independent geotechnical review of preliminary open pit

New Prosperity Topic Specific Hearings – Geology & Hydrogeology (26-27 July, 2013) 1 of 25

New Prosperity Gold-Copper Mine Project Topic Specific Hearings: Geology & Hydrogeology

Independent Geotechnical Review of Preliminary Open Pit Design and

Potential Impacts on Fish Lake Dr. Erik Eberhardt, P.Eng.

Professor, University of British Columbia


Presentation Outline

• Scope of Review Assignment

• Managing Uncertainty & State of Practice

• Key Findings, Concerns & Recommendations - Site Investigation

- Open Pit Design & Potential for Slope Failure

- Possible Impacts on Fish Lake

- Geotechnical Risk Assessment & Performance Assurance


Scope of Review Assignment Independent, non-government expert review:

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


Scope of Review Assignment Questions to Address:

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

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

3. 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.

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

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


Large Open Pits Palabora, South Africa

Bingham Canyon, Utah


Uncertainty & Design

Parameter Uncertainty: concerned with spatial variations in material properties, and the lack of data for key parameters.

Model Uncertainty: arises from gaps in theory and understanding required to make predictions on the basis of causal inference.

Human Uncertainty: ranges from subjectivity and measurement error to differing professional opinions.

Geological Uncertainty: represents the unpredictability associated with the identification, characterization and interpretation of the site geology and hydrogeology.


Uncertainty in Ground Characterization

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Geotech/Hydro Model

Assumed

Substantiated

Measured

Investigation

Behaviour Model

Hypothesized

Simulated

Observed

Testing/Monitoring


State of Practice - LOP

Read & Stacey (2009)


Findings/Concerns – Site Investigation

• 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.

• There are also a number of uncertainties in the data and related analyses that are unavoidable when dealing with geological and hydrogeological investigations (i.e., geological uncertainty, parameter uncertainty, etc.).



• Drillhole data is based on that collected in the 1990’s. There have been no new targeted boreholes to investigate the QD and East Faults (subsequent to their identification), or the ground conditions between the open pit and Fish Lake (in support of the 2012 EIS revision to the MDP to preserve Fish Lake).

• As a result, open questions persist regarding the QD and East Faults, the confined aquifers, and their potential interactions and impacts on the South and South-east pit walls, and ultimately Fish Lake.


Findings/Concerns – Site Investigation • The design equates the hydraulic conductivities of the fault

zones to those of the bedrock. Although a central fault gouge zone may be relatively impervious to flow normal to the fault, the adjacent fracture zones often serve to significantly enhance permeability parallel to the fault.

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Findings/Concerns – Site Investigation • It should also be noted that the QD and East 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.



Brehaut (2009)


Findings/Concerns – Pit Slope Design

• The preliminary design is based on a total of 148,136 m of diamond drilling completed in 379 holes (these pre-date the recognition of the QD and East Faults, although they contributed to their detection).

• Review of the design criteria and stability analysis methods confirm that they are appropriate for a preliminary design and conform to industry practices. In places, the stability analyses (1999 Feasibility Design) include advanced technical considerations, speaking to a very high level of understanding and expertise possessed by the pit slope design consultants.


Findings/Concerns – Pit Slope Design

• 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. This would result in flatter slope angles, reducing 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.


Findings/Concerns – Pit Slope Failure

• The rock mass quality ranges from FAIR to GOOD, favouring stability. Kinematic instabilities should be limited to bench and multi-bench failures, which can be managed through controlled blasting, scaling, bench cleanup, and drainage.

• Instabilities that may have a larger impact (i.e., pit wall failure), include a series of retrogressive failure in the overburden, collapse of the South wall due to a weaker rock mass being encountered because of the QD and East faults, or a progressive failure that develops as the pit deepens.


Findings/Concerns – Pit Slope Failure

Hoek et al. (2000)

Eberhardt et al. (2013)


Findings/Concerns – Impacts on Fish Lake

• 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.

• Although it can be argued that based on the investigations carried out to date, there is no evidence of a conduit providing a direct hydraulic connection between the pit and the lake, such a conclusion is based only on the pre-mining (undisturbed) ground conditions.


Findings/Concerns – Impacts on Fish Lake • 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. 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.


Findings/Concerns – Impacts on Fish Lake

Analyses should be carried out to determine how far back behind the crest tension cracks may develop.

Highland Valley Copper, BC


Findings/Concerns – Impacts on Fish Lake • 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 is 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.


Findings/Concerns – Risk & Performance • No level of drillhole data can guarantee that the pit 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.


Findings/Concerns – Risk & Performance • 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.

• If the project advances, 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.

independent geotechnical review of preliminary open pit

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