dr. erik eberhardt, p.eng. - ceaa.gc.ca · dr. erik eberhardt, p.eng. ... gold‐copper mine...

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Dr. Erik Eberhardt, P.Eng.

TechnicalNote:

“Independent geotechnical review of the proposed New Prosperity Gold‐Copper Mine Project preliminary open pit design”

Submittedto:Livain Michaud Panel Manager Federal Review Panel – New Prosperity Project Canadian Environmental Assessment Agency 160 Elgin St. Ottawa ON K1A 0H3 | 160, rue Elgin, Ottawa ON K1A 0H3

______________________________

Dr. Erik Eberhardt, P.Eng. 19 July, 2013

Erik Eberhardt Rock Engineering Consulting

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Geotechnical Review: New Prosperity Mine Project Dr. Erik Eberhardt, P.Eng.

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ExecutiveSummary

This report presents the independent expert assessment, opinion and recommendations regarding the geotechnical issues associated with Taseko’s proposed New Prosperity open pit project, as outlined in the 2012 Environmental Impact Statement and associated supporting documents. Included is a review of the open pit design, the geotechnical and hydrogeological investigations carried out in support of the open pit design, the potential for slope failure, possible impacts on Fish Lake, and the geotechnical risk assessments carried out to date.

Focus was placed on several questions provided by the Review Panel for this assignment:

‐ Is the proposed open pit design reasonable and practical?

‐ What is the potential for slope failure, what mitigation measures are provided for, in such an event, and what are the possible impacts on preserving Fish Lake?

‐ What is the effect of a confined aquifer, if encountered, on pit slope stability and the efficacy of the required mitigation measures?

‐ What effect will slope flattening have on the South pit wall, and the preservation of Fish Lake, if required during the later years of mining?

‐ What are the geotechnical risks that apply to the open pit design and the adequacy of the proposed mitigation measures and contingencies?

Overall, it was found that the level of field data collected is of the quantity and quality typically expected for a “Feasibility” level design. The pit slope design is referenced as being “Preliminary” with the expectation that further refinement and optimization will occur if the project moves to a “Detailed Design” phase. Otherwise, the design and analyses carried out are thorough and follow standard open pit design practices. In places, rather advanced technical considerations are reported that speaks to a very high level of understanding and expertise possessed by the pit slope design consultants. However, as with any large mine or geotechnical project, the geological and hydrogeological conditions can never be known exactly. This gives leave to uncertainties that may have significant impacts on the constructed open pit, its performance and its interaction with critical environmental bodies and mine infrastructure. Key concerns and potential issues raised in this report are as follows:

The borehole drilling used for the open pit site investigation was primarily carried out in the 1990’s. To date there have been no new targeted boreholes directed to investigate the geotechnical and hydrogeological characteristics of the QD and East Faults (subsequent to their identification), or to investigate the ground conditions between the open pit and Fish Lake (in support of the 2012 EIS revision to the Mine Development Plan to preserve Fish Lake).

Review of the drillhole logs for the boreholes located between the South pit wall and Fish Lake consistently indicate the presence of two significant intervals of sand and gravel varying in thicknesses from 10 to 25 m each. These would suggest the presence of significant confined aquifers in addition to the thin confined aquifer at the overburden/bedrock contact that is more frequently cited in the Preliminary Open Pit Design Report.

It is strongly suggested here that equating the hydraulic conductivities of the fault zones to those of the bedrock, as is done in the preliminary open pit design, would be counter to most experiences involving large fault zones similar in scale to the QD and East Faults. Although a


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central fault gouge zone may form a relatively impervious aquiclude that impedes flow normal to the fault, the fracture zones adjacent to the fault gouge often serve to significantly enhance the permeability parallel to the fault. It should also be noted that these faults may be in direct hydraulic contact with the confined artesian aquifer at the overburden/bedrock contact, and that there is precedence where such a scenario has significantly limited depressurization efforts due to recharge to the confined aquifers provided by the faults. Such a scenario also offers an alternative hypothesis regarding the source of leakage possibly observed in pump tests as discussed in numerous communications between the Proponent and different review bodies.

As stated in the EIS, the recommended pit slope design is reasonable and appropriate, in the context of a “preliminary” level design. The stability analyses performed and design criteria applied conform to commonly accepted industry practices. It could be argued that the acceptance criteria for the design of the South and South‐east walls should be elevated to a higher Factor of Safety (1.4 instead of 1.3) to better reflect a “high” consequence of failure.

The rock mass conditions based on the data currently available favour stability. Controlled blasting and dewatering are considered essential for achieving the stated open pit design and ensuring the safe performance of the pit wall slopes. Because open questions currently persist regarding the hydrogeological characteristics of the confined aquifer along the overburden/bedrock contact and the QD and East Faults, it is possible that the slopes in the southern part of the pit may require flatter slope angles to maintain stability.

Statements made in the EIS to the effect that the interaction between Fish Lake and the groundwater table has been assessed with respect to the pit wall designs, are not strongly supported in the EIS. It can be argued that based on the investigations carried out to date, that there is no evidence of a conduit providing a direct hydraulic connection between the pit and the lake; but it should be emphasized that this conclusion is based only on the pre‐mining site conditions. Although a major collapse of the South or South‐east wall is unlikely, and in any event can be mitigated against, slope displacements that develop in response to deep toppling movements in the South wall could potentially generate deep vertical tension cracks behind the pit crest. These could potentially breach the water control dams, or Fish Lake directly. Future analyses should be carried out to determine how far back behind the pit crest tension cracks may develop in response to slope displacements. Experiences at other large open pits where large‐scale toppling is observed suggest that tension cracks can extend more than 150‐200 m behind the slope crest.

Given the importance of dewatering to pit slope stability, very little appears to be discussed in the EIS regarding the potential for post‐closure pit slope failure after dewatering is stopped and the pit allowed to fill.

No level of drillhole investigation data can guarantee that construction will be entirely free from problems; however, the chances of encountering unexpected geological conditions can be greatly reduced. The EIS correctly recognizes that the pit design will undergo further modification and optimization as the project develops. Monitoring and updating of the geotechnical and hydrogeological models, and their implications with respect to stability of the open pit slopes is called for. The rating of likelihoods assigned in the risk assessment regarding the open pit design and its influence on Fish Lake arguably underestimate the overall risk but not significantly so. A ground control management plan should be developed outlining the open pit hazard inventory, risk reduction options, and trigger action response plan in the event that unstable pit slope movements develop or if dewatering measures are not as effective as required.


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TableofContents

Executive Summary ....................................................................................................................................... 2

Table of Contents .......................................................................................................................................... 4

1. Background ............................................................................................................................................ 6

2. Scope of Work ....................................................................................................................................... 7

2.1. First Task – Review of Documentation ......................................................................................... 7

2.2. Second Task –Report Submission ................................................................................................. 8

2.3. Third Task – Public Hearing Participation ..................................................................................... 8

3. Documents and Data Reviewed ............................................................................................................ 8

4. Review Findings .................................................................................................................................. 10

4.1. Site Investigation ......................................................................................................................... 10

4.1.1. Geological Investigation Data ............................................................................................. 11

4.1.2. Geotechnical Characterization Data ................................................................................... 13

4.1.3. Hydrogeology Investigation Data ........................................................................................ 15

4.2. Open Pit Design ........................................................................................................................... 18

4.2.1. Pit Dewatering/Depressurization ........................................................................................ 18

4.2.2. Pit Slope Angles ................................................................................................................... 20

4.3. Potential for Slope Failure .......................................................................................................... 22

4.3.1. Potential Failure Modes ...................................................................................................... 22

4.3.2. Influence of Confined Aquifers ........................................................................................... 23

4.3.3. Large Open Pit Precedence ................................................................................................. 24

4.3.4. Long‐Term Slope Performance ........................................................................................... 26

4.4. Possible Impacts on Fish Lake ..................................................................................................... 28

4.4.1. Interaction between Confined Aquifers and Fish Lake ....................................................... 28

4.4.2. Interaction between Slope Failure and Fish Lake ............................................................... 29

4.4.3. Post‐Closure Pit Stability ..................................................................................................... 30

4.5. Geotechnical Risk and Performance Assurance .......................................................................... 31

4.5.1. Risk Assessment .................................................................................................................. 32

4.5.2. Performance Assurance ...................................................................................................... 34

5. Key Concerns and Recommendations ................................................................................................ 34

6. References .......................................................................................................................................... 36


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Tables

Table A1: Site investigation reports cited in 2012 Geotechnical Site Investigation Factual Data Report, and relevant data contained within and reviewed regarding pit slope design and ground conditions between open pit and Fish Lake.. .............................................................................. 38

Table A2: Additional reports reviewed for data regarding pit slope design and ground conditions between open pit and Fish Lake. ................................................................................................. 41

Figures

Figure 1: Location of proposed open pit relative to Fish Lake.. ................................................................... 7

Figure 2: Geologic section through South wall of proposed pit. ................................................................ 12

Figure 3: a) Photo and b) schematic representation of a typical sub‐vertical fault zone intersected at depth during excavation of a drainage adit. c) Illustration of permeability anisotropy observed across the brittle fault structure. ................................................................................................ 16

Figure 4: Data compiled for a large number of open pit cases plotting slope height versus slope angle .. 25

Figure 5: Left, illustration of progressive failure involving toe shear and step‐path failure. Right, example of advanced numerical modelling of a progressive failure mechanism involving toe shear and step‐path failure up through a system of non‐persistent joints. ................................................ 27

Figure 6: Air photo of the Lornex Pit at the Highland Valley Copper mine, showing the extent of large, visible, open tension cracks behind the slope crest (requiring a road to be relocated further back) arising from deep‐seated toppling displacements. ........................................................... 30

Figure 7: Difference in mine plans with respect to open pit outline comparing 1999 Feasibility Design and 2012 Preliminary Design ....................................................................................................... 31

Figure 8: Subjective likelihood and consequence ratings for Option II (i.e., preservation of Fish Lake). ..................................................................................................................................................... 33


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1. Background

The New Prosperity Gold‐Copper Mine Project, proposed by Taseko Mines Ltd., is located approximately 125 km southwest of Williams Lake, British Columbia. The project, as planned, is to consist of a large open pit with an average mining rate of 120,000 tonne per day (70,000 tonne per day throughput to the mill) over an active pit life of 16 years [Taseko, 2012; EIS 2‐2, p. 64]. The mine site will also include support infrastructure and associated ore stockpile, waste rock and tailings areas. The submitted project is a revised version of Taseko’s March 2009 Environmental Impact Statement (EIS) for the “Prosperity Gold‐Copper Mine Project”, which was found by a federal review panel to have significant adverse environmental effects [Taseko, 2012; EIS 1‐1, p. 2]. Subsequent to this decision, Taseko undertook revisions to the mine development plan and mine site layout to address the issues identified by the panel. In November 2011, the Minister of the Environment referred the revised “New Prosperity Gold‐Copper Mine Project” to a federal review panel for environmental assessment and issued the EIS Guidelines to Taseko in March 2012. A three‐member panel was appointed for conducting the environmental assessment in May 2012, with consideration to be given to the following factors:

The environmental effects of the Project including those resulting from malfunctions or accidents, and any cumulative environmental effects that are likely to result from the Project and activities that will be carried out;

The significance of the environmental effects;

Comments from the public and Aboriginal groups that are received during the review; and

Measures that are technically and economically feasible and that would mitigate any significant adverse environmental effects of the Project.

Taseko submitted its “New Prosperity” EIS to the Panel on September 26, 2012. The open pit proposed in the 2012 EIS lies in the Fish Creek valley approximately 250‐400 m North/downstream from Fish Lake (Figure 1). The maximum depth of the proposed open pit will reach approximately 600 m [Taseko, 2012; EIS 2‐2‐4 A, p. 1], representing one of the deepest open pits in Canada. In addition to the need to adhere to legislated safety standards, a critical objective of the proposed mine plan and open pit design is to preserve Fish Lake in terms of maintaining the existing lake level, water quality, riparian and aquatic ecology. To support this objective, and others, the Review Panel has retained the services of two independent, non‐government experts in the following areas:

1. Geotechnical issues associated with open pit design, slope stability and possible confined aquifers in proximity to the proposed open pit; and

2. Hydrogeological issues associated with potential seepage and groundwater flow from the proposed tailings storage facility, and potential impacts to the receiving environment

This report presents the independent expert assessment, opinion and recommendations regarding the geotechnical issues associated with Taseko’s proposed New Prosperity open pit.


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Figure 1: Location of proposed open pit relative to Fish Lake. Modified after [Taseko, 2012; EIS 2‐2, p. 63].

2. ScopeofWork

The scope of work carried out for this independent geotechnical review follows that outlined in Annex A of the “Statement of Work” defined by the Canadian Environmental Assessment Agency in the contract agreement K4230‐13‐0007. These are summarized below.

