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7 applied to the entire landfill surface. However, consideration should be given to developing soil stockpiles on the existing landfill during the operation of the landfill expansion to provide for preloading consolidation. Also, dynamic compaction should be considered in local areas where the effects of, settlement many be more critical such as along leachate collection trench alignments. 5.0 SETTLEMENT OF HORIZONTAL EXPANSION LINER SUBGRADE The final analysis concerns the potential settlement of the geologic subgrade underlying the horizontal expansion (Phases I - IV) landfill areas.. The underlying geology of the landfill generally consists of lenticular units composed of clay, silt, and sand with minor amounts of gravel. The lenses range from a few inches to approximately 20 feet in thickness. The field investigation soil report by Twining Laboratories, Inc. for the Austin Road Landfill, October 24, 1991, showed four soil borings in the area of Phases I, II, and III. Each of these borings shows a layer of lean clay starting approximately 10 to 15 feet below the surface. Deeper soil boring and monitoring wells in the area of the vertical expansion show lenses of silty clay, sand and minor amounts of clay. Because the subsurface data is inconclusive on the nature of the geology below the horizontal expansion phases, the following assumptions were made to model the potential settlement of the horizontal expansion landfill liner. ■ The subsurface geology below the liner is assumed to consist of a 10 -foot clay layer underlain by a 30 -foot silty clay layer. ■ Primary consolidation coefficient (Cc) was estimated from the liquid limit (LL) index for the clay and silty clay. Cc for the clay layer was 0.22, Cc for the silty clay was 0.09. ■ Critical area of analysis was assumed to be the leachate collection trench alignment because of the 196' design drainage slope. ■ In-place refuse/daily cover density was assumed to be 70 lbs/ft ^ 3 Based on the settlement analysis, up to 1.7 feet of settlement might occur due to subgrade consolidation due to refuse filling through the life of the landfill. Final drainage grades in the leachate collection trench range from 0.7% to 0.97o compared to the starring design grade of 1901. It should be emphasized that the results are based on rough assumptions concerning the subgrade geology and the consolidation coefficients for the subgrade soil. It is recommended that additional geotechnical field work and settlement analysis be conducted to determine the settlement characteristics of the subgrade prior the final design of the horizontal expansion phases. WW-1811-CAl-AA/3110 Austin Road Landfill Expansion August 2,1995 M0033.DOC Landfill Settlement Page 7