)].

All classes except the LS-NAF(HC) are considered to be environmentally non-benign, with only the

LS-NAF(HC) considered benign. Only benign materials may be left exposed to the external

environment in the long-term without being capped. Note, that alluvial material is classified as

LS-NAF(HC). The zones in the NOEF (Figure 7) where these materials may be placed is summarised

in Table 1.

Table 1 Overburden types permitted in NOEF zones

Zone Waste Classification

Preferred Also allowed

BASE - MS-NAF(HC) - MS-NAF(LC)

- Alluvium

CORE - PAF(HC)

- MS-NAF(LC)

- Alluvium - MS-NAF(HC)

PAF(RE) Cell - PAF(RE)

- PAF(HC) - MS-NAF(HC) - MS-NAF(LC)

- Alluvium

HALO - MS-NAF(HC)

- Alluvium

- MS-NAF(LC)

BATTER COVER - LS-NAF(HC)

PLATEAU COVER - Alluvium - LS-NAF(HC)

The geotechnical classification of the rock overburden consists of two categories: breccia; and

shale, and are further described below:

c Shale

“Shale” is a term used at MRM to describe well bedded siltstones and mudstones that

preferentially shatter along bedding and jointing planes. They are typically fine grained, well

bedded, jointed and fissile sedimentary rocks, comprised primarily of dolomite, with bituminous

material that has variable amounts of micro-crystalline pyrite laminae.

The lithostratigraphic units that belong to the Shale group include: UdH (Upper Dolomitic Shale),

UpH (Upper Pyritic Shale), BbH (Black Bituminous Shale), LpH (Lower Pyritic Shale), LdH (Lower

Dolomitic Shale) and WFS (W-Fold Shale). Rock from these units form the majority of the

overburden materials designated for storage within the encapsulated sections of the NOEF.

Figure 4 is a photograph of MS-NAF waste rock as tipped and dozed (but not graded).

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Figure 4 An example of MS-NAF “Shale” rock

Geotechnical characterisation of the shale overburden by field or laboratory testing is difficult due

to the size and range of clast sizes and variable composition of clast material types. Historically,

material strength criteria has been based on industry experience with similar materials (URS

2008), and described with Mohr-Coulomb strength criteria as shown in Table 2.

Table 2 Shale overburden historic strength parameters (URS 2008)

Material Unit Weight

(kN/m3)

Friction Angle deg.

Cohesion kPa

All Waste Rock 20 38 0

To improve understanding, large scale direct shear tests were completed at two university testing

laboratories; the University of Queensland (UQ) and the University of Newcastle (UNC). The shear

box apparatus at UNC is significantly larger than UQ, with the 720mm x 720mm x 600mm shear

box at UNC more than twice the size of the 300mm x 300mm x 200mm sample receptacle at UQ.

The comparison of results from both apparatus provided some insight into the relative bias

sample size may have on the results, with UQ results generally reporting slightly higher

comparative shear strengths. Although the comparative results were inconclusive, as other

differences such as machine stiffness influenced results (UQ 2016).

A range of materials to be stored in the OEF’s were tested with dry (UQ and UNC) and inundated

(UNC only) testing methods. There is a clear indication that friction angles in wet samples are

slightly lower compared with those obtained for dry samples (UNC tests only), though the

difference between the dry and wet test results were not significant in most cases. Further

discussion of the testing and results is presented in Section 3.2.6.

10 20 300 40cm

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d Breccia

The term “Breccia” is used to describe clastic sedimentary rocks composed of angular to

sub-angular, randomly oriented clasts of predominantly dolomites and shales cemented in a

carbonate matrix. The Breccias constitute the hardest and most competent rocks in the MRM

sequence. The results from the large scale direct shear testing (UNC) did not show a discernible

difference between the shale rock and breccia rock. However, testing is expected to

underestimate the material strength of the Breccia due to sample preparation where larger clasts

are scalped or resized to fit into the apparatus.

With a high proportion of large clasts, the test results reported from breccia samples are

potentially an underestimate. Breccia shear strengths are therefore likely representative of a

higher bound rock strength estimates. For stability modelling purposes by comparison, strength

parameters for shale have conservatively adopted the lower bound strength envelope for shale

waste and mid bound for breccia. The testing results and adopted strength criteria are discussed

in Section 3.2.6.

Figure 5 provides an example of weathered upper breccia rock sourced from within the mine

immediately beneath alluvial cover.

Figure 5 An example of MRM “breccia” rock.

e Compacted clay liners

An important feature of the NOEF is the compacted clay liners (CCL) that will encapsulate non-

benign materials stored in the inner zones of the dump, restricting the infiltration of rainfall and

air. The CCL is an engineered layer constructed from selectively sourced materials onsite that

conform to the criteria outlined in Table 3 (MRM 2015).

Table 3 Geotechnical specification for CCL

Property Requirement

Size of largest particle Not greater than 75mm

Minimum % by weight passing 37.5 mm AS 1152 sieve 90%

10 20 300 40cm

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Minimum % by weight passing 0.075 mm AS 1152 sieve 50%

Minimum Plasticity Index (AS 1289.3.3.1) 15%

Maximum Hydraulic Conductivity (Ksat) (AS 1289 6.7.3.5.1.1

1 x 10-9 m/s

The materials used to develop CCLs must not be significantly dispersive, including under the

effects of anticipated leachate waters. Dispersive clays pose a risk of piping failure if used in the

cover CCL. To manage the risk of dispersive clays being used in inappropriate zones, ongoing

testing of clay sources is conducted using waters that are representative of the likely solute

concentrations encountered within the final landform.

Uniformity of the CCL is important to its performance. To manage conformity, rigorous testing of

both the source and liner material will be performed to the sample frequencies outlined in Table

4. The sampling frequencies will be revised (increased or decreased) upon review of CCL

construction performance.

Table 4 Indicative geotechnical testing frequency for CCL construction

Test Type Clay Liner Borrow Area

Frequency Placed Clay Liner

Frequency

Particle Size Distribution

Atterberg Limit, including Linear Shrinkage

1 test per 5,000m3 or Soil Material

Change 1 test per 20,000m

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Emerson Class

Chemical Analysis (Exchangeable Sodium Percent and Sodium Absorption Ratio)

Pinhole Dispersion

1 test per 10,000m3 or Soil

Material Change 1 test per 20,000m

Moisture Content and Dry Density Ratio 1 test per 500m

Permeability/Hydraulic Conductivity test per 10,000m

In conjunction with the selection and preparation of CCL materials, a construction methodology

will be adopted that forms a stable and uniform subgrade prior to placement of the CCL, that is

uniform, well-graded, dense, compacted surface that is free from depressions, voids and

protrusions. This will thereby provide a stable base upon which to accurately construct the CCL

layer.

MRM have developed a proven methodology (MRM 2015), located several onsite clay sources

and are maintaining a QA/QC register for CCL manufacture and installation as part of the current

NOEF construction.

In addition to the CCL that encapsulates the entire structure as the lowest layer in the cover

system, several internal advection barriers will be placed during development of the facility.

These barriers will impede the flow of oxygen into the overburden during construction. These

layers, built from fine-grained alluvials, typically with more silt and sands than the CCL

specification, and are not built to the same criteria as the final cover CCL, but will undergo

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mechanical and/or moisture conditioning as required to target a placed air permeability of less

than 10-11m3/s/m2(O’Kane 2016).

2.2 Hydrology

The NOEF is located north of Surprise and Barney Creeks and west of Emu Creek. During

significant flooding of the McArthur River, the eastern, southern and to a lesser extent the

northern and western limits of the NOEF have the potential to experience flood waters against

the batters of the facility.

Figure 6 (WRM, 2017) provides an appreciation of the scale to which flood waters may encroach

on the margins of the NOEF in a 100 year flood event

Figure 6 1% AEP McArthur River flood extents (after WRM 2017)

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2.2.1 Foundation hydrogeology

The foundation hydrogeology beneath and surrounding the NOEF is described by KCB as

consisting of three main groundwater (GW) bearing zones: alluvium, weathered bedrock and

bedrock (fractured and intact). The GW zones are regarded as generally in hydraulic connection

with each other. In the vicinity of the NOEF, the alluvial unit is relatively thin in the northwest and

increasing in thickness towards the east. Given the distance from major drainage lines, the

alluvium is primarily clay dominated, with sandy lenses located along minor drainage lines.

Towards the eastern margins of the NOEF, substantially thicker (up to 15m (Golder, 2013)) zones

of alluvium are encountered.

The alluvium overlies the Reward Dolomite and Barney Creek Formation shales to the west of the

Western Fault, and Cooley Dolomite to the east of the fault (KCB 2016). The rock units are

variably weathered, particularly where faulted.

Groundwater flow across the site trends from west to east from an approximate elevation of 34m

AHD to 28m AHD. Groundwater heads within the weathered bedrock can rise 1 to 4 metres in

response to the wet season (KCB 2016).

An appreciation of the range in measured hydraulic conductivities is provided in Figure 7. It is

important to note that most of the data points are sourced from pump testing and therefore

potentially biased toward zones of preferential horizontal flow and may not be representative (i.e.

overestimate) of the overall unit or the vertical flow component.

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Figure 7 Hydraulic conductivity ranges by unit (after KCB 2016)

2.3 Design process

The NOEF design process requires integration of multiple disciplines. Mining considerations drive

the development of a cost effective structure that optimises the energy required to place the

overburden, while geotechnical factors optimise the stability of the final landform.

Where the overburden used to form the emplacement facility is geochemically problematic,

geochemical design features such as infiltration barriers, erosion mitigation, and store and release

cover systems may be employed. These systems often also require installation of engineered

materials to manage fluid and gas movement within the facility while mitigating erosion and

encouraging vegetative growth on the surface.

Figure 8 describes the overall design process with the three main disciplines associated with

design broadly outlined. For the NOEF, geochemical considerations are foremost, with water

management and geotechnical disciplines supporting the implementation of geochemical

requirements.

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Figure 8 Simplified scientific discipline design process (modified after MRM, 2015)

2.4 NOEF Design and construction methodology

The design and construction sequence for the NOEF was devised after numerous geochemical,

hydrological, geotechnical and geomorphological studies, technical workshops, assessments and

consideration of various design options.

2.4.1 Design

The key design aspects relevant to overall geotechnical design pertain to the overall shape,

internal layout of materials and structures designed to manage hydraulic flow.

The layout (shown schematically in Figure 9) consists of:

A maximum height of 140m.

A “trilinear slope” profile, where the gradient of the slope progressively steepens with

height in three stages, to simulate the mature profile of a naturally evolving landform

based on the following configuration:

o 2.5H:1V “Upper” slope → slope height (H) = 40 m, slope length (L) = 100 m;

Design

Objective

Design

Considerations

Water

Management

Surface Water

Contaminated Water

Seepage and Groundwater

Geotechnical

Hydraulic Conductivity

Foundation Stability

Geochemical

Material

Classification

Geochemical Design

Constraints

Surface Water

Diversions

Flood Water Management

Overburden Emplacement Facility Design

SettlementSlope

Stability

Submission of design to Independent engineer

Approval

Work order to construct

Commence QA/QC processes

Sign-off and design release

Issued for construction

Rev

isio

n r

equ

ired

16


o 3.5H:1V “Mid” slope → H = 50 m, L = 175 m; and

o 4.5H:1V “Lower” slope → H = 50 m, L = 225 m.

With a cover system characterised by:

A 0.5m CCL as the barrier layer to encapsulate the non-benign materials, a 2m thick

growth media layer (inclusive of a 0.5m drainage layer on the plateau area only) to aid in

establishment of vegetation and evapotranspiration, and 0.1m of topsoil on the final

surface. A drainage network would be built into the cover system to convey peak flows

from the dump surface down to the toe of the facility.

The cover system barrier layer is tied into the prepared foundation at the toe of the slope to restrict ingress by flood waters.

An internal structure that is encapsulated by the cover system is comprised of:

A nominal, 5-20m thick “Halo zone” of MS-NAF materials placed around the Core.

A Core zone that will store PAF(HC) and MS-NAF(LC) or better materials with advection

limiting construction, such as paddock dumping or in lifts of no more than 7.5m high with

fine-grained advection barriers.

PAF(RE) cells with numerous lower permeability advection barriers, located between

stages to limit oxidation and gas movement and assist in restricting fluid movement.

A Base zone, constructed as a 5m thick layer of MS-NAFs to provide a stable base of lower

geochemical reactivity below the Core;

A foundation:

With a foundation prepared to restrict infiltration of seepage waters and remove uncontrolled preferential pathways that do not assist the underdrain network.

Where, if insufficient in-situ clay thicknesses are encountered, a 0.5m thick basal CCL will be constructed.

With a network of lined drains, formed in the foundation and graded to direct drainage paths to seepage collection points, and where natural drainage lines that do not assist the intended flow direction, they will be removed and replaced with low permeability alluvium.

Where foundation preparation near the toe of the facility will remove low strength materials that could compromise stability.

Figure 9 provides a generalised two dimensional section of the NOEF used to form the geotechnical stability assessment models. It is considered a “worst case” scenario from a stability point of view, as it includes:

An alluvium advection layer between the Halo and Core zones. This layer will only be in

placed throughout a small portion of the NOEF. It will be installed if the cover system falls

17


behind the development of the core, to limit oxygen ingress into the core until the cover

system installation catches up.

Note that in the stability sections (Figure 17 to Figure 21), the basal CCL and MS-NAF base

is not shown as a discrete layer for simplicity, as the foundation alluvium has been

assigned CCL like properties, and Base with Core properties.

18


Figure 9 Generalised stability section geometry with key features

19


2.4.2 Construction methodology

Without geochemical restrictions, the most energy efficient way to build an OEF is to create an

expanding tipping face that balances height with distance from the source and limits the use of

ancillary equipment such as dozers and graders. With a stable, level and relatively dry foundation

the stability of the slope created is a function of the strength of the material forming it. With

competent rock, stable slope angles between 35 and 38 degrees are commonly formed, with a

vertical particle grading that coarsens with depth due to gravity separation.

