CORRESPONDENCE_1992-1993 - Laserfiche WebLink

Home Browse Search

v Mr. Horton Page 4 was provided to support their conclusion that a 20% admixture will produce a soil capable of passing the permeability requirements. Test Pad - The test pad must be representative of the materials and construction techniques that will be used in cap construction. Test pad analytical results (e.g. permeability testing) can be easily biased simply because the pad is given more attention and more compactive effort than the typical area of clay cap. To overcome this problem, the test pad should be constructed at the minimum acceptable criteria with respect to moisture content, compaction, number of compactor passes, percent admixed clay and so forth. If the test pad is built under 11average" or optimum conditions, then the test results cannot be correlated to soil compacted in the cap under less ideal conditions. In other words, the test pad parameters will represent the minimum acceptable criteria when evaluating the adequacy of clay cap construction. From the CQA report descriptions, more than one soil type may be used in cap construction. The test pad program needs to include representative samples of each soil type that will be used in final cap construction. If borrow source soil types may change as construction proceeds, each soil type should be used in separate test pads. Soil Mixing. Process - As noted above, the Regional Water Board has numerous concerns about in-place mixing and compaction during cap construction. Although not described in the closure plan, the Regional Water Board's impression is that the admixture will be compacted immediately after mixing. This would not allow sufficient time for the soil to absorb the applied moisture and reduce clod strength. A waiting period of 24 hours is recommended between adding water and compaction. The potential for small scale variations in clay content, mixing efficiency, soil type and moisture content may require increased CQA testing frequency. Clod size, soil gradation and moisture content after mixing must be verified. Any cost savings gained by in-place mixing need to be weighed against CQA difficulties and the potential for large areas of placed material to fail CQA testing. Soil gradation after mixing, must comply with Chapter 15 requirements for a CL, CH or SC classification with a minimum of 30% passing the No. 200 sieve for use in clay cap construction. Turn around time due to lab testing, e.g. grain size distribution, may slow the construction progress if in-place mixing is used. The final critical factor with in-place mixing is the proposed use of loose lifts up to 16 inches thick. The Regional Water Board has significant concerns about the ability of pad-footed, self-propelled compactors to fully penetrate lifts of this thickness.