2.1. FirstTask–ReviewofDocumentation

Review of relevant sections of the September 2012 (New Prosperity) and March 2009 (Prosperity) Environmental Impact Statements, and all relevant appendices, particularly:

- 2.1 Introduction and Background - 2.2 Project Description - 2.3 Project Scoping - 2.6 Existing Environment (Geology and Geochemistry) - 2.7 Impact Assessment (Geology and Geochemistry) - 2.8 Environmental Management - 2.9 Table of Commitments - 2.2.4 A ‐ Preliminary Pit Slope Design - 2.2.4 B ‐ Waste Dumps and Stockpiles ‐ Preliminary Design - 2.2.4 C ‐ 2012 Geotechnical Site Investigation Factual Data Report - 2.2.4 D ‐ 2009 Geotechnical Site Investigation Factual Data Report - 2.6.1‐4 D ‐ A Baseline Groundwater Hydrology Assessment


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Review responses to information requests on relevant topics as well as comments submitted by other participants;

Conduct a critical review of the assumptions and findings and conclusions contained therein.

2.2. SecondTask–ReportSubmission

Submission to the panel of a concise report with recommendations and PowerPoint presentation, to be uploaded for public review on the Canadian Environmental Assessment Registry (CEAR). The report and PowerPoint presentation should address the following:

- Whether the proposed open pit design is reasonable and practical given the three main geological domains.

- The potential for slope failure and analysis of mitigation measures, if required, and their possible impact on preserving Fish Lake.

- The effect of a confined aquifer, if encountered in the excavation of the open pit, on pit slope stability and the efficacy of the required mitigation measures.

- The effect of flattening of the South pit wall, if required during later years of mining, on the preservation of Fish Lake.

- A summary of the geotechnical risks of the open pit design and the adequacy of the proposed mitigation measures and contingencies.

2.3. ThirdTask–PublicHearingParticipation

Appearance before the Panel at a public hearing in Williams Lake, BC (July 26/27, 2013), to testify in regard to the report and recommendations submitted to the Panel and made public, and provide follow‐up information if requested by the Panel.

3. DocumentsandDataReviewed

The following documents, sub‐sections, and appendices were reviewed as part of this assignment:

Taseko Mines Limited (2012). New Prosperity Gold‐Copper Mine Project British Columbia, Canada: Environmental Impact Statement. September, 2012.

- 2.1 Introduction and Background - 2.2 Project Description

- 2.2.4 A ‐ Preliminary Pit Slope Design - 2.2.4 B ‐ Waste Dumps and Stockpiles ‐ Preliminary Design - 2.2.4 C ‐ 2012 Geotechnical Site Investigation Factual Data Report - 2.2.4 E ‐ 2009 Geotechnical Site Investigation Factual Data Report

- 2.3 Project Scoping - 2.6 Existing Environment (Geology and Geochemistry)

- 2.6.1‐4 D ‐ A Baseline Groundwater Hydrology Assessment - 2.7 Impact Assessment (Geology and Geochemistry) - 2.8 Environmental Management - 2.9 Table of Commitments


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Taseko Mines Limited (2009). Taseko Prosperity Gold‐Copper Project: Environmental Impact Statement/Application. March, 2009.

- 2‐6 Assessment of Alternatives and Selection of the Proposed Project - 2‐6‐C 1998 Project Risk Assessment (Ref. 10173/13‐2, Nov. 1998)

- 3‐5 Regional and Local Geology - 3‐5‐A 1998 Geological Report (May 1998)

- 3‐6 Mine Plan - 3‐6‐C 2007 Feasibility Pit Slope Design (Ref. VA101‐00266/2‐2, Sep. 2007) - 3‐6‐E 1999 Feasibility Design of the Open Pit (Ref. 11173/12‐2, Apr. 1999) - 3‐6‐F 1994 Open Pit Design (Ref. 1736/1, Mar. 1994) - 3‐6‐G 1993 Open Pit Preliminary Hydrogeological Investigations (Ref. 1736/2, Mar. 1994) - 3‐6‐H 1994 Open Pit Investigation (Ref. 1738/2, Jan. 1995) - 3‐6‐I 1996 Open Pit Geotechnical Investigation (Ref. 1731A/7, Jun. 1997) - 3‐6‐J 1998 Geotechnical Parameters for the Plant Site Foundation Design (Ref. 10173/12‐

3, Dec. 1998) - 3‐6‐L 1994 Plant Site and Crusher Site Foundation Investigations (Ref. 1738/3, Jan. 1995) - 3‐6‐M 1991 Preliminary Geotechnical Evaluation (Ref. 1731/1, August 1991) - 3‐6‐N 1992 Preliminary Geotechnical Investigation (Ref. 1733/1, Jan. 1993) - 3‐6‐O 1994 Geotechnical and Hydrogeological Investigation for Proposed Tailings Storage

Facility (Ref. 1738/1, Jan. 1995) - 3‐6‐P 1996 Geotechnical Site Investigation for Tailings Management Options 2 and 5 (Ref.

1731A/4, Jan. 1997) - 3‐6‐Q 1996 Seismic Refraction and Reflection Investigation (Ref. FGI‐313, Jul. 1997)

- 9.2.5 Geotechnical Stability Monitoring Plan

A summary of the data reviewed contained in these reports relevant to the open pit slope design and ground conditions between the open pit and Fish Lake are provided in Appendix A. In addition, a number of technical memorandums and reports were reviewed, as provided by the Review Panel. In reverse chronological order, these include: Taseko Mines Limited (2013). Responses to the Technical Information Requests from Taseko Mines Ltd.

to the Federal Review Panel Regarding the Environmental Impact Statement for the New Prosperity Gold‐Copper Mine Project, British Columbia. Memorandum, Jul. 17, 2013.

Natural Resources Canada (2013). Evaluation of the Adequacy and Technical Merit of the Additional Information Submitted by the Proponent for the New Prosperity Gold‐Copper Mine. Memorandum, Jun. 14, 2013.

Taseko Mines Limited (2013). Responses to the Supplemental Information Requests from Taseko Mines Ltd. to the Federal Review Panel Regarding the Environmental Impact Statement for the New Prosperity Gold‐Copper Mine Project, British Columbia. Memorandum, Jun. 5, 2013.

Taseko Mines Limited (2013). Meeting Between NRCAN and Taseko. Memorandum, May 24, 2013.

Federal Review Panel (2013). Supplemental Information Requests from the Federal Review Panel to Taseko Mines Ltd. Regarding the Environmental Impact Statement for the New Prosperity Gold‐Copper Mine Project, British Columbia. Memorandum, Mar. 28, 2013.

Tsilhqot’in National Government (2013). Comments on Additional Information Submitted by the Proponent. Memorandum, Mar. 16, 2013.


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Natural Resources Canada (2013). Evaluation of the Adequacy and Technical Merit of the Additional Information Submitted by the Proponent for the New Prosperity Gold‐Copper Mine. Memorandum, Mar. 15, 2013.

Taseko Mines Limited (2013). IR Responses to Panel. Memorandum, Feb. 28, 2013.

Federal Review Panel (2012). Information Requests from the Federal Review Panel to Taseko Mines Ltd. Regarding the Environmental Impact Statement for the New Prosperity Gold‐Copper Mine Project, British Columbia. Memorandum, Dec. 10, 2012.

Tsilhqot’in National Government (2012). Deficiencies in the EIS for the New Prosperity Project. Memorandum, Nov. 11, 2012.

B.C. Ministry of Energy, Mines and Natural Gas (2012). New Prosperity Project – EMNG Comments on Adequacy of Information. Memorandum, Nov. 9, 2012.

Natural Resources Canada (2012). Adequacy of the Environmental Impact Statement (EIS) for the New Prosperity Gold Copper Project. Memorandum, Nov. 9, 2012.

4. ReviewFindings

Focus for this review was placed on the open pit slope design and ground conditions between the open pit and Fish Lake, as reported in Taseko’s 2012 Environmental Impact Statement and other supporting documents (see Section 3). Particular focus was placed on the questions provided by the Review Panel for this assignment:

- Is the proposed open pit design reasonable and practical? - What is the potential for slope failure, what mitigation measures are provided for, in such an

event, and what are the possible impacts on preserving Fish Lake? - What is the effect of a confined aquifer, if encountered, on pit slope stability and the efficacy of

the required mitigation measures? - What effect will slope flattening have on the South pit wall, and the preservation of Fish Lake, if

required during the later years of mining? - What are the geotechnical risks that apply to the open pit design and the adequacy of the

proposed mitigation measures and contingencies?

To answer these questions, the review findings are divided into several sub‐sections that comment on the site investigation data collected for the pit slope design, the appropriateness of the design, the potential for slope failure, the possible impacts on Fish Lake, the influence of the confined aquifers on slope stability, factors that may impact the long‐term performance of the pit slopes, and the assessment of the perceived geotechnical risks as reported in the 2012 EIS and associated appendices, including the 2009 EIS and its associated appendices.

4.1. SiteInvestigation

The site investigation performed for the preliminary open pit design and dewatering requirements encompasses several geotechnical and hydrogeological field campaigns carried out over a 20 year period. These are summarized in Appendix A. Each involves different levels of effort, detail, and focus characteristic of pre‐feasibility and feasibility stages of project development. Key data sets include: site


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reconnaissance and geological mapping; oriented core and detailed geotechnical logging of RQD, RMR and discontinuity orientation, spacing and surface characteristics; piezometer, in‐situ permeability and pump testing; and strength testing of overburden, intact rock and rock joints. The geological model used for the open pit design is based on 384 diamond drillholes totalling 148,000 m of core and 68 percussion drillholes totalling 6300m [Taseko, 2012; EIS 2.6, p. 210). In general, it was found that the level of field data collected is of the quantity and quality typically expected for a feasibility level design [e.g., Read & Stacey, 2009]. Several deficiencies do arise, however, as would be expected for a project of this size. These largely relate to:

the QD and East Faults, for which no new targeted investigation boreholes appear to have been drilled subsequent to their identification, and

the ground conditions between the open pit and Fish Lake, for which no dedicated drilling or testing was carried out in support of the 2012 EIS revision to preserve Fish Lake.

There are also a number of uncertainties in the data and in the subsequent analysis that is unavoidable when dealing with geological and hydrogeological investigations. These are addressed below, together with other comments/observations relevant to subsequent issues raised in the review.

4.1.1. GeologicalInvestigationData

Overburden

- The overburden in the proposed pit area is described as consisting of glacial till, basalt flows, and colluvium and lacustrine sediments [Taseko, 2012; EIS 2.6, p. 210]. The sequence is reported to increase in thickness from 10 m in the main deposit area to up to 120 m in the south area of the pit [Taseko, 2012; EIS 2.2.4 A, p. 5]. The deposit continues to thicken to a maximum of 155 m as it extends towards Fish Lake [Taseko, 2009; EIS 3.5.A, p. 8‐6]. Review of the geology logs for other drillholes in the vicinity would suggest that the overburden thins to 60 m beneath the south end of Fish Lake, and thin and pinch out to the east and west away from the main deposit area. The presence of the overburden sediments has important implications for the design of the South pit wall as discussed in subsequent sections below.

- Of special interest are the “colluvium sediments”. These are described in the Preliminary Pit Slope Design as consisting of silty, sandy gravel up to 40 m thick, with occasional inter‐bedded silts and clays [Taseko, 2012; EIS 2.2.4 A, p. 6]. “Colluvium” refers to materials that have been transported by gravity and thus are typically angular, poorly sorted and of lower hydraulic conductivity. Based on the drillhole logs, a better description would appear to be that provided in the 1998 Geological report, where these materials are described as glaciofluvial deposits (occasional gravel beds) comprised of pebbles, gravel and boulder sized rounded clasts with only a trace of sand or silt interstitial to the coarser fragments [Taseko, 2009; EIS 3.5.A, p. 8‐17]. Such materials typically have very high hydraulic conductivities, ranging from 1e‐4 to 1e‐2 m/s [Domenico & Schwartz, 1990]. Similar values were approximated for intervals described as “stratified sand and gravel” as reported in a number of drillhole logs (see next comment).

- Review of the drillhole logs for boreholes located between the South pit wall and Fish Lake (see 93‐126, 93‐127, 93‐128, 93‐129, 94‐154, 96‐212 and 96‐218, as reported in [Taseko, 2009; EIS 3‐6‐E]), consistently indicate the presence of two significant intervals of sand and gravel at approximately 20 and 50 m depth, with thicknesses varying from 10 to 25 m. The distance


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between these drillholes spans several hundred meters suggesting that these gravel beds are continuous at the local scale, but with variable thicknesses, across the entire distance separating Fish Lake from the South and South‐east walls of the pit. Further south these thick gravel beds pinch out under Fish Lake, as they do to the north towards the central pit area. For those drillholes drilled deep enough to intersect the bedrock contact (94‐154, 94‐157, 94‐159 and 96‐218), additional sand and gravel intervals were intersected, including a 2‐3 m layer along the overburden/bedrock contact. This same contact layer of sand and gravel was also intersected in drillhole 96‐205, drilled from the island in the south end of Fish Lake [Taseko, 2009; EIS 3‐6‐P], suggesting it extends north‐south under Fish Lake. Observations regarding the hydrogeological characteristics of these intervals are discussed below under “Hydrogeology Investigation Data”.

