Abstract

The amount of published literature pertaining to the field of rock mechanics has increased markedly in recent years. Whilst computer applications aid our understanding of the mechanisms of pillar instability, the applications do not take into account the effect of time; thus the determined pillar factor of safety has limited meaning. A more realistic approach would be to consider pillar stability in terms of time to failure. However such an approach would require a reasonable estimate of the in-situ pillar mass strength and a validated methodology linking the time to failure with in-situ pillar mass strength and the geometric layout of the working.

RESUME

Le volume de bibliographie au sujet de la mecanique des roches a augmente considerablement ces derniêres annees. Alors que des modeles informatiques aident a la comprehension des mecanismes d'instabilite de pilliers, ces modèles ne prennent pas en compte les effets du temps; ainsi, la determination du coefficient de surete du pillier a un sens limite. Une approche plus realiste devrait considerer la stabilite du pillier en fonction du temps de defaillance. Cependant une telle approche exigerait une estimation raisonnable de la resistance du massif rocheux du pillier ainsi qu'une methodologie qui lie le temps de defaillance avec la resistance insitu du massif rocheux du pillier et la geometrie de l'excavation souterraine.

ZUSAMMENFASSUNG

Die Anzahl der Veröffentlichungen auf dem Gebiet der Felsmechanik ist in juengster Zeit sehr gestiegen. Computerprogramme sind wertvoll um den Mechanismus der Pfeilerunbestandigkeit zu verstehen, aber sie vernachlassigen den Zeiteffekt. Daher haben die errechneten Sicherheitsfaktoren der Pfeiler nur begrenzte Bedeutung. Eine realistischere Bewertung ware, die Stabilitat der Pfeiler auf Grund der Zeit bis zum Verfall zu beurteilen. So ein Verfahren wuerde eine Schatzung der in-situ Gebirgsfestigkeit und eine validierte Methologie, die die Zeit zum Verfall in Abhangigkeit der in-situ Pfeilerfestigkeit und der geometrischen Auslage der Grube zusammenbringt, brauchen.

Introduction

Prior to 1960 coal pillar design in South African collieries was based on local mining experience and some simple rules rather than engineering judgement1. Following the catastrophic disaster at Coalbrook, described by Bryan et al.2, Salamon and Munro3 carried out a detailed investigation into the strength of coal pillars. The investigation was based on 27 collapsed and 98 intact cases of pillar geometries from the Transvaal and Orange Free State, South Africa. Using this data Salamon and Munro carried out a statistical analysis to obtain a predictive formula for coal pillar strength resulting in the well-known pillar strength equation where

(Equation in full paper)

Pillar stress is calculated on the basis of the Tributary Area theory, which assumes that the overburden weight is evenly distributed across each pillar. In essence the formula may be viewed as the ultimate strength approach where pillar failure is expected once pillar strength is exceeded. A pillar is considered stable if the factor of safety exceeds 1.6.

It should be noted that Salamon5 did not specifically state that his pillar design method would be suitable for assessing the 'permanent' stability of bord-and-pillar coal workings.

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