ABSTRACT: Much of the present knowledge about caving behavior of rock masses has been obtained from empirical observations. Additional notions about caving have been developed through inferences derived from two-dimensional finite element analyses. These analyses have indicated that a combination of one low-angle set of fractures and one nearly vertical set of fractures is the optimum fracturing configuration for ease of cavability of an orebody. This paper presents the results of two- and three-dimensional distinct element analyses which draw different conclusions than those reported from finite element studies. The distinct element method is selected for analysis of cavability because this method treats the rock mass as an assemblage of rock blocks which may interact individually. The results of the analyses are compared to a documented case history which involved a groundfall of 80,000 tons of ore in a 160-foot high pillar. The mechanics associated with these results are explained in terms of simple static stability analysis of wedges. The propensity for orebody caving is primarily a function of the number of joint sets or potential "release" surfaces in the orebody. This mechanism is influenced by the bounding weak discontinuities and intact strength of rock material.
1 INTRODUCTION
The cavability of orebodies is important to various mining methods in distinctly different ways. The block caving mining method relies on caving to extract massive ore economically, whereas other methods rely on the stability of the orebody and host rock to extract ore selectively. Cavability is a function of the geomechanical properties of the rock mass and the in-situ and mining-induced stresses. It has long been recognized that "the ability of a block to cave or fragment is a function of its strength in tension or shear and the value of applied forces" (Bucky, 1956). Numerous attempts have been made to develop classification systems for use in determining cavability. Much of the present knowledge about caving behavior has been obtained from empirical observations. For example, Mahtab and Dixon (1976) concluded from observations that the principal geomechanical features influencing cavability are in-situ stress field, rock strength, and the geometry and strengths of discontinuities in the rock mass. These authors also, by back-analysis of elastic calculations, postulated effective fracturing configurations. They concluded that "a combination of one low-angle (0° to 30° dip) set of fractures and another nearly vertical (75° to 90° dip) set of fractures is the most effective two- dimensional fracturing configuration for ease of cavability of an orebody. In an actual three-dimensional situation, one set of low angle fractures and two sets of nearly vertical fractures will be most effective in improving cavability."
These observations, concerning favorable joint orientations, may be valid for environments lacking lateral confinement (i.e., as a result of boundary slots or boundary weakening). However, results of two- and three-dimensional distinct element analysis, presented herein, suggest that caving in confined environments requires that additional release surfaces be present. Release surfaces may be either preexisting or form as a result of high horizontal stresses.
