CORRESPONDENCE_1992-1993 - Laserfiche WebLink

Home Browse Search

Mr. Horton Page 3 Clay Cap Construction Design - The lack of adequate clayey soils at Harney Lane requires the County to mix local soil with imported clay to produce an admixture capable of passing the permeability tests. The CQA plan provides two options to perform the mixing. one is to spread the imported clay over a thick lift of native soil, mix with large rotor-tilling equipment, then compact in place. The alternative mixing method is to use a mixing table arrangement where the two soils are mixed in a specified area (the 'mixing table' ) then transported to the cap area for compaction. As described by Kleinfelder in the CQA plan, the in-place mixing process raises serious doubts regarding the County's (or any subcontractor's) ability to achieve a homogeneous, low permeability cap and provide adequate construction documentation. For the reasons described below, the Regional Water Board requests that the County prohibit the use of in-place mixing and instead use a mixing table system to prepare the clay admixture. Regional Water Board staff have observed construction of a clay barrier layer at another landfill using rotor-tillers. In that case the mixing process resulted in a heterogeneous mixture of clay clods and native soil. This was due to an attempt to mix Ione clay and native soils at a moisture content which was too high for rotor-tillers to be effective mixers. The goal of mixing is to achieve a homogeneous distribution of the clay and the native soil. Moisture Content - Moisture content is specified at three percent (3%) over optimum. Kleinfelder indicated that three percent (3%) over optimum at 95% compaction will result in a liner with acceptable permeability. A range of acceptable moisture contents must be specified. Otherwise, CQA personnel will not know when clay is in an acceptable condition for installation. The rationale for this range must also be given. Borrow Area Soil Tests - The CQA plan includes a summary of permeability tests on borrow area soils mixed with Ione clay (Table 2-1) . The Regional Water Board Staff is concerned that these laboratory permeabilities are so close to the regulatory requirements that small variations in mixing ratios, moisture content or compactive effort in the field may cause the finished liner to fail CQA testing. As Kleinfelder notes, there is not much cushion in the lab test results. Experience indicates typical construction techniques result in field permeabilities that are 10 to loo times higher than lab results. Given this relationship, a conservative interpretation of the results shown on Table 2-1 suggests that only one of eight samples tested (Sample 6) will meet the field permeability requirement. Kleinfelder recommends increasing the imported clay content from 15% as used in the laboratory tests to 20% in the field. This is a good recommendation, however, no test data