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Slope Stability 2013P.M. Dight (ed)

2013 Australian Centre for Geomechanics, Perth, ISBN 978-0-9870937-5-2

Slope Stability 2013, Brisbane, Australia 1111

D. McInnes Golder Associates Pty Ltd, Australia

C. Haberfield Golder Associates Pty Ltd, Australia

P.J.H. de Graaf Rio Tinto Iron Ore, Australia

C. Colley Golder Associates Pty Ltd, Australia

Geotechnical investigations were carried out as part of the Feasibility Study Extension (FSE) and Trial Mining

program for the development of below the water table (BWT) expansion at Rio Tinto Iron Ores (RTIOs)

Marandoo Mine. The Marandoo Mine site is located in the Pilbara region of Western Australia,

1,500 kilometres north of Perth.

Previous studies have indicated that a 40 m thick, at approximately 40 m depth, layer of Tertiary clay, will

present significant challenges relating to slope and waste dump design as well as operational challenges

(trafficability and handleability). Due to uncertainty over the confidence of parameters derived from triple

tube sampling and testing, conservative design parameters were adopted for the previous studies. It was

recognised that significant upside could be realised through improved material characterisation techniques.

The main objectives of the investigation and testing program were to: 1) characterise the physical

properties and variability of the clay; and 2) obtain design parameters for the slopes for mining of the pit.

This was carried out with conventional drilling, sampling and logging procedures. Hyperspectral scanning of

diamond core was undertaken, which provided useful information on the vertical distribution of clay speciesthrough the detrital stratigraphy. Following review of the historic sample test results, it was hypothesised

that sampling and testing methods had contributed to pre-softening of samples. Subsequent testing focused

on Shelby tube and in situ testing; both resulted in significant improvement in measured shear strength

parameters over triple tube sampling.

A self-boring pressuremeter was used in two locations to carry out in situ testing of the clay at various

depths. The results of the pressuremeter testing have indicated that the permeability of the clay is

significantly lower than assumed in previous studies. An assessment of the effect of the structure of the clay

on the stability of the slopes will be an objective of further studies.

Key outcomes so far have emphasised the significant impact of sampling method, preservation, timely

testing on derived material properties, and recognition of the limitations of various techniques incharacterising heavily over consolidated clays. The material characterisation has also confirmed the critical

importance of water management in mining in relation to preserving clay strength. This work has enabled

detailed planning to be undertaken for studies covering trafficability, handleabilty, waste dump and slope

performance; with field trials scheduled in the mine plan.

Previous studies have indicated that in the area of the proposed mining there is a layer of clay, the upper

tertiary lacustrine clay (UTLC), approximately 40 m thick, at approximately 40 m depth, through which the

mine excavation will penetrate.


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Mine design for below water table clay detritals mining: Marandoo Mine, Western Australia D. McInnes et al.

1112 Slope Stability 2013, Brisbane, Australia

The main objective of the investigations and testing program were to obtain design parameters for the

slopes for the mining pit. This was carried out with conventional drilling, sampling and logging procedures.

A self-boring pressuremeter (SBPM) was used in two locations to carry out in situ testing of the clay.

The results of the pressuremeter testing have indicated that the permeability of the clay is significantly

lower than assumed in previous studies, but the effect of any structure in the clay is expected to increase

the mass permeability and this could affect the time the slopes remain stable.

The behaviour of the UTLC suggests that the significant stress reduction occurring during excavation will

reduce the pore pressures from the pre-excavation conditions and pore pressures are expected to become

negative. The negative pore pressures will initially act to preserve the effective stress state and the strength

of the UTLC. However, the Factors of Safety are expected to reduce with time as the negative pore

pressures dissipate. This behaviour is well documented for materials such as London Clay and Cambridge

Clay, which have similar engineering characteristics to the UTLC based on comparisons with available data.

Slope stability assessments have been carried out for the mining pit and indicate that an overall slope of

approximately 36 will have a Factor of Safety in excess of 1.2 for up to two years after excavation through

the UTLC. However, the computed Factor of Safety is dependent upon a number of factors which have not

been fully defined or rely on assumptions that may change as more data becomes available. Consequentlythe Factor of Safety should not be relied upon until further studies have been completed.

