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ENVIRONMENTAL ASSESSMENT <br /> 3.4.6 Resource Issue—Subsidence <br /> What would be the subsidence effects of the alternatives on natural resources and land uses within the <br /> LBA and on the structural integrity of nearby residences and other structures? <br /> 3.4.6.1 Affected Environment <br /> Subsidence is the gradual lowering of the ground surface after coal and support pillars are removed in a <br /> completed mining panel.After coal recovery,fracturing and settling of the overlying overburden may yield <br /> surface expressions of subsidence in the form of subsidence cracks and a lowering of the ground surface. <br /> At the King II Mine,the coal seam is overlain by a thick, durable sandstone layer that fractures into large <br /> blocks,minimizing void spaces.The nature of the overburden reduces the risk for surface earth movement <br /> after underground mining than would result from a less durable stratum such as siltstone or shale. Prior <br /> coal mining at the GCCE King I Mine revealed minor subsidence over 70 years of mining.At the King I Mine, <br /> surface expressions of subsidence averaged between 50 and 100 ft in length, less than 1 foot in depth, <br /> and 0.25 to 0.5 ft in width. Where these have occurred, the distance from the coal seam to the surface <br /> has ranged from 250 ft to 325 ft.The cracks themselves are vertical. Monitoring of these features by GCCE <br /> contractors determined that they filled in naturally within two seasons and have not expanded in size. <br /> Fewer than six of these subsidence features have been identified since GCCE began mining in 2004. <br /> 3.4.6.2 Environmental Effects <br /> Effects of Alternative A—Proposed Action <br /> Field measurements of subsidence cracks in the Mesa Verde Formation by Dunrud (1976) indicate <br /> subsidence cracks may develop through overburden thicknesses of up to 800 ft under unfavorable <br /> conditions.While unfavorable conditions cannot be defined exactly,they may include zones of weathered <br /> coal and overburden. Overburden thicknesses over 800 ft have been classified as having a negligible risk <br /> of surface fracturing developing.This is a conservative upper limit under normal conditions. <br /> Roof rocks primarily consisting of strong,thick sandstones of the Mesa Verde Group would cave into the <br /> mine in larger blocks than would shale roof rocks and would reduce the height of caving above the mine <br /> workings. These sandstones would generally reduce the amount of subsidence compared to shale. <br /> Sandstones at the surface would have larger displacements and may form cracks up to 1 foot wide and 25 <br /> to 50 ft deep on steep slopes. Formation of joints and fractures on steep slopes may contribute to slope <br /> instability and susceptibility to landslides and rockfalls. The proposed mine plan for the Dunn Ranch Area <br /> LBA does not include mining under steep slopes or daylighting along the steep drainages. Implementation <br /> of the Proposed Action would result in the removal of an estimated 17 mt of coal; the anticipated <br /> maximum, LPC-permitted annual production would be approximately 1,067,040 tpy (Section 2.4), which <br /> would result in a long-term, negligible to minor potential subsidence effect in the Project Area. The low- <br /> cover crossing would be backfilled at the end of the life of the mine to prevent any subsidence from <br /> occurring in The Gulch. <br /> There would be no subsidence impacts to CR 120 as none of the underground operations are beneath <br /> CR 120. <br /> Dunn Ranch Area Coal Lease by Application COC-78825 and Mine Plan Modification EA 3-29 <br />