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Mt Magnet Gold Pty Ltd

Mt Magnet Gold Mine

Preliminary Design of Geotechnical Monitoring for Raising Checkers Tailings Dams CTSF1, CTSF2 and CTSF3

21 October 2019

Preliminary Design of Geotechnical Monitoring for Raising Checkers Tailings Dams CTSF1, CTSF2 and CTSF3

(Revision 0)

Coffey Services Australia Pty Ltd ABN: 55 139 460 521

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Prepared for: Mt Magnet Gold Pty Ltd PO Box 83, Mount Magnet WA 6638

Attention: Dolly Pett

Prepared by: Coffey Services Australia Pty Ltd 1/235 St Georges Terrace Perth WA 6000 Australia t: +61 8 6218 2100 ABN: 55 139 460 521

21 October 2019

Document authorisation

Our ref: 754-PERGE232736-20 (Rev. 0)

For and on behalf of Coffey

This is a scanned signature held on file by Coffey. The person and signatory consents to its

use only for the purpose of this document.

Peter Lamb Instrumentation and Monitoring Manager; Principal Geotechnical Engineer

Quality information

Revision history

Revision Description Date Authors Reviewer Signatory

A Complete draft for MMG review. 08/10/19 PHL, PAC PHL

0 Final version (no MMG comments on Rev.A) 21/10/19 PHL PHL

Distribution

Report Status No. of copies Format Distributed to Date

Revision 0 1 PDF D. Pett, C. Dart 21/10/19

Preliminary Design of Geotechnical Monitoring for Raising Checkers Tailings Dams CTSF1, CTSF2 and CTSF3

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Coffey Services Australia Pty Ltd ABN: 55 139 460 521

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Table of Contents

1. Introduction .......................................................................................................................... 4

2. Background .......................................................................................................................... 5

3. Scope of Work ...................................................................................................................... 5

4. Purposes of Monitoring ....................................................................................................... 6

5. Instrumented Monitoring System ........................................................................................ 7

5.1. Instrumentation Types .................................................................................................. 7

5.2. Data Acquisition ........................................................................................................... 7

5.3. Preliminary Instrumentation Design .............................................................................. 9

6. Installation Programme...................................................................................................... 15

7. Closure ............................................................................................................................... 15

8. References ......................................................................................................................... 16

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Tables

Table 1 – Monitoring Data Acquisition ............................................................................................ 8

Table 2 – Proposed Instrumentation for CTSF1-3 .......................................................................... 9

Table 3 – Projected Instrumentation Installation Programme ........................................................ 15

Figures

Figure 1 – Locality Plan - Mt Magnet Gold Mine Checkers Tailings Storage Facilities

Figure 2 – CTSF1 and CTSF2 – Plan Showing Proposed Instrumented Monitoring Sections

Figure 3 – CTSF1 – Composite Instrumented Monitoring Section

Figure 4 – CTSF1 – Proposed Instrumented Monitoring Sections

Figure 5 – CTSF2 – Composite Instrumented Monitoring Section

Figure 6 – CTSF2 – Proposed Instrumented Monitoring Sections

Figure 7 – CTSF3 – Plan Showing Proposed Instrumented Monitoring Sections

Figure 8 – CTSF3 – Composite Instrumented Monitoring Section

Figure 9 – CTSF3 – Proposed Instrumented Monitoring Sections

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1. Introduction

This document presents a preliminary design for instrumented geotechnical monitoring of Checkers

Tailing Storage Facilities (CTSF) 1, 2 and 3 (CTSF1, CTSF2 and CTSF3) at the Mt Magnet Gold Mine.

Mt Magnet Gold Mine (MMGM) is located in Western Australia approximately 6km northwest of the Mt

Magnet township centre. MMGM is operated by Mt Magnet Gold Pty Ltd (MMG) and owned by

Ramelius Resources Limited (RRL).

MMGM has three paddock-type tailings storage facilities, CTSF1, CTSF2 and CTSF3 (Figure 1).

CTSF3 is active (receiving tailings on a regular basis) and CTSF1 and CTSF2 are currently inactive.

Further information on the status of the facilities and design of embankment raises is provided in Section

2.