The slope stability analyses carried out to date have assumed that the groundwater pressures in the strata

below the clay are hydrostatic until the base of the excavation reaches the Lower Calcrete and then

groundwater pressures in the lower strata will start to dissipate at a rate dependent on their

hydrogeological properties. Without dewatering of the strata below the clay, base failure could occur prior

to slope failures in the UTLC. The stability analyses have not reliably modelled the depressurisation of the

underlying strata as the hydrogeological properties of the underlying strata are not known in sufficient

detail. Further slope stability analyses will be inconclusive without additional hydrogeological data from the

underlying strata and detailed information from the dewatering model.

The primary ore zone is the Archaean Newman Member banded iron stone of the Marra Mamba Iron

Formation which is overlain by West Angelas Member weathered shales. Transported detrital units

uncomformably overly the weathered shales. The significant units within the detritals assemblage include

Tertiary lower conglomeratic breccias and pisolites (3050 m), overlain by a 4060 m thick Upper Tertiary

Lacustrine Clay (UTLC). Thin secondary calcrete lenses occur both above and below the UTLC, with

Quaternary alluvial sediments (30 m thick) exposed on the surface.

The fieldwork program comprised 16 cored geotechnical boreholes and two boreholes for the self-boring

pressuremeter testing. Eight boreholes were drilled using a Sonic drilling rig. The boreholes were designed

to sample the UTLC and to confirm the lithology of the strata immediately underlying it. Boreholes were

drilled to a maximum depth of 120 m and locations are shown on Figure 1. Eight of the boreholes were

drilled at an inclination of 85 to the north to facilitate core orientation for structural data acquisition using

an electronic (ACE) orientation tool and gyroscopic deviation surveys were subsequently carried out in

these holes.

All boreholes were logged onsite by engineering geologists and samples were collected for geotechnical

testing. Gamma surveys were carried out on all boreholes, with the exception of one borehole which was a

test hole for the Sonic drill rig.


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Hydrogeology


A summary of the geology encountered for each of the boreholes drilled is presented in Table 1.

Borehole

Reference

No.

Quaternary

Detritals

(m bgl)

Upper

Calcrete

(m bgl)

UTLC

(m bgl)

Lower

Calcrete

(m bgl)

Lower

Sediments

(m bgl)

Archaean

Units

(m bgl)

BH01 0.031.5 to 41.0 to 83.0 to 95.0 to 118.0 to 120.0*

BH02 0.024.5 to 35.0 to 47.5*

BH03 0.028.0 to 39.0 to 82.5 to 95.2 to 113.0 to 120.0*

BH04 0.025.0 to 35.0 to 82.0 to 98.1 to 108.5 to 120.0*

BH05 0.023.0 to 32.0 to 41.0*

BH06 0.033.0 to 41.5 to 89.0 to 100.0 to 113.0 to 120.0*

BH07 0.020.0 to 39.2 to 81.3 to 90.6 to 98.8 to 103.7*

BH08 0.025.7 to 38.7 to 72.6 to 80.8 to 90.5*

BH09 0.024.6 to 36.4 to 75.5 to 98.3 to 120.0*

BH10 0.034.4 to 44.3 to 64.5 to 90.5

BH11 0.029.0 to 40.5 to 66.5 to 91.5

BH13 0.028.8 to 46.0 to 67.8 to 90.0

BH14 0.028.2 to 42.3 to 85.5 to 91.5

BH15 0.027.2 to 43.8 to 85.3 to 90.0*

BH16 0.027.7 to 43.8 to 84.7 to 90.0*

BH17 0.033.0 to 43.0 to 76.0 to 89.0*

BH18 0.027.3 to 41.9 to 83.9 to 90.0*

BH19 0.035.0 to 43.0 to 73.0 to 80.0*

Notes: All depths are provided in metres below ground level (m bgl)

* indicates total depth and may not be the limit to identified strata, boreholes 2 and 5 were abandoned at shallow depth

Holes BH17 and BH19 were drilled for the SBPM.


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The Quaternary Detritals unit is typically a matrix supported silty sand which is moderately cemented, and

ranges from very dense sand to low strength rock with up to 40% angular and sub-rounded gravel clasts.

The matrix generally becomes finer grained with depth and clasts become finer and less abundant. Large

scale coarsening and fining is observed on a metre scale throughout the Quaternary Detritals. While few

true joints are present within this unit, the core tends to break sub-horizontal and parallel to the apparent

banding or bedding.

The thickness of the Upper Calcrete ranges between 6.4 m and 13.0 m. The calcrete varies from a very

weakly cemented and calcretised fine sandy silt to a well cemented, medium strength calcrete, sometimes

having a vuggy texture.

