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Subsidence and Geologic Field Observations <br />Apache Rocks and Box Canyon Mining Areas <br />Julv 12-15, 2004 <br />5.0 CONCLUSIONS <br />Of the 18 traverses made during this study, the areas of subsidence cracks observed above the <br />mined-ou[ longwall panels occur within the limits of the angle of draw and the strain projected <br />for the Apache Rocks and Box Canyon Permit areas (see Table 2 of Exhibits 60 and 60A). The <br />range in angle of draw projected in Exhibit 60 was 10° to 20°. The angle of draw, based on field <br />observations, ranges from about 10° to 15°. The tensile strain estimated in Exhibit 60 ranges <br />from 0.8 to 1.8 percent. Field estimates of strain (including increases of 10 to 20 percent in <br />2002, but no increase in 2003 and 2004) at Apache Rocks range from approximately 0.3 to 0.6 <br />percent. <br />As stated in the 2002 and 2003 reports, the effects of tension and compression caused by <br />longwall mining are more predictable and have less impact on the overburden rock than does <br />subsidence caused by room-and-pillar mining. The major reason for this is that longwall <br />' extraction is complete in the panel areas, and therefore is more uniform than room-and-pillar <br />mining practices. In longwall extraction, therefore, the rocks, alluvium, and colluvium above the <br />mining panels tend to undergo continuum downwarping as multiple plates (in three dimensions) <br />or as beams (in two dimensions). <br />Applying the concept of downwarping of rocks as multiple plates, tension cracks, for example, <br />' will close at the neutral surface of the rock unit behaving as a plate (in three dimensions) or a <br />beam (in two dimensions). Below the neutral surface the material in this plate undergoes <br />compression. Under this conceptual model of the overburden rocks downwarping as multiple <br />plates or beams, no surface water can flow a vertical distance greater than the depth of the neutral <br />' surface. This vertical distance commonly is one-half the distance of the thickness of the rock <br />unit, or composite rock materials, or surficial material behaving as a plate or beam. <br />After six years of observations, no subsidence features (cracks or bulges) were observed in Deep <br />' Creek within the area of mining influence of mined longwall panels 14 through 17 (overburden <br />depth to the BSeam-about 1,050 to 1,100 feet). No subsidence features were observed in the <br />831-032.640 Wright Water Engineers, Inc. Page 46 <br />September 2004 <br />