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Mining Plan <br />Historical Siding - Figure MP-6 shows diagrammatically the cause of the • <br />historical sliding that has occurred at this site and why, in this plan, it will no longe;r be of <br />concern. <br />The first portion of Figure MP-6 shows the general structure of the mountainside as it <br />probably existed before any mining. Within the limestone layer, which had been uplifted and <br />tilted by mountain building to the west, was a zone of clay about 6 inches thick. This clay zone, <br />of course, paralleled the bedding planes and the dip of the limestone. In effect, the clay zone <br />was a bedding plane. Originally, this clay zone was far underground and probably received little <br />if any seepage water that would cause it to become slippery. <br />The second) portion of the figure shows the structure of the mountainside after the <br />completion of the mining for the Au• Force Academy. This was the structure that exist<xi when <br />Castle Concrete moved into the site. This structure was apparently unknown until the early <br />]970's when Castle Concrete had the site core drilled to determine the reserves before their <br />formal acquisition of the site. Although all that exists of those corings are the drill logs, that is <br />sufficient to construct this picture of the mountainside structure. <br />Note that the clay zone is now much closer to the surface because of the mining of <br />limestone for the Air Force Academy. Fracturing of the surface during that mining also opened <br /> <br />small conduits all aver the mountainside and this allowed more seepage of moisture into the clay • <br />zone. <br />Since Castle Concrete began mining the site, three major slippages have occurred. Two <br />of them occurredut the mid 1970's and the last one, below Peak 3 (see Exhibit C), occurred in <br />January 1993. All three slippages occurred because the clay zone became soaked with seepage <br />water. This reduced the friction and allowed sliding of major blocks of limestone above the clay <br />zone. In all three cases the volume of rock that slid is best measured in hundreds of thousands <br />of tons. Always, the thickness of the sliding block was about 20 to 30 feet and covered surface <br />areas of from two to five acres. <br />In the latest: slippage, which was the largest, approximately 500,000 tons of rock about <br />25 feet thick slid about 400 to 500 feet down the slope. This slide was the direct result of a <br />combination of road building across the land that slid and a prolonged, high volume rain about <br />four months before the slide. The rain was produced by the remnants of a hurricane carried <br />northward on a monsoonal flow from the Gulf of California. <br />Careful examination of the slide by geotechnical engineers concluded that if part of the <br />material that slid could be removed most of the future hazard of additional sliding would also <br />be removed. A monitoring system was set up across the slide and after removal of the upper <br />several feet of the slide material no subsequent movement has been seen. The removal process • <br />Page 20 Pikeview Quarry Amendment ExhibR D <br />