The UTLC generally comprises a moist, fissured, high plasticity, very stiff, clay and silty clay. No free water

was observed on joint surfaces during field logging. Pocket penetrometer tests were conducted in the field

at intervals in the cored UTLC and in Shelby tube samples. The pocket penetrometer results indicate that

the UTLC core is generally of uniform strength and the undrained shear strength in the core and the Shelby

tubes, calculated from penetrometer readings, is typically just below 400 kPa. The UTLC can be subdivided

into three zones, an upper ferruginous clay, a middle weakly ferruginous clay and a lower non-ferruginous

clay as outlined below.

Pale grey and red brown, the red brown ferruginised material is often oriented along fissures anddiscontinuities, typically present from top of clay to between 49.6 m depth in BH07 and 58.8 m depth in

BH01, with an average thickness of about 15 m for all boreholes in this investigation.

Khaki brown mottled red brown and pale grey, the pale grey clay is often found on the discontinuity

surfaces. Observations indicate that weakly ferruginised clay was encountered between 65.0 m depth in

BH07 and 74.0 m depth in BH06, with an average thickness of about 16 m in all boreholes in this

investigation.

Khaki brown occasionally mottled red brown and pale grey. Traces of fine ferricrete nodules up to 10 mm

diameter occur within this zone. Non-ferruginised clay is observed from the base of the weakly ferruginous

material to the base of the UTLC, representing an average thickness of 11.4 m in boreholes logged as part

of this investigation.

The clay core samples appeared to desiccate quickly when exposed to the environment and rapidly

disaggregate in water (typically within an hour) from a dry or moist condition. The desiccation cracks are

often orientated along the fissures and joints, and the material shows very little resistance to shear along

these surfaces after short periods of exposure (one to two days). This effect was observed on all samples of

the UTLC exposed to the air. Samples from the middle and lower clay units generally took longer todisaggregate than the upper ferruginised clay samples. Photographs of these characteristics are presented


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Hydrogeology


as Figures 2(a) and (b). Further study into the mineralogy and soil water chemistry will be required to better

understand this behaviour.

(a) (b)

The clays displayed some jointing, generally planar, closed, and continuous across the core axis. Observed

defect surfaces were typically polished and slickensided. Core orientation was carried out within the UTLC

using an electronic (ACE) orientation tool. Orientation of the clay was problematic because orientations

could not reliably be carried through from one drilling run to the next and the assessed structural

information is ranked as unreliableaccording to RTIOs core orientation procedures.A combined defect

data set is presented as Figure 3.

Notwithstanding the confidence ranking, the data indicate structural features within the UTLC to show a

range of dip angles between 12 to 60. Four sets can be recognised from the data with dip directions

appearing to cluster at four almost equally distributed directions.


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The lower calcrete is less well defined than the upper calcrete and varies from very weakly calcretised silty

clay to clayey silt of medium to low plasticity to a well cemented, medium strength calcrete. The lower

calcrete varies in thickness between 7 and 17 m. The boundary between the UTLC and the Lower Calcrete

can be gradational and in these cases the boundary is generally identified by a decrease in plasticity,

combined with colour and textural changes. Previous studies identified that the Lower Calcrete can be

discontinuous within the investigation area and is typically absent in the west of the project area. The

Lower Calcrete is absent in BH08 (the western most of the FSE boreholes) which is consistent with the

current geological model.

The Quaternary through Tertiary sequence is bounded below by a Tertiary-Archaean unconformity and

underlain by weathered Archaean units generally comprising discontinuous horizons of:

Clay breccias with sub-rounded banded iron formation (BIF) clasts, underlain by:

completely weathered West Angela Shale, represented by firm to stiff clayey silt to silty laywith moderate to low plasticity with complete overprinting of rock fabric; and

BIF of the Archaean age Newman Group (basement).The Tertiary-Archaean unconformity is often difficult to clearly identify during field logging even when

supplemented by gamma log information.

Results of Atterberg limit testing are summarised in Figures 4, 5 and 6.

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100 120 140

Plast

icityIndex

% < 2m

PSM Data

FSE Data

FSE Nov-2010

Activity 0.75


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Hydrogeology


(a) (b)

One oedometer test was performed to assess the degree of over consolidation of the UTLC and the results

are summarised in Table 2.