However, the construction techniques to be utilised to construct the NOEF differ considerably

from conventional methods, with a carefully planned, engineered, and monitored approach

employed to manage the risks posed by oxidation of overburden. There are four fundamental

construction methods proposed to form the facility that may affect the strength and hydraulic

properties of the material.

1. Direct tip (over a tip head): Material is dumped from height over a tip head. This

technique has the effect of vertically size grading material, with the level of segregation

proportional to the height of the tip head. The possible result is an increase in material

strength and permeability with depth as coarser and more mechanically competent

material finds its way to the base of the pile. The degree of particle segregation is

proportional to the tip height; therefore low tip heights will have little to no particle

segregation.

2. Direct tip and doze (paddock dump): Material is tipped in a single lift (approximately 2m

high) and dozed level. This forms a relatively even particle grading of material with truck

rolled compacted surfaces that form a horizontal layering that is finer, thereby creating a

media with lower hydraulic permeability that potentially impedes fluid flow vertically.

3. Tip, doze and grade: A similar process to direct tip and doze with grading and more

accurate survey control to prepare a subgrade for liner placement or final landform. This

method may also utilise the addition of selected material, such as alluvials, to improve the

performance of the subgrade. Drains, high traffic routes and CCL subgrade will typically

use this method.

4. Source, condition, place and compact: This process requires the sourcing, manufacture

and placing of an engineered material such as a barrier layer (e.g. CCL). Careful material

selection, preparation and testing is required to ensure that the foundation is stable. The

material may also require rapid covering or ongoing maintenance such as irrigation for

moisture management.

Specific construction specifications include:

For the NOEF base, low tip heads (5m) and/or direct tip and doze will be used for the

construction;

For the NOEF core, two potential construction methods will be used, as they afford

equivalent geochemical controls over oxygen ingress:

o low tip heads (5.5m) over a direct tip layer (ca 2m) to form a 7.5m lift, in

conjunction with regular advection layers at every lift.

20


o Direct tip and doze in low lifts.

For PAF(RE) cells, all PAF(RE) will be placed by direct tip and doze in low (ca 2m) lifts.

The Halo will be constructed in lifts as required by the construction of the cover system,

to a maximum of 7.5m lifts.

2.4.3 Stage sequencing

The NOEF is staged with 7 phases progressively developing in a clockwise direction from the

current Central West (CW) facility. Advection barrier layers, consisting of fine alluvial material

covered by a layer of MS-NAF, are placed at the interface of each stage and additional horizontal

layers may be installed to reduce oxidation over wet seasons or if stages of the dump will be

dormant for extended periods of time. Representative advection layers have been incorporated

in the stability analyses.

Figure 10 NOEF design with indicative stage boundaries (2016 aerial photo background)

The staging allows progressive rehabilitation, so the performance of each stage can be assessed

and monitored as a final landform, enabling any improvements to be remedied and incorporated

into subsequent stages.

Stages1 and 2

Stage 3

Stage 4

Stage 5

Stage 6

Stage 7

21


2.4.4 Surface drainage and the Cover System

The NOEF drainage system design is a critical consideration to ensure the long term stability of the

landform. The drainage system needs to have enough capacity to prevent water “backing up”, or

mounding, in response to the design rain event, in areas where generation of excess pore

pressure could destabilise sensitive structures within the NOEF. For slope stability the drainage

considerations are:

Adequate crest drainage to prevent water ponding at, or near, the crest of the slope for

extended periods. This would reduce the risk of elevated pore pressures developing near

the crest, which have the potential to destabilise the slope.

The cover system media is of sufficient capacity to manage the store and release of water

along the slope, to limit excessive runoff that could scour the slope, and sufficiently low

permeability to limit infiltration that could create excessive pore pressures and

destabilise the cover system.

The drainage system of the NOEF has been devised to limit the crest catchment, by directing

runoff to one of six engineered drainage lines (Figure 11). Inter-slope drainage is also limited, as

the main trunk drains traverse obliquely down the slope, limiting the overall area of NOEF slopes

that are exposed to runoff from the entire length of the slope.

Figure 11 Drainage plan (after O’Kane 20161)

22


The drains are carefully engineered to ensure long-term performance. They will be rip rap

armoured, and lined with a low permeability liner system to enhance long term integrity and limit

infiltration into the underlying strata via the drain.

The cover system and growth media used for the plateau and slopes is also designed to limit

erosion and enhance long-term stability. Erosion modelling complete by O’Kane Consultants

(O’Kane 20161) assesses two materials suitable as growth media, namely alluvium and breccia.

An erosion classification system proposed by O’Kane describes 5 categories, shown in Table 5.

Given the importance of ensuring a stable cover system is developed, only erosion rates that do

not compromise performance are considered acceptable (i.e. very low or low).

Table 5 Erosion Classification (after O’Kane 20161)

Erosion Classification

Average Erosion

Rate (t/ha/yr)

Maximum Annual

Rate (t/ha/yr)

Maximum Discrete Rate by Position (t/ha/yr)

Effect on Sediment Yield

Effect on Cover System Performance

Very Low < 1 10 5 No effect on surface water sources

No effect on cover system performance

Low 1 to 10 100 50

Within capacity of sediment control measures to manage sediment yield to surface water sources to tolerable levels

No effect on cover system performance

Moderate 10 to 25 250 125

At capacity of sediment control measures to manage sediment yield to surface water sources to tolerable levels

Limited adverse effect on overall cover system performance, increased effect in areas of gully development

High 25 to 50 500 250

Greater than capacity of control measures, adverse effects on surface water sources

Minor adverse effect on overall cover performance, increased effect in areas of gully development

Very High > 50 > 500 > 250 Adverse effects on surface water sources

Substantial adverse effect on cover system performance

The potential erosion rates are influenced by vegetation, with substantially less erosion occurring

with even low levels of vegetation. However, despite revegetation of the cover, the NOEF

landform in its sub-tropical setting is not a geomorphically stable landform over a 1,000 year time

frame without intervention. MRM recognise this and have planned for an ongoing regimen of

landform and vegetation maintenance (referred to as Adaptive Management and Reactive

Management) to periodically, or on an as-needed basis, repair erosion and gullying.

Of the two growth media materials, alluvium is susceptible to excessive erosion and suitable only

for flatter areas such as the plateau. Breccia is well suited for all slopes with very little potential

for erosion, but at the expense of a lower likelihood for vegetation establishment and

evapotranspiration.

23


Table 6 summarises the results of erosion modelling completed by O’Kane and shows the

modelled erosive performance of material types by slope aspect and vegetative coverage. Cells in

the Slope Configuration column that are shaded green are to be used in the Project.

Table 6 Adopted growth media by slope aspect (after O’Kane 20161)

Slope Configuration Bare Surface

(No Veg.) (t/ha/yr)

Low Surface Coverage (t/ha/yr)

Medium Surface Coverage (t/ha/yr)

High Surface Coverage (t/ha/yr)

Alluvium

Base Case (Plateau) 1.2 0.5 0.1 0.1

Base Case (Full Slope) 54 35 19 1.0

Upper & Mid Slope (Above Ramp) 33 20 9.7 0.5

Mid & Lower Slope (Below Ramp) 27 15 7.0 0.4

Upper Slope 11 5.9 2.5 0.2

Lower Slope 14 7.1 2.9 0.2

Breccia

Base Case (Plateau) 0.1 0.1 0.1 <0.1

Base Case (Full Slope) 14 7.4 2.7 <0.1

Upper & Mid Slope (Above Ramp) 7.8 3.8 1.2 <0.1

Mid & Lower Slope (Below Ramp) 5.9 2.6 0.7 <0.1

Upper Slope 2.4 0.9 0.3 <0.1

Lower Slope 2.9 1.0 0.3 <0.1

Erosion has the potential to compromise the integrity of the NOEF if it were to breach the CCL and

allow significant amounts of water to infiltrate and potentially “wet up” portions of the facility.

This may trigger localised areas of instability, which in turn may alter drainage pathways,

increasing the exposed area and further exacerbating the potential for infiltration.

Given this sensitivity, a conservative approach is adopted where only breccia is used on slopes

and alluvium is only to be used on the plateau.

Cover system field trials will be required to calibrate the modelled material performance to actual

performance.

2.4.5 Foundation preparation

The foundation needs to be prepared to provide a stable and consistent surface to ensure that a

sound base is established. Foundation preparation will aim to prepare a surface free of topsoil

with limited organic material (including tree roots or stumps), and have soft and loose materials

worked or replaced.

A foundation drainage system will also be installed to manage any potential leachate from the

NOEF and aid in pore pressure management at the toe of the facility. An outline of the drainage

system, shown in Figure 11, is designed to direct foundation water to collection systems on the

24


eastern side of the NOEF. The foundation drainage system and its influence on stability

considerations is further discussed in Sections 3 and 4.

Figure 12 Foundation drainage system layout (KCB 2016)

3 NOEF Stability analyses

3.1 Standards and guidelines

In the absence of recognised standards for mine overburden storage structures such as the NOEF

in Australia, previous studies (URS 2008, UQ 2016) have utilised ANCOLD’s tailings dam

embankment guidelines for minimum design criteria.

Table 7 Design Criteria

Stability Assessment Case Assessments Stability Assessment Case Minimum

Acceptable Factor of Safety

Long term static (drained) Min FOS ≥ 1.5

25


3.2 Shear strength parameters

3.2.1 Cover system materials

The LS-NAF(HC) planned for use in the cover system is of the Breccia type. Breccia is regarded as

geochemically stable, free draining with limited fines and highly resistant to erosion. However,

given the slope cover is a fundamental component of the system protecting the CCL from erosion

and dehydration, a mid-bound overburden rock strength envelope has been adopted (Figure 14)

for the analysis works. As the Halo zone is constructed from similar materials to the Core, it is

assigned a conservative lower bound shear strength envelope (Figure 14).

3.2.2 Natural alluvium

Based on the previously described construction methods, triaxial test results from the 2013

Golder investigation (Golder 2013) have been adopted as representative of the strength

characteristics of the foundation.

Figure 13 presents a charted summary of triaxial test results of peak shear strength for clayey

samples taken as part of clay resource and foundation alluvium testing across the site. The chart

is not intended to show any correlation between effective friction angle and cohesion, but does

provide an appreciation of the spread of results and selected strengths for modelling. The

adopted values are considered to be lower-bound values for the site materials.

26


Figure 13 Summary of triaxial test results from various foundation and clay resource programs.

3.2.3 CCL, advection and basal clay layers

MRM have a proven site capability in constructing and installing CCLs at a large scale, as evident in

the current CW NOEF operations. CCL strength parameters have been adopted from the current

iteration of the MRM CW NOEF operations manual (MRM 2015).

Advection layers will be created from site sourced clayey alluvium materials, but may have a

higher sand and silt content. They will be up to approximately 1.2m thick, traffic and/or dozer

compacted, then covered by approximately 1.5m of MS-NAF material to limit erosion and

diffusion of the Core. They are not intended to be as impermeable as the CCL but are designed to

restrict air flow. Soft alluvial materials are not suitable for use in advection layers. For the

stability analysis, advection layers have been assigned the same strength parameters as the CCL.

This is regarded as a conservative estimate. With a potential for higher silt and sand content,

advection layers are likely to exhibit higher shear strengths than the cover system CCL.

27


3.2.4 Residual shear strength

Residual shear strengths can develop as a result of the soil particles undergoing displacement

(strain) such that soils with a relatively high content of plate-like particles (e.g. clays) experience a

realignment of those particles to be parallel with the direction of shear. Of the materials used to

construct the NOEF, the clay based units comprising the CCL, advection and basal clay layers are

regarded as having the highest risk (although still very unlikely) to develop softened or ultimately

residual shear strengths.

Estimating residual shear strength is difficult. The extent to which the clay layers weaken is a

function of soil properties. For example, where a higher proportion of larger particles (e.g. silt or

sand) exist the strength reduction may not be as significant. Soils with higher clay content could

experience significant reductions in shear strength. Empirically, soils with clay contents (particles

<2μm) greater than 50% may have a significant reduction and are prone to developing a “sliding

shear”. Soils with clay contents greater than 20% may develop residual shear strengths less than

the peak, while those with less than 20% are unlikely to exhibit significant reductions. (Fell and

Jeffery, 1987).

Particle Size Distribution (PSD) analysis of samples taken from site alluvial materials that have

been used to form clay layers typically show approximately 45% of samples are comprised of

particles greater than 0.002mm. Of the remainder, only 15% contain clay contents (<0.002mm)

greater than 50%, indicating that a majority of soils have substantial silt content.

The asperity of the interface between the CCL and the adjoining layers (breccia and halo), while

formed, rolled and stable, are likely to be relatively rough, with irregularities, that while

consistent with controlled formation of a continuous and regular layer, may not be amenable to

facilitate formation of a discrete and continuous slip surface over significant extents.

Based on a rationale recommended by Fell and Jeffery, an estimate of residual shear strength

based on triaxial test data considered an effective friction angle of 16° and cohesion of 0kPa

plausible for clay based units. However, the likelihood of these units developing softened

strengths post installation is considered to be very low. Further discussion on the application of

residual strengths is explored in Section 4.2.6.

3.2.5 Bedrock

The underlying bedrock consists of weathered to weakly weathered rock identified as the Cooley

and Reward Dolomites, and Barney Creek Formation dolomitic shales. Bedrock strengths have

been adopted for consistency with an estimate of overall bedrock strength derived from 2008 site

investigations.

3.2.6 Overburden rock

Studies conducted prior to 2016 have used widely recognised generic values for competent mine

spoil rock using a Mohr-Coulomb approach to characterise shear strength with values of 38

degrees for the effective internal angle of friction and zero for effective cohesion, with a unit

weight of 20kN/m3.

