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i <br />`.• vary from amatrix-supported material that behaves like a cohesive soil, as described above, to <br />~ a granular, but not free-draining soil, where nested oversized materials are in contact with each <br />other. Since the slide mass material will necessarily be variable, the best course of action is to <br />assume worst-case strength conditions, which are represented by the clay matrix in the long <br />term. Further, for over-consolidated clays, consolidated-drained (CD) testing is often specified. <br />CD testing is exceedingly slow for these types of materials. Our approach was to conduct W <br />tests, review the results, and determine if CD testing would likely obtain different or more <br />conservative results. We decided that the W test results were satisfactory. <br />Bedrock: Siltstone-shale bedrock of the Mancos Shale was encountered in Boring 7 at a depth <br />of 72 feet. The hole was drilled with mud-rotary methods. A drive sample of the material was <br />a dusky yellow-gtay color, very hard, and dry to slightly moist. Borings 18 and 23 penetrated <br />. sandstone, shale and coal bedrock from the Mesaverde Formation at depths of 90 and 80 feet, <br />respectively. NX wireline coring was done in the bedrock zone of Borings 18 and 23. The core <br />was predominantly hard, thickly siltstone and sandstone with some cemented (calcareous) shale <br />and 1-3 foot-[luck, good quality coal layers. Occasionally, a layer of highly plastic clay less <br />than a foot thick was noted, which would explain the poor stability of rock slopes in the area. <br />The plastic clay layers are the most likely failure surfaces that moved upon increased pore water <br />pressure and strength losses that would occur from ancient earthquakes, as in the case of the <br />main slide mass, or recent water accumulation in deep joint systems, as was probably the case <br />for the recent rock slope failure east of the portal area. With the possible exception of the haul <br />road, the bedrock units at this site are unlikely to be encountered for this project. <br />• 11 <br />