Test

Depth(m bgl)

Moisture

Content(%)

Liquid

Limit(%)

Plastic

Limit(%)

Overburden

Pressure(MPa)

Yield

Stress(MPa)

Over-Consolidation

Ratio

65 27 89 23 1.1 >3 2.7

The maximum vertical stress of 4 MPa applied by the oedometer apparatus was insufficient to fully define

the post-yield pressure to void ratio relationship. The yield stress interpreted from the test (3 MPa) is

therefore a lower bound to the actual value. The corresponding OCR value (3) is also a lower bound to the

actual OCR which the SBPM results indicate to be in the range 4 to 8.

For the stability assessment the effective stress parameters, effective cohesion c and effective friction

angle may be obtained from the triaxial testing. The shear strength results from the ten samples testedare summarised in Table 3.

Where noted above: (Joint) indicates that the result may have been affected by incipient failure surfaces

within the sample. All tests were carried out on single samples using multi-stage (three stage) testing

procedures (refer Section 4.3.3). Results of two stages of testing have been obtained from the samples

from BH06 at 81 m depth.

There is a range of results, c varies between 0 and 71 kPa, and varies between 6.6 and 15.9. The

factors that may have affected the results are discussed below.

20

30

40

50

60

70

80

90

100

0 50 100

Depth(m)

Plasticity Index

PSM Data

FSE Data

FSE Nov-2010

Below UTLC

20

30

40

50

60

70

80

90

100

-1 -0.5 0 0.5 1

Depth(m)

Liquidity Index

PSM Data

FSE Data

FSE Nov-2010

Below UTLC


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Borehole

Number

Test

R.L.

(m)

Test

Depth

(m bgl)

Moisture

Content

(%)

Liquid

Limit

(%)

Plastic

Limit

(%)

Liquidity

Index

Effective

Cohesion

(kPa)

Friction

Angle

(deg)

BH01 649.6 65 27 89 23 0.2 0 9 (Joint)

BH03 640.4 71 38 110 43 -0.1 56 11.1

BH03 631.4 80 40 133 43 -0.1 69 6.6 (Joint)

BH04 639.5 74 39.7 132 31 0.3 44 11.1

BH04 632.5 81 41.7 111 38 0.1 34 7.9

BH06 644.1 71 38 122 41 -0.1 71 13.6

BH06 641.1 74 40 133 41 0 49 12.6

BH06 634.1 81 34 107 30 0.1 38 182

BH20 661.4 56 38.8 111 43 -0.1 11 17

BH20 643.4 74 29.6 112 36 -0.2 56 15.9

1 = all samples classify as Hard Clay (CH) under the USCS.

2 = the 18 degree result was produced using an alternate triaxial sample consolidation methodrefer Section 4.3.2.

The low friction angle reported for the sample from BH01 at 65 m depth is due to the sample failing during

the first load stage, along an inclined joint plane. Testing of a joint was specifically targeted in this test with

the specimen cut from the sample such that the joint plane was in the centre of the specimen.

The sample from BH03 at 80 m depth exhibited failure at the end of the specimen, with the shear plane

passing through the top of the specimen. This failure mode is associated with non-homogeneity, as end

restraint encourages a specimen to fail in the central part of the specimen. Given the similarity of thefriction angle to the value obtained on a joint from BH01 it is possible that the failure of this sample also

was joint controlled. The relatively high c value may reflect the effect of the end restraint and multi-stage

testing. For very stiff clays a shear plane usually forms at the first stage and this can impact on the results of

the second and third stages, usually resulting in a high cohesion and lower friction angle.

A modified test procedure was applied to the sample from BH06 at 81 m depth, whereby the specimen was

reconsolidated to a saturated condition using external cell pressure rather than by applying internal back

pressure. Pressure saturation is preferred by some research orientated laboratories (such as the Norwegian

Geotechnical Institute) as the moisture content of the specimen is not changed. The back pressure

technique used for the other samples has the advantage that it is quicker, but may lead to results that

underestimate the shear strength of some clays. The result of this test, c = 38kPa = 18, gave the

highest friction angle in this test series.

There appears to be a large range in the effective strength parameters as there are two components that

are not wholly independent; that is, high c, low can give the same shear strength as low c, high

.There are a number of valid techniques for deriving a realistic value for use in design of which taking a

numeric average is the simplest and is considered reasonable for discussing this set of results.

The samples tested are taken from the lower two thirds of the stratum, i.e. below the upper ferruginous

zone. Averaging the eight test results in Table 3 which are thought not to be structurally controlled,provides effective strength parameters c= 47.5 kPa, and = 14.7. Stability studies to date have been


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Hydrogeology


based on: c= 24 kPa, = 16.5 which appear to be reasonably supported by the current data set. Note

that the friction angle used in plain strain slope stability analysis is 10% greater than the measured triaxial

shear value to account for stress path differences.

