Pre-calculation of Ground Displacement

Abstract

Pre-calculation of the horizontal components of ground movement causes more difficulty, even today, than the calculation of subsidence. There are a number of reasons for this. For one thing, the measurement of linear change along lines of markers, unlike the more frequent operation of taking levels, can rarely be pursued over an extended period, as it requires a great deal of time and trouble, and many markers become lost in the course of time as a result of earth works. The values obtained in practice cannot yet therefore be regarded as statistically sound for all shapes and sizes of extraction area. Above all, the magnitude of critical displacement V_Max, which cannot be deduced as easily as critical subsidence from mining concepts such as seam thickness and type of stowage, is known only approximately. This is because a half or whole critical area, over the ribside of which this peak displacement value occurs, is frequently interrupted at greater depths by fault zones. Depending on the direction of the line being measured, the displacement component v_x or v_y measured is larger or smaller, and it agrees with the resultant displacement v only when observation lines run diametrically across the working (Fig. 121). This fact seriously complicates the comparison and analysis of measurement results. By contrast, the subsidence of points is independent of direction. A further difficulty is that displacement values are very small, not only at the trough margin but even at its centre, and are thus particularly sensitive to errors in measurement or to being overlain by secondary movements of the marker pegs, unconnected with mining.