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block movement in mines using narrower yield pillars 30 to 40 ft wide based on these <br />measurements. On the contrary, wider pillars used in the Raton basin influenced surface <br />movements and showed minor humps in the subsidence profiles (King 1981). <br />BRL has successfully used yield abutment pillars in the D Seam and is planning to <br />use similar designs consisting of 37- and 114-ft-wide pillars in the Dove Gulch area. <br />Table 2 presents a summary of subsidence measured from select monuments near the <br />gateroads. With the exception of monument 11 B at shallow cover, most deep-cover <br />monuments subsided approximately 2 to 3.6 ft. The maximum subsidence occurred at <br />monument 5B toward the center of this longwall block. Below this monument, at the D4 <br />headgate, BRL used an abutment pillar width of 104 ft. At a comparable position over the <br />D8 headgate (20A), the subsidence was lower and perhaps influenced by 114-ft-wide <br />abutment pillars. Over the gate pillars, the subsidence increased with depth, approaching <br />0.4 times the extraction height. <br />Is Table 2. Maximum measured subsidence over the gate roads for different depths <br />Monument Depth, ft Subsidence, ft Location <br />11A 400 .23 D3 Headgate <br />14E 800 1.7 D6 Headgate <br />18G 1,000 1.9 D7 Headgate <br />12E 1,350 2.9 D4 Headgate <br />513 1,350 3.6 D4 Headgate <br />20A 1,350 3.25 D8 Headgate <br />4.4 Variable topography <br />Surface topography plays an important role, influencing surface movement and <br />fracturing. Studies by the Colorado School of Mines over the Raton basin (King 1981), <br />New Mexico, for instance, indicated high subsidence under topographic highs and low <br />subsidence under topographic lows, i.e., the "pile-up" effect. Surface cracks were also <br />concentrated on the topographic highs. <br />r? <br />Maleki Technologies, Inc. Page 19