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zones at the surface. The development of tensile or compressive <br />• strain is a function of curvature. Calculation of discrete <br />values for the magnitude of tensile strain is also a function of <br />formation parameters and overburden thickness. In the particular <br />environment on this site with rapidly changing and steep <br />topography, the development of procedures for the prediction of <br />actual magnitudes of surface ground strain is felt to be beyond <br />the scope of this investigation. Yet, due to its importance with <br />regard to stability, it was felt that some treatment of <br />curvature/tensile-strain development would be necessary. <br />Therefore, for the purposes of this study, a simplified procedure <br />was developed for the exa mination of curvature (rate of change of <br />tilt) for the computed subsidence profiles. The procedure <br />• involved the exa mination of each grid point assuming that a <br /> circular arc passes through that grid point and both adjoining <br />grid points. It then determines the height of a circular segment <br />formed by the three grid points, which is of course a function of <br />the radius of curvature. This provides a simplified means of <br />obtaining relative magnitudes of tensile and compressive strains. <br />Patterns of tensile strain development were then examined <br />qualitatively on the basis of low, medium and high relative <br />levels of tensile strain development. <br />7.4 TROUGH VERSUS CHININ EY SUBSIDENCE <br />Beam theory was used to determine the extent of the rubblization <br />of the rock mass due to the removal of the coal. Using the <br />. procedures described in works by H. G. Denkhaus (South Africa) <br />19 <br />