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Diaphragm walls are used to construct vertical walls in the ground (for basement/pit construction or to provide an impermeable cut-off barrier). Excavation is carried out either by using a trench cutter, or a grab. During excavation the open trench in the ground is stabilised by a support fluid, usually a bentonite suspension.
An essential characteristic of cutter diaphragm and cut-off walls is the process of excavation of the soil by a single insertion of the cutter into an unobstructed trench, supported by bentonite slurry. The excavated soil material is continuously transferred by a mud pump to the surface as a mixture of spoil or cuttings suspended in the bentonite slurry. The soil particles and cuttings are then separated from the slurry by desanding in a slurry treatment plant. The treated slurry is then returned to storage tanks for re-use before eventual disposal.
The main difference between the grab and the cutter technique is the sequence of operation. Whilst the cutter is inserted into the trench only once and excavation is continuous, the grab is inserted repeatedly and excavation continues cyclically until the required depth is achieved. The grab is inserted into the trench, lowered to the base of the trench; soil is excavated and then lifted out of the trench by the grab and deposited on the surface. The grab is then re-inserted into the trench and the process repeated until the design depth is achieved. As this process does not create extensive mixing of the excavated soil material with the support slurry, regeneration/desanding of the support slurry is only required to be carried out once, immediately prior to placing the concrete.