The capital expenditure required to bring a property from exploration to production is substantial, and if a mining company wants to be successful in navigating this long and potentially prosperous journey, every effort must be made to maximize the value of each dollar spent. In the early stages of the project development lifecycle, financial resources are primarily allocated to determine the extent and value of the mineral resource; as results from the maiden resource and preliminary economic assessment will govern whether the project attracts additional funds for progression to the next stage of evaluation. However, for companies with a long-term vision for their project, implementing a properly structured geotechnical data collection programme, concurrently with exploration drilling, will contribute to maximizing returns from their investment and reduce financial risk.
Figure 1 Aspects of mining studies reliant on geotechnical data
Although unrecognized by many mining companies, geotechnical data is substantially integrated in the mining method selection process and development of the mine plan (Figure 1). Insufficient and / or inappropriate geotechnical data, leading to gross assumptions of the geotechnical parameters in the design and planning process, may delay projects for months to years, while suitable data is collected. The financial implications may be enormous when technical deficiencies equire additional capital expenditure, mine re-design (including mining method changes) or result in production delays (Table 1). These delays may be of sufficient consequence to precipitate the termination of a project in a volatile and investor-cautious market.
|Costly mine redesigns directly related to inadequate geotechnical understanding|
|· Change in mining method.
· Mine development re-orientation to manage instability.
· Isolation of deep ore due to unachievable pit slope angles or increased stripping ratio requirements.
· Reduced stope sizes/increased ground support to manage dilution.
· Inability to manage dilution due to incorrect sublevel selection.
· Having to leave additional pillars in ore to manage stability problems.
· Extensive lateral and vertical development rehabilitation to manage poorer than anticipated ground conditions leading to additional costs and production delays.
· Mine layout redesign to avoid bad ground or manage intersection spans.
o Increased ground support requirements to manage bad ground (often can’t reduce heading sizes due to equipment fleet already purchased).
So how does a mining company avoid financial impairment due to an incorrect geotechnical assumption, without compromising the project budget? The best way to accomplish this is to engage a competent rock engineer during the planning phase of the exploration drilling programme, who can develop a site-specific geotechnical data collection plan that details the geotechnical data to be collected at each stage of project development, based on the following principles:
- Collect appropriate data for your project
Rock failures occur when a unique combination of applied stress and rock mass structure exceeds the limiting strength parameters of a rock mass. Therefore, it is important that information from the geotechnical data collection programme provides a spatial representation of the stress conditions, structural characteristics and intact rock and joint strength properties. Once a baseline level of geotechnical understanding has been acquired (+/- 35% degree of uncertainty), additional data collection can be targeted on both highly variable parameters identified during the baseline data collection as well as the limiting strength parameter(s) that govern failure. A targeted data collection programme can save valuable time and resources.
- Time data collection to avoid using risky assumptions
No stability analysis, regardless of how intricate and theoretically precise it may appear to be, can usefully contribute to the mine design if an incorrect assumption has been made. During the early stages of a project it is inevitable that some design assumptions will need to be made. Therefore, it is important to understand what the critical geotechnical parameters for the project will be (e.g. rock mass strength in the conglomerates, shear strength of the foliation, cohesiveness of the faults, etc.) and to use the available resources to characterize them first. At the Prefeasibility Study (PFS) the critical geotechnical parameters need to be adequately understood (+/- 20% degree of uncertainty), because at more advanced stages of study there is generally limited flexibility in the mining strategy. Mining strategy changes are possible beyond the PFS, but the financial implications are usually high, so they tend to be avoided, leaving high risk assumptions in the design.
A study conducted by McCarthy (2013) concluded that 9% of mine operations that failed to implement the Feasibility Study (FS) design were the direct result of incorrect geotechnical analyses. A further 32% of cases were the result of incorrect or unrealistic scheduling, to which incorrect, inadequate or inappropriate geotechnical design assumptions and methodologies may have contributed. Statistics like these emphasize that critical parameters must be reasonably understood in early-stage studies, and refined by the FS; not collected for the first time for the FS.
- Look for opportunities to share resources
The key to maximizing benefits from your investment is to identify opportunities during the exploration drilling phase to collect geotechnical data while sharing financial resources (equipment and personnel) with the exploration programme.
Opportunities to share resources include:
- Identifying a number of exploration drillholes to perform geomechanical logging on the core; core should be oriented and logging must be completed by a competent person, prior to splitting the core. This reduces the need to drill dedicated geotechnical holes.
- Selecting samples for laboratory intact rock strength testing, prior to storing core. These samples should be protected from moisture and large temperature fluctuations until shipped for testing. This reduces the need to re-handle core or drill additional holes to obtain useable core specimens
- Using borehole geophysical tools (optical and acoustic televiewers, calipers, etc.) on exploration drillholes to increase the spatial understanding of rock mass structural characteristics.
Hydrogeological investigations can also be conducted concurrently with exploration drilling and geotechnical data collection.
If developed and implemented correctly, a properly structured geotechnical data collection plan will provide sufficient and appropriate data for data-driven (and supported) design at each stage of project study for any mining method, thereby limiting the financial risk associated with design based on assumptions.
McCarthy, P. (2013). Why Feasibility Studies FAIL (Presentation). AusIMM Melbourne Branch, Australia.
Senior Geotechnical Engineer