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SURFACE SUBSIDENCE <br /> Subcritical Advance <br /> Critical <br /> xL x I <br /> Supercritical <br /> '' <br /> Plan <br /> Surlace SubVdenc`proliles <br /> h <br /> I ' <br /> 1 <br /> Gob�..j ' Seam <br /> A 6 <br /> Critical width <br /> Section on X—X <br /> 11%. 9.2.3 Critiud width of extraction to produce maximum subsidence(8).Courtesy U.K. <br /> National Cool Board. <br /> The length AB is called the critical width,and the circular area covered(in <br /> the plane view of Fig. 9.2.3) by the cone with apex at P is the critical area <br /> or area 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 i <br /> surface point,the maximum possible 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 possible subsidence.The width of the opening is <br /> then a supercritical width. On the other hand, if the opening is narrower j <br /> than the critical width, the maximum subsidence occurs at the midspan I <br /> but is less than the maximum possible subsidence for the seam,S The <br /> mex• <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 supercritical <br /> width at a shallow depth but becomes critical and then subcritical at <br /> greater depths. Therefore, depth and width must both be specified if the <br /> amount of subsidence is to be determined. It is common practice to use <br /> the ratio of width to depth. <br /> Types kf Supports <br /> The amount of convergence of underground openings or roof caving <br /> depends on the type of support used,which in turn determines the amount <br />