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Memo to Larry Oehler 4 October 24, 1993 Liner Sub-Base (Compacted Soil Linerl The operator has modified the approved specification, and is now placing the soil liner material at 4 to 6 percent dry of the optimum moisture content. This practice usually has the result of yielding relatively higher hydraulic conductivities for the compacted soil, however, in situ permeability testing of the soil liner is showing that acceptable Hydraulic conductivities are being attained using the lower moisture content. Given the practical benefits derived from placing the material dry of optimum, this practice is acceptable if the in situ permeability measwrements are confirmed (see discussion of permeability testing below). The operator has modified the method for tying the Phase II sub- base into the Phase I sub-base, and is placing the Phase II sub- base directly adjacent to the Phase I sub-base. The ohly concern with this practice would be with the bonding of the two sub-base layers. This bonding is imperative to achieve a continuous low hydraulic conductivity soil liner component for the composite liner system. To effectively bond the materials together, the Phase I sub-base should be laid back to a minimum 45 degrees, and roughened, so that the newly placed sub-base can effectively blend into the surface of the old sub-base. The operator must provide evidence that proper sub-base bonding has been achieved The compacted soil liner component of the composite liner system is not in line with a number of accepted criteria usually included in soil liner design. These include: 1. Soils selected for low permeability liners would typically have indices of plasticity (P.I.) between 10 and 35 percent. The soil liner at San Luis exhibits P.I.'s of less than 10 percent. 2. No stones or rocks larger than 1 to 2 inches in diameter should be present in the liner material (the operator is committed to removal of rocks larger than 3/4 im. diameter from the surface of the soil liner). Division inspectors noted many rocks larger than 2 inches within the borrow material placed for the compacted soil liner. The inspection and acceptance protocol described to us to remove these rocks would be effective in removing rocks from the prepared surface, but could miss rock in the lowest 2/3 of the soil liner. Since the time of the division's inspection, I understand that a more sophisticated rock removal method has been implemented, however, this has not been done until after a significant percentage of the Phase II soil limer, not to mention all of the Phase I liner, has been covered by geomembrane. 3. As mentioned previously, it is usually preferable to compact