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Mr. Bryce Romig Page 2 July 25, 1996 <br /> During the construction of the cut-off wall, field conditions were encountered which <br /> required modification of the original design. The design modifications are outlined our <br /> letter to Climax dated January 30, 1996. A copy of the letter is attached for ready <br /> reference. <br /> While excavating the cut-off wall trench, it was determined that the bedrock surface was <br /> highly variable with alternating ridges and valleys of hard (competent) and soft <br /> (weathered) bedrock. The ridges of hard bedrock were found to be very competent and <br /> could not be cut using a conventional trencher. (An example of one of these ridges is <br /> shown in Photograph 4.) <br /> Due to the difficulty in trenching the keyway into the competent bedrock ridges, the <br /> original design was modified to include an excavated bedrock keyway where possible; <br /> where the bedrock could not be cut, the keyway would be replaced with a 24-inch-wide <br /> reinforced concrete footer constructed directly onto the competent bedrock. The 24-inch <br /> footer was also extended to cover the entire length of the cut-off wall. The purpose of the <br /> footer was to provide a positive seal against the surface of the bedrock as well as to <br /> provide a stable platform for construction of the 8-inch reinforced concrete cut-off wall. <br /> To provide a positive seal between the 24-inch footer and the 8-inch cut-off wall, a PVC <br /> Water-Stop seal was placed between the footer and the wall. Photograph 5 shows a view <br /> inside the 24-inch footer form and the condition of the bedrock. Photograph 6 shows the <br /> footer after the forms have been stripped. This photograph also shows the PVC Water- <br /> Stop seal prior to construction of the cut-off wall. <br /> During the curing of the 24-inch footer, several small vertical cracks formed. The cracks <br /> generally formed where the footer section was the thinnest (i.e., along the bedrock <br /> ridges). These cracks were inspected on April 4, 1996, by TEC personnel. (Photograph 7 <br /> shows one of the footer cracks.) The footer cracks were repaired by coring a vertical hole <br /> along the axis of the crack and injecting a lean grout under pressure to fill the crack void <br /> and to provide a positive seal. A letter dated April 10, 1996, detailing the sealing <br /> procedure is attached. <br /> The 8-inch reinforced concrete wall was formed and poured after the 24-inch footer cured <br /> and the cracks were sealed. Photograph 8 shows a view of the 8-inch reinforced concrete <br /> cut-off wall with the forms removed. After the forms were stripped from the cut-off wall, <br /> three small vertical cracks were observed in the 8-inch wall. These cracks were visually <br /> inspected on May 7, 1996. It was noted during this inspection that the cracks generally <br /> occurred at the seams between two form panels. Photograph 9 shows one of the observed <br /> cracks. The cracks were sealed using an elastomeric sealant (shown in Photograph 9). In <br /> addition, a panel of CLAYMAXT" was placed over the crack to complete the seal. <br /> The perforated drainage collection pipe was placed in the excavated trench upgradient of <br /> the cut-off wall and attached to two collection sumps. Photographs 11 and 12 show the <br /> - - - - -- • <br /> CTROJECTSl5101-004U.152510I 004 - - ----Environmental <br />