Construction Sequence of Diaphragm Wall


Fixing of alignment


Guide wall construction




Trench cleaning


Stop ends fixing


Reinforcement cage lowering


Placing of concrete


Withdrawal of stop ends

Trenching Operation with Bentonite Fluid

Trenching usually carried out under bentonite slurry
Bentonite Slurry – Key Component.

  • This Slurry acts as shoring to prevent collapse by hydraulic pressure and thyrotrophic property
  • Bentonite Slurry
  • Used as a support fluid
  • The bentonite suspension used in bore holes is basically a clay of montmorillonite group having exchangeable sodium cations (Na+).
  • The action of bentonite in stabilizing the sides of bore holes is primarily due to the thixotropic property of bentonite suspension.
  • The bentonite suspension when undisturbed forms a jelly which when agitated becomes a fluid again.
  • Functions of Bentonite
  • In case of granular soils, the bentonite suspension penetrates into the sides under positive pressure and after a while forms a jelly.
  • The bentonite suspension gets deposited on the sides of the hole resulting in the formation of a filter cake in contact with soil against which the fluid pressure acts.
  • In case of impervious clay, the bentonite does not penetrate into the soil, but deposits only a thin film on the surface of the hole.
  • Factors affecting stability of trench
  • Level of the supporting fluid
  • Level of the bentonite slurry should be maintained preferably at least 1.5m above the ground water level to avoid problem of instability
  • Density and Viscosity of supporting fluid
  • Used as a support fluid
  • Loss of shear strength with time
  • Suction effect during trenching
  • Specification of Bentonite (as per IS:2911-Part1:Sec2 Liquid limit : 300 to 450 % (in accordance with IS:2720-part V)
  • Sand content : Less than 7 %
  • Density: 1.12
  • Marsh viscosity : About 37 second, tested by marsh cone
  • Swelling index: Swelling index at least 2 times of dry volume.
  • pH value : Less than 11.5
  • Tests and compliance values for support fluid prepared from bentonite (as per IS9556-1980)
Property Test Method As Supplied to the Bore Before Concreting
Density Mud Balance 1.04-1.10g/ml<1 <1.15 gm/ml
Viscosity Marsh Cone 30 – 90 seconds
Shear Strength Shearometer / Vane Shear Apparatus 1.4 to 10 N/sq.m
pH Digital pH Meter 9.5-12