Pit Slopes as Construction Elements in Mining

Rock slopes are found both in civil and mining applications. Open pit rock slopes are, however, in many ways different from rock slopes in civil engineering projects. The location of the slope is often more fixed in an open pit mine, whereas, for example roads can be re-routed if difficulties are encountered when constructing a road cut. The only design parameter which can be varied is the shape of the slope and the slope angle, and to some extent the slope height. On the other hand, the life of an open pit slope is usually far shorter than the required life of slopes in civil engineering constructions (e.g., roadcuts). The economics of the pit operation are, to a much larger extent than in civil engineering applications, closely linked to the slope geometry. The consequences of failure are also vastly different and require different approaches, for example, very safe (over-designed) slopes in civil projects in urban areas compared to pit slopes with an often accepted risk of instability (Ross-Brown, 1972).

Open pit slopes share several characteristics with other construction elements in underground mining. The mechanical stability of the pit slopes determines the maximum achievable overall pit slope angle, and thus the maximum ore recovery and minimum stripping ratio. Compare this with pillars in an underground mine, defined as a portion of the mineable orebody which is left only to maintain stability in the mine. Consequently, in pillar design one strives to

minimize the pillar area, whereas in pit slope design one strives to increase pit slope angles.

The approach to design of both these construction elements is thus in many ways very similar, although the mechanics of underground construction elements differ significantly from that of pit slopes.

From the above discussion, one realizes that the manner in which a slope is designed to some extent depends on how failure is defined. For an open pit mine slope, a general definition of failure should reflect the function of the slope. Hoek and Pentz (1968) suggested the

following: "Slope failure in an open pit mine may be defined as that rate of displacement of the rock mass surrounding the open pit which would render the recovery of ore uneconomic if the pit was being actively mined". This definition emphasizes the rate of displacement and the time of failure as design considerations for an open pit slope. One needs, however, to

distinguish between this type of failure and mechanical failure of the slope. In this report, the following terminology is being used (the various aspects of slope failures and slope collapses will be discussed in detail in the Chapter 3):

Failure Failure occurs when the loads or stresses acting on the rock material (intact rock or discontinuity) exceed the strength (compressive or tensile) of the rock. Failure could also occur through destressing of the rock. The term failure is used to describe failure on a small (but not microscopic) scale involving failure of the intact rock material, but the same term is also used to describe failure of the entire construction element. For this case, failure occurs when the construction element (in this case the pit slope) has exceeded its capacity to carry more of the loads and forces that are acting upon the slope. This means that the construction element still can carry some load after failure, but less than before failure.

Slope collapse Slope collapse corresponds to economic failure of the slope, meaning that the consequences of the failure developing in the slope are so serious that they render the slope impossible to mine. A slope collapse could thus be both of local scale, involving one or a few benches, or of

global scale involving the entire slope, but in both cases causing interruptions of the mining production in that area.

Failure mode Failure mode is a macroscopic description of the manner in which failure occurs, for example the shape and appearance of the resulting failure surface. The failure mode can be regarded as a geometric description of the failure development.

Failure mechanism Failure mechanism is a description of the physical process that takes place in the rock mass as the load increases and failure initiates and propagates through the rock.

Failure kinematics Failure kinematics is simply a geometrical description of the motion or movements which results from a failure (Meriam, 1980).

Failure kinetics Failure kinetics relates the action of forces and loads on the slope to the resulting motions, and is thus closely linked to the failure mechanisms (Meriam, 1980).

Now that some of the basic terminology has been defined, it will be possible to consider the different components of the construction element  the open pit slope. The overall pit slope in an open pit mine, going from the pit bottom (toe of slope) to the crest of the pit, is made up of benches, interramp areas and haulage roads (Figure 1.1). Consequently, the maximum interramp angle is a function of the bench face angle, the bench height, and the width of the bench. The maximum achievable overall slope angle is in turn a function of the interramp angle and the width and number of haulage roads. It is important to realize that different failure modes can prevail at different scales and thus affect different portions of the pit.

Separate design for different slope units and slope heights are therefore necessary. The geomechanical environment may also vary in horizontal direction, requiring different slope angles in different regions of the pit.

An interesting aspect of the definition of slope angles in Figure 1.1 is that both the bench face angle and the overall slope angle are defined from the toe to the crest of the slope face, whereas the interramp angle is not. This has a very practical advantage since interramp areas then have the same interramp angle as the height is being increased, which facilitates mine planning considerably. From a stability perspective, however, the slope is likely to become more prone to failure as the height is increased, which should be reflected by a decreasing slope angle. This could be accomplished if the angle was defined from toe to crest (Hustrulid

and Kuchta, 1995), as shown in Figure 2.2. Since the previous definition (Figure 1.1) is currently in use at Aitik, this definition of slope angles will also be used through this report.

The possible differences in stability of different interramp heights must be considered separately in the design process.

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Figure 2.2 Different definitions of interramp slope angle.

Finally, considerations must be given as to what governs the stability of an open pit slope.

Stacey (1968) provided the following list of important factors:

• Geological structure

• Rock stresses and ground water conditions

• Strength of discontinuities and intact rock

• Pit geometry including both slope angles and slope curvature

• Vibrations from blasting or seismic events

• Climatic conditions

• Time

The above list is probably not complete; however it serves to show the difficulty in assessing rock slope stability. These factors also determine the mode of failure of a pit slope. It is only logical that the design of pit slopes should be based on the failure modes expected to occur and the governing failure mechanisms. This is, as will be shown, not always the case, and it is therefore necessary before the design methods are presented to describe the mechanics of rock slopes.