The Influence of Previous Workings and Mining Practice on Strata Movement

Abstract

Discussion so far has assumed a virgin rock mass that is being worked through for the first time on a large scale in the course of extracting a seam. In most cases, however, the subsidence engineer is concerned with strata that have been mined repeatedly — as, for example, when there have been previous workings in the same seam, or when a deposit consisting of several seams has been worked through from top to bottom, seam by seam, for decades. In a rock mass that has been fragmented to a greater or lesser degree in this way, the strata overlying a working will undoubtedly move differently — more intensely, and also more rapidly — than over a first working, for which these pre-calculation methods were devised (Fig. 102). The degree of previous working thus affects both the development of movement and the field of stress in the rock mass as a whole. This “degree of previous working” is determined by the number and area of seams worked, the amount of convergence in the panels, the perpendicular distance between workings, and the spatial distribution of unmined parts of the deposit (peripheral zones, panel ribs, residual coal pillars). With increasing mining, the structure of the rock mass is altered and the overlying strata weakened and broken up. Furthermore, this structure is affected by the mining practices adopted: a rapid sequence of workings concentrated on one block of strata, a rapid face advance, and caving are all conducive to fragmentation of the rock mass, and all accelerate the development of movement.