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were selected for use. The first was a program known as SPASID, <br />• developed by Penn State University in conjunction with the U.S. <br /> Bureau of Mines. The program allows the use of two influence <br />functions, the first being the Knothe influence functions, and <br />the second being an influence function whose shape factors are <br />controlled by the elastic constants of the material. The second <br />computer program utilized was a program written by Kenneth Myers <br />of Rocky Mountain Geotechnical and is based on the zone area <br />method, as described by Marr, 1975. This program allows the rise <br />of complementary influence functions which correct a subsided <br />profile for the presence of pillars. This program will also <br />handle inclined coal seams and steep, irregular topography. <br />7.2 SUBSIDENCE MODELS <br />• 7.2.1 SPASID <br />SPASID is a computer program for predicting subsidence due to <br />underground mining developed by Penn State University in <br />conjunction with the U.S. Bureau of Mines. <br />SPASID uses the influence method for determining subsidence. The <br />basic premise of this method is that the relationship between the <br />extraction of material and the resulting ground movement is <br />linear so that the subsidence at any point on the surface can be <br />obtained by summing the contributions of all the extraction <br />volumes within the mine panel. Two influence functions can be <br />used with SPASID, either the Knothe function, or a function <br />• derived from traversally isotropic elasticity theory, known as <br />the Elastic Influence Function. <br />16 <br />