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<br />The soil-bentonite backfill will be mixed on a separate mixing pad or alongside the slurry <br />trench to improve consistency of the mixture. The backfill material will be prepared so <br />that all soil particles will be coated with slurry and clods will be broken down as <br />appropriate for the mixture <br />Material excavated from the trench is not expected to meet the above requirements for <br />backfill. Excess sand and gravel excavated from the slurry trench will need to be <br />"wasted" and not included in the final backfill mix. The "wasted" sand and gravel shall be <br />replaced with approved supplemental clay soils. Supplemental clay soils may have to <br />be stockpiled along the slurry wall alignment as directed by the Engineer. Samples of <br />the borrow material to be used in the backfill will need to be tested well in advance of <br />slurry wall wnstruction to insure an adequate supply of acceptable material is on hand. <br />The five test running average permeability shall be 5x10"' cm/sec, or less. The <br />maximum permissible value shall be 1x10b cm/sec. Laboratory results of proposed <br />slurry-backfill materials along the proposed slurry wall alignment has indicated an <br />addition of dry bentonite at a rate of up to 3.5 percent of the dry weight of backfill <br />material may be necessary to obtain an adequate permeability rate. <br />4.6 Key Trench and Sluny Wall Cap <br />The slurry wall trench shall be a minimum of three feet into the underlying shale bedrock <br />formation. If the shale is weathered, fractured or contains sand seams, the depth of key <br />shall commence from the first competent bedrock encountered. Trench cuttings and <br />observation of excavation difficulty shall be compared to exploratory drill logs to <br />determine the depth of competent bedrock. Depth measurements shall be made every <br />twenty feet, to ensure that the minimum key depth is achieved. The toe of the trench <br />excavation shall be a minimum of fifty feet from the toe of the backfill toe, to allow <br />suitable distance for depth measurements and cleaning of the trench bottom if needed. <br />The Applicant shall place temporary slurry wall protection of uncompacted fill along the <br />backfilled trench, within one day of backfilling. <br />4.7 Post Construction Test <br />