Pile foundation is actually a slender column or long cylinder constructed by driving preformed units to the desired founding level, or by driving or drilling-in tubes to the required depth. Pile foundations are usually used to support structures with heavy, concentrated loads by transferring the load at the desired depth. Their load capacity derives from their end bearing and skin friction resistance.
Pile foundations are of three main types based on structural behaviour: End Bearing Piles, Friction Piles and combinations of the two.
Pile foundations are used in the following situations:
- When there is a weak soil at the surface & the bearing capacity of soil is less to take loads.
- When a building has very heavy, concentrated loads, such as in a high rise structure, bridge, or water tank.
Procedure of Construction of Bored Cast-in-situ concrete piles as explained below:
- Fixing pile location
- Preparation of bentonite slurry for using as a drilling fluid during the pile boring operation
- Installation of temporary casing
- Drilling by Rotary Rig
- Discharge of spoil
- Visual samples and sounding check
- Flushing the bore
- Fabrication and lowering of the rebar cage
- Pile concreting
- Pile dismantling up to the cut off level
a) Fixing pile location
Survey shall be carried out from main grid lines and pile points shall be marked with iron pegs of 150mm long, 10mm diameter rods, covered with concrete and the tip of the rod shall be painted before commencing the work. The Rig machine shall be positioned as per the marked pile point.
b) Installation of temporary casing
A guide casing of 3 to 4m length or to the required depth based on the soil strata from top of bore shall invariably be provided. In cases where side soil can fall into the hole, the side of the borehole shall be stabilized with a suitable steel casing / liner.
c) Drilling by Rotary Rig
The pile drilling shall be carried out by means of Rotary drilling rig / winch. Boring shall proceed
Figure – Piling
with the auger or bucket or bailer / chisel until the required depth is reached. The bentonite slurry shall be kept in the pile bore hole during the process to stabilize the soil and avoid soil collapse inside the bore in case if casing is not provided.
d) Visual samples and sounding check
Soil or rock samples shall be obtained for visual examination. These samples shall be stored in plastic bags together with clear markings of date, pile number and depth. Sounding check (measuring of total bore depth by lowering down a chain) shall be done immediately after boring is over and before reinforcement cage lowering to check the bore soundness. The density of the bentonite slurry shall be checked prior to lowering of the rebar cage / prior to concreting of the pile.
e) Fabrication and lowering of the rebar cage
Prefabricated reinforcement cage shall be cleaned and made free from any loose materials. Cage shall be made in two or three parts. Cage lowering shall be done from bottom to top (one after another) by welding the main vertical reinforcement. The main vertical rods and the rings of the rebar cage shall be welded together to maintain rigidity. Concrete cover blocks of same grade of concrete to be used for constructing the pile shall be produced of the required size and shape and shall be fixed to the external ring of the pile rebar cage to provide a uniform concrete cover to the rebar. Care shall be taken while handling the rebar cage during loading, transporting, unloading and during the final lowering of the rebar cage into the bore of the pile.
Figure- Cage Lowering
f) Tremie pipe fixing
After completion of cage lowering, tremie pipes of 200mm diameter shall be inserted into the pile shaft. Tremie pipes shall be fixed one after another by thread & socket joints and shall be lowered into the pile shaft till the tip has reached the bottom end with only minimum clearance from the toe of the pile bore to allow flow of concrete. All tremie joints shall be made air tight. The inside of the tremie pipe surface shall be cleaned thoroughly in order to remove any hardened concrete / slurry which is sticking to the surface so that the hardened concrete /slurry do not impede the free flow of concrete or present a condition wherein the concrete could segregate.
g) Flushing (in case casing is not provided)
Flushing head shall be fixed to the tremie pipe for cleaning of pile shaft by flushing of bentonite slurry. The density of Bentonite slurry in the shaft shall be checked and fresh bentonite slurry of density 1.034 to 1.1 gm/ml. shall be pumped into the borehole from tank for cleaning of excess slush from pile shaft through tremie pipe. Flushing shall be continued till the exit density of the bentonite slurry is brought down to below 1.25 gm/ml.
h) Pile concreting
After completion of flushing, funnel shall be fixed to the tremie pipes for concreting. The time interval between completion of boring and start of concreting shall be minimized. Concrete pouring shall be continuous through tremie pipes. The concrete shall be supplied from the existing set up of batching plant. In the process of concreting, care shall be taken that the tremie is never empty and at least 2-3m length of tremie is always embedded in the concrete. Total concrete depth shall be more than cut-off level. The requirements / operation of tremie concreting shall be as per IS: 2911. The concrete at the time of placing using tremie pipes shall
Figure- Pile Concreting
have a slump of 175±25 mm and the concrete mix shall be cohesive. In case the tremie pipe get choked due to some reason the concreting shall recommence as per the procedure laid down in IS:2911- Part-1. Adequate care shall be taken during concreting of the pile so that there is no interruption of the concreting process.
i) Final level of pile
Concreting shall be stopped after confirmation of pile depth by sounding checking (by chain or rod). Concrete shall be sound at cut-off level. After concreting is completed smooth pull & push movements shall be given to extract the temporary guide casing by simultaneous moderate tamping of the casing.
j) Disposal of bentonite slurry
The bentonite slurry after its use in pile boring would be collected in a sedimentation tank in order to allow the sedimentation process, which would help in allowing the water to evaporate and the slurry in the powder form can be extracted from the sedimentation tank and then disposed off at a far off location away from any habitation.