28


To improve the understanding of the shear strength properties of MRM overburden rocks, direct

shear testing was undertaken in 2015, using large scale direct shear box apparatus at the

University of Newcastle (UNC) and the University of Queensland (UQ). MRM commissioned

Prof. D Williams of UQ to report on the testing and recommend suitable parameters for use in

stability assessments of the expanded NOEF (UQ/Williams 2016).

Of note is the observation by Williams that the results of the shear box testing show no

discernible difference by material type. However, Williams recommended application of a range

of material strengths based on setting and sensitivity. Table 8 and Figure 14 summarise the UQ

recommended Mohr-Coulomb shear strength parameters from the results of both UNC and UQ

testing programs.

Table 8 UQ recommended strength parameters

Application Apparent cohesion Friction angle

Near the surface 50±25kPa 40±3 degrees

Within the overburden facility

100±50kPa 35±3 degrees

Rock/CCL interfaces 20±10kPa 33±3 degrees

For the purposes of NOEF stability modelling, lower bound parameter estimates have been

adopted to identify areas of potential instability and conservatively allow for parameter

uncertainty (see Table 9 for adopted parameters).

For applications where the construction life is limited or where the rock mass is actively

monitored and managed, such as working interfaces or in-pit facilities a mid-range curve is

adopted.

29


Figure 14 UNC and UQ data points with recommended shear strength envelopes.

3.2.7 Summary of adopted parameters

The adopted parameters are presented in Table 9 with scenario specific values presented on

modelled results in Appendix A.

Table 9 Summary of adopted parameters for stability analyses

Material Type (designation) Unit Weight

(KN/m3)

Cohesion (kPa)

Friction Angle (degrees)

Source

Breccia – cover 20 UQ mid-bound shear strength function UQ 2016

CCL drained 18 10 22 Golder 2013/GHD 2015

Advection layer 18 10 22 Adopted as

conservative after GHD 2015

Natural ground 18 10 22 Golder 2013

Weathered bedrock dolomite

20 100 35 URS 2008

Overburden (NOEF)*

Overburden (In-Pit) 20

UQ lower-bound shear strength function

UQ mid-range shear strength function UQ 2016

30


Sensitivity assessment parameters

Compacted growth media alluvial based cover

20 5 30 URS 2008 data

Residual strength

CCL-rock interface Advection layer interfaces

18 18

0 0

16 16

Estimate Estimate

* The overburden material type includes all broken rock material sourced from the mine. Termed in models with

descriptors PAF Cell, Core, Central West, Halo, R PAF).

The overburden rock, once emplaced, may undergo weathering effects over time. Degradation of

minerals could weaken the strength, but precipitation products may cement and strengthen the

mass (UQ/Williams 2015). Although as the facility is specifically designed to limit infiltration of

water and gas. Should weathering occur, it is expected to be spatially variable. Selection of a

lower bound strength envelope partially captures the potential degradation weathering may

have, but ignores any potential improvement due to cementation.

Shear strengths for alluvial based materials (CCL, advection layers, and foundation alluvials) have

been adopted from previous studies.

3.3 Pore pressure characterisation

An understanding of the transient behaviour of pore pressure is required to assess the NOEF slope

stability, as changes in pore pressure can affect material shear strength and therefore, slope

stability.

3.3.1 Conceptual pore pressure model

The CCL and associated cover system is designed to maintain a growth media and prevent

excessive infiltration into the overburden Core. For the underlying groundwater system, the

restriction of infiltration significantly reduces the gradual rise of groundwater levels (i.e.

mounding) beneath the facility, as a significant volume of water that would normally percolate

through, is shed from the facility via down-gradient foundation drainage lines. However, with the

infiltration reduction, the cover system is required to manage large volumes of water in intense

rain events. This introduces an elevated risk of pore pressure within the CCL driving instability of

the batter cover system and potentially the Halo and associated advection barriers.

Potential development of pore pressures in the waste dump as a result of rainfall percolation or

flooding was investigated by O’Kane Consultants (O’Kane 20163) in association with the cover

system design (O’Kane 20162). The O’Kane conceptual model and parameter estimates have been

adopted and incorporated into this revised stability assessment.

A schematic of the conceptual model for the NOEF batter is provided in Figure 15 to simply

outline the conceptual behaviour of rainfall during a wet season event. It is shown that rainfall

saturates the cover, and once saturated any additional water reports to surface runoff, flowing

down slope to the ground or intercepted by the lined haul ramp drains. Note that frequent

saturation of the cover also maintains the required moisture content of the CCL to retain its

function as a low permeability water and oxygen barrier.

31


Figure 15 Conceptual model of CCL and cover system of the slope during the wet season

The maintenance of a saturated or partially saturated cover, while fundamental to the

geochemical performance of the CCL, introduces an elevated pore pressure within the cover

system that may pose a risk of instability. This mechanism also has the potential to occur within

and above advection layers.

At its foundation, the majority of the NOEF is underlain by thin (<4m) layers of alluvium, with the

exception of the eastern extents where significantly thicker zones of clay (up to 15m), have been

encountered (Golder 2015). The alluvium, comprised predominately of silty clays, may act as a

hydraulic barrier for any water percolating through the NOEF. If unable to drain via the

underlying groundwater system, elevated pores pressures may develop at the base, which if

allowed to build significantly at the toe, could pose a risk for localised instability at the toe. The

foundation drainage system is designed to mitigate this mechanism.

The groundwater system beneath the facility is not expected to mound significantly in the long

term, as recharge via vertical infiltration into the facility will be very limited under the expected

cover system performance, and the construction of the sub-soil drainage system beneath the

NOEF will reduce the potential for a build-up of water in the base. There may however be short

term mounding during construction until the cover system is in place.

3.4 Design Sections

A total of five sections were selected for analysis. Sections A-A, B-B, C-C South and D-D were

selected by the MRM multi-disciplinary technical team as representing the highest risk for

instability. Section C-C North has been added to provide a representative section through the

northern slope and complete a full transect through the facility when combined with C-C South.

Cover (Breccia)

Rainfall

CCL

Halo (MS-NAF’s))

Advection Layer

Core

32


The sections are also on the same alignment as previous studies completed by O’Kane (O’Kane

20163).

Figure 16 presents a plan view location of the five sections against a June 2016 air photograph of

the site, and Figure 17 to Figure 21 show sectional outputs from Slide 7TM of the 5 stability models

with the adopted material strength parameters for the base case simulations.

Figure 16 Location of 2D stability analysis sections

Assessment of interim stage designs has not been completed, as overall inter-stage slopes

typically have slopes of 1:4.5 or shallower. With their limited life at that geometry, they are not

regarded as a significant slope stability risk. However, their stability should be assessed as part of

the detailed design phase, which should also consider the final construction schedule and detailed

drainage design.

3.4.1 Model framework

Figure 17 to Figure 21 show the general layout and boundary conditions for each of the sectional

models. Note dimensions and model details have been omitted for simplicity but are presented in

Appendix A.

33


Internal complexity associated with stage boundaries, the Basal CCL, 5m MS-NAF Base and

individual lifts within the core are not specifically shown in the model sections as their properties

are captured within the model generalisation. For example, the Basal CCL and foundation

alluvium have been assigned the same properties. A similar approach is adopted for the MS-NAF

Base with the Core, PAF(RE) cells and Halo all assigned the same properties. The advection layer

between the Core and Halo is an exception to this approach. It is modelled as a complete layer to

simulate a “worst case” scenario where a complete advection layer is required due to delays in

forming the outer Halo and cover.

All models have boundaries with values adopted from O’Kane cover system modelling (O’Kane2

and O’Kane3) and WRM flood modelling:

A surface vertical infiltration rate of 2.16x10-4m/day or ~79mm/yr for steady state

simulations, to simulate the net infiltration (precipitation minus evapotranspiration).

A simulated extreme wet season with approximately 13mm/day for 180 days.

Groundwater levels are modelled to within 1m of historic groundwater highs, with the

highest level applied as a total head boundary condition at the base of the model (KCB

2016).

Lateral boundary’ conditions are simulated as no flow boundaries.

A seepage function was applied to all upper boundaries, preventing pressures above the

natural surface developing at the surface. This assumes that the cover system surface

will be free-draining.

34


Figure 17 Section A-A (5600N)

35


Figure 18 Section B-B (4400N)

36


Figure 19 Section C-C North (6800E)

37


Figure 20 Section C-C South (6800E)

Vertical infiltration (rainfall)

Total head (groundwater)

38


Figure 21 Section D-D (oblique NW sector)

Total head (groundwater)

Vertical infiltration (rainfall)

39


3.4.2 Precursor conditions

The starting conditions for each of the models are:

Steady state simulations using the annual average rainfall and groundwater within 1m of

the natural surface at the toe of the facility.

Transient wet season simulations using the pore pressure response of the steady state

simulation as a starting condition.

3.4.3 Adopted parameters

Pore water pressures were simulated with steady state and transient 2D finite element modelling

with Slide 7.0TM. Saturated and unsaturated behaviour was modelled with the aid of water

content and permeability curves developed by O’Kane Consultants. O’Kane classified the units of

the NOEF into four categories:

Intermediate-textured alluvium (COVER);

Compacted clay layer (CCL);

Natural surface (NATURAL GROUND); and,

Waste rock (OVERBURDEN –PAF, NAF, CORE, HALO).

Figure 22 and Figure 23 show water content retention against matric suction and permeability

against matric suction respectively.

Figure 22 Water content retention curves (after O’Kane 20163)

40


Figure 23 Permeability functions (after O’Kane 20163)

In addition to the four categories, two additional classifications were included to incorporate

basement and advection units. Basement and advection layers were assigned the following

parameters:

Basement: Ksat = 1x10-7m/s, Kv/Kh = 0.5;

Advection layers: Ksat=5x10-8m/s (assumes compaction by overlying waste layers), Kv/Kh

= 1. Unsaturated conditions using a Van Genuchten “silty clay” type curve.

The basement typically consists of variably weathered dolomite, overlain by alluvial sediments

that primarily consist of silty to sandy clays with minor sands and gravels. A lower bound

estimate of weathered basement (KCB 2016) hydraulic conductivity was selected as a

conservative estimate.

Advection layers will be placed within the facility where interim surfaces are exposed for

extended periods, such as stage faces and between each lift within the reactive PAF cell. To

simulate the advection layers a Kv/Kh of 0.01 was applied to simulate advection layering within

this cell.

For all other units, a Kv/Kh of 1 was applied.

3.4.4 Pore pressure model setup

The pore pressure models were constructed to be consistent with parameters and scenarios

developed by O’Kane for the cover system design (O’Kane 20161&3). Table 10 outlines the model

41


boundary conditions. The groundwater height is an exception and it has been set to the highest

groundwater level intersected on section as interpreted by historic groundwater monitoring of

foundation strata (KCB 2016), summarised in Table 11.

Table 10 Pore pressure model boundary setup approach

Scenario Upper boundary

condition Lower boundary

condition*

Lateral boundary conditions

Steady State Annualised infiltration rate with

seepage face condition at 78mm/yr.

Fixed total head No flow

Transient (Steady state starting conditions)

6 month historic high wet season Daily infiltration rates infiltration rate with seepage face condition

at 13mm/day.

Fixed total head Identical to steady

state No flow

* Lower boundary fixed head conditions are conservatively placed to simulate a background water table just below the natural surface

at the toe of the facility.

Table 11 Pore pressure modelling, lower boundary condition.

Scenario Lower boundary condition level (mRL)

A-A 10024

B-B 10024

C-C North 10032

C-C South 10032

D-D 10036

The foundation drainage network has not been incorporated into the base case models. The

foundation drains are designed to reduce the potential for groundwater mounding within the

facility, particularly near the toe. Omission of the drains from the base case simulates a potential

long-term scenario where drains clog or loose effectiveness.

3.4.5 Discussion of pore pressure response

Pore pressure modelling indicates that:

The cover system CCL acts to restrict water from rainfall infiltrating into the NOEF.

The cover system on the slopes is effectively saturated in steady state runs and

completely saturated on both the plateau near the crest and slopes in wet season

simulations, thereby applying a positive pore pressure to the cover system in stability

models.

The advection layers above the core and below the PAF(RE) have the potential to impede

flow and could develop areas where saturated conditions occur.

Groundwater levels have the potential to mound within the facility, albeit modestly, and

are likely to be overestimated due to elevated starting conditions.

42


Negative pore pressures develop throughout the core of the facility.

Pore pressures with foundation alluvium beneath the facility have the potential to

increase above surrounding groundwater pressures if hydraulic conductivities are low

(~10-7m/s). Note that this observation does not include the mitigating effects of the

foundation drainage network.

The pore pressure simulations show that the cover CCL is likely to be saturated and/or maintain

high water contents.

3.5 Seismic loading

The site is located on the northern part of the Indo-Australian tectonic plate, regarded as a low to

moderate seismic region and at large distance from the plate margin. Geoscience Australia has

been collecting seismic data for the region since 1934. No events have been recorded with

epicentres within 150km of the site and events no greater than magnitude 4.0 within 300km of

the site.

The large scale lineaments (faults) have been identified within the geology of the project area.

Although these structures are regarded as aseismic, there is the possibility that if favourably

oriented with respect to the prevailing stress regime, reactivation could occur. Recent studies

regard the maximum credible earth quake within Australia’s stable continental regions as

between Mw 7.0 and 7.5±0.2 (Allen et al, 2011).

The landform response to an earthquake has been assessed in previous studies (URS 2008) with

application of a directional peak horizontal ground acceleration to simulate the seismic load of

0.05g (based on AS1170.4, Minimum Design Loads on Structures, Part 4: Earthquake Loads

adopted). With an acceptance criteria of factor of safety (FOS)>1.1 or seismic displacement

<0.5m (USACE, 1984).

Subsequent to these studies a probabilistic seismic hazard analysis (PHSA) was completed to

assess Tailings Storage Facility (TSF) stability (GHD 2015). The outcome of the study provided

recommended values for peak ground acceleration (PGA) and event magnitude for return periods

of 1,000 years to assess an operating basis event (OBE)), 10,000 years to represent a maximum

design event (MDE)) and maximum credible event (MCE).

