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11111 <br />• HISTORICAL SLIDING <br />Figure 5 shows diagrammatically the cause of the historical sliding that <br />occurred prior to 1994 at the site; Fig. 5 was taken from the 1994 Reclamation Plan <br />Amendment. The first portion of Fig. 5 shows the general structure of the <br />mountainside as it probably existed before any mining. Within the limestone layer, <br />which had been uplifted and tilted by mountain building to the west, was a zone of <br />clay about 6 inches thick. This clay zone paralleled the bedding planes and the dip <br />of the limestone. The clay zone was a bedding plane, as described in the <br />description of the Manitou Formation above. Originally, this clay zone was far <br />underground and probably received little, if any, seepage water that would cause it <br />to become lubricated. <br />The second portion of the figure shows the structure of the mountainside <br />that existed when Castle Concrete moved into the site. This structure was <br />apparently unknown until the early 1970's when Castle Concrete had the site core <br />drilled to determine the reserves before their formal acquisition of the site. <br />Although all that exists of those borings are the descriptive drill logs, they are <br />sufficient to construct this picture of the mountainside structure. <br />Note that in this depiction, the clay zone was now much closer to the surface <br />because of the mining of limestone for the Air Force Academy. Fracturing of the <br />surface during that mining also opened additional small conduits all over the <br />mountainside and this allowed more seepage of moisture into the clay zone. Since <br />Castle Concrete began mining the site, three major slippages occurred prior to <br />1994. Two of them occurred in the mid 1970's and the last one, below the South <br />Peak, occurred in January 1993. All three slippages likely occurred because the <br />clay zone became softened with seepage water. <br />Wetting of the clay layer likely reduced the friction and allowed sliding of <br />major blocks of limestone above the clay zone. In all three historic cases, the <br />volume of rock that slid is best measured in hundreds of thousands of tons. <br />• <br />TRANSIT MIX AGGREGATES 14 <br />PIKEVIEW QUARRY SLOPE <br />CTLIT PROJECT NO. CS17341-125 <br />S:1CS17000-174991CS17341.0 0 011 2 512. Reports1CS17341-125-R1-Final.doc