Bedrock Lithology

- The bedrock lithology below the overburden is reported to be comprised of the Cretaceous Fish Lake intrusives, hosted in Cretaceous volcaniclastic and volcanic rocks, which transition to a sequence of Cretaceous sedimentary rocks including mudstones, siltstones, sandstones and conglomerates at shallower depths along the southern boundaries of the proposed pit [Taseko, 2012; EIS 2.2.4 A, p. 6]. Quartz feldspar porphyry dikes occur as an east‐west trending, steeply south dipping swarm that cross‐cut all of the volcanic and sedimentary rocks identified in the deposit [Taseko, 2012; EIS 2.6, p. 213]. Figure 2 depicts the different rock types that will intersect the South wall of the proposed pit, as reported in [Taseko, 2013b; SIR, p. 10/11‐23].

Figure 2: Geologic section through South wall of proposed pit. Modified after [Taseko, 2013b; SIR, p. 10/11‐23].


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Bedrock Alteration & Gypsum Line

- Five bedrock alteration types are reported, with varying significance with respect to the pit slope design. Argillic alteration is described as being localized along fault zones and overprints earlier alteration assemblages. This type is characterized as involving the alteration of feldspar to soft clay, which alters the strength characteristics of the rocks affected by it [Taseko, 2009; EIS 3.5.A, p. 9‐29].

- Gypsum is strongly associated with the potassic alteration and is present in healed fractures. Gypsum concentrations are described as being very low to non‐existent outside of the potassic alteration zone. A gypsum line has been interpreted in the open pit geology model, separating the lower three‐quarters of the deposit where gypsum infilling of discontinuities is prevalent, and the upper one quarter, where it has typically been dissolved from the potassically altered rocks [Taseko, 2012; EIS 2.2, p. 97]. Where groundwater flow is controlled by fracture permeability, rocks above the gypsum line can be expected to have higher hydraulic conductivities.

Faults

- Two major faults have been identified to pass within the pit limits: the QD and the East Faults. These structures are sub‐parallel, trend roughly north‐south through the centre of the deposit, and are steeply dipping to vertical (see Figure 2). Both faults are interpreted as being of limited thickness and are described as often being identified by lithological breaks rather than a high degree of localized fracturing [Taseko, 2012; EIS 2.2, p. 98]. In the 1998 Geological Report, the East Fault has been interpreted as being associated with a greater degree of clay gouge than the QD Fault [Taseko, 2009; EIS 3.5.A, p. 11‐1]. The presence of gouge would suggest that the fault has undergone significant offset, implying that a certain degree of tectonic brittle fracture damage adjacent to the gouge zones can be expected.

- It should be noted that the QD and East Faults appear to have been identified based on a detailed analysis of the open pit geology carried out in 1998 [Taseko, 2009; EIS 3.5.A]. This post‐dates the geotechnical investigation drilling campaigns carried out in the proposed pit area. Although several geotechnical boreholes do penetrate these faults (leading to their identification), to date there have been no new targeted boreholes directed to investigate the geotechnical and hydrogeological characteristics of the faults and adjacent tectonic damage zones. This is discussed in more detail in the later sections of thereport.

4.1.2. GeotechnicalCharacterizationData

Overburden Properties

- The overburden materials that form the upper slopes of the pit (specifically the glacial till) are reported in the 2012 EIS to be over‐consolidated, due to recent glacial activity, and of high strength [Taseko, 2012; EIS 2.2.4 A, p. 8]. A small amount of cohesion is also reported based on laboratory testing. Although the high strength is favorable with respect to pit slope stability, the material may also exhibit a brittle deformation behavior. The significance of a brittle response is explored further under the comments regarding “Potential Impacts on Fish Lake”.


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Rock Mass Structure

- The 2012 EIS reports that the predominant structural features in the open pit area include veining, open jointing, a limited number of shear zones, and the sub‐vertical QD and East faults [Taseko, 2012; EIS 2.2, p. 98]. Stereonets provided in the Preliminary Pit Slope Design Report, together with the structural model reported in the 1998 Geological Report, suggest that three major discontinuity sets are present, including a sub‐vertical set and two west/southwest dipping sets [Taseko, 2012; EIS 2.2.4 A, p. 8].

- The frequency of occurrence of open joints is reported to be significantly greater above the gypsum line [e.g., Taseko, 2012; EIS 2.2, p. 98]. The majority of discontinuity surfaces are characterized as being smooth and planar [Taseko, 2012; EIS 2.2.4 A, p. 9]. These have important implications with respect to both their hydraulic conductivities and their shear strengths.

- The discontinuities are considered to be semi‐continuous over the bench scale (10‐20 m) [Taseko, 2012; EIS 2.2.4 A, p. 8]. If so, this would limit their adverse influence on kinematic stability to bench scale failures. At the inter‐ramp and pit slope scale, discontinuities will contribute to slightly lower rock mass strengths (relative to the intact rock strengths). Their limited persistence will promote rock slope deformations that require step‐path and/or progressive failure mechanisms to evolve to a more critical state. These are discussed in more detail under “Pit Slope Potential for Slope Failure”.

Material Properties

- A review of the rock mass conditions reported for the open pit area indicates that the bedrock is of FAIR to GOOD quality [Taseko, 2012; EIS 2.2.4 A, p. 9]. This suggests favourable conditions with respect to pit slope stability. Similarly, the intact rock strengths are equally favorable, with reported UCS values ranging between 50 and 175 MPa [Taseko, 2012; EIS 2.2.4 A, p. 9]. A review of the summary table cited compiling data from past pit slope investigations, between 1992 to 1996, confirm a range of best estimates between 40 and 140 MPa [Taseko, 2009; EIS 3.6.E, tab. 2.2]. It should be noted that these UCS values are heavily weighted towards less reliable point load measurements, but nevertheless, the rock types involved are generally found to be strong unless they are exposed to hydrothermal alteration or weathering, which does not appear to be the case here.

- Intact rock strength is an important consideration, because as noted in the 2009 EIS, given the limited persistence of the rock mass discontinuities, many potential failure surfaces are not completely developed and would require some failure of intact rock. The moderate to high rock strengths at the New Prosperity site is therefore beneficial in light of the high stresses that are expected to develop in the pit slopes during later stages of mining [Taseko, 2009; EIS 3.6, p. 6‐11].

- Exceptions to the high rock strengths reported in older pit slope investigation reports include sections of argillically altered rock, typically associated with shears and faulting. The strong rock conditions imply that behaviour will be structurally controlled, except in areas of localized shearing and faulting, where both structurally controlled behaviour and potential failure through the weaker argillically altered rock should be considered [Taseko, 2009; EIS 3.6‐I, p. 25).

- As previously noted, dedicated drilling to test the geotechnical characteristics of the rock types within and adjacent to the QD and East Faults has not been carried out, and the rock mass and intact rock strengths reported do not consider any potential weakening effects resulting from tectonic damage to the rock. Reduced rock mass quality can be expected not only along the fault structures but also in adjacent zones. In a study of brittle fault zones in crystalline rock, [Laws et


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al., 2003] showed that intact rock strengths alone can decrease by 20 to 60% due to tectonic damage occurring in rocks adjacent to a fault.

4.1.3. HydrogeologyInvestigationData

Hydrogeology Characterization

- The hydrogeology of the open pit is characterized as encompassing a shallow water table, unconfined flow near surface and pressurized, preferential flow in permeable aquifers, confined at depth. The confined conditions are reported as occurring at various depths in the thick sequence of overburden where low permeability glacial till, siltstone and fine sandstone act to confine groundwater flow, most notably at the bedrock contact where high artesian pressures were observed in the sands, gravel and fractured rock located along the contact. Drilling investigations also identified low artesian pressure conditions in the glaciofluvial sand and gravel intervals and fractured basalts that exist at various depths within the overburden sediments [Taseko, 2012; EIS 2.2.4 A, p. 10]. Although a specific recharge source is not specified for these confined aquifers, the general groundwater flow in the Fish Creek valley system is reported as being driven by recharge in the higher ridge areas west and east of the valley towards Fish Creek [Taseko, 2012; EIS 2.6, p. 286]. As previously noted, some of these intervals reach thicknesses of up to 25 m and extend several hundred meters between the southern pit limit and Fish Lake, indicating significant aquifers. However, they also pinch out to the east, west, north and south as the bedrock paleo‐surface depression infilled by the sediments shallows.

- The hydraulic conductivity of the rock mass below the overburden is characterized in the Preliminary Pit Slope Design Report as being low to very low, consistent with tight infilled discontinuities. Exceptions are noted with respect to anticipated zones of highly fractured rock related to the faults [Taseko, 2012; EIS 2.2.4 A, p. 10‐11]. This is discussed separately in the comment that follows. With respect to the rock mass hydraulic conductivities, supporting documents for the 2009 EIS report low hydraulic conductivities on the order of 1e‐4 to 1e‐6 m/s for competent rock below the gypsum line (characterized by infilling along discontinuities), and values of 1e‐2 to 1e‐4 m/s in the upper portion of the rock mass where the dissolution of gypsum provides preferential paths for groundwater flow [Taseko, 2009; EIS 3.6‐E, tab. 3.6]. Values from this table, which summarize the permeabilities recorded from all tests up to 1999, are repeated in the Preliminary Pit Slope Design Report [Taseko, 2012; EIS 2.2.4 A, tab. 3.2]. Higher hydraulic conductivities can be expected in the upper rock mass above the gypsum line due both to the dissolution of gypsum in the fractures and the increased fracture apertures in response to lower confining stresses and increased relaxation compared to the deeper rock below the gypsum line.

- Dedicated drilling and testing of the hydrogeological characteristics of the rock types within and adjacent to the QD and East Faults has not been carried out. The 2012 EIS notes this limitation but suggests that the limited hydraulic data available indicates that the permeability of these structures is similar to the bedrock hydraulic conductivity [Taseko, 2012; EIS 2.6, p. 286]. This statement was questioned in the Tsilhqot’in Nation memo of Nov. 11, 2012 regarding deficiencies in the EIS [TNG, 2012; p. 36]. It is strongly emphasized here that equating the hydraulic conductivities of the fault zones and bedrock would be counter to most experiences involving large fault zones similar in scale to the QD and East Faults. When intersected at depth in alpine tunnels driven in low permeability crystalline rock, sub‐vertical brittle fault zones are observed to act as major conduits, with individual faults in some cases producing tunnel inflows of up to 150 l/s [Loew et al., 2007]. In numerous studies of the influence of fracture flow on deep seated slope


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movements in crystalline rock slopes, piezometer data shows that although a central fault gouge zone may form a relatively impervious aquiclude that impedes flow normal to the fault, the fracture zones adjacent to the fault gouge serve to significantly enhance the permeability parallel to the fault [e.g. Eberhardt et al., 2007]. This is illustrated in Figure 3. It is common in such deep drainage projects involving tightly jointed crystalline rock masses cut by faults, to see water pressures varying appreciably from one borehole to the next, depending on whether conductive fractures are intersected, and water levels varying significantly on different sides of a fault (often described as hydraulic compartmentalization).

Figure 3: a) Photo and b) schematic representation of a typical sub‐vertical fault zone intersected at depth during excavation of a drainage adit. c) Illustration of permeability anisotropy observed across the brittle fault structure.

[Eberhardt et al., 2007].

Packer/Permeability and Pump Tests

- Results from the packer permeability testing echo several of the comments made above. A summary of tests performed in 1992, 1993 and 1994 indicate a log average permeability of 2e‐3 m/s above the gypsum line, with the permeability dropping to a log average of 6e‐5 m/s below the gypsum line [Taseko, 2009; EIS 3.6‐E, p. 10]. It is also reported that results indicate in some locations, low permeability fault gouge may act as a barrier to groundwater flow across the fault structure, which results in higher water pressures on one side of the fault. Fractured rock along the fault structures can also provide relatively higher permeability materials that allow increased flow parallel to the structure. In other areas, higher permeability fracture zones may also form local aquifers that are confined by less pervious rock. Artesian pressures may develop in these confined aquifers if the faults are hydraulically connected to groundwater recharge areas with elevated topography [Taseko, 2009; EIS 3.6‐E, p. 8‐9].

- Several reports, including the Preliminary Pit Slope Design, recount the observation that continuous pumping from a production well (94‐164) completed within the confined artesian aquifer in the sand and gravel unit above the bedrock contact, showed negligible piezometric drawdown in adjacent observation wells (94‐154, 94‐159) confirming the relatively high recharge capacity of this unit [Taseko, 2012; EIS 2.2.4 A, p. 10]. Examination of the well completion details reported in the appendix to [Taseko, 2009; EIS 3.6‐H] show the borehole being perforated at 36, 62 and 158 m depth, each with 4 m screened intervals. Although drillhole geology logs could not be found for this borehole, the perforated depths can be compared to the 10‐25 m thick sand and


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gravel aquifers observed at 20 and 50 m depth in the neighboring boreholes, together with the artesian aquifer detected at the overburden/bedrock contact. Depending on how the pumping test was carried out, pumping may have encompassed all three intervals. Given the significant transmissivities of the thicker sand and gravel intervals (up to 25 m thick), it would not be surprising that continuous pumping over an extended period had a negligible effect on neighbouring wellbores. As noted in the Preliminary Pit Slope Design Report, aggressive dewatering will likely be required to limit pit inflows and avoid related instabilities in the southern area of the pit [Taseko, 2012; EIS 2.2.4 A, p. 10].