Drained and undrained shear strength are related by Mohr Circle relationships, the effective stressparameters cand , and the excess pore pressure coefficient A fwhich expresses the pore pressure change

caused by the change in shear stress: u/(13). The Afcoefficient is closely related to OCR, reducing

with increasing OCR to approach zero at OCR 5. Adopting Af= 0 for simplicity, the relationship between

undrained shear strength, cu, and the effective stress strength parameters, c, , is:

cu = (c.cos+ v0.k0.sin)/(1sin) (1)

Taking the average values:

c = 47.5 kPa.

= 14.7.

k0 = 1.2.

v0 = 1,150 kPa.

Corresponding to the lower part of the UTLC, results in cu = 535 kPa. This is reasonably consistent with the

undrained shear strength obtained from Self Boring Pressuremeter (SBPM) tests in the lower part of the

UTLC. If Af is slightly positive at 0.1, the corresponding cu = 510 kPa. Based on the likely range of OCR = 4 to

8, and the associated range of Af= 0 to 0.1, there is fairly good agreement between the effective stress and

total stress shear strength parameters for the UTLC.

Difficulties in drilling through the UTLC have been attributed to the expansion of the clay after release ofthe confining stress with core swelling by up to 25% in length in some instances. In order to understand the

mechanism of the swelling some non-routine tests were carried out.

Mineralogical characterisation and petrographic examination were carried out on samples of drill core from

UTLC. A complete section of the UTLC between 35.8 and 81.0 m bgl from BH3 was scanned by infrared

scanner to assist in identification of minerals within the clay sequence. Complimentary to this, four samples

were analysed by X ray diffraction (XRD) analysis and X ray fluorescence (XRF) assay and a further five

samples sent for petrographic study.

The infrared scanner collates the result of reflectance scans with high definition photographs taken at

10 mm intervals down the core column. The reflectance from the scans is compared against a spectral data

base from known mineralogy. The best two reflectance matches are recorded. Some reflectances, whichare not clear or have not been defined, are presented as unknown. No depth correction has been provided.

Four categories of minerals from the processed scan results were plotted with respect to hole depth. The

categories are Iron oxides, Carbonates, Magnesian Clays and Kaolin. There is an apparent relationship

between mineral suites and the sub-layering observed within the UTLC. A summary of the observed

relationship is shown in Table 4.

The mineralogy appears to vary within the UTLC and further study of the mineralogy may be required to

determine if this will significantly affect the performance of the cut slopes during excavation. The behaviour

of the trial slope should assist in refining material handling techniques, taking into account the proportions

of the above mineralogy.


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Observed Sub-layers Data Sourced from CSIRO Infrared Scanning Test

Possible

Sub-stratum

Depth

(m bgl)

Depth

(m bgl) Comments

Upper CALC 22.836.8 35.838.7 Near kaolin free dolomite, palygorskite rich zone

UTLC A 38.755.45 41.256.6 Hematite rich zone

UTLC B 55.4565.4 56.664.8 Mixture of goethite and hematite rich zones

UTLC C 65.482.4 64.881.0 Goethite rich zone

Lower CALC 8288 Near kaolin free dolomite, palygorskite rich zone

The Self Boring Pressuremeter (SBPM) was used in two boreholes drilled with the sonic rig. The testing

targeted the UTLC, to provide information for assessing pit wall stability. The SBPM device used in the

investigation represents the state of the art in pressuremeter testing. Its self-boring capability minimisesground disturbance, which facilitates measurement of critical parameters including the in situ lateral stress,

shear strength and permeability. The latter requires pore water pressure sensors and advanced test control

software which are features of the SBPM used in the investigation. The pre-failure stiffness of the ground is

also measured. A summary of the SBPM test data is given in Table 5.

