Diaphragm wall is a continuous wall constructed in ground in to facilitate certain construction activities, such as:

a) As a retaining wall.

b) As a cut-off provision to support deep excavation.

c) As the final wall for basement or other underground structure.

c) As a separating structure between major underground facilities.

d) As a form of foundation.

Diaphragm wall is a reinforced concrete structure constructed in-situ panel by panel. The wall is usually designed to reach very great depth, sometimes up to 50m.

The following are the steps:

• Guide wall – These are two parallel concrete beams constructed along the side of the wall as a guide to the clamshell which is used for the excavation of the diaphragm wall trenches.

• Trench excavation – In normal soil condition excavation is done using a clamshell or grab suspended by cables to a crane. The grab can easily cut through soft ground. In case of encountering boulders, a gravity hammer (chisel) will be used to break the rock and then take the spoil out using the grab.

• Excavation support – The sides inside the trench cut can collapse easily. Bentonite slurry is used to protect the sides of soil. Bentonite is a specially selected fine clay.The slurry will produce a great lateral pressure sufficient enough to retain the vertical soil.

• Reinforcement – Reinforcement is inserted in the form of a steel cage, but may be required to lap a few sections in order to reach the required length.

• Concreting – The placing of concrete is done using tremie pipes to avoid the segregation of concrete. As Concrete being poured down, bentonite will be displaced due to its lower density than
  concrete.

• Joining for the diaphragm wall panel – Diaphragm wall cannot be constructed continually for a very long section due to limitation and size of the mechanical plant. The wall is usually constructed in alternative section. Two stop end tubes will be placed at the ends of the excavated trench before concreting. The tubes are withdrawn at the same time of concreting so that a semi-circular end section is formed. Wall sections are formed alternatively leaving an intermediate section in between. The in-between sections are built similarly afterward but without the end tube. At the end a continual diaphragm wall is constructed with the panel sections tightly joined by the semi-circular groove.