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Subsidence Evaluation For Exhibit 60 The Apache Rocks And The Box Canyon Mining Areas Page 18 Cracks are projected to be the widest and deepest on the steep slopes, cliffs, and ridges flanking the west side of West Flatiron. [n the Apache Rocks mining azea, maximum crack depth on these steep slopes and cliffs is estimated to locally be from 75 to as much as 1>0 feet deep, and as much as 200 feet deep in the Box Canyon mining area. Due to the lack of lateral constraint, these cracks may remain open until they are filled by processes such as mass wasting or sedimentation. 6.2 Block Glide This type of movement occurs where the shear strength, or frictional resistance, along a plane or planes of weakness (commonly bedding planes) is less than the shear stress generated by the block mass above them. The material above the plane of weakness then slides as a block (block glide; decollement-type movement). 6.2.1 Block Glide Potential in fhe Oliver No. 2 Mine Area The Oliver No. 2 mine was an E/D-Seam coal mine operated east of Sylvester Gulch during the 1940s and early 1950s (see Map 5 of the permit). Based on information contained in the Oliver No. 2 Mine closure report dated October 1953, water- and methane-filled fractures within the mined E/DO-Seam were encountered in the floor of the Oliver No. 2 Ivtine. (Both water and • methane were reportedly under high pressure). The fractures (with a reported trend N77°E) were encountered during pillar development before any fractures produced by floor-heave would likely have occurred. For block glide to occur, the shear strength must be less than the shear stress generated by overburden load and any unbalanced lateral stresses. At the very gentle dip of 2.2 to 3.2 degrees (Figure 9), a high pore-fluid pressure would be necessary to cause the shear strength to be less than the shear stress. It is very unlikely that shear strength along bedding planes in the coal seam (or seams) mined at Oliver No. 2 could be reduced to this extent because: There is no high pre-fluid pressure; the water can drain out the Oliver No. 2 Mine portals even though seals of cinder blocks, arranged in double rows, were constructed 700 to 800 feet south of the portal area. This is evidenced by the lack of spring development in the overburden. 2. The dip of bedding is 2 to 3 degrees (Figure 9;-much less than the 8 [0 17 degree angle of friction for any clays that might be present in the coal-bearing rocks of the [vlesaverde Formation. 3. No indication of block glide has been observed in the 42 years since the mine was closed. 4. All of the Oliver No. 2 Mine workings are outside the angle of draw of planned B-Seam mining, with the exception of the first Box Canyon panel. 831-032.181 Wright Wafer Engineers, Inc.