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�3 �URF'Al:k SUUSIU�NC:Iu <br /> 9.2 DESCRIPTIVE THEORIES <br /> 9.2.1 Trough Subsidence <br /> Descriptive theories include those of Lane and Roberts (1). Briggs (2), <br /> Grond (3). and Wardell (4). Since they are derived solely from surface <br /> observations, these theories are subject to continuous modification as <br /> more data are obtained from the field. A recent finding is that surfaces <br /> directly above excavated openings usually subside in a trough,the area of <br /> which extends beyond the limits of the excavated underground opening. <br /> Figure 9.2.1 illustrates the concept of trough subsidence over a horizontal <br /> longwall face. It is symmetrical about the mining geometry with maximum <br /> subsidence S at the midspan of the opening. It gradually reduces outward <br /> unction of Advance <br /> l CI! .. <br /> 1 Par. <br /> �B <br /> � l <br /> Plan <br /> o A 1 Orgwl Surface g <br /> s s,�, O <br /> s .of \ ' "� s"`` I / <br /> Suinderog <br /> Ifu A"of Draw <br /> \ I <br /> Mf <br /> Coal Seam 4fl 7hck <br /> S.w,on on A—B <br /> �R• 9.2.1 Subsidence over a h► awwi fuse M. Courte.y U.K. Naliumd Cual Niwj. <br />