CTSF3 is currently monitored via a network of piezometers, monitoring bores and one borehole

inclinometer. Inactive CTSF1 and CTSF2 are not currently monitored with instrumentation. This

document presents preliminary design of instrumented monitoring for recommissioning of CTSF1 and

CTSF2 and for continued operation of CTSF3. The report also summarises the purposes of the

geotechnical monitoring and presents options and recommendations for data acquisition and monitoring

data management.

Figure 1 – Locality Plan - Mt Magnet Gold Mine Checkers Tailings Storage Facilities

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2. Background

Of the three TSFs at MMGM, CTSF3 is active (receiving tailings on a regular basis) and CTSF1 and

CTSF2 are currently inactive.

Stage 8 and Stage 9 embankment raises of CTSF3 were designed by Coffey in August 2017 and the

Stage 9 embankment raise to RL485m is currently under construction. CTSF3 embankments have

been raised by upstream construction methods using tailings from CTSF1. Refer to Coffey (2016)

‘Construction Report CTSF3 Stage 7 Embankment raising by 2.5m to RL480.5m’ (Reference 1).

Coffey recently undertook an options study to assess future management of tailings at MMGM (‘2019

TSF Options Study’) (Reference 2) based on a 5 and 10-year Life of Mine (LoM), at a proposed mill

throughput rate of 2Mtpa. The study outcomes included a recommendation to recommission CTSF1

and CTSF2. Coffey then recently completed design of the next embankment raises for CTSF1

(RL483m to RL488m), CTSF2 (RL476m to RL488m) and CTSF3 (RL485m to RL490m) to store

additional tailings until the current projected end of operation for the TSFs. Refer to Coffey (2019)

‘CTSF 1,2 and 3 Embankment Raise Design Report’ (Reference 3). Details of the proposed TSF lifts

are shown in drawings 754-PERGE223063-02 and 03 in that report.

MMG subsequently requested the services of Coffey’s instrumentation and monitoring group to advise

geotechnical instrumented monitoring requirements for CTSF1 and CTSF2 (D. Pett email 28 August

2019) and then CTSF3 (D. Pett email 11 September 2019).

Coffey previously designed and installed an instrumented monitoring system at CTSF3 (refer to Coffey

(2017) ‘CTSF3 Geotechnical Instrumentation Installation Report’ (Reference 4), and currently provides

ongoing monitoring services to MMG for CTSF3, including monthly site support and reporting, and

monitoring data interpretation and reporting for the annual TSF audit (Coffey (2019) ‘Mt Magnet Gold

Pty Ltd, TSF Audit 2019’, Reference 4).

3. Scope of Work

The following scope of work for preliminary design of CTSF1, CTSF2 and CTSF3 geotechnical

instrumented monitoring was prepared by Coffey based on our understanding of MMG’s requirements

(D. Pett email and subsequent telephone call, 28 August 2019).

• Document background of Mt Magnet TSF operations and proposed recommissioning of CTSF1 and

CTSF 2.

• Explain purposes of geotechnical monitoring.

• Recommend types of geotechnical monitoring instrumentation required for CTSF1, CTSF2 and

CTSF3 monitoring. These will include vibrating wire piezometers (VWPs), standpipe piezometers,

borehole inclinometers and surface displacement survey targets.

• Prepare a preliminary design of an instrumented monitoring system for CTSF1, CTSF2 and CTSF3,

including proposed locations shown in plan and section.

• Present options for data acquisition and data management, including: data logging, telemetry and

monitoring data management software.

• Prepare a cost estimate (refer separate document – Reference 5) for provision of:

◼ Detailed design of instrumented monitoring system,

◼ Supply of instrumentation, equipment and consumables,

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◼ Drilling services for installation of borehole instrumentation,

◼ Installation, testing and commissioning of instrumentation and equipment, including technical

supervision of the drilling contractor,

◼ Set-up and configuration of the monitoring databases, including for a web-based monitoring

data management software option, and

◼ Preparation of a factual installation report.

• Present a programme estimate to deliver the above scope of works.

Detailed design of the instrumented monitoring for CTSF1, CTSF2 and CTSF3 is to be undertaken

following review and approval of the embankment raise designs (Reference 3) and following review of

this preliminary monitoring design.

4. Purposes of Monitoring

Geotechnical monitoring of tailings storage facilities is undertaken for a variety of purposes, many

essential to help ensure the safe operation of the facilities.

