Diaphragm walls refers to the construction of in-situ retaining vertical walls by deep trench excavation method.
Stability of the sides of the excavation is ensured by bentonite slurry. Hence these types of walls are also termed at times as ‘slurry walls’
The wall is constructed in panels and the length of typical panel is between 2.5 to 7m.
Standard methods of constructing retaining walls require temporary form work and supports. These become uneconomical after a certain depth.
The construction of the work above ground cannot proceed till the basement work is complete. This involves more time for construction.
Use of diaphragm wall construction eliminates the need for formwork and temporary support and also allows above ground construction to proceed along with basement construction
Purpose built grabs or milling machines called as hydromills are used to do the excavation.
Since the wall is cast in panels special care has to be taken to make the joints between the panels watertight.
Water bars are constructed within the construction joints to prevent water leakage through them.
Standard widths of the walls range from 600,800, 1000 and 1200mm.
Bentonite slurry is a clay mixed with water which possesses ‘Thicksotropic’ property. That means that when it is left undisturbed it acts as a ‘gel’ and when it is moved it acts like a liquid. The bentonite slurry is poured into the excavation and it seeps into the adjoining soil forming a cake which prevents the sides from caving in.The bentonite is removed while concreting by displacement.
The depth of diaphragm walls can reach 50 to 80m.
The tolerance for verticality is normally 1:200
This type of construction method is called ‘Top down basement construction’
This allows for above ground construction simultaneously with excavation of basement.
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