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described by Hall and Dowding (1981). Similarly, the <br />subsidence profile tends to be narrower, limit angles are <br />steeper, and the risk of subsidence cracks appearing at <br />surface is probably greater in the U.S. <br />There is no way of taking account of varying geological <br />conditions using the SEH. Since the predictions have been <br />developed based on observations under British conditions, <br />the method should be used for predicting subsidence in the <br />U.S. only with considerable caution and a thorough <br />understanding of its basis and limitations. <br />Empirical Equations: <br />These include influence functions, in which the <br />principle of superposition is used to sum the effects of the <br />various elements which comprise an extraction area, and <br />profile functions, in which a mathematical equation, <br />generally a hyperbolic function, is used to describe the <br />subsidence profile. These are described in detail by Brauner <br />(1973). <br />In theory, empirical equations have the advantage of <br />being able to include parameters which represent the <br />characteristics of the overburden strata. In practice, <br />• little guidance and experience is available in the selection <br />of these parameters. The method therefore offers little <br />advantage over using the SEH in most practical cases. <br />Significant limitations are inherent in all subsidence <br />prediction methods when applied to areas and geological <br />conditions where there is little or no previous experience <br />in subsidence prediction. Realistic estimates of subsidence <br />can be made, although the accuracy of the prediction will be <br />less certain than that of estimates made i^ areas where <br />there has been adequate documentation of previous <br />subsidence. By considering past experience with the model <br />used, it is possible to make a best estimate of subsidence <br />as well as to estimate limits within which the actual <br />subsidence should lie. <br />After consideration of the limitations inherent in the <br />alternative methods available for subsidence prediction, the <br />6EH method has been selected as the principal method for <br />subsidence prediction at the Grchard Valley Dtine. Its <br />application has been modified, where appropriate, to account <br />for the specific geotechnical and mining conditions at the <br />mine. <br />• <br />- 27 - <br />