ABSTRACT:

The stability of joint defined wedges in the backs of underground openings is often a key concern for support design. Assessments of roof support demand typically consider the primary or average orientations of ubiquitous structure (sets or oriented clusters of joints or shear swarms) within the rockmass. These representative sets are then examined for mutual intersections that form wedges which, in turn, are assumed to reach a maximum dimension limited only by the span, assuming full continuity and complete ubiquity of the structure. This full-span or ubiquitous wedge approach represents a worst-case analysis and often leads to highly conservative support design recommendations. This paper introduces a new hazard assessment, the "viability index" approach, to scale full-span wedge predictions, accounting for both occurrence and instability potential of possible wedge geometries. Predictions for wedge failure potential and support demand are generated resulting in more economical support design.

INTRODUCTION

It is of considerable importance in today's mining climate to reduce operating expenditures, of which support costs make up a significant component. This can be done by designing systems with sufficient capacity for the most viable instability case instead of the worst possible case. This implies the acceptance of a minimum potential for wedge falls instead of zero risk. The "worst case" for wedge instability is the full-span wedge, or the largest wedge that can form across a given span from the intersection of ubiquitous structure. In most cases, however, the probability of occurrence of three or more joints mutually intersecting to create a full-span wedge is low, due to the discontinuous nature of joints and the variability of spacing. In addition, any discrete wedges which do form may possess internal stability due to clamping or to tensile strength of rock bridges.

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