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~~ ~~ Mr. Steve Magnuson <br />May 6, 1987 <br />Page 2 <br />analyses on the same: specimen before and after the Proctor test, and one • <br />triaxial test. Testing was conducted at the Lincoln-DeVore, Inc. laboratory in <br />Colorado Springs, Co rrado. <br />The test results indicate that the Standard Proctor dry density of the refuse <br />is approximately 90 {~cf with an optimum moisture content of approximately 11 <br />percent. A small amount of particle breakdown was measured before and after <br />the Proctor test (4ihich resulted in a slight breakdown in gravel-sized <br />particles). <br />A staged triaxial test under consolidated, undrained conditions (with pore <br />pressure measurement's was conducted on a specimen remolded to at least 90 <br />percent of Standard Proctor density and at optimum moisture content. The <br />specimen was saturated, then consolidated and sheared at confining pressures of <br />3.6, 7.2, and 13.1 ksf. <br />The effective shear strength reported by Lincoln-DeVore from this test was an <br />angle of internal friction of 32 degrees and 0.10 ksf cohesion. Evaluation of <br />the stress path diagnim from the test by WWL (Figure 2) resulted in a slightly <br />different shear strength (30 degrees with 0.66 ksf cohesion), and was used in <br />the stability analyser. Comparison of these shear strengths is shown in Figure <br />3. , <br />METHOD OF STABILITY ANALYSIS <br />Stability analyses were conducted along a two-dimensional section through the . <br />waste pile in its cortipleted configuration. The section used in the previous <br />analyses (shown in Figure 1) was used as the critical section for slope <br />stability. <br />Slope stability was evaluated by the modified Bishop method (Bishop, 1955), <br />which is a..widely used method and generally yields canservative results (Huang, <br />1983). Since the modified Bishop method employs circular failure surfaces, <br />confirmation analyses were conducted using the Janbu method for non-circular <br />surfaces (Janbu, 1973), in the event that non-circular surfaces would represent <br />more critical stabiiity conditions. Factors of safety calculated with the <br />Modified Bishop method were slightly lower than those calculated with the Janbu <br />method, and results presented in the remainder of this letter are from the <br />Modified Bishop method calculations. <br />The slope Stability calculations were actually performed by the STABL2 computer <br />program (Siegel, 1977). Modified Bishop and Janbu methods of evaluation are <br />incorporated into the: program. The STABL2 program uses a random failure <br />surface generation routine, then calculates the factor of safety along the <br />failure surface. For the results given below, the factor of safety shown is <br />the minimum of at least 100 failure surfaces generated for each case. <br />Seismic conditions were represented with pseudo-static analyses by an <br />equivalent horizontal acceleration or seismic coefficient of 0.05 g. <br />Pseudostatic analyses <ire appropriate for embankments that would not liquefy or <br />exhibit a significant loss of shear strength with seismic shaking (Seed, 1979). . <br />The seismic coefficient; of 0.05 was selected as a conservative value based upon <br />a brief review of the seismicity of the site area from published literature <br />