Embankment monitoring is undertaken principally to measure the performance of the embankments in

response to construction, tailings emplacement and external factors such as weather and seismicity,

and to provide warnings of potential instability. Other key reasons for monitoring include allowing

evaluation of critical design assumptions (design verification and optimisation), for construction control,

as an input into risk management and to help devise remedial measures in the event of any instability.

A broader list of reasons for monitoring can be summarised as:

• Allow indication of conditions that could cause instability. Reveal unknowns.

• Provide early warning of instability or impending failure.

• Measure performance of facility for safety during life of structure.

• Evaluate critical design assumptions – for design verification and design optimisation.

• Calibration of stability assessments and numerical models.

• Embankment construction control.

• Process control – help inform decisions integral to TSF operation.

• Minimise influence upon adjacent structures and landforms.

• Target performance improvement.

• Quality assurance – materials, methodology, integrity and durability, process compliance

• Devise remedial measures in the event of instability.

• Input to risk assessment.

• Inform stakeholders.

• Legislative and regulatory compliance.

• Reduce litigation.

• Advance state of knowledge and assist research.

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5. Instrumented Monitoring System

5.1. Instrumentation Types

In consideration of the above monitoring objectives, the design of the proposed embankment raises

and the intended life of the facilities, the following monitoring instrumentation is proposed for CTSF1,

CTSF2 and CTSF3, as shown in Figures 2 to 9:

1. Vibrating wire piezometers (VWPs) installed in vertical boreholes, with sensors at multiple levels

within each borehole, to measure pore pressure and thus pressure/piezometric head within the

emplaced tailings and (at some locations) within the TSF foundation.

2. Vibrating wire piezometers installed in shallow excavations (or in a short vertical pipe housing) in

the tailings beach just upstream of the ‘current’1 embankment toe, to measure pore pressure and

thus pressure/piezometric head within the emplaced tailings.

3. Standpipe piezometers (vertical boreholes) to measure pressure/piezometric head within the

emplaced tailings, and to allow a comparison to data from adjacent/nearby VWPs.

4. Borehole inclinometers (vertical boreholes) to measure subsurface lateral deformation and

displacement, and thus identify potential failure surfaces.

5. Surface monitoring prisms, surveyed via total stations, to measure 3D surface displacement.

In addition to the above monitoring, any observed surface tension cracking, when detected, is to be

carefully monitored, preferably using instrumentation to allow accurate measurement of displacement

across each significant crack. Until significant cracking (i.e. not surface drying cracking) is observed

the location and specific design of appropriate crack monitoring devices cannot be determined, but

MMG should be prepared to be able to implement crack monitoring as soon as is feasible after cracking

is detected.

The proposed preliminary design of the CTSF1-3 monitoring instrumentation is detailed in Section 5.3.

5.2. Data Acquisition

At a minimum, data is to be acquired from the monitoring instrumentation by the methods and at the

frequencies shown in column 2 in Table 1.

However, considering the quantity of monitoring instruments (existing and proposed), the limited

resources on site to read/survey/download the instruments, and the numerous benefits of automation

such as improved data quality and frequency, Coffey recommends MMG considers the additional

automation as shown in column 3 in Table 1.

Automation of data acquisition can be applied partially or progressively across the monitoring system,

according to monitoring priorities, actual monitoring results and available finances. However Coffey

recommends that each new VWP be data logged regardless, as is the case for the existing CTSF3

VWPs.

1 ‘Current’ refers to the existing completed embankment raise at the time of instrument installation.

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Note that the frequencies of reading/survey shown are for monitoring under normal operating

conditions. Should there be a deviation from normal conditions, such as a major rainfall event or

detection of instability, then all monitoring frequencies are to be reviewed and increased as determined

to be appropriate by engineering assessment.

Table 1 – Monitoring Data Acquisition

Type of Instrument

Data Acquisition: Minimum Data Acquisition: Recommended for Consideration

Vibrating wire piezometers

Connected to a (multi-channel)

data logger to be automatically

read at a pre-determined interval

(e.g. hourly), as per current CTSF3

VWPs.

Add telemetry (by adding radio devices to data

loggers or replacing data loggers with combined

logging/telemetry devices) for automatic data

transmission to monitoring data server.

