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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 BRL. The minimum angle <br />of draw is calculated at monument station 5E to be 15°. Considering all available data <br />from the basin, an average angle of draw of 15° to 17° is expected, although higher values <br />are locally possible at greater depths. A subsidence factor of 0.60 to 0.75 is used to <br />estimate ground movement over the longwall panels at Dove Gulch. At one monument <br />(22D), the subsidence factor approaches 0.75. The greater subsidence at this location is <br />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 />9 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 western 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 />indicating uniform subsidence over a portion of a 40-ft-wide barrier separating two room- <br />and-pillar panels (Dunrud 1975). Thus overburden response is characterized by uniform <br />Maleki Technologies, Inc. Page 18