In sublevel stoping, drilling and blasting is undertaken from drilling drives located on sublevels strategically placed over the height of a stope. Because of the limited cablebolt reinforcement that can be provided at the exposed stope walls, the excavations must be designed to be inherently stable. In this regard, experience has shown that, in general, it is possible to achieve stope wall stability with minimal dilution by creating openings having high vertical and short horizontal dimensions. An example would be a stable, vertical raisebore that is extended laterally, until it becomes unstable.

Stability is also achieved by forming openings having long horizontal and short vertical dimensions. An example would be a long, stable tunnel, whose height is increased until it becomes unstable. Square-shaped stopes are the most ineffective in terms of potentially stable volumes.

The shape of the conceptual transition curve in Figure 1.8 is hyperbolic and indicates that for multiple lift sublevel open stopes (excavations with walls that have high vertical and short horizontal dimensions) the critical spans are either the exposed horizontal lengths or the stope widths. Length and width, that is, dimensions in plan view, are the critical stope dimensions as they also control the dimensions of the stope crowns. Bench stopes are excavations where the longest dimension is the strike length and the critical spans are usually the exposed heights, as the orebody width is usually narrow. Figure 1.9 shows an example of hangingwall performance for single- and double-lift stopes extracted in a similar geotechnical domain.

The case study data show that for the single-lift stopes, stope performance is not controlled by geometry, as the depth of failure is not correlated with stope dimensions. However, as the stope height is increased, the depth of failure increases with an increase in stope strike length. An immediate conclusion is that a reduction in stope size may not necessarily result in better stope performance. Another case study is shown in Figure 1.10, in which stope performance is clearly related to stope geometry.

Rock Mass Strength

It is generally accepted that the behavior of the stope walls is largely controlled by the strength of the rock mass surrounding the stope. This rock mass strength depends upon the geometrical nature and strength of the geological discontinuities as well as the physical properties of the intact rock bridges. Single or combinations of major discontinuities (usually continuous on the scale of a stoping block) such as faults, shears, and dykes usually have very low shear strengths and, if oriented unfavorably, provide failure surfaces when exposed by the stope walls (Figure 1.11). Such geological discontinuities largely control overbreak and stability around exposed stope walls. This is particularly the case for those discontinuities having platy and water-susceptible mineral infill such as talc, chlorite, and sericite.

In some cases, instability can be linked to activities in concurrent voids along the strikes or dips of major geological features such as fault zones (Logan et al., 1993). Ideally, the location of large-scale geological discontinuities is well defined and most open stoping mines have a threedimensional model of the local fault/shear network (Figure 1.12). These features can also be seismically active, further increasing falloff at the excavation boundaries, especially in narrow orebodies. When large-scale structures are exposed, stope wall overbreak is usually very difficult to control, regardless of the blasting practices used, and can only be minimized by stope sequencing.

Stope wall behavior is also a function of the number, size, frequency, and orientation of the minor-scale geological discontinuities. Such discontinuity networks usually control the nature and amount of overbreak at the stope boundaries. Rock mass characterization techniques can be used to estimate the shapes and sizes of blocks likely to be exposed at the final stope walls. The geometrical discontinuity set characteristics (size, frequency, orientation, persistence, surface strength, etc.) relative to the stope walls largely control the amount of dilution experienced at those walls (Figure 1.13). Individual joints have a limited size and they may either terminate in intact rock, forming an intact rock bridge, or against another structure within a discontinuity network. These intact rock bridges are significantly stronger than the naturally occurring discontinuities and provide a higher resistance to failure within a rock mass.

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