Standpipe piezometers

Manually read using electrical dip

meter, at a minimum once per

month.

Add submersible transducers to automate water level

readings. Transducers to be connected to data

loggers, with further option for telemetry as for VWPs

above.

Borehole inclinometers

Manually surveyed using traversing

probe (MMG already owns DGSI

Digitilt AT system), at a minimum

once per month.

Consider automating any inclinometers recording

significant displacement, by installing in-place

inclinometer chain/array, connected to data logger,

reading e.g. hourly. Could also further automate by

adding telemetry as for VWPs above.

Monitoring prisms

Manually surveyed from total

station mounted on network of

survey pillars situated outside toe

of TSFs, at a minimum once per

month.

Survey could be automated by permanently mounting

total station operating in robotic mode (i.e. automated

total station, ATS) but that would only be warranted if

significant slope displacements were recorded and

thus increased monitoring survey coverage and

frequency was warranted.

Monitoring data management

Data logger download data and

manual readings processed (for

inclinometers and prisms this is in

proprietary software) and stored in

Microsoft Excel database files.

Presentation and reporting of data

is carried out manually.

All data uploaded (manually, or automatically if

telemetry is implemented) to a web-based monitoring

data management system (MDMS). The MDMS

handles import, validation, processing, storage,

presentation, reporting and export of all monitoring

data. Each user accesses the MDMS (via secure

login) on any web browser without needing to install

any local software.

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5.3. Preliminary Instrumentation Design

Based on the above monitoring objectives and types of instrumentation, a preliminary design for

monitoring instrumentation at each of the three CTSFs has been prepared, as presented in Table 2 and

in Figures 2 to 9.

Table 2 – Proposed Instrumentation for CTSF1-3

Intrumentation Section

Existing Instrumentation

Proposed Instrumentation

Borehole VWPs

Beach VWP

Standpipe Piezometer

Inclino-meter

Monitoring Prisms

CTSF1

Section 1 None. 130m south of toe monitoring bore T3MB1

⚫⚫⚫ ⚫ ⚫ ⚫⚫⚫

Section 2 None ⚫⚫⚫ ⚫ ⚫ ⚫⚫⚫

Section 3 None ⚫⚫⚫ ⚫ ⚫ ⚫ ⚫⚫⚫

CTSF2

Section 1 None ⚫⚫⚫ ⚫ ⚫ ⚫⚫⚫

Section 2 None ⚫⚫⚫ ⚫ ⚫ ⚫⚫⚫

Section 3 None ⚫⚫⚫ ⚫ ⚫ ⚫⚫⚫

Section 4 None ⚫⚫⚫ ⚫ ⚫ ⚫⚫⚫

CTSF3

Section 1 None ⚫⚫⚫ ⚫ ⚫⚫⚫⚫

Section 2 VWPs installed from Stage 7 crest and Stage 5 berm

⚫⚫⚫ ⚫ ⚫ ⚫⚫⚫⚫

Section 3 VWPs installed from Stage 7 crest and Stage 5 berm. Inclinometer installed from Stage 7 crest.

⚫⚫⚫ ⚫ ⚫⚫⚫⚫

Section 4 None. 120m west of section with VWPs installed from Stage 7 crest and Stage 5 berm

⚫⚫⚫ ⚫ ⚫ ⚫⚫⚫⚫

Section 5 VWP installed from Stage 7 crest ⚫⚫⚫ ⚫ ⚫ ⚫⚫⚫⚫

Section 6 None ⚫⚫⚫ ⚫ ⚫⚫⚫⚫

Proposed monitoring instrumentation sections at CTSF1 and CTSF2 are shown in Figure 2. Each of

these monitoring sections is entirely new as there is currently no geotechnical monitoring

instrumentation at either CTSF1 or CTSF2.

Proposed monitoring instrumentation sections at CTSF3 are shown in Figure 7. Sections 2, 3 and 5

are designed to be aligned at or near existing monitoring instrumentation, which is shown in orange in

Figures 8 and 9.

