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u 3.3 Variable topography Surface topography plays an important role, influencing surface movement and fracturing. To examine the influence of surface topography on ground movements at BRL, the authors have superimposed subsidence contours and overburden thickness contours for two monitoring time frames over the longwall block as illustrated in figure 5. In addition, the location of geologic structures is shown together with horizontal displacement vectors. The subsidence contours are influenced by wide, irregular monument locations selected to accommo- date steep topographies; nevertheless, we consider the contours useful for these comparisons. Eiccluding multiple-seam mining areas, the maxi- mum subsidence of 2.3 m (7.5 ft) occurred in panel D8 at monument location 22D at the higher topog- raphies (figure SGT. In contrast, at lowertopograph- ies along panel D7, subsidence was much reduced, to 1.7 m (5.5 ft) at monument 14D. This pattern of localized movement is influenced by topography and the orientation of a steep slope into the mined- outlongwall panel. Results clearly show horizontal displacement concentrating at topographic highs. Over the western panel boundaries at topographic highs, ground moved downhill to the east and movement diminished at topographic lows nearHubbard Creek (figure SA). Similazly, horizontal movement increased significantly toward the Freeman Gulch drainage as the monuments to the north and south of the gulch moved toward the drainage at low elevations. This creates compression at the bottom of the drainage, an assertion that is supported by the lack of much observed surface cracking. The topography dominates patterns of horizontal move- ment. Figure SB presents similar results after the 2003 survey. During this period, the operator extracted four longwall panels (D5, D6, D7 and D8). The ground moved eastward at the western boundary of the longwall panels and became insignificant near Hubbard Creek. Maximum horizontal displacement is now toward Sheep Corral Gulch. 1n this area, the authors mapped several short faults. These faults do not extend to the surface and thus are not believed to have influenced horizontal movement significantly. 3.4 Surface fracturing An annual subsidence crack survey is completed over active longwall panel areas. The survey includes the location, orientation, horizontal length, and crack width. Mining-induced cracks form at topographic highs over the panels, over the gateroads, and at longwall boundaries. Repeated inspections show many areas without surface cracking. Overall, a higher percentage of cracks are created over areas of shallow cover (less than 300 m [ 1000 ft]). Seven percent of the cracks are reported at higher topographies. As illustrated in figure 6, crack orientation varies considerably. Some are near-parallel with the longwall boundaries, while others are influenced by premining geologic structures, including N70°E joints, N60°E normal faults, and perhaps east-west- oriented strike-slip faults. Tensile cracks are generally 2.5 to IS cm (1 to 6 in) wide and form at an estimated tensile strain level of 0.003. Monitoring indicates that cracks form in front of the face and close as the surface goes from tension to compression. Observations also suggest closing and gradual healing of cracks in soils and weathered bedrocks by natural caving of crack walls, in-filling by sediments and organic material, and effects of storm events. Cracks remain more noticeable in brittle sandstones. aso o i° 29i zsa zr9 zs9 25i 190 180 tI0 Fig. 6. Rose diagram showing the orientation of surface cracks A -7 eo 90 mo