2008-12-09_PERMIT FILE - C1996083 (2)

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2 years after mining, depending on the production schedule. Subsidence is defined by the subsidence factor and angle of draw. The subsidence factor is the ratio of maximum vertical surface movement to seam mining height and is often expressed as a percentage of mining height. The angle of draw identifies the limits of surface movement beyond the mining boundaries underground. Angle of critical deformation is similar to the angle of draw, but is measured to a point of critical deformation with respect to existing structures. It is preferred by many practitioners because it avoids shortfalls with the accuracy of surveying equipment. Based on subsidence data from 40 longwall panels, Peng (1992) found that it is 10° less than the angle of draw. From a review of subsidence in the North Fork Valley and the analysis prepared by Stewart (2006), a subsidence factor of 0.60 to 0.75 and an angle of draw of 15° to 30° were chosen for modeling purposes at the Dove Gulch study area so that the dependency of subsidence parameters to topographic conditions could be simulated realistically. Significant subsidence beyond excavations is controlled by the angle of critical deformation of 17°. The predicted maximum subsidence is 9 ft near the center of the B8 panel. It approaches 4 ft over the deeper gateroads. Predicted tensile strains reach levels that can cause surface fractures. These are more likely to occur over panel boundaries and gate roads. Many of the fractures should heal due to compression and settlement of the surface, but some of the fractures will remain open, particularly to the east under shallow cover. A few cracks could also form at topographic highs. Expected significant surface movement beyond the underground mining boundaries varies from 275 ft to the east to 700 ft to the northwest, with some variations to be expected locally. Maleki Technologies, Inc. Page 7