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Figure 2 – CTSF1 and CTSF2 – Plan Showing Proposed Instrumented Monitoring Sections

Figure 3 – CTSF1 – Composite Instrumented Monitoring Section

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Figure 4 – CTSF1 – Proposed Instrumented Monitoring Sections

Figure 5 – CTSF2 – Composite Instrumented Monitoring Section

Section 1

Section 2

Section 3

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Figure 6 – CTSF2 – Proposed Instrumented Monitoring Sections

Figure 7 – CTSF3 – Plan Showing Proposed Instrumented Monitoring Sections

Sections 1 and 3

Sections 2 and 4

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Figure 8 – CTSF3 – Composite Instrumented Monitoring Section

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Figure 9 – CTSF3 – Proposed Instrumented Monitoring Sections [for legend refer to Figure 7] [monitoring prisms not shown]

Section 1 Section 2

Section 4 Section 3

Section 5 Section 6

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6. Installation Programme

The preliminary instrumentation design presented above is for monitoring of each of the Checkers TSFs through

to completion of embankment construction and tailings deposition, to the designs presented by Coffey in design

report Coffey (2019) ‘CTSF 1,2 and 3 Embankment Raise Design Report’ (Reference 3). However depending on

results from the proposed CTSF2 instrumentation it may be advisable to install additional instrumentation from

the upper embankment raises of CTSF2 after they are constructed.

The preliminary instrumentation design presented in this report is all – apart from the uppermost monitoring prisms

– to be installed from either the next embankment raise (CTSF1 and CTSF2) or immediately following the current

embankment raise (Stage 9 raise under construction at CTSF3). Therefore according to the embankment

construction schedule proposed in report Coffey (2019) ‘2019 TSF Options Study’ (Reference 2), the proposed

instrumentation would be installed at approximately the time intervals listed in Table 3.

Table 3 – Projected Instrumentation Installation Programme

TSF Projected Monitoring Instrumentation Installation Period

CTSF1: Next lift 2m lift to RL485m November 2019 to February 2020.

Lower prisms (and prisms at RL483m) can be installed now.

CTSF2: Next lift 3m to RL479m February 2021 to April 2021.

Lower prisms (and prisms at RL476m) can be installed now.

CTSF3: Current lift 2m to RL485m

January 2022 to March 2022.

Lower prisms can be installed now and prisms at RL485m can be

installed as soon as Stage 9 earthworks are complete.

Alternatively the CTSF1 and CTSF2 instrumentation could be installed from the current embankment crest and

dry tailings surface, prior to the first re-commissioning embankment raises to RL485m and RL479m respectively

That would have the advantage over the design presented above of saving some drilling costs (shorter boreholes),

and being able to be undertaken at any time from now – subject to MMG’s resources – rather than waiting for

completion of an embankment raise at CTSF1 and CTSF2. Should MMG wish to pursue that option then Coffey

would adjust and resubmit the preliminary design presented in this report.

7. Closure

This report presents a preliminary design for geotechnical instrumented monitoring of CTSF1, CTSF2 and CTSF3

to completion, based on the embankment raise and tailings deposition design also prepared by Coffey.

We trust that the report meets MMG’s requirements but should you require any clarification or any further

information please contact Peter Lamb in Coffey’s Perth office.

A process of detailed monitoring design is to be undertaken following review of this preliminary design. Detailed

design is to include detailed borehole and instrumentation design and specification, preparation of issued for

construction (IFC) drawings, preparation of scopes of work and method statements for drilling services and

instrumentation installation and preparation of a detailed schedule for staged implementation of the monitoring

systems.

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8. References

1. Coffey Mining Pty Ltd (2016). ‘Construction Report: CTSF 3 Stage 7 Embankment Raising by 2.5m to RL480.5m’, Ref: MINEWPER00310AN Rev 0.

2. Coffey (2019). ‘2019 TSF Options Study’, Ref: 754-PERGE222411.

3. Coffey (2019). ‘CTSF 1,2 and 3 Embankment Raise Design Report’, Ref: 754-PERGE233063 Rev B.

4. Coffey (2019). ‘Mt Magnet Gold Pty Ltd, TSF Audit 2019’. Ref: 754-PERGE230284.

5. Coffey (2019). ‘Preliminary Cost Estimate for Geotechnical Monitoring of Raising of Checkers Tailings Dams CTSF1-3’. Ref: 754-PERGE232736-21 (Rev A).