Diaphragm wall

A continuous reinforced-concrete wall cast underground, in a trench excavated and held open by bentonite slurry. Panel after panel it forms a deep, stiff and watertight wall, able to support very deep excavations next to buildings and to become the permanent basement wall. It is the system for stations, car parks and large basements with a high water table.

A diaphragm wall is a continuous reinforced-concrete wall cast underground, in a trench excavated and held open by bentonite slurry. Panel after panel it forms a deep, stiff and watertight wall, able to support very deep excavations next to buildings and to become the permanent basement wall.

The secret is the bentonite slurry: a suspension of clay and water that fills the trench as it is dug and, by its pressure, supports the sides of the cut and stops them collapsing — with no formwork or props. The trench stays open, full of slurry, until it is ready to receive the cage and the concrete.

Into the slurry-filled trench the reinforcement cage is lowered, then the concrete is poured from the bottom through a pipe (tremie pour): the concrete, being heavier, rises and pushes the slurry out, which is recovered and reused. Panel after panel, with joints that link and seal them, a continuous wall is formed.

Deep (tens of metres), stiff and watertight with the right joints, the diaphragm wall is the choice for deep excavations with a high water table — stations, car parks, large basements — where it also becomes the permanent perimeter wall. For great heights the head is held back with tie-backs or propping slabs; the verticality of the panels, the quality of the under-slurry pour and the settlement around must be controlled.