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,, ~~ Memo to Larry Oehler August 6, 1993 tailing surface, or some other acceptable method. Monitoring stations should be established at one hundred foot intervals along the axial length of the dam, and consolidation should be measured daily until movement levels off. Control stations should also be established on the upstream face of the main embankmerit, adjacent to half of the stations founded on tailings (i.e. ever 200 feet). Any significant consolidation, or differential consolidation, may necessitate re-compaction of materials subjected to unacceptable strains. LIQUEFACTION AND STABILITY ANALYSES The SRK memorandum dated July 28, 1993, presents arguments regarding the adequacy of a minimum factor of safety of 1. 1. The logic behind the arguments is that conservative values $or material strength parameters were utilized as inputs to stability analyses, and that if more realistic (less conservative) material strength parameters are utilized, a higher numerical factor of Safety would result. The Division was aware of the derivation of tine material strength parameters utilized in the calculations when design changes to provide a higher factor of safety (1.3) were requested.. In the absence of a statistically significant number ofl laboratory tests of material strength properties, it is only ,prudent to utilize conservative, or even very conservative strength parameters for typical materials, that can be fond in the literature. This is particularly true for the shearin resistance at the soil/liner interfaces within the system, as rec t research has indicated that shearing resistance at these interfaces is more complex than was previously assumed, and is depende t on such factors as soil water content, density, plasticity, angularity, and uniformity, as well as the normal stress applied to the interface and the flexibility and elasticity of the geomembrane, ~n addition to its surface roughness. Simply utilizing less conservative input parameters in order to provide a higher factor of safety, in the absence of laboratory test data, is not acceptable in this case. More compelling, are the arguments presented in the SRK memorandum regarding the buttressing effect provided by tailing placement subsequent to installation of the upstream raise. The mass of tailing placed against the upstream face of the raise will provide a factor of safety in excess of 1.3 prior to the point that there would be any release of tailing in the event of any failure of the dam raise. For this reason, the Division can accept the design, as submitted, as it relates to the potential failure surfaces depicted in Figure 4.1 of the SRK design report. In reviewing item 2, under the heading "Liquefaction and Stability Analyses", contained within the SRK memorandum dated July 28, 1993, I have concluded that I was not sufficiently clear in attempting to