- Considerable discussion between Taseko and NRCan has transpired regarding the interpretation of the pump test carried out in 94‐164 and the validity of the hydraulic conductivity values derived. This exchange and the positions held are summarized in various communications, including [Federal Review Panel, 2013; SIR, p. 5‐6] and [Taseko, 2013b; SIR, p. 10/11‐2]. In short, Taseko explains that the pump test data from wells 94‐154, 94‐157 and 94‐159 were not relied on by BGC Engineering because the pumping and observation wells were screened across multiple hydrogeological units, the pumping rate was not recorded throughout the test duration, and the observed pumping water levels suggest that a steady state pumping condition was not achieved [Taseko, 2013b; SIR, p. 10/11‐2]. A review of the groundwater monitoring well completion details, provided in [Taseko, 2009; EIS 3‐6‐H], and as noted in the previous comment, confirm that the wellbores are screened across multiple levels including the three sand and gravel intervals in which artesian conditions were recorded. In a separate communication, Taseko further explains that the majority of groundwater is likely coming from the overburden/bedrock contact and from isolated sand and gravel seams interlayered within the low‐permeability overburden sediments. They go on to report that the interlayered sand and gravel seams are generally thin and are not interpreted to be continuous across the site [Taseko, 2013a; IR, p. 10‐7]. As previously noted, the drillhole data shows these sand and gravel beds to be significant (10‐25 m thick) and continuous across several hundred meters between the southern pit limit and Fish Lake, before pinching out further to the south, as well as to the north, east and west of the deposit area.

- Further to this discussion, NRCan’s interpretation of the data from the pump test in 94‐164 is that the pressure response potentially indicates leakage across the confining layers, and suggests that this may be demonstrating a hydraulic connection between the pump well and Fish Lake [Federal Review Panel, 2013; SIR, p. 5‐6]. An alternative explanation offered here, at least with respect to leakage, is that leakage is more likely derived from the QD and East Faults, which would intersect the thin confined aquifer and fractured bedrock immediately above the bedrock contact, than through communication with Fish Lake 150 m above. Although the latter may also be possible, the QD and East faults represent major conductive features (parallel to their structures). At the South East Prongs open pit at the Mount Tom Price mine in the Pilbara region of Western Australia, recharge of underlying confined aquifer units encountered in the pit floor was found to be primarily sourced via faults [Brehaut, 2009]. This study goes on to report that after more than 13 years of active dewatering, there has been little to no effect on the perched groundwater pressures; investigative drilling and monitoring has shown that two major faults were acting as flow conduits (recharge) to the confined aquifer [Brehaut, 2009; p. 65‐67].

- Given the above cited case, a targeted investigation of the geotechnical and hydrogeological characteristics of the QD and East Faults in future is critical to understanding the potential effect these geological structures may have on the open pit hydrogeology and that of the overburden material between the South pit wall and Fish Lake. At the same time, it is appreciated that detailed targeted investigations of this kind are more typically carried out during the design and


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construction phases of an open pit project [see also Read & Stacey, 2009; p. 13]. As remarked by Taseko in their May 24, 2013 meeting notes summarizing the meeting between NRCan and Taseko, any difference in interpretation of the technical data can be resolved by a specifically focussed pump test program, to be carried out during the detailed engineering design stage [Taseko, 2013b; SIR, p. 10/11‐8].

4.2. OpenPitDesign

One of the questions posed by the Review Panel was to comment on whether the proposed open pit design is reasonable and practical, given the three main geological domains (overburden, and rock above and below the gypsum line). The preliminary design calls for a large open pit 1200 to 1600 m in diameter at the pit rim, and reaching depths between 525 and 600 m [Taseko, 2012; EIS 2‐2, p. 64]. Construction of the open pit will involve several phases where a smaller starter pit will be sequentially enlarged (“push backs”) over a 16 year period creating a progressively deeper pit [Taseko, 2012; EIS 2‐2, p. 81]. The minimum pushback width is 80 m; however, in general the expansions are in excess of 100 m widths [Taseko, 2012; EIS 2‐2, p. 96]. The maximum wall height of 600 m will coincide with the South‐west wall of the pit. The preliminary design is based on a total of 148,136 m of diamond drilling completed in 379 holes at the Prosperity Site in the 1990’s [Taseko, 2012; EIS 2.2.4 A, p. i]. As previously noted, these drilling programs pre‐date the recognition of the QD and East Faults, although they contributed to its detection. The pit slope design was based on seven main design sectors that were identified for pit development. Sub‐sectors were defined for the pit walls to differentiate the overburden, fractured rock above the gypsum line and competent rock below the gypsum line [Taseko, 2012; EIS 2.2.4 A, p. 12]. Taseko’s position in the 2012 EIS is that all currently available drilling and discontinuity mapping data and stability analyses suggest the recommended pit slope design is reasonable and appropriate [Taseko, 2012; EIS 2‐2, p. 85]. A review of the proposed pit slope design confirms this, at least in the context of it being a “preliminary” design as part of a project in the Feasibility stage that has not yet transitioned to Detailed Design and Construction. Below are several comments/observations relevant to the preliminary pit slope design and its main components.

4.2.1. PitDewatering/Depressurization

- Pit dewatering will begin with depressurization wells around the perimeter of the pit, and eventually evolve into an in‐pit dewatering system [Taseko, 2012; EIS 2‐2, p. 81]. Dewatering is essential for achieving the stated open pit design and ensuring the safe performance of the pit wall slopes. The full design will involve a combination of depressurization techniques including vertical wells, in‐pit horizontal drains and collection systems implemented in a staged approach during pit development. The QD and East Faults are specified as requiring targeted deep dewatering in order to minimize the potential for slope failure on the North and South walls [Taseko, 2012; EIS 2.2, p. 102‐103].

- The Preliminary Pit Slope Design assumes that pit inflows will likely be dominated by unconfined flow in the upper 150 to 300 m of fractured rock mass above the gypsum line [Taseko, 2012; EIS 2.2.4 A, p. 18]. As previously noted, the dissolution of gypsum from the discontinuities above the


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gypsum line will result in higher hydraulic conductivities. The Preliminary Pit Slope Design further states that inflows from good quality, low permeability rock below and peripheral to the gypsum line are expected to be low [Taseko, 2012; EIS 2.2.4 A, p. 18]. An exception should be made with respect to the intersection of the pit walls and floor with the QD and East Faults. This is discussed in more detail in a separate comment below.

- As stated in the 2012 EIS, pit inflows will also be dominated by localized confined aquifers in the southern area of the pit [Taseko, 2012; EIS 2.2, p. 102]. These will likely be considerable and may be underestimated in the pit slope depressurization plan. As previously noted, two of the confined aquifers involve gravel beds between 10 and 25 m thick that extend several hundred meters from east to west across what is proposed to be the South wall. The north‐south extent of these beds appears to only be 250‐300 m before they pinch out. This may mean that the pit walls may not intersect these confined aquifers until a later pushback when the slope heights are in a more critical state. It is also equally possible that the pit walls may not intersect these units at all.

- Additional consideration must also be given to the lowermost confined aquifer along the overburden/bedrock contact. Although thinner in terms of the sand and gravel layer intersected (2‐3 m), the presence of more highly fractured rock below the overburden sediments may equally contribute to the inflows expected. Similar arguments (as above) may be made as to whether the sands and gravels contributing to this confined aquifer pinch out before they would intersect the southern pit walls as the depression infilled by the overburden sediments shallows towards the centre of pit. Nevertheless, they may still contribute to significant inflows through either leakage through the QD and East Faults and/or the upper skin of highly fractured bedrock observed in places along the overburden/bedrock contact.

- Considerable discussion between Taseko and NRCan has transpired regarding the estimate of inflows expected in the pit (including whether a portion of these will be communicated via Fish Lake). This exchange and the positions held are summarized in various communications, including those for Information Request (IR) 11 in [Federal Review Panel, 2013; SIR, p. 6‐7] and Taseko’s last response regarding 11 [Taseko, 2013b; SIR, p. 10/11‐2‐4]. Estimates provided in the Preliminary Pit Slope Design suggest a range on the order of 58 to 153 l/s [Taseko, 2012; EIS 2.2.4 A, p. 21]. These values are slightly lower than earlier estimates of 85‐169 l/s reported in the 2007 Feasibility Pit Slope Design using the same 1999 data and assessment results [Taseko, 2009; EIS 3.6‐C, p. 22]. It should be noted that although the upper bound values reported consider in part the inflows from the unconfined aquifers, it does not appear they account for the potentially sizeable inflows that may occur along the North and South wall intersections with the QD and East Faults.

- Although inflows derived from the QD and East Faults do not appear to be considered in the pit inflow estimates, the Preliminary Pit Slope Design does include plans for deep dewatering of the faults for groundwater depressurization to minimize the potential for slope failure along the North and South walls [Taseko, 2012; EIS 2.2.4 A, p. 19]. The design recommends the drilling of multiple 350 m deep vertical pumping wells into the fault zones in Years 1, 6 and 11 as the pit bottom deepens, where the latter will actively dewater the pit to the end of mining operations [Taseko, 2012; EIS 2.2.4 A, p. 19]. Such plans despite the lack of detailed data are reasonable considering what is typically expected for a preliminary pit design, and can be revised and refined once more dedicated testing data is obtained for the QD and East Faults. It should also be emphasized that as previously noted regarding the experiences at the Mount Tom Price mine, that even with active dewatering of a confined aquifer over extended periods of time (13 years in the Tom Price case), it is possible that the desired effect may not be achieved due complexities in the flow paths and relationships between major faults and regional recharge sources.


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4.2.2. PitSlopeAngles

Design Criteria and Stability Analysis Methods

- Review of the design criteria and stability analysis methods applied confirm that they are appropriate for a preliminary level pit design. The design cites the use of the 2009 Guidelines for Open Pit Design [Read & Stacey, 2009], which are considered industry standards for large open pits.

- The acceptance criteria selected correspond to those suggested where the consequence of failure is considered “moderate” [Taseko, 2012; EIS 2.2.4 A, p. 4]. This involves designing to a Factor of Safety (FoS) of 1.3 for the overall slope height. It’s important to note that this was the same value targeted in the 2007 Feasibility Pit Design when the mine plan did not include the preservation of Fish Lake [Taseko, 2009; EIS 3‐6‐C, p. 18]. Given the revised design in the 2012 EIS and criteria that Fish Lake must not be adversely impacted, it may be argued whether the South and South‐east walls should be designed to a higher FoS more reflective of a “high” consequence of failure (FoS = 1.4). The added uncertainty regarding the geotechnical and hydrogeological characteristics of the QD and East Faults, and therefore lower confidence, may also justify a higher factor of safety. At the same time, it may be argued that Fish Lake is more than 250 m away from the proposed southern limits of the open pit and that a FoS = 1.3 is appropriate. The potential for slope failure and its impact on Fish Lake are discussed in further detail in later sections of this report.

- The design procedures followed conform to standard practice for a preliminary‐level pit slope design. Geotechnical design sectors are identified differentiating sectors with similar geology, geomechanical characteristics, and wall orientations. Stereonet analyses were carried out to assess kinematic stability (i.e. potential for planar, wedge, and toppling failure), which in turn was used to select suitable bench and inter‐ramp slope angles. Conventional 2‐D limit equilibrium analyses assuming a circular mode of failure (Method of Slices) were applied to three representative pit walls (North, South, and West) to assess the overall stability of the full pit slope height for given slope angles. Rock mass properties, which account for the weakening effect of discontinuities, were assessed for these limit equilibrium analyses using the geotechnical properties derived from the field investigations and applying appropriate weighting factors to account for blast damage and slope relaxation. A separate limit equilibrium analysis was carried out for the overburden portion of the South‐east wall where the slope height in the overburden material will reach 120 m. The analyses were performed using standard software commonly used for such analyses (Rocscience’s DIPS and Geo‐Slope’s SLOPE/W).

- The Preliminary Pit Slope Design emphasizes that the design basis for the recommended maximum overall slope angles assumes that careful controlled blasting practices will be implemented and that comprehensive groundwater depressurization measures are implemented [Taseko, 2012; EIS 2.2.4 A, p. 24]. This will require closely monitored quality control measures be implemented during construction and design implementation.

Overburden Slope Angles

- The maximum overburden slope heights along the upper South‐east and South‐west walls of the proposed pit will be on the order of 120 m; overburden exposures on the rest of slopes are less significant [Taseko, 2012; EIS 2.2.4 A, p. 12]. Based on the limit equilibrium analyses performed, an overall slope angle of 30 degrees was recommended for slopes excavated in the overburden materials provided that sufficient groundwater depressurization can be maintained 30 m into the pit wall.


