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2 . 05 . 6 Page 20 <br /> SUBSIDENCE CONTROL <br /> SURFACE SUBSIDENCE <br /> Underground mining of horizontal or gently dipping coal <br /> seams always causes ground subsidence when coal pillars are <br /> extracted. It is highly desireable for the mine roof to cave <br /> after pillars are removed to relieve stress concentration in <br /> active mine workings . Ground subsidence resulting from pillar <br /> extraction may rupture to the surface causing disruption of the <br /> surface. Such disruptions are commonly called surface <br /> subsidence . <br /> Surface subsidence may take the form of tension cracks , <br /> troughs , or shear planes . Surface strata may settle gently so <br /> that troughs are not readily noticeable because no cracks or <br /> joints are visible. This type of surface subsidence, which is <br /> preferable , leaves no unattractive visual scars on the surface. <br /> Damage may, nevertheless , occur to buildings , aquifers , and <br /> surface waterways . If subsidence damage occurs , National King <br /> Coal , Inc. has the necessary personnel and equipment to effect <br /> repairs . Surface owners have and will allow entry to complete <br /> such repairs . <br />