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KLEINFELDER <br /> File No. 24-2206-O1.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 li her over compacted <br /> native materials. <br /> ° Ccnstruction of a one fcct thick low <br /> ce=eabillty "clay" l..ner us4n- clay from an <br /> off-site source mixed with native soil. <br /> ° Construction of a one foot thick "clav" liner <br /> using commercial additives . <br /> HDPE Diner 017er Native Ma tsrial <br /> We understand that a plastic liner underlain by soils with a <br /> coefficient of 1 X 10-4 or 1 X 1-0-5 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 />