Successes and Needs for Large Open Pit Mines – the LOP Project

Practices and Challenges

For Open Pit Geotechnical Characterization, Design and Execution

Enabling Technologies Workshop - SLC, March 2013 2

Practices and Challenges for Geotechnical Characterization,

Design and Execution

Geomechanics Risk Management Perspective - Mining Context

Sizing up geomechanics risk in mining:

Mining is considered to be a high risk (potential) business in terms of

both safety and economics. There is always the possibility that an

excavated slope, or underground excavation, may not perform as

predicted and could fail with significant and even catastrophic results.


Practices and Challenges for Geotechnical characterization,

Design and Execution Deep Open Pits

Robotham, 2011



Design and Execution Deep Open Pits

Batu-Hijau – Phase 7 orig 1,050m @37º rev 930m @34º

11 now to 43 LOM

31 now




Factors effecting opening stability

- Depth

- Stress levels due to depth (Virgin stress)

- Stress levels associated with mining

- De-stressing after mining

- Rock mass strength

- Major structures

- Size

- Stress direction

- How close are other openings (interaction distance)

- Ground Support requirements

- Seismic activity



Design and Execution SWOT Analysis – Observations Open Pits

Threats and Weakness

Geological Context for Slope Design

Groundwater and Surface Water Management – Seasonal Changes

Executing the Mine Plan

- mine the plan, satisfy design assumptions

- operational controls – cleanup, blasting

Strengths and Opportunities

Mine Planning

- optimization process

- site specific opportunities

Relevant Experience – Operations and People

Analysis Tools – analytical and numerical

Performance Monitoring instrumentation


Practices and Challenges for Geotechnical Characterization, Design

and Execution SWOT Analysis – Observations Underground Mines

Threats and Weakness

Geological context and uncertainty – material influence on reserve model/ mine plan

Structural geology – changes to mine layouts; ad-hoc design changes are common

Mine sequence changes to compensate for production issues

- stopes designed beyond feasible limits; standup time excedances;

- delayed backfilling

- blasting usually not optimized (ability and tools are there; but not a priority)

Educated, skilled and experienced personnel

Strengths and Opportunities

Ground (Rock) Support

Backfill for operational flexibility; can allow for mining in the worst

ground (UH cut/fill)

Mine design using 3D methods

Performance Monitoring instrumentation

Automation that reduces exposure to Operators; Safety




Is enough factual data available for geomechanics

design and reliable operational performance?

- Some examples…




Batu Hijau


Geomechanics – Practice vs Performance xxx

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5 6

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Design and Execution Batu Hijau – Domain 1 mapping vs oriented core




Gold – Copper Project at

Feasibility Study Level




Example of a structural model

For an ultimate design pit for a

Copper Project at the FS level.

Design final pit > 1 km deep;

Interpretive focus on orebody;

Model of major structure

representative of the ore

zone, but what about the

waste rock zone to be

excavated to access ore?

14 Enabling Technologies Workshop - SLC, March 2013

SUGGESTED PERCENTAGE OF CORED BORE HOLES TO BE

GEOTECHNICALLY LOGGED

Stage of mine development Suggested percentage geotechnically logged

Prefeasibility study Feasibility study Operating mine

25 - 50 % 50 - 100 % 25 - 75 %

Modern down hole geophysical logging methods may be used to extract some

geotechnical data from diamond drill and, less optimally, RC hole walls. These down

hole logging techniques should be calibrated in known ground conditions by comparing

the results obtained from conventional geotechnical logging of whole diamond drill core

with those obtained from down hole geophysical logging.

Regardless of the actual number of holes geotechnically logged, what is of

fundamental importance is that those holes that are geotechnically logged

constitute a representative sample of the ground conditions found in the ore

zone(s) and the wall rocks of a potentially mineable deposit.

From: Geotechnical Considerations in Underground Mines; Guideline;

MOSHAB Doc #ZME723QT






Hard Rock?



Design and Execution Groundwater – Open Pits

Risk Factors - Open Pits

Pore pressure increases reduce effective stress

Contributes to deformation response during mine development

Contributes to block deformation during blasts

In combination with deficient surface water management; contributes to:

Uncontrolled pit inflows

Pit flooding

Wet blasting

Impacts to access, production delays, and increased mining costs




Mine dewatering/ slope depressurization

Weak-deformable rock and soil masses are

susceptible to instability due to groundwater

pressurization.

East Wall -

KCGM

Oct 2011 80 mm rain

June 2011 nearby blast


Fiji is the basal fault of the

October 2011 slip, previously

unknown

- it took 9 months to

stabilize the wall, mine out

failed debris and put the

ramp back into service

Reliance fault is interpreted

from intersection of 4 core

holes

- modeled to undercut the final

east wall


Structural Control,

previously unknown

Structural Control?


Challenges associated with

mining into voids can be

mitigated

GPR appears to be promising, but

reliability using GPR falls short of

requirements, where probe drilling

is currently the standard.






Low probability - high consequence events such as slope instabilities and

ground fall in underground mines will continue to occur in the mining

industry; the most common reasons:

Decision to accept risks associated with the selected design option

Inadequate geological characterization

Inadequate design consideration (long term vs. short term)

Assumptions associated with mine design recommendations not adequately

implemented (groundwater, blasting, scaling, clean-up)

Failure to mine the plan

Poor operating practices and QC (blasting, scaling, cleanup, backfill,

deficient ground support)

etc., etc.

Summary




Enlightened companies are improving in geotechnical effort as design and

operational components of their business.

We as geotechnical practitioners will continue to be challenged to determine

the level of data and analysis required to achieve adequate level of design

and performance reliability; and be able to communicate and sell those needs

to Mine Operations and Project Managers.

Conclusions




Big meeting – end report

Successes and Needs for Large Open Pit Mines – the LOP Project

Documents

Transcript of Successes and Needs for Large Open Pit Mines – the LOP Project