Coring of Soft Soil-like Rock Material

A new coring technique is described for the sampling of highly altered rock. This and an innovative field survey method were developed to supply valid and representative data for numerical modelling of openings in this soil-like material. The results were not only reflective of in situ behaviour, but successful enough to be adopted for the sampling and modelling of other highly altered surface crown pillar masses, mining structures in need of better design methods

Lettre de L. Bétournay à ______

2 page

Lettre de L. Bétournay à Moreau Leblanc & Cassidy

2 page

Lettre de L. Bétournay à C. A Leblanc

1 pag

The stability of shallow stopes of hard rock mines

Canadian hard rock mine extraction practices have commonly created shallow stopes, 12% of which have caved to surface, from instabilities originating from stope hangingwalls, crowns or footwalls. To date, however, mine operators have applied few of the available data gathering and design tools to strike a balance between maximum economic excavation dimension and stope viability. The preference has been to use personal mining experience.Several common rock mass environments are surveyed as well as the various ways in which shallow stopes have failed. It has been found that these rock masses develop gravity-induced movement in the form of plug failures, ravelling of rock blocks, strata failures, chimneying disintegration, and rock mass block caving.New failure-specific analytical equations are developed here for these common failure mechanisms. They address the mechanics of the failure process and incorporate the capability to arrive at the ultimate failure outline, for comparison to the location of the bedrock surface.This research has shown that: plug failures occur along steep, uninterrupted discontinuities bounding large blocks. Plug failure potential reduces substantially with confining compressive stress, discontinuity inclination, the absence of low friction surfaces and shearing of intact rock interrupting the discontinuity. Ravelling requires little peripheral confining stress for stabilization and prevention of block falls or slides. However, inherent conditions such as shallow dipping or vertically dipping joints can cause block falls to develop to surface. Low confining stresses, resulting from multiple stope extraction in orthogonal horizontal directions, would offer conditions suitable for plug and ravelling failures. Strata failures are caused by excessive stope spans, but the limited loading received from above strata is such, that the failure cavity created is of limited vertical extent, some 25% of the stope width. Chimneying disintegration occurs in weak rock masses with low cohesion, over narrow openings that can be as deep as 275 m. The onset of chimneying disintegration can be created by compressive stresses, but develops as a result of mobilization of the rock mass by gravity in active shear. Block caving requires large spans to develop, and stabilizing could be overcome from arching stresses overcoming bulk arching strength. Controlling instability elements are tabulated for these failure mechanisms. A limit equilibrium correlation between span and cohesion for chimneying disintegration is presented, and the controlling limits between the occurrence of chimneying disintegration and block caving is discussed. Ravelling and chimneying disintegration are the most expected failure mechanisms for shallow stopes of hard rock mines. Although failure of the shallow stope may start around its periphery, stope failure to surface would likely occur in or close to the surface crown pillar.Although the analytical equations developed require input of in situ stresses defined by numerical modelling in order to yield a precise answer, conventional numerical modelling or empirical methods are shown to be unable to predict stope failures as the analytical equations have. A step-by-step stability analysis procedure is presented, incorporating rockmass environment, expected failure mechanism(s), and applicable data gathering and anlytical methods. (Abstract shortened by UMI.