Borehole

Test Depth

(m bgl)

Limit

Pressure

(kPa)

Shear

Strength

(kPa)

Shear

Modulus

(MPa)

Lateral

Stress

Ratio (k0)

Horizontal

Permeability

(m/s 1011

)

BH17 44.5 9,100 1,500 275 8.0

BH19 51.4 7,100 860 120 1.35 2.6

BH19 54.5 6,300 785 115 1.4 4.3

BH19 55.5 6,300 800 1.2

BH19 60.0 5,300 610 75 1.25 1.9

BH19 62.9 5,000 550 65 1.1 1.0

BH19 64.3 5,100 615 70 1.1 2.0

BH17 65.5 5,200 620 60 1.5

BH17 66.9 5,500 715 55 1.0

BH19 69.0 5,200 545 75 1.15 3.7

BH19 70.5 5,300 555 80 1.1 2.5

BH19 72.0 5,300 555 2.0

BH17 75.1 5,300 475 85 1.25 2.1

Note: Derivation of the permeability assumes = 0.2

The ground surface elevation was taken as RL 717.7 m, and the UTLC was present between 43 and

77 m bgl. The groundwater level was approximately 38 m bgl. The first test, at 44.5 m bgl, was performed in

the ferruginous zone at the top of the UTLC, close to the interface with the overlying calcrete layer at43 m bgl. The results from this test are quite different from those of the deeper tests and are


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Hydrogeology


representative only of the top of the UTLC stratum. The remaining tests show a general reduction in shear

strength and shear modulus with depth with a more distinct change below the ferruginous zone in the

upper third of the stratum.

Values for the in situ lateral stress coefficient (k 0)are related to over-consolidation ratio (OCR), which is the

ratio of the effective yield stress to the current effective overburden stress. Generally k0 is found to be

about 0.5OCR, which leads to OCR ranging from 4 to 8 for the k 0values of 1 to 1.4 measured in the UTLC.

CPTs were performed in 2009 near the SBPM test boreholes. The cone resistance in the UTLC varied

between 6,000 kPa and 10,000 kPa, averaging about 8,000 kPa. The ratio of cone resistance to undrained

shear strength for clays is termed the cone factor. From international geotechnical literature an

appropriate cone factor for stiff clay is unlikely to be less than 14, which leads to an undrained shear

strength range of 430 to 715 kPa with an average of approximately 570 kPa.

The undrained shear strengths deduced from the SBPM tests below the ferruginous zone are typically

550 to 620 kPa, which are reasonably consistent with the earlier CPT data.

More generally, it is known that the undrained shear strength obtained from pressuremeter tests in very

stiff fissured clay can overestimate operational values separately determined by large scale in situ testing

(Marsland and Powell, 1990). Marsland and Randolph (1977) indicate that the reduction factor depends on

the nature and scale of the fissuring relative to the size of the pressuremeter. Where the fissuring is

intense, as at Marandoo, the theoretical pressuremeter shear strength approaches the mass value.

Marsland and Randolph (1977) take the undrained shear strength as about 15% of the difference betweenthe limit pressure and the lateral stress, based on cavity expansion theory. Clarke (1996) observes that the

undrained shear strength of London Clay is observed to be not less than 10% of the limit pressure from the

SBPM. The empirical Clarke criterion provides the more conservative interpretation, and would lead to

shear strengths in the range 500 to 700 kPa, excluding the initial limit pressure value in Table 5. This range

is reasonably consistent with the results of the SBPM below the ferruginous zone and the shear strengths

derived from the CPTs.

The shear modulus derived from the SBPM results decreases from 275 MPa near the upper surface of the

UTLC to approximately 80 MPa towards the base. However, the shear modulus is reasonably consistent atapproximately 120 MPa in the upper third, i.e. in the ferruginous zone, and between 60 and 80 MPa in the

lower two thirds of the strata.

A critical stiffness parameter used in finite element slope stability analyses is the undrained Youngs

modulus, E, at a reference effective stress, typically 100 kPa. Taking a realistic, but conservative, value of

70 MPa for the shear modulus for the lower horizons of the UTLC this gives an equivalent E of 170 MPa.

The measured stiffness value is scaled down to the reference stress based on proportionality with the

square root of the effective stress ratio, as is appropriate for stiff, over-consolidated clay. The effective

stress in the lower part of the UTLC is about 1,200 kPa. Consequently, the scaling factor is (100/1,200)0.5

or

about 0.3, which leads to Eref50 MPa.


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In summary, based on the information available from this program and previous investigations, it is

considered reasonable to conclude that an undrained shear strength of 500 kPa is conservative for the

UTLC. The measured stiffness corresponds to a drained Youngs modulus of about 50MPa at a reference

stress of 100 kPa. The SBPM indicates an in situ lateral stress coefficient of just above unity, which

corresponds to an over-consolidation ratio of about 6.

The horizontal permeability given by the SBPM is typically less than 5 10-11

m/s, and may be

approximately 2 10-11

m/s (2 10-6

m/d) in the lower half of the UTLC. This value is significantly less than

assumed by the stability analyses carried out previously.

