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1. Cave zone-broken and fragmented rocks that fill mined space. The immediate <br />roof rocks fracture into blocks often controlled by preexisting structure, filling the mined <br />space. Bulking and rotation of individual roof rocks eventually limits the upward growth <br />of failure. The thickness of this zone is estimated to be two to eight times seam thickness, <br />depending on the bulking characteristics of the immediate roof rocks. <br />2. Fracture zone fractured rocks that fail because of shear stresses near the ribs <br />and delamination toward the center of the panel. This zone is located directly above the <br />cave zone. The strata within this zone move downward, usually in large blocks, but <br />without major rotation, to rest on the caved zone below. The permeability of the rocks is <br />increased within this zone, which is estimated to extend twenty to fifty times seam <br />thickness above the mine roof depending on geologic conditions and the strength of the <br />rocks. At the North Fork valley, this zone is estimated to extend twenty to thirty times <br />seam thickness (WWE 2004). Some researchers report higher values reaching as much as <br />fifty times seam thickness (Peng 1992). <br />3. Continuous deformation zone-deformation zone from the top of the fractured <br />zone to the surface soils. The strata flex downward without significant fracturing, <br />gradually settling over the fracture zone. In the absence of soils, this zone extends to the <br />surface, forming compression zones at the surface to the center of the panel and tension <br />zones at the edge of excavations. <br />4. Soil zone-This zone is an extension of the continuous deformation zone, which, <br />depending on site-specific conditions, generally consists of soils and weathered rocks. <br />Because of the less-brittle nature of soils, tensile cracks associated with transient <br />subsidence may not be detected easily in front of the face and any existing fractures tend <br />to heal quickly. Tensile fractures forming at panel boundaries last longer, but eventually <br />get closed due to caving of fracture walls. <br />Three subsidence phases are associated with trough subsidence. These are shown in <br />Figure 3. <br />Maleki Technologies, Ina Page 10 <br />