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terized by uniform block movement in mines using <br />narrower yield pitlazs 9.1 to 12 m (30 to 40 ft) wide <br />based on these measurements. On the contrary, <br />wider pillars used in one western U.S. mine <br />influenced surface movements and showed minor <br />humps in the subsidence profiles [7]. <br />BRL has used yield-abutment pillars in the D Seam. <br />Table 2 presents a summary of subsidence measured <br />from selected monuments near the gateroads. With <br />the exception of one monument under shallow <br />cover, most deep-cover monuments subsided <br />approximately 0.6 to 1 m (2 to 3.6 fr). The maxi- <br />mum subsidence occurred at monument SB toward <br />the center of this longwali block. Below this monu- <br />ment, at the D4 headgate, BRL used an abutment <br />pillar width of 32 m (104 fr). At a comparable <br />position over the D8 headgate (20A), subsidence <br />was lower and perhaps influenced by 35-m- (I 14-fr) <br />wide abutment pillars. Over the gate pillars, the <br />subsidence increased with depth, approaching 0.4 <br />times extraction height. <br />Table 2. Maximttm measured subsidence over the gate roads <br />for different depths <br />Monttment Depth, ft Subsidence, m Location <br /> (ft) <br />11A 400 0.07 (0.23) D3 headgate <br />14E 800 0.5 (1.7) D6 headgate <br />18G 1000 0.6 (1.9) D7 headgate <br />12E 1350 0.9 (2.9) D4 headgate <br />SB ]350 ].1 (3.6) D4 headgate <br />20A 1350 1.0 (3.25) D8 headgate <br />15 <br />r <br />`~ <br /> <br />Subsidence (ft) <br />-- <br />(2 ft) <br />Horizontal <br />Displacement (ft) <br />--~. <br />Fault <br />I <br />Fig. 5. Subsidence contours, horizontal deformation vectors. and mining geometry for two surveys <br />A -6 <br />