1993-11-03_REVISION - M1988112 (2)

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F". need for oversight by the design engineer, the Division, and now the third party engineer. COMPACTED SOIL LINER (Liner Sub-Base) BMRI 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 permeability testing below). (see discussion of BMRI has modified the method for tying the Phase II sut~-base into the Phase I sub-base, and is placing the Phase II sub-base directly adjacent to the Phase I sub-base. The only 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. BMRI must Arovide 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 (BMRI is committed to removal of rocks larger than 3/4 in. 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 tkle soil liner. Since the time of the division's inspection, a more sophisticated rock removal method has been implemented, however, this has not been done until after a significant percentage of tkle Phase II soil liner, not to mention all of the Phase I liner, has been covered by geomembrane. 3. As mentioned previously, it is usually preferable to compact soil wet of the optimum moisture content to achieve minimum hydraulic conductivity. The soil liner at San Luis is being compacted dry of optimum. BMRI points to the low permeabilities measured (one in situ permeability test per acre of soil liner) for approved soil liner as the critical parameter that assures adequate soil liner performance.