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SubTena, Inc. CYVCC-Foidel Creek Mine-Subsidence .. Mazch 23,1993 <br />• 1.0" MINE SUBSIDENCE and RELATED DISPLACEMENTS <br />1.1 Mechanics of Mining Subsidence due to Longwall Undermining <br />1.1.1 Surface Subsidence <br />The surface manifestation of longwall coal extraction is a gently sloping trough which is typically <br />symmetrical about the area of excavation. Maximum vertical displacements, within this trough, <br />are a function of the width (W)and depth (h) of the excavation and the nature of the overlying <br />strata. Maximum horizontal displacements are typically located over the excavation centerline <br />(compression) and within O.OSh and 0.25h of the panel rib (extension). <br />Four unique elements of the surface subsidence trough are typically evaluated during the <br />subsidence prediction exercise. <br />The transverse profile is calculated for across-section of the excavation, normal to <br />the direction of face advance/retreat, located a distance greater than depth (h) from <br />the panel start room. Beyond this region, the transverse profile is considered to <br />be fully developed and maximum subsidence (conesponding to the panel's width <br />to depth ratio (W/h)) will occur. <br />2. The start profile is calculated far across-section of the excavation parallel to face <br />advance/retreat located over the centerline of the longwall panel. This section <br />extends forward a distance equivalent to depth from the start room, in the <br />• du•ection of face advance/retreat, and a distance (d) defined by depth (h) and the <br />start profile draw angle (a) behind the start room (d = h.TAN(a)). <br />3. The end profile is calculated in the same manner as described for the start profile <br />using its characteristic parameters (e.g., the end profile draw angle). <br />4. The fourth element evaluated during subsidence prediction is the dynamic or <br />travelling subsidence profile. This profile defines characteristics of the subsidence <br />wave that follows the advancing/retreating longwall face and is only used when <br />transient effects are expected to resultin impacts (e.g., tilting in transmission lines). <br />1.1.2 Overburden Deformation <br />Deformations in the intervening overburden consist of near seam caving, bed separation and the <br />development of tensional frachves (or existing fracture dilation) in overlying bedrock strata. The <br />nature and extent of these deformations are a function of the mining dimensions (W, h) and <br />characteristics of the overburden strata. <br />Numerous studies (e.g., Rauch, 1989; Coe and Stowe, 1984) have attempted to define the limits <br />of zones exhibiting different levels of mechanical deformation as a precursor to determining <br />hydrologic impacts. Four zones have typically been identified and chazacterized as follows: <br />Caved Zone: This zone is mechanically characterized as rubbilized and extends from the <br />• coal seam a distance 4-6 times the mined coal thickness. <br />