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_~ ~~~ Ms. Catherine W. Begej October 31, 1991 Page Two • With the possible exception of active working benches, actual stockpile sideslopes have been established at 3H:1V or greater compared with the 2H:iV slopes used in stability modeling. • Refuse material is being placed in lifts consisting of a lift of fine material overlain by one or more lifts of coarse material as opposed to the original assumption of a 3:1 mixture of coarse and fine refuse. • The proportional mix of coarse and fine refuse varies dependant on changes in geologic and mining conditions. • Given current layered fill construction methods, the potential for circular failure is significantly reduced and a sliding block failure through the fine refuse material may be the more critical failure mechanism. • Actual strength testing of fine refuse materials indicates a much higher effective friction angle, at the densities documented by field sampling, than assumed for previous stability modeling. Given these variances between actual placement practice and the design basis, and in consideration of the Division's concerns, EFCI proposes to evaluate refuse stockpile stability using current placement practices as the design basis. Since current practice involves placement of refuse as a layered fill, the overall proportion of coarse and fine refuse can vary, and the fine refuse is the lower strength material, the stability evaluation will focus on the fine refuse layers as the primary structural units influencing overall stability. EFCI has material strength values from laboratory testing and in-place density data from field testing for the fine refuse material. It is not anticipated that additional field evaluations or laboratory testing would be required. EFCI proposes to complete a stability evaluation for current refuse placement practices based on the following assumptions and design factors: • Maximum refuse stockpile height of 75 feet; • Maximum long-term sideslopes of 3H:1V; • Placement of alternating lifts of fine and coarse refuse, with 2 foot maximum lift thickness and compaction to at least 90 percent of maximum dry density (ASTM D-698);