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1100318 R0T0 0811 JRW1 29 <br />5.2.3 Geologic Characterization <br />The results of the field mapping and two phases of subsurface investigation carried out at <br />Pikeview Quarry provided insights into the geology of the site that were relevant to <br />understanding the cause of the Pikeview Landslide that commenced on December 2, 2008. Key <br />findings include the following: <br /> Data from the core borings indicate that the weakest materials in the slope are <br />clay beds present in the upper, fine-grained member of the Sawatch <br />Formation. These beds were encountered in core borings EXC-1 and EXC-4 <br />located at the crest of the quarry, and in EXC-2and trench TR-1, located in <br />Area H. The uppermost clay bed was encountered at a depth of 80 feet in <br />EXC-2. Cross sectional reconstructions of the slope indicate that the western <br />edge of the Area H excavation extended to within about 20 feet of this <br />interval (see cross section D-D', Appendix 39). To the south, stratigraphic <br />reconstructions indicate that the upper clay bed is located closer to the <br />surface (see cross section A-A', Appendix 39). As mining progressed, the <br />clay bed was likely exposed in the quarry wall at the south end of the quarry <br />prior to the placement of fill in this area circa 2008 (see cross section E-E', <br />Appendix 39). Two important observations are relevant to this likely <br />exposure of the clay bed. First, the clay bed was an unknown condition that, <br />based on its small size (thickness) would have been easily obscured and <br />difficult, if not impossible, to identify during mining operations. Second, <br />based on the reviewed documents and analysis, the mining of Area H had <br />progressed substantively according the approved Mining Plan. <br /> These observations, coupled with the observation that one of the clay beds <br />was present at the base of landslide debris in Trench 1, indicates that the clay <br />beds in the Sawatch Formation formed the plane of weakness along which <br />the Pikeview Landslide occurred. The location and geometry of the Pikeview <br />Landslide indicate that the failure was initiated when a sufficient length of <br />the overlying limestone material, which was acting to buttress the slope <br />above the clay beds, was removed during the implementation of the approved