2011-03-14_REVISION - M1977493 (54)

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• • Mr. Bryce Romig CMC001605 Page 2 July 25, 1996 During the construction of the cut-off wall, field conditions were encountered which required modification of the original design. The design modifications are outlined our letter to Climax dated January 30, 1996. A copy of the letter is attached for ready reference. While excavating the cut -off wall trench, it was determined that the bedrock surface was highly variable with altemating ridges and valleys of hard (competent) and soft (weathered) bedrock. The ridges of hard bedrock were found to be very competent an d could not be cut using a conventional trencher. (An example of one of these ridges is shown in Photograph 4.) Due to the difficulty in trenching the keyway into the competent bedrock ridges, the original design was modified to include an excavated bedrock keyway where possible; where the bedrock could not be cut, the keyway would be replaced with a 24- inch -wide reinforced concrete footer constructed directly onto the competent bedrock. The 24 -inch footer was also extended to cover the entire length of the cut -off wall. The purpose of the footer was to provide a positive seal against the surface of the bedrock as well as to provide a stable platform for construction of the 8 -inch reinforced concrete cut -off wall. To provide a positive seal between the 24 -inch footer and the 8 -inch cut -off wall, a PVC Water -Stop seal was placed between the footer and the wall. Photograph 5 shows a view inside the 24 -inch footer form and the condition of the bedrock. Photograph 6 shows the footer after the forms have been stripped. This photograph also shows the PVC Water - Stop seal prior to construction of the cut -off wall. During the curing of the 24 -inch footer, several small vertical cracks formed. The cracks generally formed where the footer section was the thinnest (i.e., along the bedrock ridges). These cracks were inspected on April 4, 1996, by THC personnel. (Photograph 7 shows one of the footer cracks.) The footer cracks were repaired by coring a vertical hole along the axis of the crack and injecting a lean grout under pressure to fill the crack void and to provide a positive seal. A letter dated April 10, 1996, detailing the sealing procedure is attached. The 8 -inch reinforced concrete wall was formed and poured after the 24 -inch footer cured and the cracks were sealed. Photograph 8 shows a view of the 8 -inch reinforced concrete cut -off wall with the forms removed. After the forms were stripped from the cut -off wall, three small vertical cracks were observed in the $ -inch wall. These cracks were visually inspected on May 7, 1996. It was noted during this inspection that the cracks generally occurred at the seams between two form panels. Photograph 9 shows one of the observed cracks. The cracks were sealed using an elastomeric sealant (shown in Photograph 9). In addition, a panel of CLAYMAX' was placed over the crack to complete the seal. The perforated drainage collection pipe was placed in the excavated trench upgradient of the cut -off wall and attached to two collection sumps. Photographs 11 and 12 show the G = =G == c :vROhcrsnio1-0ont,1szsioi.ow �— — — � .� M Environmental