Piles can either be driven into the ground (driven piles) or be installed in a predrilled hole (bored piles or drilled shafts). The construction of bore cast in situ concrete pile consists of 4 primary phases
2.Reinforcement cage lowering,
Now we will discuss each phase one by one.
1. Pile Boring
Hydraulic rig/manually operable auger should be mobilized at the required location
Four reference points (making two lines perpendicular to each other) should be marked for checking centre of pile bore during boring of pile.
Initial boring of about 2.0 meters is to be done using cutting tool of desired diameter of pile
Then boring will be carried out according to the sub-soil investigation report of that location. It will be done using liner, bentonite or both.
The temporary guide casing, approximately 2.0 meter length with outside diameter equivalent to nominal diameter of the pile, may then lowered in the bore hole. In such a case dia of cutting tool will be little less, maximum 75 mm less than outside dia of casing for free movement in the casing pipe during operation.
Position / centerline of the guide casing pipe with reference to pile reference points already fixed around the pile location shall be checked to shift/adjust the casing pipe to ensure proceeding of drilling at exact pile location without any deviation.
Boring has to be done up to the founding strata as per drawings/ pre decided depth using intermittently bentonite slurry as per requirement. In case of requirement the bore hole is then supplied with bentonite slurry, from bentonite installation. Bentonite circulation channel will be made from bore hole to bentonite tank and fresh bentonite slurry will be pumped to bore hole through hose pipes. 24 hours prior to start of pile boring, ensure that bentonite is completely dispersed I the water and attains required density to stabilize the sides of bore hole during drilling. Bentonite slurry of specified quality should be circulated continuously during boring process.
Bentonite used to stabilize the sides of bore hole should be conforming to requirements as listed in inspection and test plan. Density of bentonite solution should be checked during boring operation to ensure that the density is about 1.05 g/cc to 1.10 g/cc, marsh cone viscosity 30 to 40 and pH value 9.5 to 12.
Bentonite slurry is pumped by high pressure reciprocating pumps/ vertical pump into the bore hole and the same is allowed to overflow the bore hole. The overflow slurry with bored mud/soil etc that comes out along with bentonite slurry is passed through channels and is collected in sediment tanks where sediments settle and bentonite can be re used. If necessary, the bentonite may be passed through the de sander tank to remove sand particles before it is re used.
Depth of pile shall be checked with sounding chain and exact depth shall be recorded in the pile report.
After boring upto required depth underreaming will be done using underreamer of desirable diameter. Completion of desired bulb cutting will be ascertained by (i) vertical movement of the handle and (ii) using L shaped rod of length enough to reach upto bulb location from approximately 2 feet above ground level and horizontal dimension equal to 0.5 of bulb dia minus pile dia.
2. Reinforcement Cage Lowering
Prefabricated reinforcement cage prepared as per the drawings and approved depths, is brought and kept near pile location while boring is in progress.
After getting the permission from the engineer, the reinforcement cage will be gently lifted and lowered by crane/manually into the bored hole. Necessary concrete cover will be obtained by using the circular cover blocks already made of the same strength as of pile.
If the reinforcement cage is very long i.e. not possible to handle in one lift, the cage will be lifted one by one and spot welded at the joints and then lowered inside the bored hole.
It is to be checked whether the reinforcement cage has reached up to bottom of the pile by measuring from the top of the cage to the ground level.
After cage lowering, 200 mm diameter tremie pipes in suitable lengths are to be lowered in the hole. The operation is done by lowering one tremie pipe after another and connecting them threading to maintain water tightness throughout its length till the gap between the pile base and Tremie is between75 – 100 mm. the tremie pipe is locked/supported from top to maintain the level and funnel is attached on top.
The tremie head to be provided to the tremie pipe for the flushing activity. The bore is flushed by fresh bentonite slurry through the tremie head. The pumping for flushing is done by use of mud circulation pump. Flushing will be done to remove all the loose sediments which might have accumulated on the founding strata. Further, the flushing operation shall be continued till the consistency of inflowing and out flowing slurry is similar.
4. Pile Concreting
The concrete placing shall not proceed if density of fluid near about the bottom of borehole exceeds 1250 kg/m3.
Determination of the density of the drilling mud from the base of the borehole shall be carried out by taking samples of fluid by suitable slurry sample approved by the engineer in charge, in first few piles and at suitable interval of piles thereafter and the results recorded.
After flushing is completed, tremie head should be removed and funnel should be attached to the tremie pipe.
The slump of the concrete will be maintained at 150 mm to 200 mm.
Concreting operation will be carried out using the 200 mm diameter trmie pipes.
Initial charge of concrete should be given in the funnel using a plug. Total concrete quantity in the funnel should be more than the volume of the entire pipe plus free space below the tremie. This will ensure a water tight concrete pouring through tremie.
Lifting and lowering is repeated keeping sufficient concrete in funnel all the time. As the concreting proceeds the tremie pipe are to be removed one by one, taking care that the tremie pipe has sufficient embedment in the concrete until the whole pipe is concreted. Sufficient head of green concrete shall be maintained to prevent inflow of soil or water in to concrete. Placing of concrete shall be a continuous process from the toe level to top of pile.
The concrete is poured in the funnel. As the concrete reaches the top of the funnel, the plug is lifted up to allow the concrete to flow corresponding to the placing of each batch of concrete.
The concreting of pile is to be done up to minimum of 300 mm above the cut off level to get good and sound concrete at cut off level.
After completion of concreting tremie, funnel and other accessories are to be washed properly and kept greased in proper stacking condition near next pile location.
While doing under water concreting 10% extra cement over and above the design mix requirement should be added in each batch.