Isaac River Coal Project

Progressive Rehabilitation & Closure Plan

Geotechnical Assessment Report

BMS-BCC-IRP-202008-01-V1.0

August 2020

Isaac River Project Progressive Rehabilitation & Closure Plan Geotechnical Assessment

DOCUMENT CONTROL SHEET

Document Details

Project Isaac River Coal Project

Progressive Rehabilitation & Closure Plan Geotechnical Assessment

Client Xenith Consulting/Bowen Coking Coal Ltd

Report No. BMS-BCC-IRP-202008-01-V1.0

Prepared By T. Sullivan (Senior Engineering Geologist)

Reviewed By T. Grantham (Senior Geotechnical Engineer/Associate)

Approved By T. Grantham (Senior Geotechnical Engineer/Associate)

Document History

Version Date Status Comments Issued To

0.1 01/07/2020 Draft Preliminary Draft J. Porter (Xenith), Tim Rohde

1.0 04/08/2020 Final Final James Porter (Xenith), Tim

Rohde

Ty Grantham Tom Sullivan

Senior Geotechnical Engineer BAppSc, MEngSc, CP (Geotech), RPEQ #18482 Blackrock Mining Solutions Pty Ltd

Senior Engineering Geologist BSc (Geology Hons 1) MAusIMM Blackrock Mining Solutions Pty Ltd

M: +61 437 881 075 M: +61 418 613 610 E: t.grantham@blackrockmining.net E: t.sullivan@blackrockmining.net

Isaac River Project Page i Progressive Rehabilitation & Closure Plan Geotechnical Assessment

TABLE OF CONTENTS

DOCUMENT CONTROL SHEET .............................................................................................. i

Document Details ......................................................................................................................... i

Document History ......................................................................................................................... i

TABLE OF CONTENTS............................................................................................................... i

LIST OF FIGURES ...................................................................................................................... ii

LIST OF TABLES ........................................................................................................................ ii

ACRONYMS & ABBREVIATIONS ......................................................................................... iii

1 INTRODUCTION .................................................................................................................. 1

2 PRCP REQUIREMENTS ..................................................................................................... 1

3 FINAL LANDFORM GEOMETRY .................................................................................... 1

3.1 Excavated Pit Slopes ........................................................................................................ 1

3.2 Final Landform ................................................................................................................. 2

4 SLOPE STABILITY ANALYSIS ......................................................................................... 2

4.1 Methodology and Assumptions ........................................................................................ 2

4.2 Material Properties ........................................................................................................... 3

4.3 Limit Equilibrium Analysis and Results .......................................................................... 4

4.3.1 Leichhardt (LHD) Pit ................................................................................................ 4

4.3.2 Vermont (VEM) Pit .................................................................................................. 5

4.3.3 Out-of-Pit Dump ....................................................................................................... 6

5 CONCLUSIONS ..................................................................................................................... 7

6 REFERENCES ....................................................................................................................... 8

Isaac River Project Page ii Progressive Rehabilitation & Closure Plan Geotechnical Assessment

LIST OF FIGURES

Figure 3-1: Isaac River Project Final Landform Design (Xenith, 2020) ........................................ 2

Figure 4-1: Leichhardt (LHD) Pit Lowwall Final Landform – Circular Failure Analysis ............. 5 Figure 4-2: Leichhardt (LHD) Pit Highwall Final Landform – Circular Failure Analysis............. 5 Figure 4-3: Vermont (VEM) Pit Lowwall Final Landform – Circular Failure Analysis ................ 6 Figure 4-4: Vermont (VEM) Pit Highwall Final Landform – Circular Failure Analysis ............... 6 Figure 4-5: Out-of-Pit Final Landform (West) – Circular Failure Analysis ................................... 7

Figure 4-6: Out-of-Pit Final Landform (East) – Circular Failure Analysis .................................... 7

LIST OF TABLES

Table 3-1: Isaac River Project Recommended Excavated Slope Design Parameters (after GeoTek

Solutions, 2020) .............................................................................................................................. 1

Table 4-1: Isaac River Project Mohr-Coulomb Shear Strength Parameters (after GeoTek

Solutions, 2020) .............................................................................................................................. 3 Table 4-2: Isaac River Final Landform 2D Limit Equilibrium Analysis Results ........................... 4

Isaac River Project Page iii Progressive Rehabilitation & Closure Plan Geotechnical Assessment

ACRONYMS & ABBREVIATIONS

Term Description

BCC Bowen Coking Coal Ltd

BHWE Base of Weathering

BMS Blackrock Mining Solutions Pty Ltd

BUTE Base of Tertiary

FoS Factor of Safety

GLE General Limit Equilibrium Method

LHD Leichhardt

PRCP Progressive Rehabilitation and Closure Plan

RL Reduced Level

SWL Standing Water Level

VEM Vermont

Isaac River Project Page 1 Progressive Rehabilitation & Closure Plan Geotechnical Assessment

1 INTRODUCTION

This report presents the results of a geotechnical assessment for the final landforms, i.e. rehabilitated open cut pits and ex-pit spoil dump, for the proposed Isaac River Coal Project located in the Bowen Basin, Queensland.

