The mining industry, particularly in old legacy mining areas, such as Ontario, in Canada is currently dealing with a considerable volume of closure work. One of the main issues that often needs to be dealt with regarding the physical aspects of mine closure involves the assessment of the stability of crown pillars over stopes which have been left unfilled. The Voussoir beam analogue is a quick and efficient method for evaluating stability of certain types of slabby and/or laminated crown pillars and hangingwall rockmasses in order to aid in estimating possible requirements for remediation. As it is always best for such cases to undertake multiple analyses using different methodologies, numerical analyses were undertaken for comparison with the Voussoir approach for a number of crown pillar thickness to span ratios to confirm the reliability of the method for this purpose. A number of observations were noted from the numerical results which prompted improvements to the Voussoir beam analogue. In all the past reported Voussoir formulations, including the more recent improvements, the thickness of the compression arch has been assumed constant across the beam. The numerical analyses have shown that, while the thickness of the compression arch at the abutments is a function of the thickness to span ratio and the elastic modulus of the rock, at midspan, the arch is always equal to half the thickness of the beam. This finding has prompted a change to the equilibrium equations defining the compression arch.

1. INTRODUCTION

The mining industry in Ontario (as well as in several other jurisdictions across Canada and the US) is currently dealing with a considerable volume of closure work, some of it involving legacy mine workings. One of the main aspects of expediting such closure work is ranking severity of problems on a case by case basis. Assessing the stability of crown pillars over stopes and old workings which have been left unfilled is often a particular concern. There are a number of tools available for this purpose, some more complex than others.

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