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• Table 1 summarizes estimates for the angle of draw and the subsidence factor for <br />selected western operations in the North Fork valley. Because of great similarities <br />between geologic conditions in these selected operations, we have used subsidence <br />parameters based heavily on measurements from the Somerset basin. West Elk <br />measurements have identified an angle of draw of 15° to 17°, although a more <br />conservative angle of draw of 25° has also been used in some reports. These <br />measurements are in general agreement with USGS measurements (Dunrud 1976) in the <br />range of 10° to 21° considering accuracy of surveying equipment used in those early days. <br />Stover (2003) reports a conservative angle of draw of 25° for B]tI,. The minimum angle <br />of draw is calculated at monument station SE to be 15°. Considering all available data <br />from the basin (Stewart 2006), an average angle of draw of 15° to 17° is expected, <br />although higher values are locally possible at greater depths. A subsidence factor of 0.60 <br />to 0.75 is used to estimate ground movement over the longwall panels at Dove Gulch. At <br />one monument (22D), the subsidence factor approaches 0.75. The greater subsidence at <br />this location is believed to be influenced by local topography as described in section 4.4. <br />• Table 1. Subsidence parameters for selected North Fork Valley sites. <br />Site Subsidence factor Angle of draw, Reference <br />degrees <br />West Elk .70 15 to 17 WWE 1998-2004 <br />U.S. Steel 10 to 21 Dunrud 1976 <br />Bowie .60-.75 15 to 25* J.E.Stover 2003 <br />* A conservative upper value of 30° was used for deep cover areas for modeling purposes <br />4.3 Gate Pillar Behavior <br />Because gate pillar designs influence surface subsidence, some recent investigations <br />have focused on evaluating subsidence above gate pillars. The westem U.S. <br />measurements show different overburden deformation characteristics influenced by the <br />choice of pillar designs. Based on a comprehensive case study by the USBM in 1991, <br />Dyni showed that the narrow 30-ft-wide yield pillars commonly used in the two-entry <br />Utah reserves crushed completely with no influence (or subsidence humps) above the <br />• gateroads. This is in general agreement with the measurements over the U.S. Steel mine <br />Maleki Technologies, Inc. Page 18 <br />