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Jrne t992 CyPrrc TCC "Prediction o/Subridmre Effecrr • 1 ' 1.0 MECHANICS OF MINING SUBSIDENCE DUE TO LONGWALL UNDERMINING 1.1 SURFACE SUBSIDENCE The surface manifestation of longwall coal extraction is a gently sloping trough which is typially symmetrical about the area of exnvaion. Maximum vertical displacements within this trough ue a function of the width (Q(~ and depth (h) of the excavation, the extracted coal height (m), and the nature of the overlying strata. Maximum horizontal displacements are typically located over the exavation centerline (compression) and within O.OSh and 0.25h of the panel rib (extension). The limit of subsidence is traditionally designated by the'draw ankle', which is the an¢le between a line drawn vertically from the edge of the longwall panel (rib, start room or end room) and a line drawn between the panel edge (rib, scan room_ or end_roo_m_) a_n_d_ the limit of subsidence.' - Four unique elements of the surface subsidence trough are typically evaluated during the subsidence prediction exerrise. 1. The transverse profile is calculated for across-section of the excavation, normal to the direction of face advance/retreat, located a distance greater than depth (h) from the panel star room. Beyond this region, the transverse profile is considered to be fully developed and maximum subsidence (corresponding to the . panel's width to depth ratio (W/h)) will occur. 2. The start profile is calculated for across-section of the excavation parallel to face advance/retreat located over [he centerline of the longwall panel. This section extends forward a distance equivalent to depth from the scan room, in the direction of face advance/retreat, and a distance (d) defined by depth (h) and the scan profile draw angle (a) behind the start room (d s h.TAN(a)). 3. The end profile is calculated in the same manner as described for the start profile using its charatteristic parameters (e.g., the end profile draw angle). 4. The fourth element evaluated during subsidence prediction is the dynamic or~ travelling subsidence profile. This profile defines charatteristia of the subsidence wave that follows the advancing/retreating longwall face and is only used whet' 1 transient etts_ar~expected to result in impacts. / J 7.2 OVERBURDEN DEFORMATION Deformations in the intervening overburden consist of near seam caving, bed separation and the development of tensional fractures (or existing fracture dilation) in overlying bedrock strata. The nature and extent of these deformations are a function of the mining dimensions (W, h and m) and characteristia of the overburden strata. Numerous studies {e.g., Rauch, 1989; Coe and Stowe, 1984) have attempted to define the limits . of zones exhibiting different levels of mechanical deformation as a precursor to determining hydrologic impacts. Four zones have typically been identified and characterized as follows: ACZ lru. "P.O. Box 77~0r8 "Srmmboat Springy, Colorado 80177 "(303J879b160