These values have been utilised in application of a simplified method of estimating earthquake

induced deformation devised by Bray and Travasarou (2007). Application of this method is

described in Section 3.6.

3.6 Analysis methods

Consistent with previous studies, a 2 dimensional limit equilibrium slope stability tool (Slide 7.0TM

by Rocscience) was utilised to assess representative sectional models. Bishop simplified and

GLE/Morgenstern-Price methods of slices were adopted to calculate the Factors of Safety (FOS)

against both circular and path specified block (non-circular) failures. The finite element

groundwater package available in Slide 7.0TM was utilised to generate pore pressure grids for both

steady state and transient simulations.

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The potential response to seismic loading has been assessed utilising the seismic function in Slide

to determine the seismic yield coefficient (Ky) for limiting failure surfaces at a FOS=1. The

coefficient is subsequently used to estimate a deviatoric slope displacement using a simplified

method offered by Bray and Travasarou (2007):

ln(𝐷) = −0.22 − 2.83 ln(𝑘𝑦) − 0.333(𝑙𝑛(𝑘𝑦))2 0.566𝑙𝑛(𝑘𝑦) ln(𝑃𝐺𝐴) + 3.04(ln 𝑃𝐺𝐴)1 −

0.244(ln(PGA)))2 + 0.278(𝑀 − 7) ∓ 𝜀

Where

P(D=0) = probability (as a decimal number) of occurrence of zero displacement

D = seismic displacement in cm

Φ = standard normal cumulative distribution function

ky = yield coefficient in units of g

PGA=peak ground acceleration

ε = normally distributed random variable with zero mean and standard deviation σ=0.67

3.7 Stability model scenarios

The base scenario for all sections is considered representative of a conservative long term stability

case. It is defined by:

Drained soil shear strength parameters, to simulate long-term, post construction

conditions.

A foundation water table that is set as a total head boundary condition at the base with

levels to match historically high levels.

The foundation drainage network has been omitted in the base case.

Circular and block slip surface search methods with block search paths set to follow CCL,

advection and alluvial foundation layers.

Annual infiltration rates (78mm/yr) to simulate overall steady state pore water pressures.

Transient infiltration of 13mm/day for 180 days to simulate an extreme wet season

infiltration on the cover system – effectively fully saturating the cover system above the

CCL.

A number of ranged sensitivity scenarios were considered to examine the potential implications

for failure of key controls, undetected conditions and variation in parameter estimation:

Undrained conditions in the foundation and advection layers. Undrained conditions

could develop in the foundation alluvial and advection layers during, and potentially soon

after, construction if their permeabilities are low enough to prevent drainage. However,

with the majority of the NOEF foundation consisting of a relatively thin (<5m) alluvium

which is underlain by permeable weathered bedrock and a foundation drain network, it is

highly unlikely that undrained conditions will develop within the foundation to the full

extent of the footprint (if at all) during construction. This mechanism is not considered a

risk for long term stability.

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Use of alluvial based cover material as a substitute for breccia based media for the slope

cover system on all slope aspects.

CCL and advection layer permeability sensitivity, where greater infiltration rates could be

realised if the effective CCL and advection layer permeabilities are not achieved (i.e. more

permeable). Conceptually this allows more water to flow through the CCL and advection

layer into the core of the facility; however, given the permeabilities of the CCL and

advection layers are still relatively low, the majority of the additional water infiltrating

through the CCL on the batters is directed within the Halo to accumulate at the facilities

toe. Without the facilities foundation drainage system, and if unable to readily flow into

the underlying groundwater system, pore pressure could build within the facility, behind

the CCL.

Shear strength reduction (residual shear strength) between the overlying breccia cover

and CCL, and between the advection and basal clay layers, with surrounding rock, have

the potential to result in residual shear strengths developing, if subjected to post

installation displacements of sufficient magnitude (e.g. greater than 0.5m) to generate a

significantly lower strength than peak shear strengths measured with triaxial testing.

Slickensides have not been observed in test pits across the site and therefore residual

strength conditions have not been considered for foundation alluvials.

Lower bound strengths have been adopted as a base case for all rock overburden materials in all

scenarios.

Block and circular search functions were used for both base and residual strength cases. Paths for

the block searches were set to direct search paths within lower strength CCL, advection and

foundation layers.

4 NOEF Analysis results

4.1 General

The analyses are specifically intended to identify areas of sensitivity in the design to inform

detailed investigations, monitoring and management plans. Parameters selected for the base

cases are lower bound estimates from existing datasets and studies and as such are considered

conservative.

4.2 Results

The results of the cases are summarised in Table 12, with graphical results presented in

Appendix A.

Table 12 Base case summary of results (FOS)

Design acceptance criteria: FOS≥1.5

45


Section Base

Annual rainfall (Circular)

Base Annual rainfall

(Block)

Base case Wet season

(Circular)

Base case Wet season

(Block)

A-A 1.62 1.58 1.61 1.62

B-B 2.1 1.91 2.1 1.91

C-C north 1.95 1.85 1.95 1.84

C-C south 1.71 1.7 1.67 1.7

D-D 1.79 1.74 1.78 1.74

Table 13 Base case summary of results for critical seismic coefficient (Ky for FOS=1)

Section


(Circular) ms

-2


(Block) ms

-2

A-A 0.13 0.16

B-B 0.25 0.24

C-C north 0.21 0.19

C-C south 0.21 0.21

D-D 0.2 0.21

Table 14 Sensitivity cases - liner conductivity– summary of results (FOS)

Section

Clay layers (10E-8 m/s)

Annual rainfall (Circular)


Annual rainfall (Block)


Wet season (Circular)


Wet season (Block)

A-A with toe drains

1.63 1.58 1.62 1.62

4.2.1 Pore pressure

General observations of the pore pressure distributions indicate that:

The majority of the facility’s core develops suction pore pressures;

The cover system is typically simulated as saturated in the lower half of the slope for the

annualised case and, almost the entire slope simulated as saturated during wet season

case;

Advection layers have the potential to impede vertical flow such that positive pore

pressures could develop. Saturated conditions did not develop in any of the models

however, it could be possible during construction that “perched” water tables develop on

advection layer boundaries.

46


With a modelled low permeability foundation (Ksat~10-9m/s), and without foundation

drains, groundwater has the potential to mound within the facility post construction,

especially if groundwater tables rise close to the surface during construction.

A poorly performing foundation drain network could result in elevated pore pressures

within the halo, that if allowed to accumulate between the advection, CCL and

foundation layers could impact stability. However with an effective cover system

predicted FOS achieves design criteria (i.e. FOS greater than 1.5).

4.2.2 Overall slope stability

The base case modelling predicts that the overall slope is very stable with both circular and

directed block search analyses reporting FOS>1.5 for all sections, indicating long-term stability.

4.2.3 Cover system stability

The cover system stability is governed by the CCL shear strength, and more importantly the

erosive resistance of the growth media. Breccia was conservatively modelled with the UQ

mid-bound shear strength envelope for rock overburden. All sections report FOS greater than 1.5

for the base case. Sensitivity analyses performed with alluvium shear strengths applied to cover

materials did not achieve the minimum FoS, indicating that these materials are not recommended

for use on the batters.

4.2.4 CCL permeability sensitivity

Section A-A, with the thickest basement alluvials was selected to conceptually explore the

scenario of altered CCL permeability. Sensitivity runs with permeabilities an order of magnitude

higher than the minimum design acceptance criteria, for both the CCL and advection layers, show

potential for an increase in pore pressures at the toe of the facility when compared with the base

cases.

The increase in pore pressure could cause localised instability at the toe. However, with the

addition of a toe drainage system, situated within the foundation and the base of the facility with

collection drains on the perimeter, pressures do not excessively accumulate and modelled

stability results exceed acceptance criteria (i.e. FOS>1.5).

4.2.5 Seismic loading

At the Maximum Design Event(MDE) Peak Ground Acceleration (PGA) load calculated by GHD for

the region, potential deviatoric displacements as calculated with the Bray and Travasarou, (2007)

method, with yield accelerations (Ky) calculated from base case stability runs (see Appendix A) are

very low, at less than 2cm. This demonstrates a low risk of large deformations in the event of a

sizable earthquake and within tolerance for acceptance criteria adopted for previous studies (URS

2008). Notwithstanding this assessment, the monitoring plan described in Section 5.3.2 enables

timely, remote detection of problematic damage to the facility in the event of a significant

earthquake.

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Figure 24 Maximum Design Event chart for yield coefficient vs deviatoric displacement.

4.2.6 Residual shear strength

As described earlier in Section 3.2.4, residual shear strengths develop as a result of the soil

particles undergoing displacement (strain) such that soils with a relatively high content of plate-

like particles (e.g. clays) experience a realignment of those particles to be parallel with the

direction of shear. Circumstances applicable to the NOEF that could contribute to realisation of

this mechanism are:

Seismic activity where earthquakes of sufficient magnitude and ground acceleration cause

temporary instability to an extent that sufficient displacement occurs within the clay

based layers to develop residual strengths. Modelled displacements in the order of 10 to

50cm are typically considered worthy of further consideration.

Excessive erosion whereby a significant portion of material is progressively eroded to

undercut or undermine the slope. For the NOEF, undercut of the upper aspect of the

trilinear slope (i.e. where the slope gradient is 1V:2.5H or approximately 22 degrees)

where the breccia cover and CCL are eroded over a sufficient strike length so as to

undercut the overlying material and induce shearing at the CCL-breccia interface.

48


Of the units capable of developing residual strengths, the stability of the cover system CCL,

particularly the upper aspect of the slope, is sensitive. However, with successful implementation

of the design, where CCL moisture is managed through cover thickness and erosion limited

through careful installation of the breccia cover and engineered slope drainage (OKC 20162) and

ongoing maintenance, the trigger mechanism required cannot be realised with the exception of

an earthquake greater than the MDE (1:10,000yr), which is considered rare. Section 5.2.1

describes the sampling and testing program to support further assessment in preparation for final

detailed design.

5 NOEF findings and considerations

5.1 Key findings

The overall long-term stability of the facility as modelled with 2D limit equilibrium methods is

shown to be conceptually stable with proper foundation preparation, careful manufacture and

emplacement of materials, attentive monitoring and frequent review and reconciliation against

design assumptions and performance.

The cover system is sensitive to the strength and hydraulic properties of the growth media,

particularly in the upper third of the trilinear slope, where a strong and free draining growth

media is essential to protect the underlying CCL from breach caused by transient hydraulic loading

and erosion.

The hydraulic and geotechnical properties of the foundation and advection layers are important

for overall stability of the facility, particularly at the toe. Continued assessment of the foundation

permeability and hydraulics along with direct measurement of advection and CCL layers is

recommended to further assess transient pore pressure behaviour and calibrate the modelled

understanding.

5.2 Required work program in preparation for implementation

The current Central West NOEF (CW NOEF) facility is managed and monitored with a rigorous

testing and QA/QC program that is continually revised and improved. The change in design from

the current CW NOEF also requires a change in management and monitoring requirements.

This section describes the sampling, testing, monitoring and design recommendations to be

considered for the NOEF management plan. It is not intended as a detailed work program or

design for implementation and will require further development as part of a detailed design

process.

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5.2.1 Sampling and testing

a Foundation

Consistent with the current construction practices, foundation preparation will be required to

ensure the foundation is stable and of low risk for differential settlement. Continued

maintenance of the foundation geological model with testing and sampling of the foundation

materials will inform an understanding of the hydraulic and consolidative behaviour of the

foundation.

Ongoing evolution of the existing 3D geological model is required to expand and improve the

model, specifically:

A detailed representation of basement geology and hydrogeology that captures fault

zones and areas of preferential weathering.

A detailed representation of alluvial sediments prior and post foundation preparation, to

enable an understanding of the pre and post geomorphological system overlying the

basement.

Additional drilling, test pitting and geophysical surveys will be required to inform the model. This

is particularly important on the eastern slope where alluvial sediments are thickest.

Geotechnical sampling of representative foundation areas is recommended, with Table 15

adapted from the frequencies established for the current CW Stage construction (MRM 2015).

The sampling frequencies should be routinely reviewed as part of an overall site review process.

Table 15 Indicative foundation sampling schedule

Test Type CCL borrow area Placed CCL (m

3) or foundation

prepared (m2) sampling frequency

Particle Size Distribution

Atterberg Limit, including Linear Shrinkage



3

or 50,000m2

Emerson Class

Chemical Analysis (Exchangeable Sodium Percent and Sodium Absorption Ratio)

Pinhole Dispersion



3

or 50,000m2

Moisture Content and Dry Density Ratio

1 test per 500m3

Permeability/Hydraulic Conductivity Laboratory and (in-situ)

1 test per 10,000m

3 or

10,000m2 (50,000m

2)

Triaxial shear strength testing

Direct shear testing (for residual shear strength)

1 test per 50,000m

2

1 test per 50,000m2

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b CCL manufacture and post installation testing

MRM have a demonstrated capability in sourcing, manufacturing and placing CCLs. Processes and

quality programs already established for the CW NOEF should be applied to the expanded NOEF

cover system, with the addition of direct shear testing to complement triaxial testing and gain an

appreciation of likely residual shear strengths. Table 15 describes the recommended sampling

frequency.

In addition to the installation testing, occasional confirmatory post-installation in-situ

permeability is required to demonstrate the effectiveness of the CCL to limit infiltration and

maintain integrity. The CCL integrity is a function of the permeability and lateral continuity. The

lateral continuity is also a function of the placement process, with working widths requiring

interface points between stages that could develop linear zones of higher permeability as

moisture conditioning and placement will be hardest to achieve on the margins of each run.

Post-installation permeability of the CCL is a difficult parameter to measure directly over a large

spatial area as the size and sensitivity of the apparatus required is very difficult to install and

maintain. O’Kane Consultants (O’Kane 20164) describe in-situ permeability testing using a

borehole permeameter, which assesses the effectiveness of the manufacture and placement

process of each CCL run (vertical integrity), but may not capture the lateral integrity. If large scale

tests prove impractical, a close spaced linear network of borehole permeameters with moisture

and pore pressure monitoring may prove effective.

