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
- 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|