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- Although not specified in the 2012 Preliminary Pit Slope Design Report, details regarding the limit equilibrium analyses carried out for the maximum overburden slope scenario were reviewed in the 1999 Feasibility Design of the Open Pit Report [Taseko, 2009; EIS 3‐6‐E]. This report suggests that the analyses considered a variety of phreatic surface locations [Taseko, 2009; EIS 3‐6‐E, p. 41], but did not consider the adverse effects of encountering artesian pressures at the base of the overburden. Hence an assumption is made for these analyses that the slope will be perfectly drained to a phreatic surface parallel to and 30 m back from the slope face [Taseko, 2009; EIS 3‐6‐E, figs. 4.1‐4.2]. Given the complex geological and hydrogeological conditions previously discussed for the overburden/bedrock contact, this assumption may be overly optimistic.

Bedrock Slope Angles

- Results from the kinematic analyses are reported as suggesting that large‐scale multiple bench instabilities are not indicated for most pit design sectors [Taseko, 2012; EIS 2.2.4 A, p. 15]. Thus it is suggested that 65 degree bench face angles should be achievable in the bedrock, and that inter‐ramp angles of 50 degrees should be achievable for more competent rocks using a 30 m double bench configuration, provided that low‐damage blasting practices are implemented [Taseko, 2012; EIS 2.2.4 A, p. 15]. An inter‐ramp slope angle of 45 degrees is recommended for the less competent rocks above the gypsum line, as well as for the lower walls in the West and North‐east sectors where lower rock mass strengths and/or adverse structural features are encountered [Taseko, 2012; EIS 2.2.4 A, p. 15]. These angles seem appropriate although lower inter‐ramp angles may also be required for the North and South walls due to the presence of the QD and East Faults resulting in lower than expected rock mass strengths.

- Kinematic stability analyses also indicated the potential for bench‐scale toppling in the South, South‐west and North‐west walls [Taseko, 2012; EIS 2.2.4 A, tab. 6.1]. As noted in earlier support documents, the QD and East faults may serve to aggravate stability serving to act as potential release planes to allow toppling displacements along the South wall [Taseko, 2009; EIS 3‐6‐C, p. 15‐16]. The significance of these toppling displacements is further discussed in later comments regarding “Potential Impacts to Fish Lake”.

- Rock mass properties were derived for the stability analyses of the final overall slope heights based on the commonly used empirical relationships published by [Hoek et al., 2002]. These enable rock mass shear strength properties to be estimated accounting for the weakening effect of fractures in the rock mass relative to the intact strength of the rock. To meet the acceptance criteria (FoS = 1.3), the calculation of the rock mass properties assume a rock mass disturbance factor of D = 0.85 [Taseko, 2012; EIS 2.2.4 A, tab. 4.3]. This factor accounts for blast damage and stress relaxation, with recommended values for production blasts in large open pits being D = 1.0. The slightly more favorable value used assumes that good controlled blasting practices will be employed. The [Hoek et al., 2002] relationships also require an estimate of the confining stress levels, entered as a slope height, to account for the increase in rock mass shear strength observed with increasing confining stresses [see Eberhardt, 2012]. Although the value used in the Preliminary Pit Slope Design Report isn’t specified, it could be back‐calculated to a value of 200 m. Given the total slope heights involved ranging from 530 to 580 m, it is assumed that a degree of conservatism has been built into the analysis by using rock mass shear strengths more appropriate for the weaker rock above the gypsum line (hence the slope height of 200 m) than the stronger rock below it.

- The overall slope angles recommended based on the limit equilibrium analyses performed range from 40 degrees for the South‐west and West walls to 43 degrees for all other walls [Taseko,


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2012; EIS 2.2.4 A, tab. 6.1]. These angles assume good controlled blasting practices (D=0.85) and effective groundwater depressurization to 50 m behind the final walls. It should be noted that the overall slope angles for the North and South walls do not account for the possibly weaker rock mass conditions to be encountered due to the presence of the QD and East Faults.

4.3. PotentialforSlopeFailure

A second question posed by the Review Panel was to comment on the potential for slope failure, and in such event, what mitigation measures would be available and what possible impact would these have on preserving Fish Lake. The second part of this question is addressed separately in the section that follows. Discussed here are several comments/observations related to the potential for pit slope failure.

4.3.1. PotentialFailureModes

- As previously discussed, the rock mass quality within which the deposit is hosted ranges from FAIR to GOOD. This favours stability. In an earlier report, the discontinuities mapped in the boreholes were described as 90% joints and 6% shears [Taseko, 2009; EIS 3‐6‐I, p. 20]. The shears were characterized as having slickensided surfaces. Although small in number, the low shear strength of these shears would likely contribute to bench or multi‐bench failures if they adversely daylight into the pit. Otherwise, bench to multi‐bench stability will be governed by the orientation, spacing, persistence, shear strength and interconnectivity of the joints. Likely modes of failure identified by the kinematic analyses include minor bench‐scale translational planar/wedge instabilities, especially in the North‐east pit wall [Taseko, 2012; EIS 2.2.4 A, p. 15]. Bench backbreak, ravelling and rockfall can be expected, which can be managed together with other bench‐scale instabilities by employing controlled blasting techniques, scaling and bench cleanup to remove loose blocks, and horizontal drainage. The minimum bench width required by the British Columbia Mines Act for a 15‐metre high single bench configuration is 8 m. Increasing the bench width provides an additional option to mitigate against rockfall and spillage arising from unstable benches.

- The benches in the overburden on the southern walls will likely break back to shallower angles upon excavation and/or exhibit considerable ravelling over the long term [Taseko, 2012; EIS 2.2.4 A, p. 12]. The recommendations provided in the preliminary pit slope design is to place a wide catch berm below the bottom of the overburden slopes to provide additional allowance for ravelling cleanout and surface water diversion [Taseko, 2012; EIS 2.2.4 A, p. 12‐13].

- Kinematic analyses have also indicated a potential for bench‐scale toppling along the east‐west striking sub‐vertical discontinuity set (main veining), in the North‐west, South and South‐west walls [Taseko, 2012; EIS 2.2.4 A, p. 14‐15]. The 2007 Feasibility Pit Slope Design described these as potentially involving large‐scale toppling type displacement along the North and South walls during the later stages of mining [Taseko, 2009; EIS 3‐6‐C, p. ii]. As noted in this report, precedent practice in deep open pit mines has indicated that disruptions due to this type of failure can be minimized through appropriate groundwater depressurization with the use of dewatering wells and horizontal drains. The QD and East faults may further serve to act as potential release planes to facilitate these toppling displacements along the South wall. To mitigate the development of large‐scale toppling instabilities in the South wall, recommendations are made to install deep depressurization wells in close proximity to these fault zones together with closely spaced horizontal drains [Taseko, 2009; EIS 3‐6‐C, p. xii]. Implications arising from large‐scale toppling


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movements in response to stress‐relief as the open pit reaches greater depths are further discussed under “Possible Impacts on Fish Lake”.

- The thickening of the overburden from 10 m in the main deposit area to 155 m towards Fish Lake to the south is interpreted as involving a paleo‐surface forming a bedrock depression infilled by glacial lake sediments and basalt flows [Taseko, 2009; EIS 3.6.I, p. 11]. Kinematically, this is favourable as the bedrock interface on which the weaker overburden sediments lie dips away from the pit towards Fish Lake.

- Shear strength testing of the overburden materials, particularly the thick over‐consolidated glacio‐lacustrine silt and silty clays underlying the basalt flows in the southern area of the pit have been reported as displaying tendencies to swell and disintegrate upon hydration and exposure to atmospheric conditions [Taseko, 2009; EIS 3.6.I, p. 26]. This indicates the potential for surficial sloughing and shallow retrogressive instabilities in the overburden materials. Recommendations are made to consider covering exposed sections of these materials with geosynthetics or shotcrete. The potential for retrogressive failure in the overburden materials has minor to moderate implications with respect to Fish Lake. These are discussed in more detail under “Possible Impacts on Fish Lake”.

- Limit equilibrium results for the South wall overburden carried out using SLOPE/W indicate that the slope acceptance criteria of FoS = 1.3 can be met if effective groundwater depressurization can be achieved 50 m into the slope [Taseko, 2012; EIS 2.2.4 A, tab. 4.3]. However, these analyses do not consider the possibility of artesian conditions at the slope toe (overburden/bedrock contact) or other localized perched water tables that may not be effectively drained by the measures employed. If so, large scale rotational movements encompassing the full height of the overburden (up to 120 m in the final southern walls), may develop. Implications of a large rotational failure developing in the overburden on Fish Lake are further discussed under “Possible Impacts on Fish Lake”.

- Similarly, if the rock mass encountered between the QD and East faults is more highly fractured and weaker than expected, a large rotational instability may develop in the South wall. The most effective means to stabilize such slope movements is deep drainage. If additional mitigative measures are required, other routine practices in large open pits include slope flattening or unloading material at the top of the slope and placing it at the foot as a buttress.

4.3.2. InfluenceofConfinedAquifers

- Details regarding the presence of confined aquifers in the overburden sediments are generally limited in the Preliminary Pit Slope Design Report to general statements regarding their presence and the need for dewatering [see Taseko, 2012; EIS 2.2.4 A, p. 10]. However, addition details can be found in supporting documents to the 2009 EIS submission, including the 2009 Feasibility Design Report [Taseko, 2009; EIS 3.6.E, p. 57‐58]. This report, based on the hydrogeological investigations previously carried out, characterizes the confined aquifer(s) as exhibiting high recharge capacity. The confined artesian aquifer is reported to be a 2 m thick gravel unit located at 160 m depth. Pump tests estimate the aquifer to have an average transmissivity of 22 m2/day, a specific storage of 3.4e‐5 l/m, and a permeability of approx. 7e‐2 m/s [Taseko, 2009; EIS 3.6.E, p. 58]. Artesian pressures for this confined aquifer are reported to be 12 m above the existing level of Fish Lake. Depressurization wells are recommended to be screened along most of their lengths such that they will serve to drain other smaller confined aquifers which may be perched within the overburden units. Results from the seepage analysis performed for the 1999 feasibility design


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indicate that a total flow of approx. 10 l/s will be required to depressurize the aquifer [Taseko, 2009; EIS 3.6.E, p. 58].

- As previously discussed, the Preliminary Pit Slope Design assumes that aggressive groundwater depressurization measures will be implemented and therefore the stability analyses do not consider the possible influence of a confined aquifer on overall pit wall stability. Again, the experiences at the Mount Tom Price mine are raised here, where after more than 13 years of active dewatering of a confined aquifer, leakage from two large faults in hydraulic contact with the confined aquifer have prevented the aquifer from being properly drained [Brehaut, 2009]. Achieving proper drawdown may simply be a matter of increasing the pumping and drainage effort to contend with adverse conditions. However, it is also possible that the desired depressurization may not be achieved if complexities in the flow paths and interactions between the confined aquifer, the QD and East Faults and regional groundwater recharge sources prove more challenging than expected.

- Without proper dewatering, high pore pressures in the confined aquifer may cause floor heave as the deepening pit approaches the horizontal lying confined aquifer, or destabilize the slopes in the exposed overburden sequence. High seepage gradients may cause piping and erosion of the sand and gravel aquifer material, undermining the till and basalt layers above causing localized failures that may retrogressively work their way back until stabilization measures can be enacted. In‐rushes of water and collapsed rocks are a potential hazard, especially if the sand and gravel beds pinch out in near proximity to the pit wall without actually daylighting into it.

4.3.3. LargeOpenPitPrecedence

- The Preliminary Pit Slope Design Report presents a plot of slope height versus overall slope angle for a large number of open pit case histories compiled by various authors and supplemented with data from large open pit mines in British Columbia [Taseko, 2012; EIS 2.2.4 A, fig. 6.3]. This figure is reproduced in Figure 4, and it shows the West and North walls as plotting between two trend lines representing nominal factors of safety of FoS = 1.0 and 1.3. The South wall would likewise plot between these trend lines. However, inspection of the data reveals that there is a significant degree of scatter, as would be expected given the differing site conditions, geology, hydrogeology and climatic conditions unique to each case. Within this scatter, there are numerous instances where large open pit slopes have failed at angles less than 40 degrees with supposed nominal safety factors greater than the acceptance criteria of FoS = 1.3. This is also shown in a table comparing large open pit mines in British Columbia [Taseko, 2012; EIS 2.2.4 A, tab. 6.3]. The table shows that of the 12 cases cited, 8 (67%) have experienced some form of unstable slope displacements.

- Additional details from this data review are discussed in earlier design reports, including the observation that slope stability problems encountered are sometimes complicated by high stresses that are encountered where pits are exceptionally deep. In particular, large scale toppling type movements have been encountered at Chuqicamata in Chile (780 m deep) and in the Lornex Pit at the Highland Valley Copper mine near Kamloops (370 m deep) [Taseko, 2009; EIS 3.6.E, p. 83]. The latter case is discussed in more detail under “Possible Impacts on Fish Lake”.


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Figure 4: Data compiled for a large number of open pit cases plotting slope height versus slope angle. [Taseko, 2012; EIS 2.2.4 A, fig. 6.3].


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- The Preliminary Pit Slope Design Report concludes that almost all of the large open pit operations reviewed, including several porphyry copper mines, have encountered slope stability problems in some area of the mine [Taseko, 2012; EIS 2.2.4 A, p. 27]. It is therefore suggested that there is a significant possibility that some area of the New Prosperity pit will require slope flattening during operations in response to slope movement. Therefore, the mine plans should remain flexible so that extra stepouts/buttresses can be maintained in critical areas of the pit until the end of the mine life when lower factors of safety can be tolerated [Taseko, 2012; EIS 2.2.4 A, p. 27].