In addition to these tests, long term (6 months to 2 years) dispersion simulation testing to

complement Sodium Absorption Ratio (SAR) and Exchangeable Sodium Percentage (ESP)

laboratory testing with cover leachate waters are recommended to demonstrate the long-term

stability of the CCL and cover system.

c Cover system performance testing

Given the shear strength of breccia material easily exceeds requirements for stability in the cover

system geometry, the long-term stability of the cover system therefore relies on its resistance to

erosive mechanisms. Trial test sites are recommended to test and demonstrate the cover system

performance prior to expansive implementation.

O’Kane Consultants (O’Kane 20164) describe the requirements for erosion trials with a purpose

built field testing system consisting of an integrated weather station, sediment dam and

automated sensor system to measure the rainfall, runoff, interflow, moisture, pore pressure and

sediment loading. The data gathered from the system will provide a means to test and calibrate

the design parameters.

5.2.2 Design and analysis

To transition from the current CW NOEF design and construction process, a detailed design and

construction process is required, incorporating the requirements of:

The cover system design and monitoring plan;

Groundwater management and monitoring;

Interim stage and final design stability modelling; and

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Detailed monitoring and management plan.

5.3 Monitoring network

5.3.1 Pore pressure monitoring

Pore pressure behaviour associated with the installed fluid control layers (cover, CCL and

advection) and foundation is an important component of the NOEF construction.

O’Kane Consultants describe a comprehensive automated monitoring network for the cover

system inclusive of the CCL and underlying Halo (O’Kane 20164). This system is suitable to

monitor pore pressures that may develop within the cover system and act on the CCL.

In conjunction with the foundation permeability testing and modelling described earlier, a trial

vibrating wire piezometer array should be installed in an area where representative low

permeability alluvial sediments are within the foundation of the current CW NOEF. The array,

monitored while the facility is constructed, will provide insight into how the underlying

foundation drains and inform mine planning of the sustainable rates of vertical advance,

foundation consolidation rates and change in permeability. The results of the trial will also inform

of the requirement and location of additional foundation monitoring systems.

The trial array design consists of a section through the facility where a high rate of vertical

advance will occur, over relatively thick low permeability alluvials. An indicative array is

comprised of:

Three sites on a section from the centre (highest load) to the outer slope (toe);

Each site consists of four grouted vibrating wire piezometer gauges (e.g. Geokon 4500)

installed vertically within a borehole drilled below the base of alluvials and into the

weathered bedrock;

Gauges are sited within the borehole to measure pore pressures within representative

zones (e.g. weathered basement, lower alluvials and upper alluvials);

Gauge cabling is trenched and buried below the surface to prevent damage during

construction and terminated in a monitoring station;

The monitoring station can be automated with a similar system to the cover monitoring

network. Such a system constructed with Campbell Scientific components could consist

of a CR800, AVW200, AM16/32 and be telemetered via a 3G network or local short haul

radio network.

Elevated pore pressures could also accumulate at the toe of the facility where the cover system

CCL ties into the natural alluvial sediments of the foundation, potentially pressurising the toe of

the facility if foundation drains are not effective. Fully grouted, vibrating wire piezometer arrays

consisting of at least three gauges should be installed within the facility at approximately 50 to

100 metres from the toe of the facility with gauges sited in the alluvial foundation, Halo zone and

subgrade below the cover system CCL. At least two sites per slope (at least 8 sites) should be

installed, targeting areas where post closure interflow could be focussed (e.g. along stage

52


advection layers or relict paleo-drainage) before equilibrating with the underlying groundwater

system.

If toe pressures prove excessive, additional relief drains and associated collection systems may

need to be installed to alleviate pressures.

5.3.2 Settlement

The NOEF at its highest consists of up to 140 metres of overburden rock with alluvial foundation

thicknesses of up to 15m in the eastern extents. Most of the facility is expected to settle

relatively quickly during or soon after construction. However, should differential settlement occur

after construction of the CCL and cover system, the integrity of the system could be compromised

allowing higher than anticipated infiltration into the facility to occur.

Given the large area and difficulty predicting where differential settlement may occur, due to all

known mechanisms for differential settlement are removed in the design, a monitoring system

with the capability to monitor millimetre displacements over a long period of time with reliable

accuracy may be beneficial.

A network of survey surface monuments in conjunction with satellite, Interferometric Synthetic

Aperture Radar (InSAR) monitoring would enable detection of relative millimetric X,Y,Z movement

during and post construction without the need for site maintenance.

Figure 25 Satellite monitoring with SqueeSARTM monitoring system (http://tre-

altamira.com/mining/#pit-monitoring)

During construction, installation and maintenance of surface reflectors, in conjunction with site

managed survey monuments, provides data with a higher level of displacement accuracy that is

suitable for calibration of settlement models and effective demonstration of facility stability prior

to closure.

Monitoring frequencies are dependent on the satellite visit time, typically between 8 and 35 days

and with displacement accuracies at <1mm/yr and spatial accuracies of <10m for X-Y and <2m for

Z. Whilst it is not comparable with the spatial accuracy possible with site based survey techniques

or dedicated monitoring, it does allow for large areas to be assessed for relative displacement at

millimetre accuracy and provides a system for monitoring post closure that does not require site

maintenance.

A program of settlement monitoring in conjunction with cover CCL emplacement planning is

required on an early development stage to confirm that CCL placement has occurred after the

http://tre-altamira.com/mining/#pit-monitoring

http://tre-altamira.com/mining/#pit-monitoring

53


majority of settlement has taken place. Particular attention is required when progressing the

facility over former stage boundaries or where significant differentials in subsurface geology

occur. For example, where thicker alluvials are encountered in historic ephemeral drainage lines.

5.3.3 Infiltration rates

An understanding of infiltration rates is required for pore pressure modelling, an important

parameter in the slope stability assessment, but critical to long term geochemical performance of

the NOEF. O’Kane Consultants (O’Kane 20164) describe installation of a large scale lysimeter to

assess net percolation rates.

5.4 Independent external review

To ensure that the trial learnings and results from ongoing monitoring and assessment are

incorporated into the plan, the practice of having an independent, annual geotechnical review of

the design and monitoring program should be continued. This review will ensure that

geotechnical alignment with mine planning processes in the first 2 years of construction is

maintained, thereby reducing the risk of expensive rework through unrecognised implications of

change.

6 In-Pit Dump

6.1 Background

An In-Pit Dump (IPD) is planned for construction in the later stages of the mine life. There are

significant benefits associated with the development of this facility, including:

Reducing the volume and area/footprint disturbed by the external NOEF;

Reducing the amount of PAF stored in the NOEF;

Permanently storing non-benign materials in a sub-aqueous geochemically stable

environment;

Enabling closure and rehabilitation of the NOEF to occur during the mine’s operational

period;

Reducing quarrying/rehandling requirements for clean NOEF cover material; and

Reducing haulage requirements, with associated reductions in greenhouse gas

emissions.

The geotechnical related preliminary design objectives for the IPD are:

Appraise the viability of the concept;

Identify geotechnical sensitivities with the design; and

54


Describe the likely detailed design, operational management and monitoring

considerations required to maintain a physically stable workplace, for operations

occurring on top of, and below the toe of the IPD during the operations phase.

6.2 Description of design and construction

Several designs have been appraised to evaluate the volume of material that could be stored in

the in-pit facilities, however only the smaller and conceptually more stable design, the North IPD,

has been considered in this assessment.

Located against the northern and western walls of the pit, the dump is envisaged to be

constructed from the bottom up, with a ramp system developing up the face of the dump, as it

progresses in 16 metre lifts over a period of approximately 6 years in the final stages of mine life.

The design accommodates approximately 8Mm3 of overburden material, of which the majority is

MS-NAFs and PAF(HC) from the final stage of mining. During this final stage, some highly reactive

wastes will need to be mined: these are proposed to be hauled to the surface and placed in

alternative storages rather than being placed in the IPD, to reduce the risk of gas generation in

the open cut workings. They may be rehandled back into the IPD at the cessation of ex-pit

operations. However, small volumes of problematic material will be paddock dumped in the IPD

and immediately covered to limit oxidation of the material.

The design specifically aims to avoid dumping directly onto the footwall without buttressing

against the opposing hanging wall, thereby negating potential “daylighting” of the dump fill on

the footwall slope.

Figure 26 shows the location of the North IPD within the final mine design.

55


Figure 26 North IPD at completion of the final design

With construction from the “bottom up”, groundwater levels will need to be maintained below

the base of the mine while the dump is constructed to maintain access. Drainage off the IPD will

need to be managed to expediently direct rainfall runoff to mine dewatering systems, reducing

infiltration into the dump as much as practicable.

Periodic armouring of the southern interface between the IPD and the footwall may be required

to manage runoff and limit erosion from water directed off ramps and the footwall catchment.

6.3 Design analysis

Two sections were selected to assess the stability of the IPD. The sections were selected on

aspects that capture the steepest inter-ramp slopes, the steepest foundation aspect and the full

vertical extent. Figure 27 is a generalised plan view of the design sections with the IPD and final

mine design. Figure 28 and Figure 29 capture the spatial location of the sections.

56


Figure 27 Conceptual IPD location with stability sections.

Figure 28 Section A – Oblique section through North IPD with pit wall foundation

57


Figure 29 Section B – Oblique section through North IPD with pit wall foundation

6.3.1 Stability model setup

The two sections were analysed with the aid of a software tool, SlideTM v7.0, a 2D limit equilibrium

software package by Rocscience and by using GLE/Morgenstern-Price method of slices with a

circular failure and block path slope search.

The models were devised to include:

The pit wall as a foundation;

The IPD;

Internal advection layers to simulate the potential for clay based caps to be placed over

reactive materials if they need to be accommodated within the facility;

A water table with potential for 20 metres to temporarily pond at the bottom of the

dump. This simulates possible scenarios of short-term flooding due to operation issues

such as failed pumps or extreme storm events.

The pit slopes underlying the IPD are defined by:

The west wall that is mined as a dip slope (berm-less and following bedding), defined by

an east dipping W-Fold Shale with interbedded and weathered tuff bands.

The north wall, developed across strike in steeply south-east dipping interbedded shales

and breccias.

Figure 30 describes defect mapping completed by PSM (PSM 2011) of available exposures within

the domain that captures the northern slope.

58


Figure 30 Northern Slope- Western Fold Zone Domain (Domain 1, after PSM, 2011)

Given the absence of a dominant defect set within the pit slope that has the potential to

“daylight” in the foundation of the IPD, the shear strength model for the foundation (mine slope)

is characterised as a rock mass.

Rock mass parameters for each of the major structural domains were developed as part of

stability modelling for the current design by BFP (BFP 2004). Of the domains, the foundation of

the IPD is described by two domains, Hangingwall (HW) and Footwall (FW). Of the two, the FW

domain is the weakest and has been adopted as the foundation rock mass for the stability model.

The IPD rock mass is characterised by shear strength estimates described in Section 3.2.6 with the

mid-point selected, as the overburden will be mined and placed with little time for degradation

and the life span of the dump is limited to the operational life of the mining operation.

The interface of the IPD overburden and pit slope is assumed to be stable and hydraulically

relieved with an effective horizontal drainhole network. This reduces the risk of undrained

loading of tuff bands within the footwall.

A summary of the strength parameters used in the stability assessments is provided in Table 16.

Table 16 Base case stability model parameters

Material type Strength type Description Source

Core and PAF Shear/normal

function

Mid-point

Figure 14 UNC and UQ data points with recommended shear strength envelopes.

UQ 2016

Advection layer

Mohr-Coulomb

Unit Weight: 20 kN/m3 Strength Type: Mohr-Coulomb

Cohesion: 10 kPa Friction Angle: 22 degrees

URS 2008

Generalised Hoek-Brown Unit Weight: 27 kN/m3 BFP 2004

59


Rock Mass Strength Type: Hoek-Brown Unconfined Compressive Strength (intact): 80 MPa

m: 6 s: 0.0003

The stability model layouts are presented in Figure 31 and Figure 32. Internal PAF cells with 1

metre thick advective capping layers have been included in the lower extent of the dump at

nominally 50m vertical spacing. The cells are encapsulated within the dump, and have a cover

thickness of at least 20m, vertically and horizontally.

Figure 31 Section A - Slide model setup

Figure 32 Section B – Slide model setup

60


During the initial construction of the IPD, it is assumed active dewatering will be maintained at

levels below the base of the IPD to maintain access; however, the stability of the IPD is not

materially impacted by water below the tipping level. It is anticipated that the IPD rock mass will

be relatively free draining, with vertical restriction where PAF advection layers are located.

Drainage will need to be directed around the margins of the IPD, thereby restricting the volume of

water infiltrating through the rock mass, protecting the angle-of-repose crests from local

instability. However, it is anticipated that saturated conditions could develop within the IPD

during operation. To simulate the pore water pressure system, a water table profile that follows

the overall slope geometry close to the surface of the IPD has been incorporated into the stability

models. This approximation aims to represent a transient condition for the overall slope after the

wet season.

6.4 Discussion of results

General observations of the stability analyses show that the base case 2D limit equilibrium

stability results estimate limiting factors of safety at around 1.4 to 1.5. Individual batters define

the extent of the limiting slip surfaces. Larger scale limiting slip surfaces are defined by the IPD

and pit wall interface and show sensitivity to pore water pressure. Figure 33 and Figure 34 show

base case results with limiting slip surfaces. Slip surfaces with FOS greater than 2 have been

filtered from the results.

Figure 33 Section B – Example of stability analysis results, filtered for FOS<2

61


Figure 34 Section A – Example of stability analysis results, filtered for FOS≤2.0

A preliminary sensitivity test for block search and shear strength parameters was completed.

