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Trenching 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.
    • 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 15g/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  

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