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Ii ~ I <br />i <br />L~ <br />• As part of our evaluation, we visited the site to observe the condition of the <br />embankments. No signs of movement or distress such as cracking or toe bulges <br />were observed. No excessive erosion was observed. The eight embankments <br />appeared well maintained and in good condition. <br />Based upon our discussion with Colowyo personnel and our review of the <br />construction documentation, it is our opinion the sedimentation ponds and <br />embankments were constructed in a prudent manner in general accordance with the <br />design practices and regulations in place at the time of their construction. <br />Soil Conditions <br />The sedimentation pond embankments (with the exception of 002-Work Area <br />and 001-Streeter) were constructed across existing drainage ways. CTLIThompson, <br />Inc. has pertormed several geotechnical investigations on the mine site including <br />investigations near the East Taylor and West Pit Ponds. Soil conditions at the mine <br />site typically consist of a shallow cover of residua!/colluvial soil underlain by <br />sedimentary bedrock of the Williams Fork Formation. The bedrock consists of <br />interbedded sandstone, siltstone and shale with carbonaceous shale and coal layers. <br />In drainages, the depth of alluvial/colluvial soils can exceed 20 feet. Our experience <br />indicate these soils typically consist of clayey sand to clay soils with sand to gravel- <br />sized bedrock fragments. <br />CTL/Thompson, Inc. performed a Geotechnical Investigation for the West Pit <br />Excess Spbil disposal site, Job No. 16,516, dated March 1, 1991. At this location, <br />sandy clay soils were found to a depth of 28 feet near the center of the valley. The <br />clay soils were stiff to very stiff above ground water (depth of 28 feet) and medium <br />stiff below. This sample had 80 percent fines (passing No. 200 sieve), a liquid limit <br />of 31 and plasticity index of 14 percent. A consolidated-undrained (CU) triaxial test <br />. with pore pressure measurements was performed on a sample of clay. The test <br />f results indicated effective strength parameters of ~ = 15 degrees, cohesion C = 500 <br />COLOWYO COAL COMPANY, LP <br />STABILITY EVALUA770N -SEDIMENTATION PONOS 4 <br />CTIJr 26,753 <br />