There are six types of common failure mechanisms of shallow stopes of hard rock mines:
• rock fracturing:
Rupture of the surface crown pillar and collapse into the shallow stope.
• plug failure:
Sudden fall of the surface crown pillar, delineated by its boundary planes, by gravity into the shallow stope. The planes are well-defined, with near to vertically dipping uninterrupted discontinuities with low shear strength. The pillar thickness can vary; plugs up to 660 m high have occurred [Allen, 1934].
Failure potential depends considerably on the confinement available from redistributed stresses around the stope to resist movement; areas of numerous regional shallow stopes are prime candidates for plug failures.
Rarely do visual precursor movements occur, although smaller scale movements do open discontinuities and allow for water flow. Failure occurs suddenly and completely as a block.
Peripheral block by block rock mass failure without a self-support cavity form reached which includes delamination), ravelling failure involving sliding or buckling of thin rock layers at stope boundaries leading to the deStabilization of the surface crown pillar.
Gradual failure from an unsupported periphery of unfavorably oriented rock blocks, or those where discontinuity orientations are relatively shallow, is commonplace when the span exceeds self-support capabilities, but very low peripheral compressive stresses are required to keep blocks in place [Bétournay, 1995].
Blocks fail without the remainder of the rock mass mobilizing on a large scale unless a stable self-supported arch cavity is formed.
• strata failure:
Tensile failure of stratified rock at wall contacts or along the surface crown pillar span.
Few case studies of vertically progressing delaminations have reached surface. The poor cross-jointing normally shown by the rock masses make the strata stiffer and limits failure heights to shallow domes. In other cases, massive block failures have occurred when pillars fail in room and pillar mining. Such failures quickly choke off because of high bulking factors.
• chimneying disintegration:
The upward progression of disintegration within a weak rock mass forming a cavity with limited lateral extent (<5 m), developing from an underground opening toward surface. This condition is similar to that described for roadway intersections in room and pillar coal mining.
Failures (self-driving, and continuous if failed material is removed) normally progress upward in thick weak geological units (with low cohesion that can be sheared easily), but will continue up-dip when stronger units are encountered [Bétournay, 1998]. Equations 4 and 5 can be used respectively to evaluate the possible initiation of the failure mechanism and its maximum height. The bulking factor for weak rock units which host this failure mechanism in hard rock mines (e.g.schists, altered rock) is 1.05<k<1.2.
Ground support is only marginally effective in providing anchoring and global peripheral integrity.
• rock mass caving: Rock mass break-up and gravity mobilization of blocks (flow) towards and into an opening leading to a progressive failure front moving towards surface.
Although the initiation of rock mass caving can begin with block ravelling, the mechanical action involved in breaking and mobilizing the rock mass (Figure 29) is difficult to quantify or predict with accuracy. Rock masses with blocks of similar shapes, smaller sizes with low surface friction, and shallow angle discontinuities are likely in areas of low confining stresses to block cave. These and many other factors readily make it difficult for a rock mass to breakdown during and after mining extraction.
The dimensions of the problem, along with surface effects have been defined by Janelid and Kvapil (1966) based on silo studies. The volume that has caved into a stope is defined by the draw ellipsoid. The limit ellipsoid contains the zone of broken material that has subsequently moved and expanded under gravity to fill this volume. If the draw ellipsoid intersects surface, complete failure of the rock mass in the surface crown pillar is registered