Engineering geology

Pit slope designs

Surface Mining

Surface mining is a process whereby soil and rock overlying the mineral deposit are removed. There are two principal types of surface mining, strip mining and open-pit mining (Fig. 1,2).

Strip mining is the process of mining a seam of mineral by removing a long strip of overlying soil and rock. Strip mining is a practical type of mining when the ore body that is to be extracted is near the surface.

Open-pit mining is the process of extracting rock or minerals from the earth through their removal from an open pit or borrow.

Figure 1. Strip mining. Strip mining lignite at Liberty Coal. Mississippi (Photo by Flickr)
Figure 2. Open-pit mining. An open-pit mine in Russia. (Photo courtesy Wikimedia Commons / cc3.0)

Surface mining is usually used to develop coal seams and deposits of many other minerals, but their feasibilities vary with the nature of the body ore. Large, low-grade ore deposits are usually economically attractive because it permits high recovery of the resource and allows sufficient space for large and efficient mining equipment. An open-pit mine could recover almost 90% of the oil shale in a very thick deposit. It is said that strip mining could provide an even higher recovery.

Shown in Fig. 3 is an open-pit mine used to explain the process of mining the oil shale. The overburden is drilled and blasted loose over an area above the oil shale zone and the load is then transported to a disposal area. Once the shale beds are exposed from the blasting, the shale is then drilled and blasted and then extracted from the pit.

oil shale
Figure 3. Mining the oil shale

Open-pit design

The aim of any open-pit mine design is to provide an optimal excavation configuration in the context of safety, ore recovery and financial return. Investors and operators expect the slope design to establish walls that will be stable for the life of the open pit, which may extend beyond closure. At the very least, any instability must be manageable. This applies at every scale of the walls, from the individual benches to the overall slopes.

It is essential that a degree of stability is ensured for the slopes in large open-pit mines to minimise the risks related to the safety of operating personnel and equipment, and economic risks to the reserves. At the same time, to address the economic needs of the owners ore recovery must be maximised and waste stripping kept to a minimum throughout the mine life.

The resulting compromise is typically a balance between formulating designs that can be safely and practicably implemented in the operating environment and establishing slope angles that are as steep as possible.
As outlined in Figure 4. the slope designs form an essential input in the design of an open pit at every stage of the evaluation of a mineral deposit, from the initial conceptual designs that assess the value of further work on an exploration discovery through to the short- and long-term designs for an operating pit.

At each project level through this process, other key components include the requirements of all stakeholders. Unlike civil slopes, where the emphasis is on reliability and the performance of the design and cost/benefit is less of an issue, open-pit slopes are normally constructed to lower levels of stability, recognising the shorter operating life spans involved and the high level of monitoring, both in terms of accuracy and frequency, that is typically available in the mine.

Although this approach is fully recognised both by the mining industry and by the regulatory authorities, risk tolerance may vary between companies and between mining jurisdictions.

Uncontrolled instability, in effect failure of a slope, can have many ramifications including:
■ Safety/social factors
→ loss of life or injury;
→ loss of worker income;
→ loss of worker confidence;
→ loss of corporate credibility, both externally and with shareholders.
■ Economic factors
→ disruption of operations;
→ loss of ore;
→ loss of equipment;
→ increased stripping;

Figure 4. Schematic representation of steps in mine design

Adapted by J. Read, P. Stacey; S. Zendehboudi,A.Bahadori

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