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-8- 3. As stated in the report, the material used in file direct shea,- • testing consisted of the portion or' the sample which passed the ilo. 4 sieve. Use or` such a large percentage of slit and clay can substantially reduce the overall strength result (6 i. 4. All the samples used in 'the test ~•rere placed loosely in the shear box and leaked prier to testinn to de ermine "effective" cohesion and "effective" anole of internal friction, it should also be realized that the effective stress for a cohesionless material normally represents the lower bound strength value. In our estimation, we believe it is Guite unconservati~de to use cohesion values in the analysis of a spoil pile which consists essentially of granu- lar material. To illustrate the er""feet of cohesion on stability, ;•re re- analyzed the Colorryo slope using a cohesion of 200 psf and an angle of internal friction of 30 degrees, as presented by Huang (7). The results indicate that for a dry slope condition, (Run 1A-2-~-0) the factor of safety is 1.69. This represents a reduction in the factor of safety or- approxi- mately 6.5 percent. The major effect of the added cohesion is illustrated by analyzing the ground ~.vater conditions. For the ~,vorst case condition (the water table elevation is 250 feet above the bedrock, Run IP.-2-2-I), the calculated factor of safety was found to be 1.21. This represents approxi- mately a 40 percent increase in the factor of safety over that obtained using a friction angle cf 37 degrees ir.d no coesion. In paragraph ?, concern is stated that zones of fine fraction material • could essentially trap vrater and create a "perched" crater table condition. In our opinion, this is relatively unlikely. The gradation of the spoil is such tt;it estimated ;~~inimum permeabilities on the order or 1,000 feet per year dr2 2XpeCte d. ?erm2abilities pT tnl5 mdgnitUCe normally ind;Cate a relatively free-draining material. At the toe of eaci~ lift,. the permeabilities are estimated to be in excess of IOG,000 feet aer year. The precipitation in the !9estern Slope areas is considerably less than the average annual evaporation ~,vhich creates a semi-arid climate. Furthermore, current reclama- tion plan _= call for the placing of up to 3 feet of topsoil over the fill which trill consist essen*_ially of law permeability clays. The clay<_ will act essentially to seal the embankment aeainst moisture infiltration. These precautions, plus tfre high permeability of the fill, coupled arith the drain- age plans to direct surface runoff a~•ray from the fill should considerably reduce the probability or- a water table developing. The final comment made in the i1LRB letter refers to the underground work- ings and their effect on the stability. As stated in our original report, subsidence o-rill usually take place crithin a range of 5 to S years after the completion of mining and the pillars are pulled. The mines were abandoned many years prior to placement of the Streeter Canyon fill. Golder Associates report (.2) comments "Collapse of the underground :vine due to tie surcharge by the dump in the critical area of the Streeter Canyon itself is extre!nely unlikely". The report continues, •