The assessment was carried out by Blackrock Mining Solutions Pty Ltd (Blackrock) at the request of Xenith Consulting Pty Ltd (Xenith) as part of Bowen Coking Coal Ltd’s (BCC) Progressive Rehabilitation and Closure Plan (PRCP).

2 PRCP REQUIREMENTS

The general geotechnical PRCP requirements for voids and final landforms, as specified in the Department of Environment and Science’s guidelines, are outlined as follows:

• Specifications of the mechanisms for achieving acceptable geotechnical stability.

This includes:

o void filling, partial void filling, reshaping and void configuration through

earthwork methods such as back filling, regrading, buttressing and

benching; and

o where applicable, methods and techniques for achieving safe slopes must be

detailed.

• Consideration of long-term erosion and weathering; and

• Details of how the void and final landform slopes achieve post-closure stability

consistent with the planning and design of the post-closure mine void.

3 FINAL LANDFORM GEOMETRY

3.1 Excavated Pit Slopes

Excavated pit slope geometry for the Leichhardt (LHD) Pit, Vermont (VEM) Pit, and Out-

of-Pit Dump analyses were based on GeoTek Solutions’ (2020) recommendations, as

summarised in Table 3-1.

Table 3-1: Isaac River Project Recommended Excavated Slope Design Parameters (after GeoTek Solutions, 2020)

Area/Pit

Component

Min

Bench

Width

Max

Batter Face

Angle

Max

Batter

Height

Lowwall/Boxcut

Weathered Permian 15 m 45° 20 m

Eastern Lowwall 15 m 45° 20 m

Isaac River Project Page 2 Progressive Rehabilitation & Closure Plan Geotechnical Assessment

Area/Pit

Component

Min

Bench

Width

Max

Batter Face

Angle

Max

Batter

Height

Weathered Permian adjacent intrusion

Endwalls/Highwalls

Highly weathered Permian

Distinctly weathered to fresh Permian

15 m

15 m

45°

60°

20 m

40 m

3.2 Final Landform

Final landform geometry for the Leichhardt (LHD) Pit, Vermont (VEM) Pit, and Out-of-Pit

Dump analyses were based on Xenith’s (2020) report, as shown in as shown in Figure 3-1.

Final landform slopes were designed to 1V:6H (~9.5°). The base of the final void landform

was designed to be above the maximum void water level of approximately 195 m AHD.

All excavated slopes would be buttressed by fill material.

Figure 3-1: Isaac River Project Final Landform Design (Xenith, 2020)

4 SLOPE STABILITY ANALYSIS

4.1 Methodology and Assumptions

Two-dimensional limit equilibrium analyses were performed to determine the overall slope stability in terms of a Factor of Safety (FoS), which is a commonly employed measure in slope stability analysis to determine the likelihood of slope failure. The FoS is generally a

Isaac River Project Page 3 Progressive Rehabilitation & Closure Plan Geotechnical Assessment

measure of driving forces versus resisting forces in a system, where a FoS of 1 equates to a 50% probability that failure will occur. FoS values >1 indicate that a system is likely to be geotechnically stable.

Three final landform scenarios were examined:

• Leichhardt (LHD) Pit final landform;

• Vermont (VEM) Pit final landform; and

• Out-of-Pit Dump final landform.

The following methodology and criteria were used for the assessment:

• Models were generated from representative cross-sections cut through the proposed

final landforms;

• Models were analysed using industry standard 2D limit equilibrium methods in

Slide 2018 software (Rocscience), i.e. circular failure surfaces using the Auto-

Refine search algorithm and the General Limit Equilibrium method (GLE);

• The generally accepted FoS criteria for long-term slopes of ≥1.5 was adopted; and

• Category 2.5 spoil material is anticipated to be encountered, with a basal saturated

spoil layer of approximately 5 m thickness assumed for the Out-of-Pit Dump

scenario.

4.2 Material Properties

The current assessment is based on the same Mohr-Coulomb shear strength parameters and material types specified in GeoTek Solutions’ (2020) report, as summarised in Table 4-1. It should be noted that the shear strength parameters for ‘DW sed rock’ were adopted for ‘Weathered Permian Rock’ in the current assessment.