Table 17 summarises the stability results. A FOS of greater than 1.2 is considered appropriate for

the IPD where fixed infrastructure is not installed on or beneath the structure and FOS greater

than 1.3 where high exposure of personnel and or plant are likely (e.g. ramps). The table shows

that the IPD is conceptually stable under the range of expected parameters.

Table 17 Summary of IPD stability assessment results

Section Section A Section B Comment

Base case 1.59 1.45 Single batter

Path directed 1.6 1.39 Mid-slope where thinnest and steepest

foundation aspect

Historic (MRM, 2015) shear strengths for overburden shale c=0, phi=38, D=20kN/m

3

1.23 (1.01)

1.3 (Single batter < 1m thick)

Typically FOS >1.3 for > 1m thick slices

Sensitivity with cohesionless strength estimates (i.e. c=0) show batter scale FOS ~ 1. For angle of

repose batters. This is expected and with any hydraulic loading will be unstable, reinforcing the

need for very good drainage management.

6.5 Ground control management plan (GCMP) considerations

The IPD is a dynamically formed slope and ramp system with a high level of operational exposure.

Active movement with heavy equipment over the extent of the slope will make geotechnical

ground inspections and monitoring installation challenging.

It can be expected that areas of differential settlement will occur, although vertically advancing

with 16m lifts from the base will reduce the vertical settlement over the foundation. Higher rates

of differential settlement may occur where dumping over sharp vertical contrasts in the

foundation (i.e. open cut berm-batter crests). Control of water over these areas is very

62


important, as focussed infiltration of water could channel and/or redistribute fines along the

displaced path, creating a zone of weakness.

Notwithstanding that with good operating practices, pit wall mapping, monitoring and design

verification, the proposed IPD is predicted to be stable and will meet the stated objectives.

6.5.1 Monitoring

Standard open cut displacement monitoring tools are recommended for implementation of the

IPD option, consisting of:

Prism: Overall slope movement, sited on the IPD and in pit walls adjacent to the

advancing dump;

Radar monitoring: Focussed monitoring, particularly on footwall (western) interface.

VWP monitoring on any IPD advection layers and in the footwall selected clay or Tuff

layers beneath the foundation;

Visual observation of higher risk settlement zones and ramp crests (southern slope);

Visual observation of water related features such as: toe seeps, pooling water against

dump crests or against slope-dump interface.

6.5.2 Geotechnical management

The following management items are recommended:

Complete geotechnical detailed design with the as-built pit slope, slope pore pressure

monitoring data and updated waste and advection layer material shear strength

parameters.

Specific wet season drainage plans;

Advection cover standoff distances from southern slope; and

Schedule to ensure that overburden rock does not “daylight” into the pit and is always

self-buttressing.

7 Long-term stability – Open Pit

7.1 Background

The open cut has been planned to extend the current excavation from approximately 160m deep

to 420m deep over 20 years. The final limits of the design are unchanged from the Phase 3 design

assessed in 2011 by PSM.

63


The open cut slope design is currently planned as a series of hangingwall cutbacks. The cutbacks

advance from west to east, with the footwall (western slope) and part of the south wall, mined at

final limits from commencement. All other stage slopes form interim walls subject to further

cutbacks until the final crest limits are developed with Stage M. With the final pit deepening

further, but within the final limits defined by Stage M.

The Wozybun Quarry (WZQ) provides predominantly LS-NAF(HC) cover materials for the NOEF. It

is currently planned to be mined in at least two stages down to a depth of between 70 to 140m

depending on NOEF material requirements. The southern crest of the quarry wall will be offset at

least 50m from the levee wall.

This section simply summarises the current design and level of confidence with a brief outline of

the work program planned to calibrate the design.

Figure 35 Mine design stage progressions

Following the cessation of mining, the tailings will be reprocessed, with the spent tailings

deposited in the mined-out void over a ten year period. Additionally, non-benign materials

reclaimed from the EOEF, SOEF and general rehabilitation of operational areas will be placed in

the open cut void. After partial backfilling, the open cut void remaining would be approximately

175m deep, and then filled with water over approximately five years to become a mine pit lake.

64


The lake is planned to be gradually opened up to the external environment over time as water

quality enabled, with regular inflows of McArthur River floodwaters. This lake would not be

legally accessible to the public, nor available as a resource for use by industry or landholders.

Understanding the slope stability of the open pit, post-closure, is important for a number of

reasons:

Pit failure has the potential to undermine the levee thereby connecting the pit to the

surface water system associated with the McArthur River.

A failure of sufficient size could create a waveform in the pit void water body that results

in overtopping the pit crest and levee with void water.

The current levee is at least 250 metres from the majority of the final pit crest and at least 50

metres from the crest of the Wozybun Quarry.

7.2 Current geotechnical design

The current overall mine slope design is defined by nominated Inter-ramp Slope Angles (IRSAs) for

specified structural domains based on a combination of rock mass strength, defect strength and

geological structure.

Transient groundwater behaviour is also considered as it has the potential to influence the

effective shear strength of the material forming the slope. The design assumes that the rock mass

is effectively drained via drainholes, dewatering systems and/or surface water drainage

management (BFP 2004).

Figure 36 captures a plan view of the mine site inclusive of the mine levee wall with the extent of

the final design (without WZQ).

Figure 37 is a schematic of the conceptual geology with data confidence for each domain.

65


Figure 36 Design structural domains (PSM 2011)

66


Figure 37 Conceptual model for each design domain - ore zone shown in red (after PSM 2011)

Understandably, confidence is very good in the areas associated with the orebody where

underground mapping and the current mine exposures provide good and accurate data.

Progressing east, where the structural domains are poorly exposed in the current excavation or

within shallow cuttings such as the drainage diversion, the data set increasingly relies on drillhole

data. Figure 36 captures the collar and downhole traces of oriented diamond holes that have

been completed to collect data on structures and rock mass within the slopes of the eastern

stages.

The data collected provides enough insight into the nature of the rock mass to conservatively plan

slope configurations in the eastern, southern and northern domains.

Table 18 captures the recommended slope parameters currently in use for the stage and final

designs with a comment on the level of data confidence.

Table 18 Design slope configuration parameters (modified after PSM, 2011)

Domain Bench Angle

Height

(m)

Berm Width

(m)

Inter-Ramp Angle

Critical Mechanism Comment

2. Shallowly dipping Barney Creek formation

75° 24 10 60° Planar sliding fault Data is good – to very good.

3. Eastern Fold Zone 75° 24 18 50° Multiple Additional data to confirm IRSAs.

4. Steeply dipping bedding in eastern Fold Zone

60°

24 17

42° Planar sliding on

bedding

Additional data required to firm location and aspect of large scale structural

features. 16 10

5. Western Fault Drag Zone

60° 24 17

42° Planar sliding on

bedding

Additional data required to firm location and aspect of large scale structural

features.. 16 10

6. Cooley Dolomites 65° 24 20.5

42° Disperse set of

joints

Additional drilling recommended to confirm the presence and orientation of

major structures. 16 12.5

67


It is important to note that where final slopes are currently being constructed on the western

slope, data confidence is good. In all other domains where addition data is required, interim

slopes are being developed. The interim slopes present an excellent opportunity to gather very

good quality structural and performance data that serves to calibrate the geotechnical model and

raise confidence in subsequent stage designs and ultimately the final design.

The Wozybun Quarry is regarded as a modest extension of the current design and has been

conservatively designed with a slope configuration shallower than the current domain

(Geotechnical Domains 3,4 and 5) design specifications outlined in Table 18. The Quarry will be

developed in at least two stages. The internal stage boundaries have not be defined but will

adopt the current geotechnical design parameters for respective domains. Slope mapping and

performance assessments of the initial stage will provide data to inform validation of the final

Woyzbun Quarry design and guide installation of any additional geotechnical monitoring systems

to understand the long term interaction of the final slope and the levee system.

Currently the design is constrained to lower bound IRSAs (<3% probability of failure), regarded as

very conservative (PSM 2011).

7.3 In-pit tailings disposal

In-pit tailings disposal presents an opportunity to reduce the depth and area of the exposed

slopes, thereby reducing the overall long term risk of slope instability. There are however

operational slope stability considerations associated with utilising the pit as a tailings facility.

These are primarily associated with the change groundwater dynamics and introduction of fixed

infrastructure.

During mining the groundwater table is progressively lowered as the mine deepens. The stability

of the mine slopes is sensitive to pore water pressure, and considerable effort is spent draining

water from the slopes so as to depressurise the slope sufficiently. In-pit tailings disposal could be

viewed as the opposite of this process, where water contained in tailings is pumped into the void,

progressively raising the groundwater table within the void and the immediate surrounds.

The surrounding groundwater system is also recovering as the tailings are placed. The change in

groundwater dynamic could elevate pore pressures within the slope, particularly if passive

depressurisation systems such as historic horizontal drains are not effective. Rapid changes in pit

water level should also be avoided, particularly rapid drawdown of pit lake water, which if unable

to readily drain from the slope could generate inherently high slope pore pressures triggering

slope instability. Therefore the slope depressurisation monitoring system employed during

mining, will also be a fundamental management tool required for in-pit tails disposal.

Placement of infrastructure such as pipelines, lighting systems, reclaim pumping systems, decant

locations and access roads require careful consideration. The mine slopes are designed to

different levels based on the application. For areas such as berms and batters where there is low

exposure to personnel and plant a lower FoS is tolerated, whereas areas of higher exposure such

as ramps or where in-pit infrastructure is sited higher levels are adopted. The current in-pit

tailings conceptual plan places infrastructure in areas where the highest design rigour is applied,

i.e. haul ramps and life of mine slope limits. With the main discharge pipeline on the western

ramp, utilising the dip slope on the footwall slope to “run” the line to the base. To gain access to

68


the discharge point (if necessary), further work, informed by future slope performance is required

to install stable access. All slope monitoring infrastructure employed during mining will be

required during tails disposal, with consideration given to installation of additional automated

systems to offset the reduced direct observational monitoring performed by mining teams and

geotechnical staff during mining operation.

Overall the in-pit tailings concept presents an opportunity to improve the long-term mine slope

stability risk profile. The specific operational considerations associated with slope stability and

pore water management, and infrastructure placement will be best informed with the detailed

understanding of the slope gleaned from 20 years of slope and groundwater management.

7.4 Ongoing design program and recommendations

To finalise the design of the slope angles, a program of mapping, diamond drilling, pore pressure

monitoring and independent review is recommended in conjunction with development of Stage K

and prior to development of the Stage L crest, and also prior to commencement of the Woyzbun

Quarry second stage.

Approximately (but not limited to) eight geotechnical oriented boreholes are recommended to

assess the following:

• Define the Eastern Fold Zone (Domain 3),

• Define zones with steep bedding (Domain 4),

• Define the extents of the Western Fault Drag Zone (Domain 5),

• Confirm the orientation and type of defects within the Cooley Dolomite, and

• Confirm extrapolation of structural patterns south of the Woyzbun fault.

To assist validation of the pore pressure design inputs, a downhole pore pressure monitoring

network is also recommended to monitor the groundwater response to excavation and overlying

surface water interaction. Six to eight fully grouted VWP monitoring arrays with up to six gauges

per hole are recommended to be installed at key locations on the south, east and northern final

slope crest to monitor the groundwater pressure response to the advancing and deepening mine.

Progressive installation of shallower and less intensive (less than 100m deep with 3 VWP gauges)

on the western slope may also be installed to monitor the effectiveness of footwall drainholes.

Site locations should be finalised in conjunction with hydrogeological and geochemical

requirements.

These programs, together with detailed structural and geotechnical mapping of all exposed

slopes as they present, are considered important to form a dataset necessary to calibrate the

geotechnical model in a timely way so as to demonstrate a safe and effective long-term slope

design prior to constructing the final slope.

69


8 References

Allen et al, 2011 Development of the next generation Australian National Earthquake

Bray and Travasarou, 2007, Simplified Procedure for Estimating Earthquake-Induced Deviatoric

Slope Displacements.

Hazard Map. NZSEE PCEE 2011

BFP 2004, Geotechnical Study of the McArthur River Mine Open Pit, Internal.

Fell, R. and Jeffery, R.P. 1987, Determination of drained shear strength for slope stability analysis.

Soil Slope Instability and Stabilisation, Walker, B.F and Fell, R (eds), Balkema 53-70.

GHD 2015, Tailings Storage Facility Regional Seismic Hazard Assessment, Internal.

Golder 2004, Geotechnical Investigation Detailed Feasibility Study McArthur River Mine Expansion

Project. Internal.

Golder 2013, Geotechnical Investigation Report – Pilot Study for Material Classification – NOEF,

Internal.

Hatch 2012, Clay Borrow Investigation. Internal

KCB 2015, Clay Resource Estimate, Internal.

KCB 2016, Groundwater Impact Assessment, Internal

MRM 2011, Geotechnical – MRM Phase 3 Development, Internal

MRM 2015, Northern Overburden Emplacement Facility (Central West Phase), Internal.

O’Kane 20161, Landform Design in Support of the EIS Submission 750/15-01, Internal.

O’Kane 20162, NOEF Cover System and Landform Design, in Support of the EIS Submission,

Internal.

O’Kane, 20163, Description of pore-water pressure simulations completed for stability assessment

of North Overburden Emplacement Facility, Internal.

O’Kane 20164, NOEF Closure Monitoring System in Support of the EIS Submission, Internal

PSM 2011, McArthur River Mine Expansion Phase 3, Feasibility Study – Structural Analysis,

Internal.

REE 2014, NOEF Dams – EPROD Additional Geotechnical Data Report.

REE 2015, NOEF Dams – Clean Water Drain (CWD)Geotechnical Data Report.

REE 2015, NOEF Dams – WPROD Additional Geotechnical Data Report.

Soliman 2012, Geotechnical – MRM Phase 3 Development, Internal

UQ (Williams, DJ) 2016, McArthur River Mine Waste Rock Shear Strength, Internal.

URS July 2008, Overburden Emplacement Facility (OEF) Design Open Cut Project, Internal.

