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C <br />KLEINFELDER <br />File No. 24-2206-01.A06 <br />January 2, 1991 <br />possible use of the silty clay or clayey silt soils Within the <br />Phase I area, other possible alternatives are being considered at <br />this time. These include the following: <br />° Construction of an HDPE liner over compacted <br />native materials. <br />° Construction of a one fcet thick low <br />permeability "clay" liner using clay from an <br />off-site source mixed with native soil. <br />° Construction of a one foot thick "clan" liner <br />using commercial additives. <br />HDPE Linar OyS" Native Matarl ai <br />We understand that a plastic liner underlain by soils *Kith a <br />coefficient of 1 X 10-4 or 1 X 10_' cm/sec is being considered as an <br />engineered alternative to Chapter 15 requirements. We also <br />understand that the size of the current proposed unit is <br />approximately 9 acres. Construction of such a unit would not be <br />as sensitive to maintenance prior to filling as would a "clay" <br />liner, and could feasibly be constructed for up to a 20 acre unit. <br />Kleinfelder, Inc. will be developing a liner design report as part <br />of preliminary design (Task 2) of the Phase I engineering. Our <br />preliminary recommendation at this time is that an HDPE liner over <br />compacted native material may be a viable alternative for the <br />County. If the on-site soils do not exhibit permeabilities below <br />the 1X106 cm/sec criteria of Chapter 15, it is likely they will <br />attain levels of 1X104 ox1X10-5 cm/sec which, when overlain by an <br />HDPE liner, might be acceptable to the RWQCB. Additionally, some <br />coarse-grained, on-site materials may also be suitable when mixed <br />with clays from off-site sources. <br />Copyright 1991 Kleinfelder, Inc. 7 <br />