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<br />the discontinuities without significant rock breakage. Within this lower range the rock <br />effectively has high friction and no effective cohesive strength. Conversely, at high stress <br />' levels the rock mass strength is based on a combination of friction along the discontinuities <br />and strength of the parent rock material due to shearing through irregularities. <br />Rock mass strengths used in the calculations were based on a combination of historical data <br />for similar rock masses in similar conditions, and on back-calculations within the failed area. <br />Based on historical data the rock mass strength is expected to have a friction angle of 45 <br />' degrees at low stress. At high stress the rock mass is expected to have a cohesion of 1,000 to <br />2,000 psf and a friction angle of IS to 35 degrees. Based on these data, a friction angle of 25 <br />degrees with varying cohesive strengths was used. <br />' Using these strength ranges as a base, back-calculation of the rock mass strength within the <br />active slide area using XSTABL (see below) resulted a strength envelope defined by 45 <br />' degrees at !ow stress, and 1,000 psf cohesion with a friction angle of 25 degrees at high stress <br />levels. These strengths were used in subsequent calculations of the stability of alternative <br />slope configurations. <br />Slope stability calculations were performed using the computer program XSTABL version 4.1 <br />marketed by Interactive Software Designs, Inc. This program is the most recent version of <br />STABL which was developed at Purdue University. The program uses the method of slices to <br />' calculate slope stability safety factors for randomly generated failure surfaces. The. surfaces <br />can be specified to have any one of several different user-specified shapes with the beginning <br />and termination points falling within user-specified ranges. The program also allows for use <br />' of alternative non-linear material strength envelopes. Results of computer analyses of slope <br />stability are included in Appendix B and summarized below. <br />' D. Re ults <br /> <br />1 <br /> <br />1 <br /> <br /> <br />Using XSTABL and the rock mass strengths presented above, the safety factor against sliding <br />was calculated for alternative slope angles. Results are shown in the following table: <br />51ope Angle Slope Grade Safety Factor <br />(Degrees) (H:V) <br />40.6 (existing) 1.167:1 I.0 <br />35.4 1.41:1 1.17 <br />32.7 1.56: I 1.25 <br />30.3 1.71:1 1.34 <br />28.0 1.88:1 1.47 <br />Based on These results and using a design criteria of 1.25, it was determined that an overall <br />slope grade of approximately 1.5 H:l V or (latter is required for the interim slope. The <br />interim slope was, therefore, designed with typical benches 60 ft wide with 40 ft highwalls. <br /> <br />