70


WRM 2017, Surface water impact assessment for the McArthur River Mine Overburden

Management Project Environmental Impact Statement.

9 Limitations

This document has been submitted by Pando (Australia) Pty Ltd (“Pando”) subject to the following

limitations:

This document has been prepared for the particular purpose outlined in Pando’s proposal and/or

agreed scope of work. No responsibility is accepted for the use of this document, in whole or in

part, in other contexts or for any other purpose.

The scope and the period of Pando’s Services are as described in Pando’s proposal/and or scope,

and are subject to restrictions and limitations. Pando did not perform a complete assessment of

all possible conditions or circumstances that may exist at the site referenced in the Document. If

a service is not expressly indicated, do not assume it has been provided. If a matter is not

addressed, do not assume that any determination has been made by Pando in regards to it.

Conditions may exist which were not detected given the scale of the enquiry Pando was retained

to undertake with respect to the site. Variations in conditions may occur between study locations,

and there may be special conditions pertaining to the site which have not been revealed by the

study and which have not therefore been taken into account in the Document. Accordingly,

additional studies and actions may be required.

In addition, it is recognised that time affects the interpretation and assessment provided in this

Document. Pando’s opinions are based upon data that existed at the time the data is analysed. It

is understood that the Services provided allowed Pando to form no more than an opinion of the

actual conditions of the site at the time the site was investigated and cannot be used to assess the

effect of any subsequent changes in the site, or its surroundings, or any laws or regulations.

Any analysis’s, designs, and advice provided in this Document are based on the conditions

indicated from published sources and the investigation described. No warranty is included; either

express or implied, that the actual conditions will conform exactly to the assessments contained

in this Document.

Where data supplied by the client or other external sources, including previous site investigation

data, have been used, it has been assumed that the information is correct unless otherwise

stated. No responsibility is accepted by Pando for incomplete or inaccurate data supplied by

others.

Pando may have retained sub consultants affiliated with Pando to provide Services for the benefit

of Pando. To the maximum extent allowed by law, the Client acknowledges and agrees it will not

have any direct legal recourse to, and waives any claim, demand, or cause of action against,

Pando’s affiliated companies, and their employees, officers and directors.

71


This Document is provided for sole use by the Client and is confidential to it and its professional

advisers. No responsibility whatsoever for the contents of this Document will be accepted to any

person other than the Client. Any use which a third party makes of this Document, or any reliance

on or decisions to be made based on it, is the responsibility of such third parties. Pando accepts

no responsibility for damages, if any, suffered by any third party as a result of decisions made or

actions based on this Document.

Appendix A

0.211

0.247

0.239

0.2110.211

0.247

0.239

0.211

Critical Seismic Coefficient0.0000.0120.0250.0370.0500.0630.0750.0880.1000.1120.1250.1370.1500.1620.1750.1880.2000.2120.2250.2370.2500.2630.2750.2870.300+

Pore Pressure[kPa]

-100.000-80.000-60.000-40.000-20.0000.00020.00040.00060.00080.000

100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000 340.000 360.000

10450

10400

10350

10300

10250

10200

10150

10100

10050

10000

9950

3500 3550 3600 3650 3700 3750 3800 3850 3900 3950 4000 4050 4100 4150 4200 4250 4300 4350 4400

Company MRM/PandoScale As ShownDrawn By SBAnalysis NOEF Stability block searchDate December 2016

Descrip onSec on D-D West : Annual Rainfall : Block searchSeismic Ky (FOS=1)

Project MRM NOEF EIS

SLIDEINTERPRET 7.021

0.2050.2050.2050.205


Pore Pressure[kPa]

-120.000-100.000 -80.000 -60.000 -40.000 -20.000 0.000 20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000 340.000

10500

10400

10300

10200

10100

10000

4200 4300 4400 4500 4600 4700 4800 4900 5000 5100 5200 5300

Company MRM/PandoScale As ShownDrawn By SBAnalysis NOEF Stability Block SearchDate December 2016

Descrip onSec on C-C South : Annual rainfall : Block searchSeismic Ky (FOS=1)



0.210

0.299

0.221

0.2100.210

0.299

0.221

0.210


Pore Pressure[kPa]

-120.000-100.000-80.000-60.000-40.000-20.0000.00020.00040.00060.00080.000

100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000 340.000

10500

10400

10300

10200

10100

10000

4200 4300 4400 4500 4600 4700 4800 4900 5000 5100 5200 530


Descrip onSec on C-C South : Annual rainfall : Circular searchSeismic Ky (FOS=1)



0.187

0.232

0.283

0.285

0.266

0.1870.187

0.232

0.283

0.285

0.266

0.187


Pore Pressure[kPa]

-120.000-100.000 -80.000 -60.000 -40.000 -20.000 0.000 20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000 340.000

10500

10400

10300

10200

10100

10000

9900

5600 5700 5800 5900 6000 6100 6200 6300 6400 6500 6600

Company MRM/PandoScale As ShownDrawn By SBAnalysis NOEF StabilityDate December 2016

Descrip onSec on C-C North : Annual rainfall : Block searchSeismic Ky (FOS=1)



0.21

0.28

0.29

0.25

0.21

0.210.21

0.28

0.29

0.25

0.21

0.21


Pore Pressure[kPa]

-140.000-120.000-100.000-80.000-60.000-40.000-20.0000.00020.00040.00060.00080.000

100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000

10500

10400

10300

10200

10100

10000

9900

5600 5700 5800 5900 6000 6100 6200 6300 6400 6500 6600 6700


Descrip onSec on C-C North : Annual rainfall : Circular searchSeismic Ky (FOS=1)



0.240

0.254

0.2480.2400.240

0.254

0.2480.240


Pore Pressure[kPa]

-140.000-120.000-100.000-80.000-60.000-40.000-20.0000.00020.00040.00060.00080.000

100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000

10450

10400

10350

10300

10250

10200

10150

10100

10050

10000

9950

7800 7850 7900 7950 8000 8050 8100 8150 8200 8250 8300 8350 8400 8450 8500 8550 8600

Company MRM/PandoScale As ShownDrawn By SBAnalysis NOEF stability baseDate December 2016

Descrip onSec on B-B : Annual rainfall : Block searchSeismic Ky (FOS=1)



0.28

0.27

0.27

0.26

0.28

0.27

0.27

0.26


Pore Pressure[kPa]

-140.000-120.000-100.000-80.000-60.000-40.000-20.0000.00020.00040.00060.00080.000

100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000

106

10500

10400

10300

10200

10100

10000

7700 7800 7900 8000 8100 8200 8300 8400 8500 8600 8700 8800


Descrip onSec on B-B : Annual rainfall : Circular searchSeismic Ky (FOS=1)



1.7421.7421.7421.742Material Name Color Unit Weight(kN/m3) Strength Type Cohesion

(kPa)Phi(deg) Shear Normal Func on

3 Halo 20 Shear Normal func on UQ WR Lower Bound

2 CCL 18 Mohr‐Coulomb 10 22

1 Cover Material 20 Shear Normal func on UQ WR Mid Bound

6 Natural Ground 18 Mohr‐Coulomb 10 22

7 PAF Cell 20 Shear Normal func on UQ WR Lower Bound

10 Central West 20 Shear Normal func on UQ WR Lower Bound

11 Bedrock 20 Mohr‐Coulomb 100 35

12 Advec on layer 18 Mohr‐Coulomb 10 22

Safety Factor0.0000.2500.5000.7501.0001.2501.5001.7502.0002.2502.5002.7503.0003.2503.5003.7504.0004.2504.5004.7505.0005.2505.5005.7506.000+

Pore Pressure[kPa]

-100.000 -80.000 -60.000 -40.000 -20.000 0.000 20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000 340.000 360.000

10450

10400

10350

10300

10250

10200

10150

10100

10050

10000

9950

3500 3550 3600 3650 3700 3750 3800 3850 3900 3950 4000 4050 4100 4150 4200 4250 4300 4350


Descrip onSec on D-D West : Wet season rainfall : Block search



1.7791.7791.7791.779

Material Name Color Unit Weight(kN/m3) Strength Type Cohesion










Safety Factor0.0000.2500.5000.7501.0001.2501.5001.7502.0002.2502.5002.7503.0003.2503.5003.7504.0004.2504.5004.7505.0005.2505.5005.7506.000+

Pore Pressure[kPa]

-100.000 -80.000 -60.000 -40.000 -20.000 0.000 20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000 340.000 360.000

10450

10400

10350

10300

10250

10200

10150

10100

10050

10000

9950

3500 3550 3600 3650 3700 3750 3800 3850 3900 3950 4000 4050 4100 4150 4200 4250 4300 4350 4400


Descrip onSec on D-D West : Wet season rainfall : Circular search



0.211

0.228 0.250

0.2110.211

0.228 0.250

0.211












Pore Pressure[kPa]

-100.000-80.000-60.000-40.000-20.0000.00020.00040.00060.00080.000

100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000 340.000 360.000

10500

10450

10400

10350

10300

10250

10200

10150

10100

10050

10000

3500 3550 3600 3650 3700 3750 3800 3850 3900 3950 4000 4050 4100 4150 4200 4250 4300 4350 4400 445


Descrip onSec on D-D West : Annual Rainfall : Block searchSeismic Ky (FOS=1)



0.196

0.2290.246

0.1960.196

0.2290.246

0.196

12


Pore Pressure[kPa]

-100.000-80.000-60.000-40.000-20.0000.00020.00040.00060.00080.000

100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000 340.000 360.000

10450

10400

10350

10300

10250

10200

10150

10100

10050

10000

3500 3550 3600 3650 3700 3750 3800 3850 3900 3950 4000 4050 4100 4150 4200 4250 4300 4350


Descrip onSec on D-D West : Annual Rainfall : Circular searchSeismic Ky (FOS=1)



1.7431.7431.7431.743











Safety Factor0.0000.2500.5000.7501.0001.2501.5001.7502.0002.2502.5002.7503.0003.2503.5003.7504.0004.2504.5004.7505.0005.2505.5005.7506.000+

Pore Pressure[kPa]

-100.000 -80.000 -60.000 -40.000 -20.000 0.000 20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000 340.000 360.000

10450

10400

10350

10300

10250

10200

10150

10100

10050

10000

9950

3500 3550 3600 3650 3700 3750 3800 3850 3900 3950 4000 4050 4100 4150 4200 4250 4300 4350 4400


Descrip onSec on D-D West : Annual Rainfall : Block search



1.7871.7871.7871.787











Safety Factor0.0000.2500.5000.7501.0001.2501.5001.7502.0002.2502.5002.7503.0003.2503.5003.7504.0004.2504.5004.7505.0005.2505.5005.7506.000+

Pore Pressure[kPa]

-100.000 -80.000 -60.000 -40.000 -20.000 0.000 20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000 340.000 360.000

10450

10400

10350

10300

10250

10200

10150

10100

10050

10000

9950

3500 3550 3600 3650 3700 3750 3800 3850 3900 3950 4000 4050 4100 4150 4200 4250 4300 4350


Descrip onSec on D-D West : Annual Rainfall : Circular search



1.6991.6991.6991.699



5 RPAF Cell 20 Shear Normal func on UQ WR Lower Bound






8 MRM4 Clay Borrow 19 Mohr‐Coulomb 0 32



Safety Factor0.0000.2500.5000.7501.0001.2501.5001.7502.0002.2502.5002.7503.0003.2503.5003.7504.0004.2504.5004.7505.0005.2505.5005.7506.000+

Pore Pressure[kPa]

-120.000-100.000 -80.000 -60.000 -40.000 -20.000 0.000 20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000 340.000

10500

10400

10300

10200

10100

10000

4200 4300 4400 4500 4600 4700 4800 4900 5000 5100 5200


Descrip onSec on C-C South : Wet season rainfall : Block search



1.6691.6691.6691.669












Safety Factor0.0000.2500.5000.7501.0001.2501.5001.7502.0002.2502.5002.7503.0003.2503.5003.7504.0004.2504.5004.7505.0005.2505.5005.7506.000+

Pore Pressure[kPa]

-120.000-100.000 -80.000 -60.000 -40.000 -20.000 0.000 20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000 340.000

10500

10400

10300

10200

10100

10000

4200 4300 4400 4500 4600 4700 4800 4900 5000 5100 5200


Descrip onSec on C-C South : Wet season rainfall : Circular search



0.2050.2050.2050.205



4 Core 20 Shear Normal func on UQ WR Lower Bound









Pore Pressure[kPa]

-120.000-100.000 -80.000 -60.000 -40.000 -20.000 0.000 20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000 340.000

10550

10500

10450

10400

10350

10300

10250

10200

10150

10100

10050

10000

4250 4300 4350 4400 4450 4500 4550 4600 4650 4700 4750 4800 4850 4900 4950 5000 5050 5100 5150 5200


Descrip onSec on C-C South : Annual rainfall : Block searchSeismic Ky (FOS=1)



0.210

0.221

0.297

0.2960.2100.210

0.221

0.297

0.2960.210












Pore Pressure[kPa]

-120.000-100.000 -80.000 -60.000 -40.000 -20.000 0.000 20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000 340.000

10600

10500

10400

10300

10200

10100

10000

4200 4300 4400 4500 4600 4700 4800 4900 5000 5100 5200 5300


Descrip onSec on C-C South : Annual rainfall : Circular searchSeismic Ky (FOS=1)



1.7001.7001.7001.700












Safety Factor0.0000.2500.5000.7501.0001.2501.5001.7502.0002.2502.5002.7503.0003.2503.5003.7504.0004.2504.5004.7505.0005.2505.5005.7506.000+

Pore Pressure[kPa]

-120.000-100.000 -80.000 -60.000 -40.000 -20.000 0.000 20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000 340.000

10500

10400

10300

10200

10100

10000

4300 4400 4500 4600 4700 4800 4900 5000 5100 5200


Descrip onSec on C-C South : Annual rainfall : Block search



1.7071.7071.7071.707












Safety Factor0.0000.2500.5000.7501.0001.2501.5001.7502.0002.2502.5002.7503.0003.2503.5003.7504.0004.2504.5004.7505.0005.2505.5005.7506.000+

