2014-10-10_PERMIT FILE - C1996083 (8)

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Most ponds are located in areas covered by colluvial soils; the occurrence of cracks is less probable in these formations and healing of the cracks would be faster in them than in bedrock formations or in colluvial soils covering the ridges. Any pond disrupted by subsidence cracks could easily be repaired at low costs and in a short period of time. The occurrence of subsidence cracks on the surface may not necessarily result in direct inflows into the mine when a surface drainage is disrupted. Studies by various authors such as Orchar (1969, 1975) in Great Britain and Williamson (1978) in Australia indicated that the subsidence cracks may be limited in depth by the development of compressive stresses replacing the tensile stresses that cause surface fracturing. Changes in land surface and drainage slope may affect the run -off water quality by increasing the sediment load. The increase of drainage channel slope could result in an elevated flow velocity and therefore, increase the erosion potential. In summary, we believe that the occurrence of surface subsidence cracks comprises the most important hazard to the surface drainage regime. There occurrence is predicted principally within the areas where the overburden thickness does not exceed some 800 feet. Such areas include: • small ephemeral drainages near the outcrop • mouth of Sheep Corral Gulch • mouth of Dove Gulch • mouth of Freeman Gulch To eliminate or reduce the hazard of surface drainage disruptions would require a change of the mining plans and design of protective pillars or zones of lower recovery beneath these areas as discussed in Section 8.3. Alternatively, surface modifications of the surface drainage and ponds could be made in cases of any occurrence of subsidence cracks. Such surface modifications would not preclude a temporary disruption of the existing surface run -off regime. PRA 4 -24- 03/14