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i• L The site is in an alluvial valley setting. Geotechnical test holes were drilled along the proposed slurry wall alignment prior to construction, which indicated a bedrock low in the southwest corner of the site. This work was performed by Tetra Tech RMC in February of 2005. Observed drill cuttings and soil samples, soil types, and depth to bedrock measurements were used as reference during construction. Test hole locations and geological exploration finding of each test hole and test pit are presented on the summary logs in the original plan set (Appendix C). Depths to bedrock and total trench depths observed during construction are illustrated on the as-built construction drawing (Plate 1). The natural site stratigraphy generally consists of three main units: 1) near surface sandy clay with localized spots of silty clay, 2) sand and gravel alluvial deposits, and 3) claystone bedrock. Depth to bedrock is variable throughout the site. Bedrock depths ranged from 23 to 49 feet with an average depth of 30.8 feet. Trench depth worksheets are provided in Appendix B. In general, the depths to bedrock were greater in the southeast comer of the property. The slurry wall as-built dimensions are as follows: an average width of 3.5 feet, total trench depth 23 to 49 feet with an average value of 30.8 feet, overall slurry wall length of 6,565.7 feet, and overall curtain area of approximately 205,178 square feet. The total area encompassed by the slurry wall is approximately 58.8 acres. CONSTRUCTION SUMMARY The typical slurry wall construction crew consisted of five to seven men managed by one superintendent who oversaw and coordinated construction activities. Motorized equipment included a Komatsu PC 1100 trackhoe excavator, which excavated the trench through a water/bentonite slurry; a smaller trackhoe that provided initial soil-bentonite backfill mixing, trench backfilling, and slurry mixing pond construction; one Caterpillar dozer that provided final soil- bentonite backfill mixing, trench backfilling, and slurry mixing pond construction; a Caterpillar scraper which was used to move dirt to prepare level ground surfaces for the track hoe; two conical static mixer for preparing a bentonite slurry; two pumps used to recirculate slurry stored in the ponds and deliver slurry to the trench; and two stand alone pneumatic trailers for on-site storage of bulk bentonite. A slurry pump/desanding unit was positioned along the side of the active trench to clean the trench bottom and reduce sands suspended in slurry. Prior to construction of the slurry wall, berms were constructed along the inside of the slurry wall alignment. These berms consisted of overburden soils from on-site, as well as dry fine-grained bentonite delivered from offsite. The granular bentonite was added at a rate of approximately 1 percent of dry weight of the backfill soils. In the last approximately 600 feet of berm (147+50 to 155+50) the bentonite was reduced to 0.5 percent. A slurry mixing/holding pond was constructed near the center of the site. A reservoir of hydrated slurry was produced by combining and mixing bulk bentonite powder with water using a conical static mixer. Groundwater was pumped from an on-site sump to the slurry pond and used for making the slurry. The slurry in the holding pond was continuously circulated until thoroughly hydrated. The finished slurry product was pumped through afour-inch high-density polyethylene pipe to the open trench section under construction. Pond re-circulation and slurry delivery was accomplished using aneight-inch pump. - 2 - September 2007 Heit Construction Report