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SHANNON &WILSON, INC. <br />We considered a lower bound spoil shear strength estimate of a friction angle of 30 degrees with <br />no cohesion. A density of 110 pounds per cubic foot (pcf) was assumed. <br />6.2.2 Clay Layer <br />The clay layer below the valley floor consists of clay with occasional clayey sand seams. <br />Laboratory testing of clay index properties are summarized on Table B -1. Testing of four <br />samples of clay indicated 63 to 76 percent fines, Liquid Limits (LL) of 31 to 38 and Plasticity <br />Indicies (PI) of 16 to 21. <br />One sample of clay from a Shelby tube sample from boring C -104 (depth 24 to 26.5) was <br />selected for triaxial consolidated - undrained (CU) strength testing with pore water pressure <br />measurements. The results of the CU test are summarized on Figure 7. The results of the CU <br />test, along with the results from direct shear testing from previous investigations, are presented <br />on Figure 8. <br />We also considered Plasticity Index test results to utilize published correlations of <br />Plasticity Index to drained shear strength (Abramson, 1996). The PI test results from previous <br />direct shear testing combined with results from this study range from 16 to 25. These values <br />correlate to drained shear strength values friction angles of about 30 to 32 degrees. This <br />compares well with the results of the CU test. <br />Based on these factors we selected a drained shear strength envelope for the clay layer of <br />31 degrees to evaluate the long term stability of the spoil fill. We also analyzed a lower bound <br />strength envelope with a friction angle of 24 degrees and cohesion of 370 psf. <br />6.2.3 Carbonaceous Mudstone / Clay Shale <br />Our evaluation of the shear strength of the CMS /CS layer is based on previous testing by <br />CNI and S &W which was performed as part of studies regarding rock slope stability for the <br />South Taylor mine area and the Collom mine area. Test results are summarized on Figure 9 <br />along with references to the previous studies. The 27 test results represent samples obtained <br />from variable stratigraphic levels at South Taylor and the Collom mine areas. Considering <br />depositional factors and our experience with the rock conditions at the Colowyo mine, it is our <br />opinion the tests provide a reasonable basis to estimate the shear strength of a potential CMS /CS <br />layer below the spoil. <br />23 -1- 01101- 400- R1.docx /wp /lkn 23-1-01101400 <br />Exh. 23 -1 -16 Revision Date: 08/05/09 <br />Revision No.: PR -03 <br />