December 2015

Geotechnical considerations for Resource and Reserve estimation

  • By R J Butcher FAusIMM(CP), General Manager, Technical Services, MMG Ltd

Why geotechnical design should be seen as an integral part of the overall mine design

During last year’s Monograph 30 Roadshow a number of questions were raised regarding the need for additional guidance in connection with geotechnical input into Resource and Reserve estimations. In this respect this article advances some of the concepts given in Sullivan’s 2014 paper ‘The influence of geotechnical and groundwater factors on Ore Reserve estimation’ (Sullivan, 2014). It is important to understand that today the debate is focused on the level of geotechnical input required – as opposed to if geotechnical information is needed for a Resource and Reserve estimate. Clearly geotechnical input is required as part of the mining and infrastructure modifying factors (JORC, 2012). This article gives three key geotechnical considerations when compiling a Resource and Reserve estimation.

The first key consideration is that geotechnical input/designs for Resource and Reserve estimation should not solely be limited to pit slopes and underground excavations, but should also be extended to waste disposal and processing structures. The selection of suitable sites and designs for waste rock dumps, tailing storage facilities and heap leach pads is becoming increasingly challenging as land access and social pressures increase. The author contends that a Reserve cannot be declared if waste disposal facilities and heap leach pads have not been designed to at least pre-feasibility level. In terms of a desirable requirement for Mineral Resource declaration, potential sites suitable for waste disposal structures large enough to accommodate mining and processing waste should at least be identified.

The second key consideration relates to the actual level of geotechnical design input required for Mineral Resource and Ore Reserve estimation. In terms of an Ore Reserve the JORC Code is clear that all modifying factors have to be considered to at least pre-feasibility level and hence geotechnical inputs and designs should also be to this level as an integral part of the mining modifying factor. In practical terms this means that all geotechnical designs need be at a stage of development where they are fit for purpose. In essence this really means that geotechnical designs for Ore Reserves may not be optimal but they could be implemented. In recent years a number of good references have been produced regarding the geotechnical detail required for pre-feasibility level (Haile, 2004; Read and Stacey, 2009). However, in the author’s experience the following minimum requirements should be met to support Ore Reserve estimation:

  • sufficient site investigations and test work should be carried out to define the geotechnical/hydrogeological environment
  • a geotechnical and hydrological model should be compiled
  • geotechnical risks and failure modes should be fully defined
  • designs should be determined by some form of experiential or empirical assessment
  • final designs should be confirmed and supported by some form of deterministic and numerical analyses.

In terms of Mineral Resource estimation, some form of high-level geotechnical assessment is required to ensure there are reasonable prospects for eventual economic extraction. The author considers the following to be adequate:

  • an understanding of the potential geotechnical and hydrogeological conditions and risks based on best available information or assumptions
  • an indicative geotechnical design based on experience
  • mine waste disposal sites indicated and some form of site waste capacity calculation undertaken and preliminary design considered.

The third key consideration is concerned with the need for all geotechnical designs that support a Mineral Resource and Ore Reserve estimate to be technically sufficient and fit for purpose in terms of reporting confidence. As already described, this means that geotechnical designs that support an Ore Reserve estimate may not be optimal but they could be implemented if required. In the case of Mineral Resource estimates this means that the described designs or design assumptions appear reasonable in terms of the potential to extract the Mineral Resource. In terms of materiality, it is important that the Competent Person applies a common sense test to all designs by asking if they could see them working in terms of extraction. From the author’s experience over the last 15 years the following are considered as common examples of where designs are not geotechnically fit for reporting purposes:

  • declaring an Ore Reserve without having defined in sufficient detail the sites for, and the designs of, waste rock dumps and tailings storage facilities (TSF)
  • significantly increasing the Mineral Resource of an open pit without having given sufficient consideration to the construction of potential new waste rock dumps and TSFs
  • no consideration given at any stage of the Mineral Resource and Ore Reserve estimation process to the siting and design of heap leach pads
  • an open cut Ore Reserve estimate with no provision in the pit design for sufficient access ramps or geotechnical berms
  • designs that support open cut Ore Reserve estimates based on highly-aggressive slope angles or inter-ramp heights with little or no data to substantiate this parameter
  • in an underground mine, declaring Mineral Resources or Ore Reserves mainly comprised of small remnant pillars, stope ore skins and rib pillars
    that are difficult, dangerous or impractical to extract 
    Mineral Resource and Ore Reserve estimates for an underground mine that is comprised of water barrier pillars and regional support pillars without having given due consideration to their safe method of extraction
  • in the case where the orebody extraction is by caving, Mineral Resource or Ore Reserve estimates are defined but little or no practical/financial consideration given to the impacts of surface damage caused by the development of the glory hole or subsidence trough
  • declaring a Mineral Resource or Ore Reserve estimate for a block cave without having given due consideration to cave initiation or propagation; this is especially pertinent for caving projects with block heights greater than 500 min an underground mine, declaring a Mineral Resource estimate below a depth of 1500 m from surface without giving any consideration to the impacts of in situ rock stress and seismicity on extraction.

The purpose of this article is to give some basic guidance in terms of geotechnical inputs for Mineral Resource and Ore Reserve estimation. In this respect it is important that geotechnical design is seen as an integral part of the overall mine design that supports a potential extraction or an extraction scenario to support the Mineral Resource and Ore Reserve estimation. Therefore it is important that a basic materiality test is applied by the Competent Person (or geotechnical engineer contributing advice) to all designs by determining if they could:

  • in the case of an Ore Reserve, foresee the design being implemented without material modification; or
  • in the case of a Mineral Resource, envisage the design working in practice. 

References

Haile A, 2004. A reporting framework for geotechnical classification of mining projects, AusIMM Bulletin, Sept/Oct:30–37.

Joint Ore Reserve Committee (JORC), 2012. Australasian Code for Reporting of Exploration Results, Mineral Resources and Ore Reserves, The JORC Code, 2012 Edition.

Read J and Stacey P, 2009. Guidelines for Open Pit Slope Design (CSIRO Publishing and CRC Press/Balkema).

Sullivan T D, 2014. ‘The influence of geotechnical and groundwater factors on Ore Reserve estimation’, Mineral Resource and Ore Reserve Estimation, The AusIMM Guide to Good Practice, Second edition, pp 385–400 (The Australasian Institute of Mining and Metallurgy: Melbourne).

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  • Hendrik Grobler
    13 Jan 2016 at 8.44pm

    An excellent topic for discussion and I agree with Richard’s comments I full. I am all for fit for purpose mine designs based on known geotechnical ground conditions. I would add in the list of considerations of geotechnical fit designs:- due diligence that will include scrutiny of data resource and the then relationship between the data and the excavation area considered, be it open pit or underground. Thus, not just data confidence but also data representation, there should be no extrapolation of data or assumptions on the general condition of the rock mass by the time a project goes to FS level or gates.