Shallow stopes of hard rock mines, usually affected by inherently weak or complex rockmass conditions, can be subject to various types of gravity-driven failure mechanisms. A number of conclusions on the rockmass control and physical continuation of failure development, site condition effects, effectiveness of dedicated and generic stability analysis techniques and ground stabilization measures, are presented based on the examination of a large number of case studies. The dedicated analysis methods are shown to be effective, while taking into consideration parameter value variation, ground control, failure path and mechanics.
Mining metal-bearing ores in Canada has primarily centered around near-surface extraction from underground stopes, routinely creating shallow stopes near the boundary to overburden, bodies of water, and surface infrastructure. Of these, 12% have caved to surface. Compared to potash, coal or evaporite mining, the shallow stope subsidence which can occur in hard rock mining is localized above the affected underground working. It evolves as cave-ins compared to the regional subsidence with no breakthrough, common in soft rock mining. Analyses of shallow stope stability have been based on conventional rock mechanics methods (theory of elasticity, continuum modelling, generic empirical methods) to address rock mass environments where failure mechanisms depend primarily on the effects of discontinuities or low rock mass strength. This is seen in the predominance of gravity driven failures, distinct and causative, the common ones being (Figure 1): plug failures (sudden drop into a stope of a large but integral block which extends to the top of bedrock), ravelling failures (gradual block failures from the periphery, resulting in stope enlargement towards surface,) chimneying disintegration (upwardly disintegrating weak rock leaving behind "chimneys"), strata failures, block caving (free movement of a rock mass by its breakdown, towards an underground opening) (Bétournay, 1995). In the case of competent, relatively massive rock, only partial failures occur along shallow stope peripheries. The considerations presented here originate from over sixty site case studies, including ten detailed stability analyses of failed and unfailed shallow stopes (Roche Associates, 1984) (Roche Associates, 1985) (Strata Engineering, 1987) (Golder Associates, 1990) (Yu and Wang, 1993) (Aston, T. and Charette, F., 1993) (Bétournay, 1995) .
Table 1, Figure 1, outline the development of the common failure mechanisms and the guiding stability parameters. Most failures occur progressively. Most are related to the existence of discontinuities. The exceptions to the primary role of discontinuities occurs when rock mass strength is exceeded. Progression is fast (12-20 m per month) and uninterrupted towards surface with a width as small as 2 m (Bétournay, 1994a). Departures from this behaviour will occur when the rock mass improves it capacity to resist shearing under its own weight, the basis for its progression (Bétournay, 1994] or if the material will choke the failure process. However, the bulking factor is low (~1.05) for weak rock mass failures. This contrasts with ravelling, block caving and destratification which would choke themselves off much quicker. Ravelling is a small piecewise failure formed by persistent jointing.