The FSE investigation has provided additional information to characterise the UTLC, which is a significant

stratum in regard to pit wall stability. Examination of continuous core from the UTLC indicated the stratum

could be subdivided into three zones: an upper ferruginous zone, an intermediate zone of reduced

ferruginisation, and a lower non-ferruginous zone. There is a corresponding change in classification

characteristics of the materials; notably a reduction in plasticity with depth in the stratum matching the

visual-manual perception of the soil. The upper ferruginised material feels gritty and friable. It readily

breaks apart on discontinuities characterised by the presence of nodules. The lower non-ferruginous zone

is significantly more plastic and competent. This material is difficult to penetrate with a steel blade and is

resistant to splitting under (manual) tensile stress. Discontinuities are visible, but when the core is broken

apart it does so, on fissures rather than on the discontinuities, which contrasts with the ferruginised clay in

the upper part of the stratum.

Potential failure surfaces predicted from stability analyses and observed in slope failures in stiff fissured

clays develop within the clay at the base of the excavation. Therefore, the strength of the non-ferruginisedclay in the lower portion of the UTLC is of primary consideration with respect to a major instability.

The results of laboratory shear strength testing conforms to the trend of sequential investigations

improving shear strength parameters as sampling techniques improve and there is a reduction in soil

disturbance and storage times prior to testing. The undrained shear strength of the UTLC is in the order of

500 kPa indicating a hard soil. Conventional soil mechanics theory predicts that under undrained conditions

a vertical face in clay with a shear strength of 500 kPa could be cut to 90 m without failure. However, long

term stability is much less as the effective (i.e. fully drained) shear strength parameters are relatively low.

A permanent slope would need to be excavated at an angle of less than 15 degrees to avoid eventual

failure. Experience of stiff fissured clays elsewhere indicates that the time frame for the eventual failure ofa steeper slope could be measured in decades. The factors controlling time-dependent reduction in stability

are the in situ permeability and soil stiffness and in particular the horizontal values of both these

parameters. Both these parameters are affected by the behaviour of fissures and other discontinuities in

the clay which cannot be satisfactorily investigated by conventional borehole investigations.

The SBPM tests conducted in the UTLC at Marandoo indicate horizontal permeability values less than

5 10-11

m/sec (5 10-6

m/d), which is well below the value of 1 10-4

m/d assumed in the Marandoo BWT

pit studies to date. The SBPM device tests a zone extending up to about 0.5 m from the cylinder which

provides a more representative permeability than is associated with laboratory tests on samples recovered

from boreholes. The permeability results from the SBPM tests are at the lower limit of permeabilities

assessed from conventional soil classification systems and relationships, and correspond to massive, highly


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Hydrogeology


plastic clay formations considered to be practically impermeable. The permeability of the upper third of the

UTLC appears to be 2 to 4 times higher than the lower zones, again confirming the ferruginous has slightly

different properties.

The measured permeabilities of the UTLC are comparable to results from unweathered blue London Clay

in the UK, also a stiff fissured clay. There are documented cases of slopes in railway cuttings which have not

failed after 100 years old although similar slopes in similar material failed much sooner after excavation.Measured pore pressure recovery rates (from initial negative values caused by the excavation) in the

London Clay are of the order of 3 or 4 kPa per year, indicating that the time to a slope failure in the blue

clay could be of the order of 400 years (Vaughan and Walbancke, 1973; Chandler, 1984). These slopes are

relatively shallow; typically 3H (horizontal) to 1V (vertical), i.e. 18.5. These case histories support the

possibility that steeper slopes in similar clays, such as the UTLC, could be stable for several years.

The previous engineering parameters for stability analysis of the north wall lead to an overall slope in the

UTLC and overlying materials of 11. While the strength parameters previously used represent a reasonable

assessment of the available information, some of the estimated pore pressure conditions appear unrealisticin that they are based on existing (pre-excavation) piezometric conditions. The behaviour of the UTLC

suggests that the significant stress reduction during excavation will reduce the pore pressures from the

pre-excavation conditions and pore pressures are expected to become negative. The negative pore

pressures will initially act to preserve the effective stress state and the strength of the UTLC. However, the

Factors of Safety are expected to reduce with time as pore pressures dissipate. This behaviour is well

documented for materials that have similar engineering characteristics to the UTLC based on comparisons

of available data. Consequently, considering the thickness and low permeability of the UTLC the negative

pore pressures developed during excavation will dissipate slowly and the 11 overall slope angle proposed

is unnecessarily shallow.

