Pillar loading part i: literature survey and new hypothesis

Abstract

This research work on pillar loading is in what might be described as the area of engineering theory. As such, it is concerned largely with the combining of existing scientific theories into a rational hypothesis for predicting the loading of pillars. Hitherto, this has been possible only in a very crude way and only for horizontal workings using the tributary area theory. In previous work on this subject, some workers have recognized that rock is not a fluid material applying a dead weight to pillars but, rather there will be a structural reaction of the wall rock on the excavation of the vein rock. However, no complete analysis of the mechanics of the system in producing pillar loading had been established. The first step in this research work, therefore, has been to analyse the structural aspects of the problem by solving the statically indeterminate net deflection of the walls. This net deflection at the pillars will be a measure of the increase in pillar stress resulting from mining. The resulting equations show that not only is the extraction ratio important in determining pillar loading but that the hitherto ignored parameters of the ratio of field stress components normal and transverse to the mining zone, the height of the pillar, the location of the pillar within the mining zone, the ratio of compressibility of pillar rock to wall rock, the number of pillars across a typical section of the mining zone, the breadth of the pillar, and the ratio of the depth from the ground surface to the span of the mining zone are all of some significance. Additional analytical work has been done in examining the various alternatives to the hypothesis that is based on elastic ground. In this work it is shown that for compatible strain the loading of pillars will almost always be a phenomenon associated with elastic, or at least pre-failure, deformation in the walls. Consequently, any mechanisms based on post-failure rock properties will not be applicable to this problem, although they may be for other types of support. In this supplementary work, an elliptical arching theory has been developed that may be more valid than the various other theories hitherto available for this case of loading of yielding support.