report€for€golden€grove operations€tailings€storage facility

Oz Minerals GoldenGrove Operations

Report for Golden GroveOperations Tailings Storage

FacilityDesign for TSF 2 Raise 7B

ADDENDUM

October 2008

61/22813/82212 Golden Grove Operations Tailings Storage FacilityDesign for TSF 2 Raise 7B ADDENDUM

Contents

1. Introduction 1

2. Licence Conditions 2

3. Design Parameters 3

3.1 Capacity 3

3.2 Design Criteria 3

4. Stability Analyses 5

4.1 Background 5

4.2 Stability Assessment 6

5. Piezometers 7

6. Conclusions and Recommendations 8

7. References 9

8. Scope & Limits of Geotechnical Investigation 10

Table IndexTable 1 – TSF 2 Geometry 3Table 2 – Capacity estimation 3Table 3 – Embankment Design Criteria 4Table 4 – Material Properties TSF 2 5Table 5 Summary of Stability of TSF 2 6

Figure IndexFigure 1 Existing Piezometers and Phreatic Levels 7

AppendicesA Drawings


1. IntroductionTHIS DOCUMENT IS AN ADDENDUM TO THE GOLDEN GROVE OPERATIONS TAILINGSSTORAGE FACILITY DESIGN FOR TSF 2 RAISE 7, GHD REPORT REF 61/19305/69386 REVISION2, DATED SEPTEMBER 2007.

This report documents the additional technical design aspects of increasing the approved design heightof Tailings Storage Facility No. 2 from its approved height of RL385.0m to RL386.5m. The documentshould be read in conjunction with the original design report ref: 61/19305/69386 revision 2, datedSeptember 2007.

The proposed raise is referred to as TSF No.2 Raise 7B.


2. Licence Conditions

The current licence conditions states:

2.1.1 Dept. of Environmental (DoE)

The Oxiana operations are currently operated under licence no. 5175/8, file no. L144/88 dated 30November 2006. Conditions W2 relate specifically to the maintenance of the Tailings Storage Facility,however Condition W5 and W6 also reflect controls through the monitoring of strategically placed boresagainst a set suite of analytes. Timely reporting of any exceedances or noncompliance against theabove conditions is dictated by conditions G2. This licence is currently under review, however until suchtimes that it is upgraded 5175/8 will remain applicable.

Under Part V, Section 53 of the Environmental Protection Act 1986 it is the requirement of the occupierof a prescribed premise to be authorised in respect of certain changes leading to discharge of wastewhich includes construction or alterations to a Tailings Storage Facility. Such activities are authorised inaccordance with a works approval, a licence or a requirement contained in a closure notice or anenvironmental protection notice.

Conditions as set in the DoE licence were addressed in the 2006 Audit Report, Reference 1.

2.1.2 Dept. of Industry and Resources (DoIR)

The TSF 2 overlaps on two leases namely M59/3 and M59/227. The following compliance of TSF2 isdictated by the following Clauses sourced from respective tenement conditions:

• Clause 8: Operations to maxmise the settled density, minimise seepage and collect seepage

• Clause 9: Keep the water pond at a practical minimum.

• Clause 10: Outside walls to be covered with competent rock to prevent erosion.

• Clause 19: Construction of starter bank to be supervised.

• Clause 20: Provide a detailed construction report 1 month after completion of the works.

• Clause 21: The TSF shall be checked daily.

• Clause 22: Biennial audit is required.

• Clause 23: Decommissioning review is required prior to rehabilitation.

• Clause 24: Construction details to be provided for the starter embankment and subsequent lifts.

• Clause 25: Provision of a site specific Operations Manual is required.

• Clause 26: Dam break and emergency actions plans are required prior to deposition.

• Clause 27: Stability assessments required prior to first lift.

Conditions as set in the DoIR conditions were addressed in the 2006 Audit Report, Reference 1.


3. Design Parameters

3.1 CapacityThe storage characteristics summarised in Table 1 have been extrapolated from previous productionrates as noted in the 2006 Audit report, (reference 1). The capacity assessment is based on a 1.146Mtpaproduction rate.

Table 1 – TSF 2 Geometry

Description Value

Date of last wall raise: May 2008

Current crest level: RL 384 m

Approved crest level: RL 385 m

Final crest level: RL 386.5 m

Proposed area at final crest level: 26.9 ha (approx.)

Storage capacity available from Raise 7B: 918 000 m3 (approx.)

Maximum height of TSF: (RL 360 m to RL 386.5 m) = 26.5 m

Embankment width at crest (average) 6.4 m (including cover layer)

Overall exterior slope above starter wall (V: H) 1:4

Local upstream wall slope (V: H) 1:1.75

Local downstream wall slope (V: H) 1: 2.75

Based on the geometry of TSF 2 Raise 7B in Table 1the following estimation of the capacity can bemade:

Table 2 – Capacity estimation

Case Volume of 2.5membankmentraise (m3)

Dry Density(t/m3)

Tonnage (t) MonthsCapacity* (m)

Higher Density 602 500 1.9 1 144 750 12

Lower Density 602 500 1.6 964 000 10

* Based on 1.146M tpa. The lower density capacity should be a more realistic figure as the density ofthe tailings around the pool area will be lower than along the perimeter.

