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Chapter 3 Caved Zane -Extends an estimated two to eight times mining height vertically and directly above the mine level (see Figure 3-3). It is characterized by fractured mine roof strata formed in individual blocks that fill the gob (mined out) area. In the RFMP for the Dry Fork LBA, the average mining height is assumed to be 10 feet. This would compute to a caved zone extending 20 to 80 feet above the mined-out area vertically into the overburden. Fractured Zone -Overlies the caved zone to a height of 20 to 30 times the mine height. Fractures develop in the overburden and permeability may increase, bu[ rotation is limited and bedding orientation is retained. In the RFMP for the Dry Fork LBA, the average mining height is assumed to be 10 feet. This would compute to a fractured zone extending 220 to 330 feet above the mined-out area vertically into the overburden. Continuous Deformation Zone - Overlies fractured zone and extends to the surface or the base of the soil zone. It is characterized by downward flexing strata resulting in increased tension forces above the center of the neutral surface and compression developing below. Some shallow tensional fractures may develop on the edge of the subsidence trough extending down to the neutral surface. Soil Zone -Similar to continuous deformation zone, however effects much less obvious due to the pliable nature of soft sediment. Any extensional cracks that develop heal quickly. If, however, exposed bedrock within the soil zone were to experience surface cracking, it would take years to heal. Topography and Geology The actual leasing of the LBA tract would impose no topographic change on the area. If the tract is leased and subject to future underground longwall mining, subsidence would occur. Ground movement due to subsidence in the Dry Fork LBA could lower the land surface 5 to 7 feet over most of [he tract. Where the B and E Seams could both be extracted, as much as 14 feet of ground lowering could result in the northwest area of the tract. The subsidence effect area is expected Affected Environment and Environmental Consequences to extend from 250 to 650 feet from the edge of the longwall panels, assuming an angle of draw of 15° to 17° (Agapito 2005, see Appendix B). Ground movements associated with subsidence are expected to be complete or "mature" within 12 months of extraction. Subsidence from longwall mining in the LBA can be expected anywhere above or within the angle of draw of fully-extracted longwall panels. Based on past local experience, the angle of draw is expected to average l5° to 17°, with a conservative upper limit of 25°. If fault zones and sandstone fades are encountered while mining the B and E Seams, mine geometry may change and the rate and magnitude of subsidence may be affected. The faults, if they exist, may focus subsidence along the fault plane and cause focused movement on the land surface. Due to the uncertainty of whether or not the faults exist, it is not possible to predict the differential subsidence movement they could cause, if any. The geologic strata in the overburden would deform generally according to the zones described previously in this section. Strata in the caved zone would be permanently changed from rubblizing to fill the mine void. This would change the intrinsic permeability of the rocks by increasing permeability through fractures (see also Section 3.3 Water Resources). Strata in the fractured zone will also be permanently altered. Assuming the mining height of 10 feet, the maximum combined height of the caved zone and fracture zone that could develop during longwall panel retreat is expected to extend up to 380 feet above the top of the E Seam. The permeability of the strata will be increased through mining-induced fractures. Strata in the continuous deformation zone will also be permanently altered as they downwarp into the mine void. There may be small local changes in the dip and attitude of rock layers. Since the overburden in the tract ranges between 1,000 and 2,000 feet, subsidence-induced caving and fracturing of the strata would occur between 600 and 1,600 feet below the surface; therefore, these impacts should not be visible on the land surface. The most common subsidence-related impacts are likely to be surface tension fractures (which will be more evident on hard-competent sandstone outcroppings), although tension fractures may also Dry Fork Lease-By-Application FEIS 3-11