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2004 Geologic Hazard Field Observations <br />South of Divide Mining Area <br />• commonly is one-half the distance of the thickness of the rock unit, or units, behaving as a plate <br />or beam. <br />Using this conceptual model of the overburden downwarping as multiple plates, the maximum <br />crack depth-the depth of the neutral surface--can be calculated by dividing the maximum <br />horizontal strain values by those of maximum curvature at a given location. The calculated depth <br />of the tension zone to the neutral surface-the boundary between tension above and compression <br />below-ranges from approximately 50 to 100 feet in the West Elk Mine monitoring network. <br />Crack depth may be much less than this projected 50-100 foot range of maximum values. An <br />unpublished study for the U. S. Bureau of Mines (Engineers International) indicated that surface <br />crack depth razely is greater than 50 feet. Cracks will also be less extensive or terminate where <br />shale and claystone layers are present. <br />During the past nine years of annual observations in the West Elk Mining azea (from 1996 to <br />2004 inclusive), particularly the Apache Rocks and Box Canyon mining areas, no cracks were <br />• observed above mined-out longwall panels in colluvium more than an estimated ten feet thick. <br />No cracks have been observed in alluvium above mined-out longwall panels. <br />• <br />No cracks were observed in the alluvium and colluvium in Deep Creek (overburden depth to the <br />mined B Seam ranges from about 950 to 1,050 feet) above mined longwal] panels 15, 16, and 17 <br />during periodic field studies in the Apache Rocks and Box Canyon Mining azeas. In addition, no <br />cracks were observed in the upper reaches of Sylvester Gulch above mined longwall panels 14, <br />15 and 16 (overburden thickness ranges from 800 to 1,600 feet). <br />The neaz-surface alluvial material consists of primarily sand, silt, clay and soil in these two areas. <br />The azeas were located above rigid pillazs and panel boundaries where the overburden depth <br />ranges from 800 to 1,600 feet. A significant amount of the clay and silt in these stream azeas has <br />been derived from the Wasatch Formation during processes of erosion and deposition. Based on <br />the observations in these two areas, it is unlikely that cracks will occur in colluvium and <br />alluvium in the stream valleys of the South of Divide Mining Area, because the alluvium and <br />colluvium contains more clay and silt than is present in the Deep Creek azea (above mined <br />831-032.621 Wright Water Engineers, Inc. Page 16 <br />November 2004 <br />