diaphragm construction by zap foundation

Diaphragm Walls

Specialisation of ZAP Foundation | Dwall Construction

Diaphragm wall is a continuous reinforced concrete 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 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 during excavation using bentonite slurry
  • As a separating structure between major underground facilities
  • Lower reinforcement and placing of concrete to form the wall panel

Guide wall – guide wall is two parallel concrete beams constructed along the side of the wall as a guide to the grab or cutter, which is used for the excavation of the diaphragm wall trenches.

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

Excavation support – the sides inside the excavated trench can collapse easily. Bentonite slurry is used to protect the sides of the trench. Bentonite, a specially selected fine clay, when added to water, forms an impervious cake-like slurry with very large viscosity. The slurry will produce lateral pressure sufficient 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 – placement of concrete is done using tremie pipes to avoid the segregation of concrete. As concrete is being poured, bentonite will be displaced due to its lower density than concrete. Bentonite 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 triangular 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 triangular groove.