What is Bentonite?
Bentonite is a very fine-grained clayey material having a high percentage of montmorillonite material. Bentonite clay is a mined product. Usually, two types are there – Sodium based and Calcium based. Sodium based is suitable for soil stabilization work.
Bentonite slurry shall be used for stabilizing the sides of the pile borehole. Bentonite powder shall be mixed with water to make slurry having specified parameters, conforming to IS:6186 and procured from the established source shall be used.
Collapse of Soil:
Stabilisation of soil is essential for the quality of the pile as well as for time and cost economy. Stabilisation of soil depends on soil characteristics mainly on cohesive factor and angle of friction. There are different types of collapse observed during piling operation, which are as under.
During pile boring:
During pile boring operation, due to the collapse of the inside diameter of the borehole increases irregularly. Actual pouring of concrete in pile bore will increase to fill the over breakage volume.
Before pouring of concrete:
After flushing of the borehole bottom and before pouring of concrete into the pile bore, if side soils of the borehole collapse, again loose muck will be deposited at the pile bottom which affects the quality and end bearing capacity of pile adversely.
During pile concrete:
During concreting there is a chance of producing some soil pockets inside the build-up concrete, which may cause damage to the pile shaft and decrease the load bearing capacity of the pile.
Factor affecting collapse:
- Cohesiveness and angle of friction.
- Density of soil.
- Pore water pressure.
- Borehole depth.
- Fluid inside the pile bore.
Soil Stabilization using Bentonite slurry:
The technology of soil stabilization works only for sodium-based bentonite clay. It is montmorillonite clay mineral which swells considerably, while comes in contact with water. Suspension contains exchangeable sodium cations(Na+). These cations(Na+) together with the cations generated in water exchanged by negatively charged ions, accumulated on soil particles of the vertical cut surface of pile borehole, will form electric double layer resulting stability of the vertical surface of the soil.
The thixotropic property of bentonite suspension maintains the consistency of the slurry by virtue of electrical bonds, created due to ionization. During turbulence/disturbance in the suspension, jell materials become fluid by breaking the electrical bonds or otherwise during undisturbed condition the suspension will form a jelly.
Bentonite slurry Property and Tests:
The powder mixed with water takes normally 12 hours soaking time to get the maturity. Soaked bentonite shall work like jell material. For stabilization of vertical cut soil surface during boring of piles is based on consideration of the following properties:
- Swelling Index: 100 % or more in 12 hours.
- Liquid limit: More than 300% and less than 450%
- Marsh viscosity: Between 30-40 seconds
- The pH value: Between 9 to 12
- The density of slurry: 1.05-1.12gm /cc
Bentonite Setup Installation:
Setup Installation for piling includes the following arrangement which will be arranged/constructed as per site requirement.
- Water tank
- Mixing tank
- Swelling, curing and storing tank
- Pumping and recycling arrangement
- Return tank
Bentonite will be mixed with the help of agitator in a central mixing tank. Freshly mixed bentonite slurry transferred to the swelling tank and will be allowed for swelling minimum of 12 hrs. Once the mixed slurry is in the swelling tank, it will be further re-mixed with the help of a pump, by recycling the slurry in-to the same swelling tank. Then properly soaked and mixed slurry will be transferred to the storing tank from which it shall be pumped with the help of a vertical pump through the pipeline to the pile point for the purpose of boring.
The returned bentonite with excavated muck coming out from the pile during concerning will be transferred to the Return tank through a desander. After completion of every pile construction, the slurry from the returned tank will be checked for density, Marsh viscosity, Sand content and pH value, and if found not satisfying the requirement, it will be corrected by adding fresh bentonite slurry or otherwise, it will be disposed of. The tank will be properly cleaned and refilled with fresh bentonite slurry for the next pile.
The returned bentonite with excavated muck will join de-silting tank through an open channel. The feeding tank will be positioned such that the returned bentonite from piling location joins sedimentation and after sedimentation overflows in-to feeding/recycling/pumping tank. The water pump will feed water for making bentonite slurry and transfer to storage/feeding/recycling/pumping tanks. After the quality checking, it can be allowed for reuse.
On completion of pile bore, after cage lowering prior to concrete pouring, loose dredged muck from the pile bore bottom shall be removed by flushing with fresh bentonite slurry of requisite density. Cleaning of pile bottom shall be confirmed by collecting slurry from the pile bottom using a bottom sampler and comparing the specific gravity with the fresh slurry, being pumped into the pile borehole.