- Although not stipulated in the 2012 Preliminary Pit Slope Report, a minimum setback/buffer distance should be specified recognizing that there is the potential for a large pit slope instability to develop. In the 1999 Feasibility Design Report, a minimum buffer area of at least 200 m around the pit was recommended, noting that any critical features or facilities which cannot tolerate slope movement must not be located within this buffer area [Taseko, 2009; EIS 3.6.E, p. 83].

4.3.4. Long‐TermSlopePerformance

- The Preliminary Open Pit Design Report discusses the importance of preserving rock mass integrity during mine operations to prevent progressive deterioration (unravelling) of the bench faces to achieve the steepest bench face angles possible [Taseko, 2012; EIS 2.2.4 A, p. 24]. Controlled blasting methods are recommended to facilitate steeper final pit slopes by reducing face damage from blasting. It is also noted that interim slopes must also incorporate some level of controlled blasting to maintain safety but that the requirements in this situation are less rigorous due to the shorter operating life of these walls [Taseko, 2012; EIS 2.2.4 A, p. 24].

- It is noted that the 1999 Feasibility Design also employed advanced 2‐D numerical modelling methods in addition to the more conventional 2‐D limit equilibrium analyses reported in the 2012 Preliminary Pit Slope Design Report. The commercial finite difference program FLAC and distinct‐element program UDEC were used to assess large‐scale slope stability and potential instability mechanisms not considered by the limit equilibrium analyses. (Limit equilibrium methods require the assumption of the failure mechanism as part of the analysis; numerical models allow for the failure mechanism to develop in the model as a function of the rock mass characteristics and material properties incorporated into the model. See Stead et al., 2006). An important failure mechanism that can be assessed using advanced numerical modelling is that of “step path” and “progressive” failure [Eberhardt et al., 2004]. Step‐path and progressive failure surfaces are associated with high rock slopes in jointed rock masses where over‐stressing and crushing near the slope’s toe is followed by the progressive failure of intact rock bridges between non‐persistent discontinuities up through the rock mass towards tension cracks that develop along the crest of the slope (Figure 5).

- Progressive failure was considered as a potential pit slope instability mechanism in the numerical analyses carried out for the 1999 Feasibility Design. This was a rather advanced technical consideration at its time and speaks to a very high level of understanding and expertise possessed by the pit slope design consultants (Knight Piésold). As stated in the report, numerical models can simulate the pit excavation sequence and the development of failure modes due to progressive overstressing and step path failure surface development [Taseko, 2009; EIS 3.6.E, p. 50]. The report goes on to emphasize that unlike non‐cemented granular materials, rock masses tend to exhibit relatively high undisturbed strengths that undergo very significant strain softening at relatively low strains. In other words, the over‐stressing of materials in a region of the slope, followed by significant strength reduction in that region and the shedding of loads to adjacent


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regions, is the typical mechanism for the progressive development of a failure surface [Taseko, 2009; EIS 3.6.E, p. 50‐51]. Conclusions drawn from the numerical modelling study suggest that the proposed pit slope configurations will remain stable during operations, provided that weak adversely oriented structures are not encountered and that the slope is depressurized [Taseko, 2009; EIS 3.6.E, p. 55]. However, significant deformations may be experienced due to relaxation of the lower pit walls, with outward displacements occurring in conjunction with overstressing along shallow dipping discontinuities. The models predict zones of overstressing and yielding extending approximately 50 to 100 m into the slope over a height of approximately 200‐300 m [Taseko, 2009; EIS 3.6.E, p. 53]. The report suggests that these zones could result in stability problems over the lower portions of the slope in the absence of proper monitoring and dewatering [Taseko, 2009; EIS 3.6.E, p. 55].

Figure 5: Left, illustration of progressive failure involving toe shear and step‐path failure. After Hoek et al. (2000). Right, example of advanced numerical modelling of a progressive failure mechanism involving toe shear and step‐

path failure up through a system of non‐persistent joints. After Eberhardt et al. (2012).

- The Preliminary Pit Slope Design Report notes that the slope stability analyses undertaken did not evaluate the potential for seismic (earthquake) triggering of a large pit slope failure. The report cites [Read & Stacey, 2009] in stating that there are few recorded instances in which earthquakes triggered significant failures in hard rock open pits, instead producing small shallow slides and rock falls, but none on a scale sufficient enough to disrupt mining operations [Taseko, 2012; EIS 2.2.4 A, p. 4]. A review of historical earthquake records and regional tectonics reported in the Waste Dumps and Stockpiles Preliminary Design Report, states that the New Prosperity Project site is situated in a region of moderate seismic hazard [Taseko, 2012; EIS 2.2.4 B, p. 5]. Using the Natural Resources Canada database, a mean potential peak ground acceleration (PGA) of 0.07g is estimated for a 100 year return period event with a probability of exceedance (for a 20 year design operating life) of 18%. For a 475 year return period event (probability of exceedance = 4%), a mean PGA of 0.17g is estimated [Taseko, 2012; EIS 2.2.4 B, tab. 2.2]. Future pit slope stability analyses should consider the effects of seismic loading, even if only as a simple limit equilibrium scoping calculation to see if more rigorous numerical analyses are warranted.


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4.4. PossibleImpactsonFishLake

Another key question posed by the Review Panel was whether flattening of the South pit wall, if required, might affect the preservation of Fish Lake. Prior to the 2012 EIS, the previous mine plans and open pit designs had called for draining and backfilling Fish Lake during open pit construction. Accordingly, all geotechnical and hydrogeological field investigations were carried out without consideration being given to the need to preserve Fish Lake. Relevant data was collected in the region between the southern pit limit and Fish Lake; however there is an absence of dedicated investigation data targeting specific questions regarding what potential impacts if any the development of the open pit may have on Fish Lake. The statement made in the 2012 EIS that the interaction of Fish Lake with the groundwater table has been assessed with respect to the pit wall designs [Taseko, 2012; EIS 2.2, p. 99], is not strongly supported in the EIS, Preliminary Pit Slope Design Report, or any of the other associated reports reviewed. As noted in previous comments, there are several issues that arise from uncertainty regarding the hydrogeological characteristics of the overburden, the presence of confined aquifers, and their interaction with Fish Lake, the open pit and the QD and East Faults. These are explored in further detail below.

4.4.1. InteractionbetweenConfinedAquifersandFishLake

The presence of confined aquifers with artesian pressure conditions and relatively high recharge -capacities [Taseko, 2012; EIS 2.2.4 A, p. 10], will necessitate aggressive pit dewatering measures focussed largely between the southern end of the proposed pit and Fish Lake. A combination of the thick sequence of low permeability glacial tills separating Fish Lake from the targeted confined aquifers and the flat, horizontal stratigraphy of the overburden sediments has led to the conclusion that Fish Lake does not have a direct hydraulic connection to the deep confined aquifer [Taseko, 2012; EIS 2.2.4 A, p. 10]. This is a reasonable assumption, but not one confirmed by the limited and non‐question specific data available.

- The question of seepage from Fish Lake has seen significant discussion and exchange of opinions between Taseko and NRCan. Taseko’s position as stated in their SIR response to this question is that a number of conditions would need to be met in addition to leakage into the confining aquifer during pumping to result in a significant increase in seepage from Fish Lake [Taseko, 2013b; SIR, p. 10/11‐30]. And in such an event, this water would be collected by the pit dewatering system and could be recycled to Fish Lake so that there would be no net effect on the water level of Fish Lake. In response, NRCan acknowledges that this would appear to be feasible, at least technically [NRCan, 2013; IR 10/11, p. 7]. Taseko further acknowledges the need for additional information with respect to larger scale hydraulic conductivity in the overburden between Fish Lake and the open pit in order to proceed to construction, and that will be undertaken as part of the detailed engineering design phase of the project [Taseko, 2013b; SIR, p. 10/11‐8]. It can be confirmed that such detailed and targeted investigations are more commonly carried out in the design stage of a project than in the feasibility stage [Read & Stacey, 2013; p. 13].

- The above discussions are based on the underlying assumption that because the confined aquifer and surrounding thick deposits of low permeability sediments are horizontally bedded that they impose a significant barrier to groundwater flow. Differences of opinion can be debated regarding the hydraulic conductivity values to be assigned to these materials in seepage models; in any case,


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mitigation measures are available that can contend with such differences. What would be more serious, and potentially catastrophic, would be the potential for a more serious breach. Taseko points out that based on packer testing, single well response tests, and core logs from exploration and geotechnical drilling, there is no evidence of a structural conduit that could provide a direct hydraulic connection between the pit and the lake [Taseko, 2013b; SIR, p. 10/11‐15]. This would seem to be a reasonable conclusion, but one based on the pre‐mining site conditions. It must also be assessed whether it is possible for such a vertical conduit to develop in response to mining and pit slope failure.

4.4.2. InteractionbetweenSlopeFailureandFishLake

- The design and stability of the high overburden slopes in the South and South‐east walls nearest to Fish Lake will be largely dependent on the effectiveness of the slope dewatering efforts carried out. If a catastrophic failure were to develop in these materials, the rotational movements and back scarp would not likely extend back behind the crest more than 40‐60 m. This is supported by the critical slip surfaces calculated for the overburden, as reported in [Taseko, 2009; EIS 3.6.E, p. 83]. If drainage of the confined aquifers proves problematic, there is the possibility that subsequent retrogressive failures may develop in the overburden materials, but likely not extending more than 100‐150 m behind the slope crest in a worst case scenario.

- If a larger instability was to develop in the South‐east wall nearest Fish Lake involving the full 535 m slope height (overburden and bedrock), the calculated slip surfaces show that these likely wouldn’t extend more than 100 m behind the slope crest [Taseko, 2012; EIS 2.2.2 A, fig. 4.10]. Furthermore, most large scale pit failures are detected at an early stage of development giving opportunity for stabilization measures to be enacted. Taseko correctly notes that in the event that a 5 degree reduction in a pit wall angle was necessary, this flattening would still maintain a minimum 100 m buffer between the pit crest and the nearest stockpile, and more than 200 m between the pit crest and Fish Lake at full build‐out of the pit design [Taseko, 2013a; IR, p. 2‐2]. Numerous examples exist where slope stability problems encountered in large open pits have been successfully managed by flattening portions of the pit walls in order to control slope movements.

- Greater uncertainty exists with respect to the impact of a large slope movement developing in the South wall. The geotechnical and hydrogeological characteristics of the QD and East Faults, and the rock mass in between, have not been fully investigated. Given their potentially weaker nature, a deep‐seated failure may extend further back from the pit crest than a comparable large slope failure in the South‐east wall. Again, it is unlikely in either case that this will extend back far enough to breach Fish Lake, but the nature of the deposits between the faults beyond the pit limits have not been properly investigated. Nevertheless, as noted for the previous scenarios, mitigation measures including slope flattening can be enacted to significantly reduce any likelihood of a large‐scale slope failure.

- A scenario that may of greater concern than that of a massive slope failure developing in the South or South‐east wall is the possibility of farther reaching slope displacements developing behind the slope crests. The kinematic analyses have indicated the potential for multi‐bench scale toppling to develop in the South wall, with the QD and East Faults acting as potential release planes to facilitate deeper toppling displacements. Given the brittle nature of the bedrock, as well as that of the thick sequence of tills overlying the bedrock described as dense, over‐consolidated and stiff, any displacements that arise could potentially generate deep vertical tension cracks


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through the overburden units. Figure 6 provides an example of the tension cracks that have developed behind the crest of Highland Valley Copper’s Lornex pit as a result of large scale toppling type movements (not failure). It is strongly recommended that numerical modelling be carried out to determine how far back tension cracks may develop due to slope movements.

Figure 6: Air photo of the Lornex Pit at the Highland Valley Copper mine, showing the extent of large, visible, open tension cracks behind the slope crest (requiring a road to be relocated further back) arising from deep‐seated

toppling displacements.

4.4.3. Post‐ClosurePitStability

- Beginning in Year 17, mining is expected to be completed and the pit allowed to flood [Taseko, 2012; EIS 2.7, p. 498]. However, given the importance of dewatering to pit slope stability, very little appears to be discussed in the EIS regarding the potential for post‐closure pit slope failure and it potential impact on Fish Lake. A terrain stability assessment is proposed for the pit walls during closure to identify any mitigation or monitoring required to address terrain stability issues that may affect stability of the site, or affect successful reclamation [Taseko, 2012; EIS 2.7, p. 933].

- As noted in [Read & Stacey, 2009; p. 410], after closure pore pressures will increase as dewatering or depressurization activities cease, new fracture surfaces related to step path and progressive failure may become inundated and thereby experience further shear strength decreases, and stresses in the pit walls may alter. The overall effect is that the pit walls may unravel and/or collapse over time. It is unclear whether a series of pit slope failures involving the South and South‐east walls may develop over time so as to breach Fish Lake.