3.2 Design CriteriaThe current information relating to the tailings system and pertinent to the design of the facility issummarised below. The appropriate embankment design criteria adopted for a “Significant Hazard Dam”(refer to Section 4.1, reference 6) are shown in Table 3. Other conditions require the dam to be


designed by suitably experienced engineers and that the quality and conformance of construction beverified by suitably qualified geotechnical or engineering specialists.

Table 3 – Embankment Design Criteria

Design Parameter Value

Maximum allowable overall embankment side slope 20°

Design earthquake (Maximum Credible)

peak horizontal acceleration 1.2 m/s² (≈ 0.12 g)

Minimum tailings freeboard, below dam crest 300 mm

Target factors of Safety, in ANCOLD Guidelines for Tailings Dams

steady seepage at high pool level

construction Conditions

earthquake Loading (pseudostatic analysis)

1.5

1.3

1.1


4. Stability Analyses

4.1 BackgroundStability of the embankment slopes for Lift No. 7B on TSF 2 was analysed using the limit equilibriumsoftware package SLOPE/W.

The stability was analysed under the following conditions:

» Downstream,

o Overall stability up to final tailings level with new crest elevation RL 386.5 m, and

o Local stability up to final tailings level with new crest elevation RL 386.5 m.

» Operational and storm event water levels, and

» Static and Pseudostatic (Pseudostatic acceleration coefficient = 0.12 g) loading conditions.

The stability assessment evaluated the following areas of the TSF:

» Highest (perceived highest risk) section around the TSF, on the northeastern wall, and

» Seepage area on the southeast wall.

Refer to Appendix C for the analysis results.

The phreatic surface used in the stability analysis at the highest embankment of the TSF was obtainedfrom the piezometers readings, received from Oz Minerals.

For all longterm downstream stability cases the phreatic surface has been assumed at a high level,which simulates the expected conditions when tailings have recently been placed up to 0.3 m below thecrest level.

4.1.1 Parameters

The material parameters used in the stability analyses are summarised in Table 4. These parametersare based on results from previous investigations noted in Section 4, reference 6.

Table 4 – Material Properties TSF 2

MaterialDescription

Bulk UnitWeight,(kN/m3)

Cohesion(kPa)

UndrainedShearStrength,(kPa)

FrictionAngle,(deg)

Tailings (overall) 19 7 32

Tailings (below embankment) 19 19

Embankment Saprolite Clay 19 10 28

Silty Clay Core 18 30 28

Foundation Caprock 20 60 30


MaterialDescription

Bulk UnitWeight,(kN/m3)

Cohesion(kPa)

UndrainedShearStrength,(kPa)

FrictionAngle,(deg)

Foundation Saprolite Clay 20 10 28

Waste Rock 22 0 40

4.2 Stability AssessmentThe following stability analyses were carried out under the loading conditions discussed in Section 4.1:

» Upstream slope of the newly proposed embankment raise,

» Downstream slope of the newly proposed embankment raise, and

» Overall stability of the downstream slope.

4.2.1 Stability Analyses Results

The results of the stability analysis are included in Appendix C and are summarised in Table 5.

Table 5 Summary of Stability of TSF 2

Case No. Description Static FoS Pseudostatic FoS

Highest wall

Downstream Overall 1.96 1.30

Downstream Local 2.58 1.92

Seepage section (with buttress)

Downstream Overall 2.04 1.28

Downstream Local 1.96 1.46

The above stability results indicate that under static and pseudostatic loading conditions, acceptableshort term and longterm operational factors of safety (FoS) for a 3.5 m high upstream TSF 2embankment raise can be expected. Based on ANCOLD guidelines a shortterm FoS in excess of 1.3 isconsidered acceptable under static loading and operational conditions.

Embankment settlement under dynamic loading was not evaluated as part of the current design as theFoS is greater than 1.1 for the longterm stability scenarios.


5. Piezometers

OZ Minerals has installed piezometers on TSF 2 as per Figure 1. In addition to the existing piezometers,GHD has proposed to install additional standpipe piezometers with at least three piezometers in a line.This is required for ongoing phreatic surface monitoring on the higher walls, and potential seepageareas. It would also assist with the post closure monitoring of the TSF, as per the closure requirements.

Refer to the Drawings in Appendix A for the existing piezometer positions and standpipe details.

Figure 1 Existing Piezometers and Phreatic Levels

6

15

4

ProposedPiezometerLines

ExistingPiezometers

SeepageArea


6. Conclusions and Recommendations

The design of Lift No 7B to RL 386.5 m for TSF 2 is discussed in this report. The embankment raise hasbeen designed as an upstream construction lift. This design report needs to be read in conjunction withthe drawings in Appendix A.

