Construction Sequence of Diaphragm Wall

Stage-1

Fixing of alignment

Stage-2

Guide wall construction

Stage-3

Trenching

Stage-4

Trench cleaning

Stage-5

Stop ends fixing

Stage-6

Reinforcement cage lowering

Stage-7

Placing of concrete

Stage-8

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