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• Colluvial deposits are the source for local springs <br />within the proposed long-term lease area. These are <br />generally located where there is sufficient overburden <br />thickness that the risk of the development of surface <br />subsidence cracks is low. <br />It is not expected that the subsidence will de[eriorate <br />the stability of old landslides to the extent that their <br />deformations could be renewed. Attention will have to be <br />paid to mining close to the coal outcrop and to the design <br />of the outcrop barrier pillars. Their failure could <br />potentially cause extensive sliding of the slopes in the <br />vicinity of the outcrop. <br />Few significant buildings and structures are located <br />within the proposed long-term lease area. However, <br />structures identified which could be susceptible to damage <br />due to subsidence-induced strains include the Muckle House <br />restaurant, Morrell's ranch, the Pitkin Mesa Aqueduct, and <br />the CWI well field with its associated equipmen[ and <br />pipeline. <br />8.3 POTENTIAL MITIGATING h1EASURES <br />• Subsidence can be reduced to small amounts by leaving <br />coal pillars for permanent support in a partial extraction <br />operation. Adequate protection of surface features against <br />subsidence could be achieved by not mining pillars remaining <br />after the first stage of a conventional room-and-pillar <br />operation. Higher recoveries may be possible by mining the <br />partial extraction zone as a panel and pillar operation. In <br />either case, the ability of the pillars to provide permanent <br />support of the overburden should be insured by appropriate <br />design calculations. <br />Most features identified as requiring some form of <br />protection against the effects of subsidence lie within <br />relatively small zones. Ttiis enables the possibility of <br />providing protection against subsidence by designing <br />appropriately located partial extraction zones without <br />reducing recoverable reserves excessively. <br />The locations of proposed partial extraction zones are <br />shown on Plate 16. Tiiey are generally limited by the 800 <br />foot overburden contour, ~~hich corresponds with the limit of <br />the zone of high potential for the development of subsidence <br />cracks at surface, unless There are site-specific <br />justifications for extending the protective zone further. <br />f_ J <br />- 5= - <br />