2008-02-22_APPLICATION CORRESPONDENCE - C2008086 (39)

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CHAPTERFOUR Environmental Consequences and Mitigation The project area is not located in any published flood zone. No mapped subsurface mine workings are present in the project area; therefore, collapse or subsidence of mine workings is not a credible hazard. According to soil maps prepared by the U. S. Department of Agriculture (USDA) National Resources Conservation Service (MRCS) most of the soils derived from Mesaverde Group and Mancos Shale contain high concentrations of soluble salts (i.e., calcareous and gypsiferous soils). Soluble salts present deleterious effects to concrete; therefore, on-site materials should be evaluated for potential alkali-aggregate reaction. Soils with high soluble salt concentrations are also susceptible to collapse upon loading. Mitigation Measures Subsidence Mitigation of subsidence impacts can best be done by appropriate design of the mine plan. It is possible to mitigate somewhat the adverse impacts by varying panel width, by designing gateroad pillars between panels to yield when the first of two adjacent panels is mined and crush after the face of the second panel is mined past, and by positioning longwall panels with respect to a particularly important surface feature. Normally, if landslides or rockfalls are present in an area, constraints on design and construction may be necessary to minimize risk. Longwall panels should not be completed in overburden conditions of less than 200 feet (see Figure 13, Appendix C). The 200-foot overburden contour extends approximately 360 feet upstream from the outcrop line in Big Salt Wash and approximately 550 feet upstream from the outcrop line in Garvey Canyon. Long-term protection from chimney subsidence to the overlying ground surface can be provided in such shallow overburden by partially backfilling the entries in these two areas upon final closure of the Red Cliff Mine. The potential for draining surface water into the Red Cliff Mine is low, but probably precludes longwall mining under stream courses and water impoundments when the bedrock overburden thickness is less than 95 feet. Big Salt Wash is particularly at risk because it also contains a road and has agricultural uses. Because there is no available depth of alluvium below any of the deeply incised canyons, and due to the absence of any data on the potential fault control of the nearly trellis drainage pattern in the project area, conservatism must be used and a minimum of 200 feet of overburden required to positively prevent water loss from longwall mining under even intermittent stream courses. It is possible at least to partially mitigate tilting hazards and similar potential major toppling hazards in Big Salt Wash, Garvey Canyon, and along Munger Creek by designing the longwall panels to retreat toward these drainages from the north and from the south. Retreating toward these drainages would slightly flatten the slope of the canyon walls as opposed to advancing away from Big Salt Wash which would slightly steepen the canyon walls. A conceptual mine plan has been proposed in Section 8.2 of Appendix C that would mitigate potential subsidence impacts in the project area. The goals of the conceptual plan were to maximize safety, then mitigate to the extent possible subsidence impacts, and finally to maximize resource recovery. However this is not the only plan that may mitigate certain impacts, and CAM may develop other plans. CAM will also be required to comply with state and federal regulations regarding subsidence impacts as they prepare their mine plan and permit application. 4-74 DBMS 629