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2006 Subsidence and Geologic Field Observations <br />Box Canyon, West Flatiron, Apache Rocks, and South of Divide Mining Areas <br />This concept has an important bearing on the hydrologic consequences of longwall <br />mining. Any ground water or surface water in contact with a given crack area is <br />prevented from traveling downward beyond the neutral surface of the deformed plate. <br />Annual field observations over the last 11 yeazs verify this conceptual model in bedrock <br />and surficial material (colluvium, alluvium, mudflow, and debris flow deposits) where <br />the overburden is laterally constrained. <br />However, this concept of uniform downwarping under lateral constraint does not apply to <br />very steep slopes (greater than approximately 23 to 45 degrees) and cliff areas where <br />there is low to no lateral constraint. The rocks and unconsolidated material in these areas <br />deform as unconstrained beams, plates, or cantilevers as the longwall mining faces move <br />them. Cracks in these areas commonly extend completely through sandstones and other <br />brittle units. Any ground water or surface water present near or within cracks in these <br />areas will likely flow into to existing surface drainages. <br />2. The maximum cumulative horizontal tensile strain measured at Apache Rocks during <br />2006 compares very closely to the maximum amount determined in 2000, which was <br />shortly after the cracks occurred. The maximum range of horizontal tensile strain <br />measured in 2000 was 0.47 percent, compared with 0.46 percent measured and calculated <br />on July 18, 2006. These maximum strain values are below the median strain value of <br />0.65 percent projected in Exhibit 60. <br />' 3. The length of time that tension cracks are expected to be visible-before the effects of <br />erosion and deposition, mass wasting, infilling, and re-vegetation obliterate them <br />' (duration of cracks~is a function of their location with respect to the mine geometry and <br />type of material in which the cracks formed. However, cracks which form in the zone of <br />' temporary tensile stress, such as above moving longwall faces, commonly close again <br />when the longwall moves out of their area of influence. Crack duration in zones of <br />' permanent tensile stress, such as above mine boundaries and un-mined pillars between <br />longwall panels, commonly ranges 1) from one to three years in colluvium, 2) from three <br />' 831-032.750 Wdght Water Engineers, Inc. Page 41 <br />December 2006 <br />