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<br />' developed by the slurry, combined with the filter cake formation on the trench walls, provided <br />adequate support to hold the trench open with vertical walls. <br />The soil-bentonite backfill was monitored at the mixing location using a slump cone. Backfill samples <br />were collected for further laboratory testing, including moisture content and gradation. The field and <br />laboratory testing program monitored the backfill material's slump, moisture content, and gradation <br />(specifically fines content). Non-passing field test results resulted in construction directives which <br />specified corrective actions that were carried out immediately. Corrective actions included <br />addition/reduction of slurry added to backfill material, addition of imported fines, and adjustment to - <br />the mixing operations. <br />Visual inspection of excavated soils and bedrock were conducted during trenching to evaluate when <br />adequate bedrock was encountered. The trench depth and bedrock key depth were monitored using a <br />weighted string-line marked atone-foot intervals. Depth to bedrock and total trench depth are labeled <br />on Plate t. The trench met minimum bedrock penetration requirements of three feet in all areas <br />except for two. From Station 44+90 to 45+60 and from Station 79+25 to 79+50, a high strength <br />~~ sandstone bedrock limited key depth from six to 12 inches. <br />DISCUSSION <br />Slurry wall construction began in early September, 1997, and was completed in mid November, 1997. <br />Due to minor mechanical breakdowns and weather, slurry wall construction was delayed temporarily <br />on occasion. <br />Test holes were drilled along the proposed slurry wall alignment prior to construction. Data collected <br />during the pre{onstruction geotechnical investigation was used as a reference during construction. <br />Actual bedrock depths and total trench depths observed during construction are illustrated on the as- <br />builtconstruction drawing (Plate 1). <br />The Komatsu PC 650 LC trackhoe used to excavate the slurry trench maintained adequate power and <br />' vertical reach to excavate the slurry trench with a minimum three-foot key into bedrock with the <br />following exceptions. A sandstone cap-rock was encountered at Stations 44+90 through 45+60. <br />This cap-rock was located at the contact between the alluvial and bedrock deposits. The trench was <br />excavated to refusal in this area. Despite an exhaustive effort to excavate athree-foot key, including <br />replacement of bucket teeth, the key-way depth was excavated to only six inches in this area. <br />Similarly, a high strength sandstone bedrock was encountered at Stations 79+25 through 79+50 <br /> <br /> <br /> <br />