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• thus results may be influenced by reactivation of movement in these historic mining <br />districts. Selected monuments were surveyed annually using GPS monitoring system; <br />more frequent at some locations. <br />Subsidence engineering parameters include subsidence factor, angle of draw, angle of <br />critical deformation, and horizontal strain. The subsidence factor is the ratio of <br />maximum measured subsidence to extraction height. Because this ratio depends on <br />excavation width and overburden thickness, it should be measured in supercritical <br />excavations where caving has reached the surface on collapse of the pressure arch. <br />The angle of draw defines the limit of surface movements beyond the edge of an <br />excavation. It is measured from a vertical line drawn at the panel edge and a line <br />connecting the panel edge to the point of "no" movement on the surface. In practice, the <br />accuracy of surveying equipment defines the point of no movement. This accuracy is <br />usually about 0.1 to 0.15 ft. Considering the accuracy of the GPS equipment used at <br />• BRL, we have used a value of 0.15 ft for the limit to sensible subsidence movement <br />(Cragg 2005). Angle of critical deformation is similar to the angle of draw, but is <br />measured to a point of critical deformation with respect to existing structures; it is <br />preferred by many practitioners because it avoids the shortfalls connected with the <br />accuracy of surveying equipment. Based on subsidence data from 40 longwall panels, <br />Peng (1992) found that it is 10° less than the angle of draw. <br />Horizontal strain is the change in horizontal length of the ground divided by the <br />original length of the ground. Positive strain is used here to show tensile strain indicating <br />an increase in the horizontal length of the ground. Compressive strain (negative notation) <br />occurs when the ground is shortened or compressed. Maximum tensile strain is found in <br />supercritical excavation and maximum compressive strain occurs in subcritical <br />excavations. Horizontal strain increases with an increase in extraction height and <br />decreases at greater depths. Surface topography also influences horizontal strain. <br />L ~ <br />Maleki Technologies, Inc. Page 17 <br />