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' Subsidence Field Observations <br />West Elk Mine <br />August27-29,2002 <br />' <br />' 2.0 FIELD DIFFERENTIATION OF CRACKS IN MINING AREAS <br />' 2.1 Subsidence Cracks <br />Subsidence cracks are open cracks that occur in the zone of maximum tension above a mining <br />panel where the ground undergoes extension. This zone of maximum tension occurs <br />approximately transverse and longitudinal to the orientation of the longwall mining panel. Crack <br />orientation can be variable, as with other deformational processes in nature. Transverse and <br />longitudinal subsidence cracks often do not conform to a precise pattern as discussed above. <br />Joints in the overburden rocks, for example, may influence the orientation of both the transverse <br />and longitudinal tension cracks. <br />Transverse cracks occur above the longwall mining faces-these cracks aze located roughly <br />transverse to the long axis of the panel-thus the name transverse tension cracks. Transverse <br />cracks normally have a very dynamic history. These cracks open when the longwall face moves <br />beneath the azea of observation, then often close again when the longwall face moves out of the <br />area of its mining influence. <br />Longitudinal cracks occur along the edges of the longwall mining panel-these cracks aze <br />oriented roughly pazallel to the long axis of the mining panel-thus the name longitudinal <br />tension cracks. The longitudinal tension cracks normally remain open, because the overburden <br />permanently moves downward into the mined-out panel. <br />Pseudo subsidence cracks aze quite common on ridges and near cliffs. These cracks can look <br />very much like subsidence cracks. The extensive crack along a narrow ridge on West Flatiron is <br />a good example of a pseudo subsidence crack. In this case it was easy to determine that the crack <br />was not mine related, because no mining has occurred in the azea. The presence of pseudo <br />subsidence cracks is a good reason to do baseline studies prior to mining so that these features <br />can be recorded prior to mining. <br />831-032.570 Wright Water Engineers, Inc. Page 3 <br />December 2002 <br />