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ia] <br />C <br />9.2 DESCRIIPI'lVE THEI)RIES <br />9.2.1 Trough Subs[dence <br />SUaF'At;k ~UIUIUF:Nt.'k <br />Descriptive theories include those of l~nc and Roberts 11), Briggs (2), <br />Grand (3), and Wardell (4). Since they arc derived sulcly 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. !t is symmetrical about the mining geometry with maximum <br />aubaidence S at the midspan of the opening. It gradually reduces outward <br />DiwCliOn d Adarar <br />l ° <br />I ~ <br />~ I <br />A.... '~'~a ...8 <br />t ~ f~ •~ <br />i ~ <br />L ~ , <br />I It <br />1 <br />Plan <br />e A 1 OrgW 9v1ao• g <br />tel. a \ ~ ~ / <br />tlWaienoa <br />tw ~\ I ~ Mq. a ta.. <br />~~ ._ coal sam xn rl:d~ <br />s.d~ a A-e <br />lly. f3.1 Su6aiJcaw aver u kxylwall face lat. Cuunear U.K. NrtiwvJ Cud WwrJ. <br />• <br /> <br />