It is anticipated that the in-pit and out-of-pit spoil will comprise Category 2.5 materials, i.e. a mixture of Category 2 (matrix supported) and Category 3 (framework supported) spoil materials proposed by Simmons & McManus (2004).

Table 4-1: Isaac River Project Mohr-Coulomb Shear Strength Parameters (after GeoTek Solutions, 2020)

Material Unit Weight

γ′ (kN/m3)

Cohesion

c′ (kPa)

Friction Angle

ɸ′ (°)

Quaternary/Tertiary 20 50 30

Weathered Permian Rock 22 100 30

Fresh Permian Rock 24 450 42

Fresh Permian Coal 20 30 35

Category 2.5 Spoil (U) 18 40 29

Category 2.5 Spoil (S) 20 18 24

(U) denotes unsaturated; (S) denotes saturated

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4.3 Limit Equilibrium Analysis and Results

Two-dimensional limit equilibrium analyses were carried out for the LHD Pit, VEM Pit, and Out-of-Pit Dump final landforms, respectively. Slopes were assessed for a circular failure mechanism through Category 2.5 spoil material using the Auto-Refine search, and the Bishops Simplified, Spencer’s and General Limit Equilibrium (GLE) methods.

Analyses returned critical FoS values ranging between 3.77 and 4.50, which exceed the minimum FoS requirement of ≥1.5. These results are considered satisfactory in terms of meeting final landform design and long-term geotechnical stability criterion.

The results of the 2D limit equilibrium analyses are summarised in

Table 4-2 and presented in Figure 4-1 to Figure 4-6 in the following sections.

Table 4-2: Isaac River Final Landform 2D Limit Equilibrium Analysis Results

Final Landform Slope FoS Acceptable

Leichhardt (LHD) Pit Lowwall 4.13 Yes

Highwall 4.23 Yes

Vermont (VEM) Pit Lowwall 4.50 Yes

Highwall 3.87 Yes

Out-of-Pit Dump West 4.07 Yes

East 3.77 Yes

GLE/Morgenstern-Price FoS reported

4.3.1 Leichhardt (LHD) Pit

Analysis of the Leichhardt (LHD) Pit final landform design geometry returned a FoS of 4.13

for the lowwall side and a FoS of 4.23 for the highwall side, as shown in Figure 4-1 and

Figure 4-2, respectively.

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Figure 4-1: Leichhardt (LHD) Pit Lowwall Final Landform – Circular Failure

Analysis

Figure 4-2: Leichhardt (LHD) Pit Highwall Final Landform – Circular Failure

Analysis

4.3.2 Vermont (VEM) Pit

Analysis of the Vermont (VEM) Pit final landform design geometry returned a FoS of 4.50

for the lowwall side and FoS of 3.87 for the highwall side, as shown in Figure 4-3 and Figure

4-4, respectively.

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Figure 4-3: Vermont (VEM) Pit Lowwall Final Landform – Circular Failure Analysis

Figure 4-4: Vermont (VEM) Pit Highwall Final Landform – Circular Failure

Analysis

4.3.3 Out-of-Pit Dump

Analysis of the Out-of-Pit final landform design geometry returned a FoS of 4.07 for the

western slope and FoS of 3.07 for the eastern side, as shown in Figure 4-5 and Figure 4-6,

respectively.

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Figure 4-5: Out-of-Pit Final Landform (West) – Circular Failure Analysis

Figure 4-6: Out-of-Pit Final Landform (East) – Circular Failure Analysis

5 CONCLUSIONS

Final landform designs have been proposed that meet the requirements of the PRCP guidelines from a geotechnical perspective.

It can be concluded from the limit equilibrium analyses performed that the proposed final landform designs exceed the minimum required FoS of ≥1.5 for long-term geotechnical stability, and are therefore considered acceptable.

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6 REFERENCES

Department of Environment and Science (2019). “Guideline – Progressive Rehabilitation and Closure Plans (PRC Plans)”. ESR/2019/4964 Version 1.00.

GeoTek Solutions Pty Ltd (2020). “Initial Geotechnical Assessment of the Isaac River Project”. Report prepared for Bowen Coking Coal Ltd. Report No. 116-20-1, April 2020.

Simmons, JV & McManus DA (2004). “Shear Strength Framework for Design of Dumped Spoil Slopes for Open Pit Coal Mines”. In: Jardine, RJ, Potts, DM & Higgins, KG (eds), Advances in Geotechnical Engineering: The Skempton Conference, Imperial College, London, Vol 2, 981-991, Thomas Telford, 2004.

Xenith Consulting Pty Ltd (2020). “Development of the Isaac River Mine Plan and Final Landform”. Report prepared for Bowen Coking Coal Ltd. Document No. 1938BCCM, June 2020.