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- The 2012 preliminary pit design compared to earlier feasibility designs (i.e., 1999) involves an open pit that is shallower and therefore invokes a greater separation distance between the proposed southern limit of the pit and Fish Lake (Figure 7). This modification appears in the 2009 EIS and thus predates the decision to preserve Fish Lake, suggesting that it based on revised grade estimates and project economics. Nevertheless, TNG argues that based on news releases issued by Taseko regarding increases in mineral reserves, that there is a likely possibility that the mine plan may change to access deeper reserves, which in turn would result in a larger open pit that may possibly encroach on Fish Lake [TNG, 2012; p. 17‐18]. This would depend on whether the pit expanded uniformly or more in one direction than another (for example expanding to the north instead of the south). Given the likelihood of pressure to mine the full ore body in the future to maximize resource extraction, further details may be necessary as to what future mine expansion plans may look like with respect to impacting Fish Lake.

Figure 7: Difference in mine plans with respect to open pit outline comparing 1999 Feasibility Design and 2012 Preliminary Design. [Taseko, 2012; EIS 2.2.4 A, fig. 3.6].

4.5. GeotechnicalRiskandPerformanceAssurance

The 2012 EIS establishes that an extensive amount of data has been collected and that based on all currently available drilling, geotechnical and hydrogeological test data, and the stability analyses performed, that the recommended pit slope design is reasonable and appropriate [Taseko, 2012; EIS, 2.2, p. 85]. Nevertheless, it is understood that in any large open pit project the geological and hydrogeological conditions at depth are complex and heterogeneous and can never be known exactly. An appropriate number of carefully placed boreholes must be drilled and a high level of effort invested


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to develop as accurate a three‐dimensional picture of the geological conditions as possible; however, no level of drillhole data can guarantee that construction will be entirely free from problems, but the chances of encountering unexpected geological conditions are greatly diminished. Accordingly, geotechnical risk management and performance assurance play an important role in the design and construction of large open pits. As a project evolves from Feasibility to Detailed Design, the level of investigative effort increases as does the confidence in the geological and hydrogeological models derived [e.g., Read & Stacey, p. 13]. And that certainty is not achieved until the Construction and Operation stages where the sub‐surface conditions are exposed and can be fully ground‐truthed. The Preliminary Pit Slope Design Report correctly recognizes that the design will undergo further modification and optimization as the project develops. Monitoring and updating of the geotechnical and hydrogeological models, and their implications with respect to stability of the open pit slopes, is called for. Below are several comments that address the assessment of geotechnical risk and performance assurance in the EIS, Preliminary Open Pit Design and associated documents.

4.5.1. RiskAssessment

- A risk matrix was provided in the 2009 EIS rating the likelihood and consequences of different identified failure modes related to the open pit [Taseko, 2009; EIS 2‐6, p. 6‐42]. The assessment was carried out for three different Mine Development Plans, with “Option 2” being the most relevant to the 2012 mine plan (i.e. preservation of Fish Lake and location of the TSF to the south‐east of Fish Lake) [Taseko, 2009; EIS 2‐6, p. 6‐36]. This assessment is not reproduced in the 2012 EIS, but updates relevant to the change in mine plan to preserve Fish Lake were provided in [Taseko, 2013b; SIR, p. 48‐8], specifically the likelihood and consequences of an abrupt escape of water from Fish Lake to the open pit.

- Eight failure modes were identified for the open pit, with four considering the likelihood and consequences of a pit slope failure and the other four considering excess seepage and dewatering issues. It is noted that several of the ratings assigned, differ from those reported in the earlier 1998 risk assessment [Taseko, 2009; EIS 2.6.C], without any explanation as what the changed rating was based on. Similarly, very few details in general are given in either the 1998 or 2009 assessments justifying the ratings assigned.

- The likelihood of a large slope failure in the South wall was rated as being extremely low (Figure 8). It should be noted that the likelihoods are reported to cover the duration of construction, operation and closure periods [Taseko, 2009; EIS 2‐6, p. 6‐42]. No explanation is provided for this assessment, but given the presence of the QD and East Faults intersecting the South wall and given the uncertainty regarding their geotechnical and hydrogeological characteristics due to the absence of targeted drilling, this assessment would seem to be overly optimistic. In fact a rating of “extremely low” was assigned for the likelihood of any large pit wall failure (see Figure 8), despite the Preliminary Pit Slope Design Report establishing that almost all large open pit operations have encountered slope stability problems in some areas of the mine [Taseko, 2012; EIS 2.2.2 A, p. 27]. A distinction can possibly be made between an instability developing and total failure and collapse occurring. In either case, a likelihood of “low” would probably be more appropriate for the South wall than “extremely low”, coupled with the recognition that significant uncertainty exists given the absence of targeted drilling and investigation of these faults.


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Figure 8: Subjective likelihood and consequence ratings for Option II (i.e., preservation of Fish Lake). [Taseko, 2009; EIS 2‐6, p. 6‐48].

- The consequences of a large failure of the South wall is recognized as potentially having a significant impact on Fish Lake, rating this as “Moderate” [Taseko, 2009; EIS 2‐6, p. 6‐49]. The risk to worker safety can be mitigated through a comprehensive pit slope monitoring plan, as recommended in the Preliminary Pit Slope Design Report [Taseko, 2012; EIS 2.2.2 A, p. 25‐26].

- Excessive seepage from Fish Lake is rated as having an “extremely low” likelihood [Taseko, 2009; EIS 2‐6, p. 6‐49], as shown in Figure 8. This is reasonable based on the undisturbed geological conditions as presently known, although again a rating of “low” may be more appropriate given the absence of dedicated drilling and analysis to better resolve the hydrogeological characteristics of the overburden unit between the pit limits and Fish Lake. It is often more prudent to assume the conditions are more adverse than otherwise would be assumed based on limited data (and establish through detailed investigation that they aren’t), than to assume a best case scenario when significant uncertainty is present. An update to this failure mode, with reference to abrupt escape of water from Fish Lake to the open pit was provided in [Taseko, 2013b; SIR, p. 48‐8], with the same likelihood of “extremely low/rare” but with higher consequences (“major”), given the requirement to preserve Fish Lake in the 2012 EIS. Given that a one‐step increase in the likelihood (from “extremely low” to “low”) would elevate the risk for this failure mode to “High”, consideration should be given to the plausibility of large slope deformations developing in the South and/or South‐east walls, leading to the development of deep vertical tension cracks that could breach the water control dams or Fish Lake itself. The likelihood of this scenario can be quickly assessed through preliminary 2‐D numerical models that can serve as a quick scoping calculation.


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- Excessive seepage from a deep aquifer is similarly rated as “extremely low”. This again would seem to be an optimistic rating given the uncertainty resulting from a lack of targeted investigations to determine the properties and recharge characteristics of the confined aquifer(s). The plausibility that the confined aquifer at the overburden/bedrock contact is hydraulically connected to the QD and East Faults would give rise to a higher likelihood of occurrence (“low” to “moderate”). This would elevate the risk to operations as “High” but would not impact the risk to environmental factors assuming the reasoning for rating the consequences for this failure mode as “extremely low” is appropriate.

4.5.2. PerformanceAssurance

- Given the complexities and uncertainties present in any large open pit design, the design process should be considered an iterative one involving systematic updating of the geological and hydrogeological conditions, monitoring, and pit slope performance. This should be accompanied by a ground control management plan, as outlined in [Read & Stacey, 2009; p. 370‐379], involving the preparation of a hazard inventory, risk reduction options, and trigger action response plan (TARP) in the event that unstable pit slope movements develop or if dewatering measures are not as effective as required.

- The Preliminary Pit Slope Design Report recommends a pro‐active geotechnical monitoring plan, implemented as a staged approach, combining: detailed geotechnical mapping of the pit slopes as they are excavated; tension crack mapping to track their location, frequency, and length and aperture change; surface displacement monitoring to detect the onset of any possible pit slope movement/sliding as the open pit deepens; and piezometer installation to ensure the required depressurization of the pit walls has been achieved to provide an adequate factor of safety [Taseko, 2012; EIS 2.2.2 A, p. 25‐26]. It is essential that a detailed set of protocols (i.e. TARP) be established for identifying early warning threshold movements and deformations of the pit slopes using the instrumentation available on site. The protocol should identify which personnel are responsible for monitoring the instrumentation and exactly how responsible parties (geotechnical engineers, mine managers, etc.) will respond in the case set thresholds are exceeded.

- An example to the above comment is provided in Taseko’s response to the Review Panel’s Information Request regarding the possible abrupt escape of water from Fish Lake, in the event that a pit wall failure extends back to the location of the water control dams [Taseko, 2013a; IR, p. 48‐8]. Taseko proposes that slope monitoring will be used to detect the precursors of a large pit slope failure with sufficient time to develop and implement preventative measures. The response to this event would include continuous monitoring and mitigative measures such as additional dewatering, rock bolting, buttressing, or reduction of the mass at the top of the unstable areas as required [Taseko, 2013a; IR, p. 48‐8]. Reference to rock bolting is likely not applicable in this case, unless reference is being made to deeper cable anchors. Otherwise, the mitigative measures suggested are appropriate.

5. KeyConcernsandRecommendations

The key concerns and accompanying recommendations regarding geotechnical issues associated with open pit design, slope stability and possible confined aquifers in proximity to the proposed open pit are as follows:


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Drillhole data used for the open pit design is based solely on investigations carried out in the 1990’s. To date there have been no new targeted boreholes directed to investigate the geotechnical and hydrogeological characteristics of the QD and East Faults (subsequent to their identification), or to investigate the ground conditions between the open pit and Fish Lake (in support of the 2012 EIS revision to the Mine Development Plan to preserve Fish Lake). As a result, open questions persist regarding the QD and East Faults and confined aquifer(s) and their potential interactions and impacts on the South and South‐east walls, and ultimately Fish Lake. A detailed, targeted investigation program is recommended to support the Detailed Design if the project proceeds.

It is strongly suggested here that equating the hydraulic conductivities of the fault zones to those of the bedrock, as is done in the preliminary open pit design, would be counter to most experiences involving large fault zones similar in scale to the QD and East Faults. Although a central fault gouge zone may form a relatively impervious aquiclude that impedes flow normal to the fault, the fracture zones adjacent to the fault gouge often serve to significantly enhance the permeability parallel to the fault. It should also be noted that these faults may be in direct hydraulic contact with the confined artesian aquifer at the overburden/bedrock contact, and that there is precedence where such a scenario has significantly limited depressurization efforts due to recharge to the confined aquifers provided by the faults. Such a scenario also offers an alternative hypothesis regarding the source of leakage possibly observed in pump tests as discussed in numerous communications between the Proponent and different review bodies.

The stability analyses performed and design criteria applied conform to commonly accepted industry practices. However, it can be argued that the acceptance criteria for the design of the South and South‐east walls should be elevated to a higher Factor of Safety (1.4 instead of 1.3) to better reflect a “high” consequence of failure. This will correspond to flatter slope angles that will reduce the buffer distance between the pit limits and Fish Lake. Similarly, open questions regarding the hydrogeological characteristics of the confined aquifer along the overburden/bedrock contact and the QD and East Faults may require flatter slope angles to maintain stability.

Statements made in the EIS to the effect that the interaction between Fish Lake and the groundwater table has been assessed with respect to the pit wall designs, are not strongly supported in the EIS. It can be argued that based on the investigations carried out to date, that there is no evidence of a conduit providing a direct hydraulic connection between the pit and the lake; but it should be emphasized that this conclusion is based only on the pre‐mining site conditions. Although a major collapse of the South or South‐east wall is unlikely, and in any event can be mitigated against, slope displacements that develop in response to deep toppling movements in the South wall could potentially generate deep vertical tension cracks behind the pit crest. These could potentially breach the water control dams, or Fish Lake directly. Experiences at other large open pits where large‐scale toppling is observed suggest that tension cracks can extend more than 150‐200 m behind the slope crest. It is recommended that analyses be carried out to determine how far back behind the pit crest tension cracks may develop in response to slope displacements.

Given the importance of dewatering to pit slope stability, very little appears to be discussed in the EIS regarding the potential for post‐closure pit slope failure after dewatering is stopped and the pit allowed to fill. It is recommended that more details be provided regarding the impacts of pit closure on long‐term pit stability and that supporting stability analyses be carried out.


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No level of drillhole investigation data can guarantee that construction will be entirely free from problems; however, the chances of encountering unexpected geological conditions can be greatly reduced. The EIS correctly recognizes that the pit design will undergo further modification and optimization as the project develops. Monitoring and updating of the geotechnical and hydrogeological models, and their implications with respect to stability of the open pit slopes is called for. The rating of likelihoods assigned in the risk assessment regarding the open pit design and its influence on Fish Lake arguably underestimate the overall risk but not significantly so. A ground control management plan should be developed outlining the open pit hazard inventory, risk reduction options, and trigger action response plan in the event that unstable pit slope movements develop or if dewatering measures are not as effective as required.

6. References

Brehaut, R.J. (2009). Groundwater, Pore Pressure and Wall Slope Stability – A Model for Quantifying Pore Pressures in Current and Future Mines. M.Sc. Thesis, University of Canterbury, New Zealand.

Domenico, P.A. & Schwartz, F.W. (1990). Physical and Chemical Hydrogeology. John Wiley & Sons.

