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<br />~~ <br />3. SURFACE SUBSZLF7JCE PRIDZCTICN <br />Subsidence is defined as a local lowering of the surface paints <br />caused ty underground mining operations. Coal mire subsidencz <br />includes rot only the local lowering of the ground surface but also <br />all deformation processes within the overburden, and at the surface <br />that are produced by the troveaient of Hock into mine openings. In <br />other words, this subsidence can be defined as the vertical rrovement <br />of surface points within a subsidence trough above the coal seam <br />(Figurn 7). <br />Subsidence processes, therefore, include vertical sc-ttlcJnent, <br />• local depressions, pits, horizontal ccx~ressive and tensi).e sti uins <br />produced by flexure of the strata, and compressive strains associ- <br />ated with arching phenomena above the mine w~king5. <br />Subsideroe depends, inter olio, on the Honing method, the <br />lithological composition of the strata overlaying the coal seam, trs= <br />presence of geological structures, the horizontal and vertical <br />dimensions of the mining workings, the depth of the mining, etc. <br />Maximum subsidence can be expecter~ above to~Lly extracted coal <br />seams, but it can also be minirtuzed by leaving protective pillars <br />bena3th important surface objects and installations. In some Euro- <br />pean countries, surface objects or structures wh ch must be pro- <br />tested against subsidence aze divided into four categories a:, shown <br />• in Table 3. <br />