Structural Diaphragm walls, also known as slurry walls, are typically excavated using a Mechanical or Hydraulic clam shell bucket in vertical panels from grade to create the below grade wall to design depth. During excavation, a bentonite or polymer slurry is added to prevent caving of the side walls of the excavated trench. Once the excavated depth is reached, the reinforcing steel cage is lowered into the panel and the slurry is displaced with concrete placed through a tremie pipe from the bottom up. The concrete is allowed to set, forming the diaphragm wall. For excavations in harder materials or rock, Hydraulic Cutters are used in place of the clam shell buckets. A series of panels are connected together by overlapping the panels or by the use of end stops to form a continuous rigid wall. For non-structural applications, the walls typically do not have any reinforcement. In addition, the structural concrete is often replaced with a cement-bentonite mix or if used exclusively as a barrier or cut-off wall, soil-bentonite can also be used.
Typical walls range in thickness from 24 to 48 inches and depths of up to 150 feet. However, with the correct equipment, thicknesses up to 70 inches and depths in excess of 400 feet are possible.
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Diaphragm walls can be installed with a high degree of accuracy to depths in excess of 120 feet. A guide at ground surface ensures that each panel is accurately located and also supports the top few feet of excavation. In general vertical tolerances less than 0.5% of the panel depth can be achieved, and tolerances of 0.25% are achievable with extra care.
- Permanent or temporary support of excavation for underground garages, subway stations and access shafts.
- Cutoff walls for seepage remediation for dams and levees.
- Load bearing elements (or Barrettes) where the structural loads are transferred to deeper soil strata or bedrock.
- Sometimes used as permanent basement walls, often in conjunction with top-down construction.