What is Diaphragm Wall Construction? An Overview
Diaphragm wall is a continuous wall constructed in ground in to facilitate certain construction activities, such as:
- As a retaining wall
- As a cut-off provision to support deep excavation
- As the final wall for basement or other underground structure (e.g. tunnel and shaft)
- As a separating structure between major underground facilities
- As a form of foundation (barrette pile – rectangular pile)
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, mechanical excavating method is thus employed. Typical sequence of work includes:
- Construct the guide wall
- Excavation to form the diaphragm wall trench
- Support the trench cutting using bentonite slurry
- Inert reinforcement and placing of concrete to form the wall panel
Major advantages of diaphragm wall are listed below.
- Can be used as permanent structural wall
- Water retainable
- Can be installed to deeper depths and for load bearing element
- Less temporary propping needed
- Can be applied for top-down construction method
- Rigid structure so that ground movement induced by basement excavation is less than other flexible retaining wall
- Vibration and noise generated from installation of diaphragm wall is less than other methods
- Guide wall:
guide wall is 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. - Reinforcement & Concreting:
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. placing of oncrete is done using tremie pipes to avoid the segregation of concrete. As Concrete being poured down, bontonite will be displaced due to its lower density than concrete. Bontonite is then collected and reused. - 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