2001-12-11_PERMIT FILE - C1981019 (6)

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0 • Company, dated December 1990) designate the east slope as Torriorthents and Borolls Rock Outcrop Complex with topsoil depths of 0 to 8 inches. The west valley slope is mapped as Work Loam with a topsoil thickness of 25 to 45 inches. No evidence of seepage or springs was observed on either slope at the time of our site visit on May 2, 1995. No evidence of creep or previous slides was observed on the east or west slopes in the vicinity of the proposed pond. There are slides on both the east and west slopes of Taylor Creek, north of the site. Soil/Bedrock Conditions Two borings (TH-7 and 7A) were drilled near the location of the proposed embankment (Fig. 1) as part of a previous investigation performed by our firm {Geotechnical Investigation, West Pit Spoil Disposal Site, Job No. 16,516, dated March 1, 1991). Logs of the borings, including partial laboratory test results, are presented in Appendix A. The borings indicate sandy clay soils to a depth of 28 feet near the center of the valley. Groundwater was found in the boring at a depth of 21 feet during drilling in October of 1989. The clay soils were stiff to very stiff above the water and medium stiff below. A consolidated-undrained triaxial test wRh pore pressure measurements was performed in the previous investigation on a sample of clay from boring TH-1 at 2 feet. The test results indicated effective strength • parameters of q~ = 15°, cohesion C = 500 psf. This test sample had 80 percent fines (passing No. 200 sieve), a liquid limit of 31 and a plasticity index of 14 percent. These values are comparable to the test values for samples from borings TH-7 and 7A which indicates the strength values should be appropriate. No highly permeable sand or gravel seams were noted in the drill logs, but the possibility of thin seams of more permeable soils within the alluvial clay deposits should be considered. The thickness of overburden clay soils diminishes rapidly toward the valley side slopes with the east slope consisting of exposed bedrock, and the west slope where 2 to 4 feet of soil cover is anticipated. The bedrock consists of interlayered sandstone, siltstone and shale. The general dip of the bedrock in the West Pit areas is about 4 to 5 degrees to the north- northeast. Observation of the bedrock layers in the wall of the West Pit excavation indicate localized undulations occur. The undulations combined with the shallow dip angle could result invariable dip directions in localized areas. The exposures in this area were relatively weathered and eroded. Typically blocks of the exposed rock had slumped slightly in the downhill direction. Observations of the exposures on the east slope indicate the dip is towards the north, but visual observations could not positively confirm if the dip direction was NNE or possibly NNW. A previous slide occurred on the north-northeast facing slope at the West Pit Sedimentation Pond (Job No. 23,001 dated May 5, 1995). This slide appeared to . occur on a claystone layer with an unfavorable dip direction out the slope. Seepage, possibly blocked off by a topsoil pile, is believed to be the primary cause which initiated the slide. The unusually wet spring weather this year likely contributed. The COLOWYO COAL CO.LP. CTL? 27,OB52