The major contents of this design report can be summarised as follows:

» Lift No. 7B as discussed above comprises a 2.5 m high embankment (1.5 m higher than the licensedelevation), constructed from borrowed clay material, via upstream construction method.

» The 2.5 m high lift provides additional tailings storage for approximately 1012 months at estimatedcurrent production rate of 1.146M tonnes per annum.

» Stability analyses indicate the overall embankments to be stable under static and pseudostatic(seismic) loading conditions, utilising the material parameters in this report. The stability of theembankment will require updating once the levels of the proposed piezometers are known. Seepageon some of the embankments are being remediated at the moment with additional drainage, thisrequires regular monitoring.

» Supervision of the construction is required, especially prior and during the placement of the initiallayers of the embankment wall.

» Embankment monitoring for signs of distress and seepage. Monitoring should include visualassessment as well as instrumentation, i.e. piezometer installation. GHD has proposed four newpiezometer lines on the two highest walls and in the potential seepage areas.


7. References

1. 2006 audit report by Coffey Geosciences Pty Ltd report ref. MH00073AJABTSF Audit 6 rev 2, dated2 February 2007.

2. GHD design report entitled “Scuddles and Gossan Hill Primary Storage Facility Design Report”, reportref. 1420, job no. 611/396410/00, dated 1997.

3. TSF 2 Stage 6 design report entitled “Design Review and Report Stage 6 Embankment Raising ofTailings Storage Facility 2 Oxiana Golden Grove Operations”, report ref. PZ00073.06ACembt lift 6design, dated 18 April 2006.

4. GHD site visit letter on seepage , letter ref. 61/17820/60165, dated 16 June 2006.

5. URS preliminary draft report on TSF 2 seepage Assessment entitled, “Phase II, TSF 2 SeepageAssessment, Golden Grove Mine”, report ref. 42906118/609F8234.0, dated 14 May 2007.

6. GHD report “Golden Grove Operations Tailings Storage Facility Design For TSF 2 Raise 7”, reportref 61/19305/69386 Revision 2, dated September 2007


8. Scope & Limits of Geotechnical Investigation

This report includes the results of a geotechnical investigation prepared for the purpose of thiscommission. The data and advice provided herein relate only to the project and structures describedherein and must be reviewed by a competent geotechnical engineer before being used for any otherpurpose. GHD Pty Ltd. (GHD) accepts no responsibility for other use of the data.

Where drill hole or test pit logs, cone tests, laboratory tests, geophysical tests and similar work havebeen performed and recorded by others the data is included and used in the form provided by others.The responsibility for the accuracy of such data remains with the issuing authority, not with GHD.

The advice tendered in this report is based on information obtained from the investigation locations testspoints and sample points and is not warranted in respect to the conditions that may be encounteredacross the site at other than these locations. It is emphasised that the actual characteristics of thesubsurface materials may vary significantly between adjacent test points and sample intervals and atlocations other than where observations, explorations and investigations have been made. Subsurfaceconditions, including groundwater levels and contaminant concentrations can change in a limited time.This should be borne in mind when assessing the data.

It should be noted that because of the inherent uncertainties in subsurface evaluations, changed orunanticipated subsurface conditions may occur that could affect total project cost and/or execution. GHDdoes not accept responsibility for the consequences of significant variances in the conditions and therequirements for execution of the work.

The subsurface and surface earthworks, excavations and foundations should be examined by a suitablyqualified and experienced Engineer who shall judge whether the revealed conditions accord with both theassumptions in this report and/or the design of the works. If they do not accord, the Engineer shallmodify advice in this report and/or design of the works to accord with the circumstances that arerevealed.

An understanding of the geotechnical site conditions depends on the integration of many pieces ofinformation, some regional, some site specific, some structure specific and some experienced based.Hence this report should not be altered, amended or abbreviated, issued in part and issued incomplete inany way without prior checking and approval by GHD. GHD accepts no responsibility for anycircumstances which arise from the issue of the report which have been modified in any way as outlinedabove.


Appendix A

Drawings

Raise 7 B Drawings


GHD

GHD House, 239 Adelaide Tce. Perth, WA 6004P.O. Box Y3106, Perth WA 6832T: 61 8 6222 8222 F: 61 8 6222 8555 E: [email protected]

© GHD 2008

This document is and shall remain the property of GHD. The document may only be used for the purposeof for which it was commissioned and in accordance with the Terms of Engagement for the commission.Unauthorised use of this document in any form whatsoever is prohibited.

mailto:[email protected]

This Drawing must not beused for Construction unlesssigned as Approved

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DesignConditions of Use.This document may only be used byGHD's client (and any other person whoGHD has agreed can use this document)for the purpose for which it was preparedand must not be used by any otherperson or for any other purpose.

DO NOT SCALE

Note: * indicates signatures on original issue of drawing or last revision of drawing

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