Pore Pressure[kPa]

-120.000-100.000 -80.000 -60.000 -40.000 -20.000 0.000 20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000 340.000

10600

10500

10400

10300

10200

10100

10000

4200 4300 4400 4500 4600 4700 4800 4900 5000 5100 5200 5300


Descrip onSec on C-C South : Annual rainfall : Circular search



1.8441.8441.8441.844



5 PAF(RE) Cell 20 Shear Normal func on UQ WR Lower Bound








Safety Factor0.1000.2210.3420.4630.5830.7040.8250.9461.0671.1881.3081.4291.5501.6711.7921.9132.0332.1542.2752.3962.5172.6382.7582.8793.000+

Pore Pressure[kPa]

-120.000-100.000 -80.000 -60.000 -40.000 -20.000 0.000 20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000 340.000

10500

10400

10300

10200

10100

10000

5600 5700 5800 5900 6000 6100 6200 6300 6400 6500 6600


Descrip onSec on C-C North : Wet Season rainfall : Block search



1.945

1.963

1.9451.945

1.963

1.945











Safety Factor0.1000.2210.3420.4630.5830.7040.8250.9461.0671.1881.3081.4291.5501.6711.7921.9132.0332.1542.2752.3962.5172.6382.7582.8793.000+

Pore Pressure[kPa]

-120.000-100.000 -80.000 -60.000 -40.000 -20.000 0.000 20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000 340.000

10500

10400

10300

10200

10100

10000

9900

5600 5700 5800 5900 6000 6100 6200 6300 6400 6500 6600 670


Descrip onSec on C-C North : Wet Season rainfall : Circular search



0.187

0.274

0.290

0.2130.1870.187

0.274

0.290

0.2130.187












Pore Pressure[kPa]

-120.000-100.000 -80.000 -60.000 -40.000 -20.000 0.000 20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000 340.000

10500

10400

10300

10200

10100

10000

5600 5700 5800 5900 6000 6100 6200 6300 6400 6500 6600


Descrip onSec on C-C North : Annual rainfall : Block searchSeismic Ky (FOS=1)



0.213

0.278

0.300

0.299

0.2130.213

0.278

0.300

0.299

0.213












Pore Pressure[kPa]

-140.000-120.000-100.000 -80.000 -60.000 -40.000 -20.000 0.000 20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000

10500

10400

10300

10200

10100

10000

5600 5700 5800 5900 6000 6100 6200 6300 6400 6500 6600


Descrip onSec on C-C North : Annual rainfall : Circular searchSeismic Ky (FOS=1)



1.8541.8541.8541.854Material Name Color Unit Weight

(kN/m3) Strength Type Cohesion(kPa)

Phi(deg) Shear Normal Func on









Safety Factor0.1000.2210.3420.4630.5830.7040.8250.9461.0671.1881.3081.4291.5501.6711.7921.9132.0332.1542.2752.3962.5172.6382.7582.8793.000+

Pore Pressure[kPa]

-120.000-100.000 -80.000 -60.000 -40.000 -20.000 0.000 20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000 340.000

10500

10400

10300

10200

10100

10000

5600 5650 5700 5750 5800 5850 5900 5950 6000 6050 6100 6150 6200 6250 6300 6350 6400 6450 6500 6550 6600


Descrip onSec on C-C North : Annual rainfall : Block search



1.9491.9491.9491.949











Safety Factor0.1000.2210.3420.4630.5830.7040.8250.9461.0671.1881.3081.4291.5501.6711.7921.9132.0332.1542.2752.3962.5172.6382.7582.8793.000+

Pore Pressure[kPa]

-140.000-120.000-100.000 -80.000 -60.000 -40.000 -20.000 0.000 20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000

10500

10400

10300

10200

10100

10000

5600 5650 5700 5750 5800 5850 5900 5950 6000 6050 6100 6150 6200 6250 6300 6350 6400 6450 6500 6550 660


Descrip onSec on C-C North : Annual rainfall : Circular search



1.9091.9091.9091.909










Safety Factor0.0000.2500.5000.7501.0001.2501.5001.7502.0002.2502.5002.7503.0003.2503.5003.7504.0004.2504.5004.7505.0005.2505.5005.7506.000+

Pore Pressure[kPa]

-120.000-100.000 -80.000 -60.000 -40.000 -20.000 0.000 20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000 340.000

1040

10350

10300

10250

10200

10150

10100

10050

10000

9950

9900

7750 7800 7850 7900 7950 8000 8050 8100 8150 8200 8250 8300 8350 8400 8450 8500 8550 8600


Descrip onSec on B-B : Wet season rainfall : Block search



2.0972.0972.0972.097










Safety Factor0.0000.2500.5000.7501.0001.2501.5001.7502.0002.2502.5002.7503.0003.2503.5003.7504.0004.2504.5004.7505.0005.2505.5005.7506.000+

Pore Pressure[kPa]

-140.000-120.000-100.000 -80.000 -60.000 -40.000 -20.000 0.000 20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000

10450

10400

10350

10300

10250

10200

10150

10100

10050

10000

7800 7850 7900 7950 8000 8050 8100 8150 8200 8250 8300 8350 8400 8450 8500 8550 8600


Descrip onSec on B-B : Annual rainfall : Circular search



2.0962.0962.0962.096










Safety Factor0.0000.2500.5000.7501.0001.2501.5001.7502.0002.2502.5002.7503.0003.2503.5003.7504.0004.2504.5004.7505.0005.2505.5005.7506.000+

Pore Pressure[kPa]

-120.000-100.000 -80.000 -60.000 -40.000 -20.000 0.000 20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000 340.000

10350

10300

10250

10200

10150

10100

10050

10000

9950

9900

7800 7850 7900 7950 8000 8050 8100 8150 8200 8250 8300 8350 8400 8450 8500 8550


Descrip onSec on B-B : Wet season rainfall : Circular search



0.240

0.254

0.2400.240

0.254

0.240 Material Name Color Unit Weight(kN/m3) Strength Type Cohesion










Pore Pressure[kPa]

-140.000-120.000-100.000 -80.000 -60.000 -40.000 -20.000 0.000 20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000

10400

10350

10300

10250

10200

10150

10100

10050

10000

9950

7850 7900 7950 8000 8050 8100 8150 8200 8250 8300 8350 8400 8450 8500 8550 8600


Descrip onSec on B-B : Annual rainfall : Block searchSeismic Ky (FOS=1)



0.280

0.2710.268

0.280

0.2710.268











Pore Pressure[kPa]

-140.000-120.000-100.000 -80.000 -60.000 -40.000 -20.000 0.000 20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000

10400

10350

10300

10250

10200

10150

10100

10050

10000

7850 7900 7950 8000 8050 8100 8150 8200 8250 8300 8350 8400 8450 8500 8550


Descrip onSec on B-B : Annual rainfall : Circular searchSeismic Ky (FOS=1)



1.9131.9131.9131.913Material Name Color Unit Weight

(kN/m3) Strength Type Cohesion(kPa)

Phi(deg) Shear Normal Func on








Safety Factor0.0000.2500.5000.7501.0001.2501.5001.7502.0002.2502.5002.7503.0003.2503.5003.7504.0004.2504.5004.7505.0005.2505.5005.7506.000+

Pore Pressure[kPa]

-140.000-120.000-100.000 -80.000 -60.000 -40.000 -20.000 0.000 20.000 40.000 60.000 80.000 100.000 120.000 140.000 160.000 180.000 200.000 220.000 240.000 260.000 280.000 300.000 320.000

10300

10250

10200

10150

10100

10050

10000

9950

7850 7900 7950 8000 8050 8100 8150 8200 8250 8300 8350 8400 8450 8500 8550


Descrip onSec on B-B : Annual rainfall : Block search



1.5781.5781.5781.578

Material Name Color (kN/m3) Strength Type (kPa) (deg) Shear Normal Func on









Safety Factor0.0000.2500.5000.7501.0001.2501.5001.7502.0002.2502.5002.7503.0003.2503.5003.7504.0004.2504.5004.7505.0005.2505.5005.7506.000+

Pore Pressure[kPa]

-150.000-135.000-120.000-105.000 -90.000 -75.000 -60.000 -45.000 -30.000 -15.000 0.000 15.000 30.000 45.000 60.000 75.000 90.000 105.000 120.000 135.000 150.000 165.000 180.000 195.000

10450

10400

10350

10300

10250

10200

10150

10100

10050

10000

9950

9900

7500 7600 7700 7800 7900 8000 8100 8200 8300 8400

Company MRM/PandoScale As ShownDrawn By SBAnalysis NOEF stability ‐ BaseDate December 2016

Descrip onSec on A-A : Annual rainfall : Block search



1.6181.6181.6181.618









Safety Factor0.0000.2500.5000.7501.0001.2501.5001.7502.0002.2502.5002.7503.0003.2503.5003.7504.0004.2504.5004.7505.0005.2505.5005.7506.000+

Pore Pressure[kPa]

-150.000-135.000-120.000-105.000 -90.000 -75.000 -60.000 -45.000 -30.000 -15.000 0.000 15.000 30.000 45.000 60.000 75.000 90.000 105.000 120.000 135.000 150.000 165.000 180.000 195.000

10450

10400

10350

10300

10250

10200

10150

10100

10050

10000

9950

9900

7400 7450 7500 7550 7600 7650 7700 7750 7800 7850 7900 7950 8000 8050 8100 8150 8200 8250 8300 8350


Descrip onSec on A-A : Annual rainfall : Circular search



0.157

0.197

0.196

0.173

0.1570.157

0.197

0.196

0.173

0.157












Pore Pressure[kPa]

-150.000-135.000-120.000-105.000 -90.000 -75.000 -60.000 -45.000 -30.000 -15.000 0.000 15.000 30.000 45.000 60.000 75.000 90.000 105.000 120.000 135.000 150.000 165.000 180.000 195.000

10450

10400

10350

10300

10250

10200

10150

10100

10050

10000

9950

9900

7450 7500 7550 7600 7650 7700 7750 7800 7850 7900 7950 8000 8050 8100 8150 8200 8250 8300 8350 8400


Descrip onSec on A-A : Seismic (Ky FOS=1) : Block search



0.134

0.255

0.278

0.250

0.1340.134

0.255

0.278

0.250

0.134












Pore Pressure[kPa]

-150.000-135.000-120.000-105.000 -90.000 -75.000 -60.000 -45.000 -30.000 -15.000 0.000 15.000 30.000 45.000 60.000 75.000 90.000 105.000 120.000 135.000 150.000 165.000 180.000 195.000

10400

10300

10200

10100

10000

9900

7400 7450 7500 7550 7600 7650 7700 7750 7800 7850 7900 7950 8000 8050 8100 8150 8200 8250 8300 8350 8400


Descrip onSec on A-A : Seismic (Ky FOS=1) : Circular search



1.6171.6171.6171.617











Safety Factor0.0000.2500.5000.7501.0001.2501.5001.7502.0002.2502.5002.7503.0003.2503.5003.7504.0004.2504.5004.7505.0005.2505.5005.7506.000+

Pore Pressure[kPa]

-150.000-135.000-120.000-105.000 -90.000 -75.000 -60.000 -45.000 -30.000 -15.000 0.000 15.000 30.000 45.000 60.000 75.000 90.000 105.000 120.000 135.000 150.000 165.000 180.000 195.000

10400

10300

10200

10100

10000

9900

9800

7400 7500 7600 7700 7800 7900 8000 8100 8200 8300 8400 8500


Descrip onSec on A-A : Wet season rainfall : Block search



1.6071.6071.6071.607











Safety Factor0.0000.2500.5000.7501.0001.2501.5001.7502.0002.2502.5002.7503.0003.2503.5003.7504.0004.2504.5004.7505.0005.2505.5005.7506.000+

Pore Pressure[kPa]

-150.000-135.000-120.000-105.000 -90.000 -75.000 -60.000 -45.000 -30.000 -15.000 0.000 15.000 30.000 45.000 60.000 75.000 90.000 105.000 120.000 135.000 150.000 165.000 180.000 195.000

10450

10400

10350

10300

10250

10200

10150

10100

10050

10000

9950

9900

7450 7500 7550 7600 7650 7700 7750 7800 7850 7900 7950 8000 8050 8100 8150 8200 8250 8300 8350 8400


Descrip onSec on A-A : Wet Season rainfall : Circular search



0.16

0.20

0.20

0.17

0.160.16

0.20

0.20

0.17

0.16


Pore Pressure[kPa]

-150.000-135.000-120.000-105.000 -90.000 -75.000 -60.000 -45.000 -30.000 -15.000 0.000 15.000 30.000 45.000 60.000 75.000 90.000 105.000 120.000 135.000 150.000 165.000 180.000 195.000

10500

10450

10400

10350

10300

10250

10200

10150

10100

10050

10000

9950

7450 7500 7550 7600 7650 7700 7750 7800 7850 7900 7950 8000 8050 8100 8150 8200 8250 8300 8350 8400


Descrip onSec on A-A : Seismic (Ky FOS=1) : Block search



0.134

0.233

0.277

0.278

0.1340.134

0.233

0.277

0.278

0.134


Pore Pressure[kPa]

-150.000-135.000-120.000-105.000 -90.000 -75.000 -60.000 -45.000 -30.000 -15.000 0.000 15.000 30.000 45.000 60.000 75.000 90.000 105.000 120.000 135.000 150.000 165.000 180.000 195.000

10600

10500

10400

10300

10200

10100

10000

7400 7500 7600 7700 7800 7900 8000 8100 8200 8300 8400


Descrip onSec on A-A : Seismic (Ky FOS=1) : Circular search



Documents

Appendix N Geotechnical Assessment Report · 2017-03-21 · McArthur River Mine Overburden Management Project Draft Environmental Impact Statement N Appendix N Geotechnical Assessment