Guide Lines for Using BentoniteFor preparation of bentonite suspension for pile boring, only potable water shall be used. If potable water is not available for mixing with bentonite power and saline or chemically contaminated ground water is required to be used for making the bentonite slurry, special additives as indicated in IS : 9556 / 1980, may be used.Maintenance of the specific gravity of bentonite suspension between 1.05 to 1.12 is very important for retention of the vertical cut soil surface of pile borehole. If the specific gravity of the bentonite suspension is less than 1.05, the developed cohesiveness and surface tension thereof amongst the soil particles of the surface soil will be reduced resulting collapse of the vertical cut soil surface. The specific gravity (between 1.05 to 1.12) of bentonite solution shall be maintained depending on the cohesion factor of the soil. For less cohesive soil, the specific gravity of bentonite solution should be more to prevent collapse of soil.To achieve the requisite specific gravity of the bentonite solution, the concentration of bentonite is calculated as under:s = 1 + 0.006 x CsWhere s is the density of the bentonite suspension &Cs is the concentration of bentonite by weight.Concentration of bentonite means percentage of bentonite (by weight), to be mixed with potable water to get requisite density / Sp.Gr. of bentonite solution.So, Cs = 1/6 x ( s 1 ) x 1000 ..(7)For example, required Sp.Gr. of the bentonite suspension = 1.05Cs= 1/6 x (1.05 1) x 1000 = 8.33 % by weight.i.e. In 1 m3 of water 83.3 kg bentonite shall be mixed to get 1.05 Sp.Gr. of the bentonite solution.There are two stages of use of bentonite suspension in cast-in-situ vertical bored piles. For making pile bore, bentonite suspension of specific gravity between 1.05 to 1.12 is to be maintained. After completion of pile boring, for cleaning of the bored pile bottom and flushing of the contaminated bentonite solution in the borehole, fresh bentonite solution of specific gravity between 1.10 to 1.12 is to be used.On completion of pile boring upto the requisite depth, the bore is flushed with fresh bentonite solution of proper Sp.Gr. To clean the dredged loose soil from the bottom of the pile bore, fresh bentonite solution is pumped into the bore hole. This bottom loose muck cleaning should be ensured by collecting slurry from the pile bottom using a slurry sampler and comparing the specific gravity with the fresh bentonite solution, being pumped into the pile bore. The bentonite solution remaining in the borehole, after completion of boring of the pile, contains huge dredged muck in suspended condition. This suspended muck precipitates after a lapse of time, which is very harmful for the pile. The suspended muck, mixed with bentonite solution, therefore, is required to be flushed out and replaced by fresh bentonite solution to avoid any significant sedimentation at the borehole bottom. This flushing operation can also be ensured by comparing the specific gravity with the fresh bentonite solution, being pumped into the pile bore from fresh bentonite tank.Such flushing / cleaning of muck from pile bottom and muck-mixed bentonite solution from pile bore is operated with fresh bentonite solution of higher specific gravity .The matured bentonite jelly shall be prepared by thoroughly mixing sodium based bentonite powder with potable water in proper concentration and allowing this mixed solution to be undisturbed for minimum 12 hours to form jelly. This matured bentonite jelly is poured into the fresh bentonite tank and mixed with potable water to prepare fresh bentonite solution. Fresh bentonite solution must be well agitated by re-circulation system or by agitator, prior to pumping it to the pile borehole for flushing, as because the bentonite solution, if not agitated, will get formation of jelly due to thixotropic properties. The agitation of fresh bentonite solution shall have to be continued while this solution is in use.The commencement of pile boring shall be done with this fresh bentonite solution, until sufficient quantity of rejuvenated bentonite is collected or accumulated in the rejuvenated tank for continuing the boring of the pile.However there were no any guidelines available on the volume of bentonite and time to be kept in the bore hole.For more details IS 2911 may be seen.USE OF BENTONITE IN BORED CAST IN SITU PILESUSE OF BENTONITE IN BORED CAST IN SITU PILES

During boring for pile, the side of the bore hole is subjected to various types of forces & pressure like active earth pressure, overburden pressure, hydrostatic pressure, pore pressure due to underground water, etc. due to which the side of bore becomes unstable and start collapsing if the soil strata is weak.In order to stabilize the side of bore the supporting fluid is to be used to counter the above pressure. The commonlyUSED SUPPORTING FLUID FOR STABILIZING ISBENTONITE.BENTONITEBentonite is commonly used as support fluid.In its natural form as sodium montmorillonite exhibits thixotropic properties, where by it forms a gel under quiescent /static conditions & regains its fluidity under dynamic conditions.PROPERTIESThe bentonite suspension used in bore holes is basically clay of montmorillonite group having exchangeable sodium cations, Because of the presence of sodium cations, bentonite on dispersion will breakdown into small plate like particles having a negative charge on the surfaces & positive charge on the edges. When the dispersion is left to stand undisturbed the particle become oriented building up a mechanical structure at its own. This mechanical structure held by electrical bonds is observable as a jelly like mass or gel material .When the gel as agitated the weak electrical bonds are broken & the dispersion becomes fluid.FUNCTIONThe action of bentonite in stabilizing the side of bore holes is primarily due to thixotropic property of bentonite suspension, it permits the material to have the consistency of a fluid when introduced into the excavation & when undisturbed forms a jelly which when agitated becomes a fluid again.Specification;(As per IS; 2911 Part 1 / sec 2)The bentonite suspension used for pilling work shall satisfy the following requirements.(a)Density of freshly prepared bentonite:Between 1.0325 to 1.1 g / ml(b) Liquid limit:More than 300 % & less than 450%(c) Marsh viscosity:30 to 60 seconds(d) Sculling index:At least 200% of dry volume(e) PH Value:Between 9 & 11.5(f) Sand content:Less than 7%(g) Silt content:Less than 1%Bentonite Circulation System;The preparation, stocking & the recovery of the bentonite mud requires the following.(1) Bentonite feed tank(2) Recovery & Desilting tankBentonite feed tank;In this tank fresh / recovered bentonote slurry will be stored for pilling work.Recovery & Desilting Unit;In this tank the return slurry will be received, & after desilting the usable bentonite slurry will be recovered & transferred to bentonite tank for reuse.Mixing;Bentonite slurry with a concentration of bentonite in the range of 3 5% is normally used to support the bore.Bentonite powder is manually spread in bentonite tank & is mixed / agitated with air compressor & left for 12 hrs for curing, beside this other method are also available like bentonite is filled in funnel like container and at the bottom of funnel high pressure water pipeline (using water motor) is attached and pipe leads to bentonite tank..Transportation / Circulation;From bentonite feed tank the fresh bentonite slurry will be pumped into the bore by vacseal pump / mud pump through heavy duty GI pipes / canvas hose. Near the boring location a temporary pit of size 1m x 1m x 1m will be excavated and fitted with vertical pump, the slurry which will follow out of bore will be collected into this pit & the same will be pumped to recovery tank by vertical pump. Around the bore sand bags will be places & channel will made for slurry to flow from bore to pit.Positioning of bentonite tank;The bentonite tank & recovery tank will be placed in such a way that it will cater minimum four foundation locations on either side i.e. from one location it will feed around eight foundation locations.Precautionary Measures when working on road or highway:(a)Barricading: Full height barricading boards without any gap at the bottom will be erected & one side of barricading will be sealed with sand bags and on this side the pipeline for Bentonite will be laid.(b)Pipeline joints: Throughout the boring work it will be ensured that all joints and fittings of tank & pipeline are properly tightened & regular inspection will be done to ensure that there is no spill of Bentonite.