Diaphragm walls are a used to construct retaining walls for deep basements, underpasses and the like, with the walls being constructed before excavation. The basic process consists of cutting a deep, narrow trench in the ground. To support the sides during excavation it is filled with a bentonite suspension, coninued excavation working through the suspension. (Bentonite is a thixotropic clay, mixed with water to form a slurry.)
Once any loose material has been removed from the bottom of the trench, the reinforcement cage for the wall is lowered into position through the bentonite and fixed. Concrete is then placed using a tremie pipe, displacing the bentonite as the trench is filled. Once the concrete has gained sufficient strength the material, now contained by the walls can be excavated.
Some form of temporary propping system will generally be required, to resist the lateral pressure of the soil behind the wall, until the permanent floor or roof structure has been completed. The exposed face of the diaphragm wall will be irregular, mirroring the side of the trench, and may require some secondary lining.