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SubTerra, Inc. Peabody Twentymile Coal, LLC April 11, 2016 <br />Subsidence Mitigation During 9 -East Undermining of the Energy Spur at the Foidel Creek Mine <br />2. Mine Subsidence <br />2.1. Subsidence Mechanics <br />Although an extensive review of subsidence mechanics is beyond the scope of this report, it is <br />appropriate to present an understanding of the way in which the void created by longwall mining is <br />transmitted to the surface. In general, as a longwall face retreats, the immediate roof strata <br />separate at bedding planes and pre-existing sub -vertical discontinuities and cave behind the face <br />supports. Caving propagates until it is arrested through bulking and/or intersection with more <br />competent overlying beds. As the longwall face retreats further beyond the section, the overlying <br />beds fail in bending and deform towards the centroid of the excavation ultimately resulting in <br />subsidence at the ground surface. During this deformation, shearing occurs along bedding planes <br />and bed separation occurs where more competent strata overly less competent members. These <br />actions affect both the limit and magnitude of subsidence that is transmitted to the surface. <br />The layout of development entries and the size of headgate and tailgate pillars also influence the <br />shape of the final transverse subsidence curve and related deformations (tilt, curvature, and strain) <br />both in single and subsequent panel mining. Figure 1 illustrates the change in vertical stress and <br />deformations that occur during entry development and longwall mining and the resulting zones of <br />horizontal extension and compression at the ground surface. <br />Panel <br />Ribside <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />Vertical Stress <br />Re -distribution <br />/ I <br />Yield ' "Rigid' or <br />Pillar Abutment <br />Pillar <br />(a) After Development <br />Panel <br />Figure 1 <br />Generalized Vertical Stress Change and <br />Displacement during Longwall Mining <br />(Derived using information from Mark, 1992. <br />Bienawski, 1992, Shadbolt.. 1977) <br />Pillar Yield and Floor <br />Punching <br />(load transfer to cure) <br />go 3 <br />too <br />Floor Heave <br />(b) After Longwall Extraction <br />Panel <br />i <br />Extension Compression <br />�I <br />I \ <br />I \ <br />I ♦ <br />Zone of Vertical Extension <br />(t)ed separation) <br />I <br />I <br />O Zone of Bed Separation <br />01 Zone of Caving <br />iQ 3-sm <br />Zone of Incomplete <br />Convergence <br />(c) Zones of Fracturing <br />(Singh, 1986) <br />EXPLANATION: <br />M = Extracted seam thickness <br />S = Subsidence <br />Qy = In-situ vertical stress <br />6u =Side abutment stress <br />+r = Extension <br />-E = Compression <br />3 PN: 2016-10 <br />