PLANT CONSTRUCTION gic elevations to allow for safe construction. The bottom level of bracing was removed after a thickened mud mat was poured to allow for construction of the structural mat and the wall up to the level of the first construction joint. Once the bottom section of each wall was in place, the second level of bracing was removed to allow construction to continue. This process continued until construction of the dike wall was completed. Figure 7 shows the completed cofferdam when rebar was placed for the base mat and lower sections of the wall. At that time the bottom level (fourth level) of internal bracing had already been removed after the thickened mud mat had been allowed to cure. Intake Pump House The intake pump house for the new plant was set at a depth of about 20 m below finished plant grade. The pump house is a sixsided structure that is approximately 45 m long and 39 m wide. Bechtel elected to use a concrete diaphragm slurry wall with multiple layers of tieback earth anchors to support the walls. The slurry wall construction with tiebacks allowed for an entirely open excavation. This made for easier construction of the complicated internal structure of the pump house (Figure 8). Construction of the slurry wall consisted of first building a concrete guide wall around the entire exterior of the foundation. The wall panels were then excavated using a grab bucket and slurry, reinforcement in the wall was placed, and then the concrete was placed by " tremie method " to displace the slurry. In the tremie method, concrete is poured by inserting the tremie pipe to the bottom of the slurry. The lower end of the tremie pipe is kept immersed in the fresh concrete so that the rising concrete from the bottom displaces the slurry but without the cement content being washed out. The panels were constructed in a sequence where one panel was placed and the next panel was skipped. The intermediate panel was poured after allowing sufficient time for the two outside panels to have initial set but while they were still green enough to enable the grab bucket to scarify the surface of the completed panels to allow for some locking of adjacent panels. Once the wall was completed, excavation inside the walls was started until the depth of the first level of anchors was reached and the 64 multi-strand anchors were installed. Excavation then continued, with each level of anchors being grouted in place, posttensioned, and locked off at the face of the wall. Installation of the anchors included load testing and proof testing. This sequence continued until the bottom of the excavation was reached. 78 www.powermag.com POWER | October 2010 7. Holding back the lake. A dike wall was constructed across the width of the existing intake structure to shut off the flow of water into the plant through the original intake channel. The dike wall was constructed with four levels of bracing, starting from the highest level. Shown is the rebar base mat and final level of shoring in the cofferdam. Courtesy: Bechtel Power Corp. 5. The ring beam and lagging are added to the third shaft. Courtesy: Bechtel Power Corp. 6. A new pump house will be constructed later on top of the finished third shaft. Courtesy: Bechtel Power Corp. 8. Slurry walls. The slurry wall construction with tiebacks allowed for an entirely open excavation that reduced the difficulty of constructing the complicated internal structure of the pump house. Courtesy: Bechtel Power Corp.http://www.powermag.com