2008-12-09_PERMIT FILE - C1996083 (2)

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indicating uniform subsidence over a portion of a 40-ft-wide barrier separating two room- and-pillar panels (Dunrud 1975). Thus overburden response is characterized by uniform block movement in mines using narrower yield pillars 30 to 40 ft wide based on these measurements. On the contrary, wider pillars used in the Raton basin influenced surface movements and showed minor humps in the subsidence profiles (King 1981). BRL has successfully used yield abutment pillars in the D Seam and LBS and is planning to use similar designs consisting of 47- and 127-ft-wide pillars in the Dove Gulch area. Table 2 presents a summary of subsidence measured from select monuments near the gateroads. With the exception of monument 11 B at shallow cover, most deep- cover monuments subsided approximately 2 to 3.6 ft. The maximum subsidence occurred at monument SB toward the center of this longwall block. Below this monument, at the D4 headgate, BRL used an abutment pillar width of 104 ft. At a comparable position over the D8 headgate (20A), the subsidence was lower and perhaps influenced by 114-ft-wide abutment pillars. Over the gate pillars, the subsidence increased with depth, approaching • 0.4 times the extraction height. Table 2. Maximum measured subsidence over the gate roads for different depths Monument Depth, ft Subsidence, ft Location 11A 400 .23 D3 Headgate 14E 800 1.7 D6 Headgate 18G 1,000 1.9 D7 Headgate 12E 1,350 2.9 D4 Headgate 56 1,350 3.6 D4 Headgate 20A 1,350 3.25 D8 Headgate 4.4 Variable topography Surface topography plays an important role, influencing surface movement and fracturing. Studies by the Colorado School of Mines over the Raton basin (King 1981), New Mexico, for instance, indicated high subsidence under topographic highs and low subsidence under topographic lows, i.e., the "pile-up" effect. Surface cracks were also concentrated on the topographic highs. Maleki Technologies, Inc. Page 19