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�- Section 5.0 Engineering Analyses and Recommendations 5.3.1 In Situ Construction Some examples of the in situ construction option are slurry walls or secant pile walls. Constructing a slurry wall consists of 1) excavation of a deep trench with introduction of bentonite slurry as the digging proceeds; 2) insertion of steel reinforcement; and 3) placement of tremie concrete,which displaces the slurry. Constructing a secant pile wall consists of 1) drilling large diameter drilled shafts approximately 1.75 diameters apart using either slurry or casing to support the walls of the hole;2) placement of steel reinforcement;3) placement of tremie concrete; and 4) repeating this process in between each complete pile to form a continuous wall. The main advantage of the in situ construction option is the elimination of the temporary excavation with associated cost and challenges (discussed subsequently). In addition,the overall construction would likely be quicker than the second alternative. The main disadvantage of the in situ construction option is the risks of damaging the two 6-inch-diameter pipes connecting the wet pit with the pump station have to be in i' service during construction. It is likely to be very difficult to avoid damage using the equipment typically used for in situ construction. Therefore,we judge that the in situ construction option is not likely to be feasible and • are not providing more detailed recommendations herein. In the event a contractor proposes an in situ alternative with significant cost savings,we will be pleased to work with the contractor and the City to develop recommendations specific to the contractor's proposed alternative. 5.3.2 Temporary Shoring Some examples of temporary shoring are soldier pile and timber lagging walls and sheet pile walls. Since the depth of the temporary excavation is about 30 feet,some form of internal or external bracing will be required for lateral support. The shoring will need to be relatively impervious to minimize dewatering thereby eliminating timber lagging from consideration. The following must be considered for the temporary shoring option: ■ Groundwater-The temporary excavation support system must be relatively impervious to minimize dewatering and must extend into the lower stiff clay to minimize underflow. The shoring will need to be designed to retain the hydrostatic force of the water behind the shoring. ,., In addition,some form of groundwater cut off will be required to restrict inflow through the pervious sandy layer beneath the pump station and wet pit (between Elevation 52 and Elevation 57) and through pervious pipe bedding. f.. Groundwater seeping through the sand or bedding could cause piping in the open excavation and,consequently, settlement. CDM cmp D.&mcK.1,, 5-2 025W29248-GE47hReponSBr kside Pump Stadon.0oc I.n