Findings indicate that the slopes could be excavated to an overall slope of approximately 36 (1.33H:1V)

with a bench face angle of 65 and have a Factor of Safety greater than 1.2 after approximately two years.

These analyses used slightly less favourable shear strength parameters and considerably less favourable

permeability characteristics than now appear appropriate based on the investigation results. Consequently,

the Factor of Safety may be slightly higher and the time to failure is likely to be delayed. The investigations

have confirmed that steeper slopes (than the 11 previously adopted) will be stable for a period that will

allow excavation to the full depth. However, it has not been possible to take into account the mine

dewatering and predict with a sufficient degree of accuracy the time to the onset of slope failure.

The transition from above water table (AWT) to below water table (BWT) mining occurs in 2013 and 2014.

The AWT pit is largely complete (red pit crest line, Figure 6), and initial stripping has commenced, from

west to east, for the nearly 8 km long up to 180 m deep, ultimate BWT pit (Figure 6).

During the pre-feasibility study, the planned mine development schedules (with BWT approvals and

de-watering infrastructure all on the critical path) precluded any consideration for a BWT Trail Pit. As by the

time the necessary approvals and de-watering infrastructure would be in place, the BWT mining would be

ready to commence. A shallow (80 m deep) clay Trial Pit was considered during the feasibility study, which

would not require extensive dewatering as the primary aquifer would not be intercepted, and still provide

important in situ clay performance and characteristics information. Delays in various legislative approvals,

and significant escalation in contractor mining costs, resulted in the project being deferred.

A staged or Interim Trial Slope option has subsequently been adopted. The proposed location is shown in

Figure 6. This approach allows for the important slope performance information to be provided before


14/14



significant final detrital walls are exposed (to the east of the trial slope); and also provides opportunity for

trafficability and material handling trials to be undertaken before significant clay intercepts are exposed.

The dewatering measures to be adopted during the excavation of the Trial Slope are dependent upon the

understanding of the progress of the mine dewatering. Work is currently underway to finalise the Trial

Slope design and respective mining trials. Confirmation of important hydrogeological properties of the

strata underlying the UTLC are also currently under review as the primary aquifer dewatering is advancing;this will assist with the design of the Trial Slope dewatering scheme.

Clay mining trails (trafficability and handleability) are planned and the Trial Slope being integrated into the

mine plan. This includes planning and installation of the Trial Slope instrumentation programme, focussing

on monitoring of slope performance and measurement of the development (and persistence) of negative

pore pressures in response to mining.

Important learnings from the recent Marandoo clay characterisation studies so far have emphasised the

significant impact of sampling method, preservation, timely testing on derived material properties, andrecognition of the limitations of various techniques in characterising heavily over consolidated clays. The

material characterisation has also confirmed the critical importance of water management in mining in

relation to preserving clay strength. This work has enabled detailed planning to be undertaken for future

studies covering trafficability, handleabilty, waste dump and slope performance; with field trials scheduled

in the mine plan. This work is fundamental to developing and implementing and effective below water

table mining strategy.

The authors acknowledge the contributions to this work by Dr Bob Semple (formerly Golder Associates) and

Philip Hawkins (Cambridge Insitu) and the many other people involved whose inputs are much appreciated.

Chandler, R.J. (1984) Recent European experience of landslides in over-consolidated clays and soft rocks, in Proceedings 4th

International Symposium on Landslides, 1621 September 1984, Toronto, Canada, Canadian Geotechnical Society,

Richmond, Vol. 1, pp. 6181.

Clarke, B.G. (1996) Pressuremeters in Geotechnical Design, Blackie, London, 218 p.

Marsland, A. and Randolph, M.F. (1977) Comparisons of the results from pressuremeter tests and large in-situ plate tests in London

Clay, Geotechnique, Vol. 27, No. 2, pp. 217243.

Marsland, A. and Powell, J.J.M. (1990) Pressuremeter tests on stiff clays and soft rocks: factors affecting measurements and their

interpretation, in Field Testing in Engineering Geology, Geological Society Engineering Geology Special Publication, No. 6,

pp. 91110.

Vaughan, P.R. and Walbancke, H.J. (1973) Pore pressure changes and the delayed failure of cutting slopes in overconsolidated clay,

Geotechnique, Vol. 23, No. 4, pp. 531539.

Proposed

Trial Slope

location

Above water

table pit

crest

Below water table

(life of mine) pit

Documents

78_degraaf.pdf