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<br />l <br />>k <br />Advanar. <br />fu~rrl Ilral <br />(~ ll... <br />:V <br />SURFACE SUBSIDENCE <br />Crlrleal <br />ivpnrlllrel <br />Hla. f.7.7 CrlUul width otexlnelwn w produce mrlimum ~ubsidenes (tl). Counay U.K. <br />Nuiaul Cwt Bond. <br />The length AB u called the critical width, and the circular area wvered (in <br />the plane view of Fig. 9.2.3) by the cone with apex at P is the critical area <br />or arcs of influence. Clearly, as the depth of the coal seam increases, the <br />critical width AB increases. For a coal seam at a fixed depth, if an <br />underground opening is as wide as the critical width with respect to a <br />surface point, the maximum possiblc subsidence occurs at this point only. <br />If the opening is wider than the critical width, an area in the midspan will <br />experience the maximum possiblc subsidence. The width of the opening is <br />then a supercritical width. On the other hand, if the opening is narrower <br />than the critical width, the maximum subsidence occurs al the midspan <br />but is leas than the maximum possible subsidence for the seam, S„~. The <br />opening width is then called a subcritical width. The concept of critical <br />width with respect to surface subsidence is summarized in Fig. 9.2.3. <br />It must be noted that an opening of a particular width is a aupercrilical <br />width at a shallow depth but becomes critical and then subcritical al <br />gtrster depths. Therefore, depth and width must both be specified if the <br />amount of subsidence is to be determined. It is common practice to usa <br />the ratio of width to depth. <br />Tjpes of Supports <br />Tho amount of convergenw of underground openings or roof caving <br />dependaon We type of support used, which in turn determines the amount <br /> <br />~\ <br />S~rflon M l~ l <br />