_GENERAL DOCUMENTS - C1981017 (237)

Home Browse Search

4 - fracturing and hummocked profiles . This is probably due to the highly heterogeneous nature of the sediments over the coal beds , the absence of a strongly-developed fracture pattern and the great depth of cover. The horizontal distance away from the edge of a mined-out area to which subsidence effects are detectable is deter- mined by the angle of draw. This angle is influenced by the thickness of the coal removed, rock strength, lithology and structural features within the rock sequence such as joints, faults and folds. Topography, and in particular extensive abrupt slope changes relative to mine depth, also influences the angle of draw. In the Somerset area, Dunrud calculated draw angles of from less than 100 to over 20 , the variations apparently depend- ent on the nature of the overburden strata and depth of cover. In its original application, MCR adopted 150 as an average for the worst case situation. CMLRD rejec8ed this angle and stipulated that MCR use an angle of 20 .5 for determining subsidence-affected-area boundaries. MCR accepted this stipulation. Data acquired since the subsid- ence monitoring program began in 1981 now allow a more accurate approximation of this angle. The equation used in the permit to determine the horizontal distance from the edge of a mined-out area to the limit of subsidence on the surface is : x = Htan8cos20 where x = the horizontal distance to the limit of subsidence corrected for topographic slope; H = the vertical depth of cover at the edge of the mined-out area; 8 = the angle of draw; and 0 = the slope angle from the intersec- tion of H with the surface to the limit of subsidence. In order to determine the angle of draw from subsidence moni- toring points to mined-out areas, this equation is resolved for 9: 6 = tan-1 x Hcos2o. H is determined from mine and surface topographic maps , x is the distance measured from the edge of the mined-out area to the monitoring point, and 0 is calculated using the equation