2015-02-26_REPORT - C1982056 (3)

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Subsidence Experience at Twentymile Coal Company R. Mills Twentymile Coal Company, Colorado, USA C.D. Breeds SubTerra, Inc., Washington, USA S. Archibeque Twentymile Coal Company, Colorado, USA G. Dowling Twentymile Coal Company, Colorado, USA ABSTRACT: Twentymile Coal Company used the National Coal board model for predicting maximum subsidence and angle of draw resulting from its longwall mining. Predictions were verified by installation of a monitoring network over selected longwall panels. These data are used in subsidence planning for the railroad entering the mine. 1 INTRODUCTION The Twentymile mine is located approximately 20 miles southwest of Steamboat Springs in northwest Colorado. The mine was opened in 1982 and has been operated by Cyprus Coal as a longwall mine since 1989. The initial longwall mining district, and first nine panels, were retreated from 1989 to 1996 and the mined face was widened from 640 -ft to 840 - ft for the last four panels (Buchan, 1999). During this time, coal extraction has affected county roads; a 1 -mile section of the Twentymile Sandstone cliff located above one of the county roads; a creek and its alluvial valley floor (AVF); three sets of powerlines using lattice girder and H -frame pole structures and rated at 340, 240, and 135 kV; and the railroad that transports coal from the mine. This paper describes Twentymile Coal company's approach to monitoring and predicting mine subsidence and summary results from undermining the Energy Spur railroad. 2 SUBSIDENCE MECHANICS AND PREDICTION METHODS Although an extensive review of subsidence mechanics and prediction methods is beyond the scope of this paper, it is appropriate to present an understanding of the way in which the void created by longwall mining is transmitted to the surface and the development of methods that model this process.. In general, as the longwall face retreats, the immediate roof strata separate at bedding planes and pre - existing sub- vertical discontinuities and cave behind the face supports. Caving propagates until it is arrested through bulking and /or intersection with more competent overlying beds. As the longwall face retreats further beyond the section, the overlying beds fail in bending and deform towards the centroid of the excavation ultimately resulting in subsidence at the ground surface. During this deformation, shearing occurs along bedding planes and bed separation occurs where more competent strata overly less competent members. These actions affect both the limit and magnitude of subsidence that is transmitted to the surface. The layout of development entries and the size of headgate and tailgate pillars will also influence the shape of the final transverse subsidence curve and related deformations (tilt, curvature, and strain) both in single and subsequent panel mining. Figure 1 illustrates the change in vertical stress and deformations that occur during entry development and longwall mining and the resulting zones of horizontal extension and compression at the ground surface. Theoretical interpretations of subsidence and related deformations have been the subject of research for more than a century. Early work has been extensively documented by Shadbolt (1977), and Breeds (1976); more contemporary studies have been summarized by Singh (1992). Theoretical studies, and related prediction methods, can be grouped into two broad categories: