It is clear some mining companies, wary of added costs and lengthened schedules, are willing to forego comprehensive geotechnical work during the mine design process. This risks the loss of ore reserves through poor ground conditions and ore sterilization; schedule disruptions, equipment damage and personnel injury due to rock falls; dilution of grades due to overbreak; and increased mining costs through unanticipated rock conditions.

In many cases, the geotechnical input needed to produce a successful mine design is relegated to the “nice to have but not needed now” funding category and the importance of sound geotechnical input to both the geology and mining disciplines is overlooked. This tendency to neglect geotechnical risk has become even more prevalent in the current economic climate where the frugal allocation of funds is done on a “needs-must” basis.

Not addressing mine design geotechnical issues early enough or, worse still, not at all, can lead to those same issues hitting company wallets hard later in the project’s life. We need to look closely at some of the cause and effect logic that creates this elevated risk for a project further downstream.

The problem

A lack of consistent, quality data early on can lead to major reassessments of the mine design down the road. For example, on a recent project I worked on, the implications of time dependent strength behaviour was detected and noted in core but not correlated with mining parameters such as stope sizing, stand up times and ground support requirements. This became problematic when the mining engineers drafted a mine design that was overly optimistic about the size of stopes that could be achieved over a prolonged extraction period.

I’ve also seen a project where ground was characterized as being fair to poor from core logging, and the structural information from the same core was measured but reported separately. These data sets were not combined to assess what the main implications of the major structure really were. The result was a failure that caused operational disruption but luckily no injury or loss of life.

Operations sometimes assume that geotechnical conditions remain the same for newly explored regions of the same deposit. Conversely they assume that recently collected data can be applied to older areas where data may be scant. Both are false assumptions that lead to the identification of negative changes in ground conditions only after development has commenced.

In each of these cases the projects required a great deal of additional work at a later point and at a significant extra cost.

The solution

Sound geotechnical engineering is based on good data gathering, interpretation, analysis and geotechnical recommendations. The latter three rely on the quality of the first. And this process has to continue throughout the life cycle of a project. Mine workings sometimes collapse decades after the mine has closed and this aspect is now commonly a focus for post closure and regulated closure plans.

Timely and specific input is needed to provide cost effective geotechnical recommendations. This input is reliant on the progressive development of the knowledge base with regards to the geology (including structure) and hydrogeological and geotechnical parameters. The combination of these inputs provides the geotechnical model, which in turn assists the analysis process to provide the foundation for geotechnical design recommendations for mine development.

With diamond drilling costs ranging between $100 and $300 per metre and drilling programs seeing thousands of metres of core drilled each year, it makes sense to optimize the data obtained from each and every borehole.

Well-trained technicians can easily obtain good geotechnical data, which can be addressed early in the project life cycle at minimal cost to the overall program. Such information provides much-needed data for the focused collection of more targeted and sophisticated geotechnical programs later in the project’s life. As later programs will normally require more costly drilling, it makes sense to utilise cheaper options early on to help reduce this expense.

Good geotechnical data can and should start to be collected at the outset of exploration drilling in order to support the development of a good resource model and optimised mine plans. The development and refinement of the geotechnical model should then be continued throughout the life of the project. The collection of good initial data will help guide, streamline and refine future geotechnical needs, which, along with the management of geotechnical risk over the life of the project will ultimately result in a reduced geotechnical cost overall.

Will Pitman, M.Sc., is the principal geotechnical engineer and general manager of the AMC Consultants office in Toronto.

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