Eberhardt, E. (2012). ISRM Suggested Method for Rock Failure Criteria: The Hoek‐Brown Failure Criterion. Rock Mechanics and Rock Engineering: 45(6), 981‐988.

Eberhardt, E., Bonzanigo, L. & Loew, S. (2007). Long‐term investigation of a deep‐seated creeping landslide in crystalline rock – Part 2: Mitigation measures and numerical modelling of deep drainage at Campo Vallemaggia. Canadian Geotechnical Journal: 44(10), 1181‐1199.

Eberhardt, E., Fisher, B., Burden, J. & Hungr, O. (2012). Limon Dam Stability Evaluation: North‐South Slope Expert Opinion. Technical Report to Proyecto Especial Olmos Tinajones (PEOT), Chiclayo, Peru, Report# 166AZ, 155 pp.

Eberhardt, E., Stead, D. & Coggan, J.S. (2004). Numerical analysis of initiation and progressive failure in natural rock slopes – the 1991 Randa rockslide. International Journal of Rock Mechanics and Mining Sciences: 41(1), 69‐87.

Federal Review Panel (2013). Supplemental Information Requests from the Federal Review Panel to Taseko Mines Ltd. Regarding the Environmental Impact Statement for the New Prosperity Gold‐Copper Mine Project, British Columbia. Memorandum, Mar. 28, 2013.

Hoek, E., Carranza‐Torres, C.T. & Corkum, B. (2002). Hoek‐Brown failure criterion ‐ 2002 edition. In Hammah et al. (eds.), Proceedings of the Fifth North American Rock Mechanics Symposium (NARMS‐TAC), Toronto. University of Toronto Press, vol. 1, pp. 267‐273.

Hoek, E., Read, J., Karzulovic, A. & Chen Z.Y. (2000). Rock slopes in civil and mining engineering. In Proceedings of the International Conference on Geotechnical and Geological Engineering, GeoEng2000, Melbourne. Technomic Publishing Co., Lancaster.

Laws, S., Eberhardt, E., Loew, S. & Descoeudres, F. (2003). Geomechanical properties of shear zones in the Eastern Aar Massif, Switzerland and their implication on tunnelling. Rock Mechanics and Rock Engineering: 36(4), 271‐303.


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Loew, S., Luetzenkirchen, V., Ofterdinger, U., Zangerl, C., Eberhardt, E. & Evans, K. (2007). Environmental impacts of tunnels in fractured crystalline rocks of the Central Alps. In Krásný & Sharp (eds.), Groundwater in Fractured Rocks: IAH Selected Papers 9. Taylor & Francis, ch. 34, pp. 507‐526.

Natural Resources Canada (2013). Evaluation of the Adequacy and Technical Merit of the Additional Information Submitted by the Proponent for the New Prosperity Gold‐Copper Mine. Memorandum, Jun. 14, 2013.

Read, J. & Stacey, P., eds. (2009). Guidelines for Open Pit Slope Design. CSIRO Publishing.

Stead, D., Eberhardt, E. & Coggan, J.S. (2006). Developments in the characterization of complex rock slope deformation and failure using numerical modelling techniques. Engineering Geology: 83(1‐3), 217‐235.

Taseko Mines Limited (2009). Taseko Prosperity Gold‐Copper Project: Environmental Impact Statement/Application. March, 2009.

Taseko Mines Limited (2012). New Prosperity Gold‐Copper Mine Project British Columbia, Canada: Environmental Impact Statement. September, 2012.

Taseko Mines Limited (2013a). IR Responses to Panel. Memorandum, Feb. 28, 2013.

Taseko Mines Limited (2013b). Responses to the Supplemental Information Requests from Taseko Mines Ltd. to the Federal Review Panel Regarding the Environmental Impact Statement for the New Prosperity Gold‐Copper Mine Project, British Columbia. Memorandum, Jun. 5, 2013.

Tsilhqot’in National Government (2012). Deficiencies in the EIS for the New Prosperity Project. Memorandum, Nov. 11, 2012.


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Table A1: Site investigation reports cited in 2012 Geotechnical Site Investigation Factual Data Report, and relevant data contained within and reviewed regarding pit slope design and ground conditions between open pit and Fish Lake. Table continued on next page.

Year Report Data Summary and Data Reviewed Relevant to Assignment Source

1991 Preliminary Geotechnical Evaluation (Ref. 1731/1, August 1991)

• Preliminary site survey with focus on possible locations for the tailings storage facility. • No data provided related to open pit design or sub‐surface conditions between open pit and Fish Lake.

2009 EIS, Appendix 3‐6‐M

1992 Preliminary Hydrogeological Investigations (Ref. 1732/2, May 1992)

• Report could not be located. N/a

1992 Preliminary Geotechnical Investigation (Ref. 1733/1, Jan. 1993)

• 5 new geotechnical boreholes drilled in footprint of proposed tailings impoundment structures, with packer/permeability test data. • No data provided related to open pit design or sub‐surface conditions between open pit and Fish Lake.

2009 EIS, Appendix 3‐6‐N

1993 Open Pit Preliminary Hydrogeological Investigations (Ref. 1736/2, Mar. 1994)

• 4 new vertical boreholes drilled between open pit and Fish Lake, with geology logs, well completion details, and packer/permeability test data (93‐126, 127, 128, 129). • 4 new inclined boreholes drilled within the open pit limits. Geological logs not provided. • Hydrogeological sections constructed from borehole data.

2009 EIS, Appendix 3‐6‐G

1994 Geotechnical and Hydrogeological Investigation for Proposed Tailings Storage Facility (Ref. 1738/1, Jan. 1995)

• 6 new geotechnical boreholes drilled in footprint of proposed tailings impoundment structures, with packer/permeability and rock strength test data. • No data provided related to open pit design or sub‐surface conditions between open pit and Fish Lake.

2009 EIS, Appendix 3‐6‐O

1994 Open Pit Investigation (Ref. 1738/2, Jan. 1995)

• 3 new vertical boreholes drilled between open pit and Fish Lake, with geology logs, well completion details, and pump test data (monitoring wells: 94‐154, 157, 159; production well: 94‐164). • 19 new inclined boreholes within open pit limits with oriented core for geotechnical logging of RQD and discontinuity orientation, spacing and characteristics. • 10 packer/permeability tests in inclined borehole 94‐153, located within open pit limits. • Point load data (124 tests) from 2 boreholes located within pit limits (94‐152, 153).

2009 EIS, Appendix 3‐6‐H


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Table A1 (cont.). Site investigation reports cited in 2012 Geotechnical Site Investigation Factual Data Report, and relevant data contained within and reviewed regarding pit slope design and ground conditions between open pit and Fish Lake. Table continued on next page.


1994 Plant Site and Crusher Site Foundation Investigations (Ref. 1738/3, Jan. 1995)

• 4 new geotechnical boreholes drilled at the plant and primary crusher site locations, assessing rock quality, rock strength (point load) and discontinuity characteristics. • No data provided related to open pit design or sub‐surface conditions between open pit and Fish Lake.

2009 EIS, Appendix 3‐6‐L

1996 Geotechnical Site Investigation for Tailings Management Options 2 and 5 (Ref. 1731A/4, Jan. 1997)

• 15 new geotechnical boreholes drilled and 64 test pits excavated in footprints of tailings impoundment structures for alternative Tailings Storage Facility (TSF) site options, with packer/permeability, groundwater monitoring and laboratory test data. • 1 new borehole (96‐205) drilled in middle of Fish Lake, allowing comparison of overburden thickness and geology with boreholes located between Fish Lake and open pit limits.

2009 EIS, Appendix 3‐6‐P

1996 Open Pit Geotechnical Investigation (Ref. 1731A/7, Jun. 1997)

• 5 new geotechnical boreholes with drilled within open pit limits or between pit and Fish Lake: 2 inclined boreholes in South wall (96‐180, 196), 1 inclined borehole in East wall (96‐207), and one inclined (96‐212) and one vertical (96‐218) borehole between open pit limit and Fish Lake. • Detailed bedrock logs to 600 m depth for 96‐180; logs for other boreholes missing. • Packer/permeability test data. • Strength data for bedrock in southern pit area: point load, UCS and triaxial testing of intact rock, direct shear testing of rock joints. • Strength data for overburden soils in southern pit area: triaxial compression and direct shear.

2009 EIS, Appendix 3‐6‐I

1996 Seismic Refraction and Reflection Investigation (Ref. FGI‐313, Jul. 1997)

• Seismic refraction survey to determine thickness and composition of overburden materials at proposed locations of two tailings impoundment structures. • No data provided related to open pit design or sub‐surface conditions between open pit and Fish Lake.

2009 EIS, Appendix 3‐6‐Q


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Table A1 (cont.). Site investigation reports cited in 2012 Geotechnical Site Investigation Factual Data Report, and relevant data contained within and reviewed regarding pit slope design and ground conditions between open pit and Fish Lake.


1998 Geotechnical Parameters for the Plant Site Foundation Design (Ref. 10173/12‐3, Dec. 1998)

• 5 new geotechnical boreholes with standpipe piezometers, 64 test pits, SPT testing and laboratory testing, conducted to determine the geotechnical conditions for the plant site and conveyor route. • No data provided related to open pit design or sub‐surface conditions between open pit and Fish Lake.

2009 EIS, Appendix 3‐6‐J

1998 Geotechnical and hydrogeological investigations

• Report could not be located (no report reference number cited). N/a

2007 Geotechnical investigations at the proposed Primary Crusher site

• Report could not be located (no report reference number cited). N/a

2009 2009 Geotechnical Site Investigation Factual Data Report (Ref. VA101‐266/10‐1, Jan. 2010)

• 13 new geotechnical drillholes, largely focused on original tailings dam (Prosperity Lake) and plant site, with packer/permeability testing, SPT and laboratory testing, test pits for concrete aggregate assessment, and geophysical resistivity survey at plant site. • Only one new borehole in vicinity of pit ‐ 2009‐308 (primary crusher location). • No new data added within pit boundaries, or between pit and Fish Lake.

2012 EIS, Appendix 2.2.4‐E

2012 2012 Geotechnical Site Investigation Factual Data Report (Ref. VA101‐266/26‐1, Aug. 2012)

• 8 geotechnical boreholes with standpipe piezometers, 40 test pits, SPT and laboratory testing, and 13 km of resistivity, IP and seismic refraction surveys, focussed on revised location of Tailings Storage Facility (TSF) South, West and Main Embankments, and to a lesser degree, the Ore Stockpile Area. • No new data added within pit boundaries, or between pit and Fish Lake.

2012 EIS, Appendix 2.2.4‐C


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Table A2: Additional reports reviewed for data regarding pit slope design and ground conditions between open pit and Fish Lake.


1994 Open Pit Design (Ref. 1736/1, Mar. 1994) • Prefeasibility design of open pit. Appendix includes bedrock logs for boreholes within limits of open pit and between open pit and Fish Lake, which were not included in earlier reports reviewed.

2009 EIS, Appendix 3‐6‐F

1998 Geological Report (May 1998) • Detailed interpretation of open pit geology, including the identification of two major N‐S faults transecting the proposed pit (QD and East Faults), together with descriptions of the surficial geology (overburden) and rock alteration, veining and structures. • Detailed geological level plan maps and cross‐sections.

2009 EIS, Appendix 3‐5‐A

1998 Project Risk Assessment (Ref. 10173/13‐2, Nov. 1998)

• Preliminary qualitative risk assessment comparing different mine development options in terms of likelihood of occurrence of a potential failure of a project component and their consequences on human life, water quality, fisheries, wildlife and operations.

2009 EIS, Appendix 2‐6‐C

1999 Feasibility Design of the Open Pit (Ref. 11173/12‐2, Apr. 1999)

• Feasibility design of open pit. Appendix includes summary of strength testing to date and bedrock logs for boreholes within limits of open pit and between open pit and Fish Lake, which were not included in earlier reports reviewed. • Summary of rock strengths, overburden laboratory results, and direct shear tests on bedrock joints. • Summary of shut‐in pressures (intervals at depth with artesian pressure conditions), and permeability test results. • Appendix includes report by DeLong (1999) on the discontinuities around the proposed ultimate pit.

2009 EIS, Appendix 3‐6‐E

2007 Feasibility Pit Slope Design (Ref. VA101‐00266/2‐2, Sep. 2007)

• Updated feasibility design of open pit. Includes figures of revised open pit outline and delineation of 7 geotechnical pit slope design sectors.

2009 EIS, Appendix 3‐6‐C

2012 Preliminary Pit Slope Design (Ref. VA101‐266/27‐1, Aug. 2012)

• Preliminary design of open pit. Includes summary of geotechnical parameters used in pit slope design, results from kinematic and limit equilibrium analyses, and corresponding bench, inter‐ramp and overall pit slope angles for each design sectors.

2012 EIS, Appendix 2.2.4‐A

2012 Preliminary Pit Slope Design (Ref. VA101‐266/27‐4, Aug. 2012)

• Preliminary design of waste and stockpiles. Includes discussion of seismic hazard present at the Project site.

2012 EIS, Appendix 2.2.4‐B


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