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Subsidence Evacuation For Exhibit 606 South of Divide Mining Area Page 7 Also, with increasing height in this zone, and as lateral and vertical constraints increase, fracturing that wuld impact water bearing zones will tend to occur more in zones of convex upwazd curvature, along separated bedding planes toward the center of the panel, and along Iocal cracks in zones of convex downward curvature (Figure 2). Fracturing within the expected zone of fracture may cease completely where sofa shales and claystanes occur as altema[ing sequences with sandstones. Drainage, however, may cease after mining is complete and any water bearing zones present may be restored. This is particulazly likely in the upper part of the fractured zone in shale sequences between sandstone Layers, once subsidence is wmpleted and the separated beds re- compress and close in response to overburden load (Figure 2). Evidence of restored water levels has been measured and reported in some wells in the West Elk Mine subsidence monitoring area after mining and subsidence were complete. 5.3 Continuous Deformation Zone and Neal Surface Zone These two zones are discussed together because the ground surface is where neazly all measurements are made that monitor subsidence processes active in the zone of continuous deformation. The neaz surface zone, which typically consists of weathered bedrock, colluvium, alluvium, and soil a few feet to a few tens of feet thick, may deform differently than the underlying bedrock . (Figure 2). Field studies by the author indicate that near-surface colluvium and alluvium, which consist of predominantly clay and silt, can undergo significantly more extension without rupturing than can the underlying material. Ia both the Somerset, Colorado and Sheridan, Wyoming areas colluvium and alluvium 5 to 10 feet thick were observed to cover cracks as much as 10 to 14 inches wide so that there was no indication of the underlying ruptures. The zone of continuous deformation, which is transitional to the overlying near-surface zone and also to the underlying zone of fracturing, undergoes differential vertical lowering and flexure as laterally-constrained plates (in three dimensions) or beams (in two dimensions). With flexure, sheaz occurs at the boundaries of rock units with different strength and stiffness, characteristics, such as sandstones and shales. Zones of tension above the neutral surfaces of a rock unit, for example, become compressive above the boundary with another rock unit and below its neutral surface (Figure 2, Enlargement 2). Any cracks, therefore, wfiich occur in the tension zone of a rock unit, terminate at the neutral surface, because the unit is in compression below this point. 5.3.1 Vertical and Horizontal Displacement Tilt, and Horizontal Strain Differential vertical lowering of the continuous deformation and neaz surface zones causes vertical displacement (S), horizontal displacement (S~J, tilt (-Vl), and horizontal strain (E). In flat or gently sloping terrain (slopes less than about 30 percent), surface profiles of subsidence depressions are similar to flexure of fared-end, laterally constrained beams. Tensile stresses are present in areas of positive curvature, which become zero downward at the neutral surface, then reverse to compressive stresses below the neutral surface. 831-032.690 WiightWaterEngineers, /nc.