Piles are usually transferred axial loads from a structure to subsurface soil strata having sufficient bearing capacity. The load transfer mechanism from structure to surrounding soil strata through the pile is complicated. However, the application of piled foundations is in usage over many decades. Mainly, piles are designed considering friction along its shaft and/or the end bearing capacity of foundation soil. Selection of pile foundations requires the subsoil conditions, the load characteristics of a structure, settlement criteria and any other specific requirement of a particular project. Construction of pile foundation demands careful control on position, alignment and depth, and require specialized skill and experience. In this section, we know more about the construction of pile foundation by Rotary Method.
Construction of pile foundation:
Pile Point Survey Before Start of Piling :
With respect to longitudinal and transverse centerlines, co-ordinates for individual pile shall be worked out. Pile points shall be marked at exact pile locations as per relevant structural drawings with Theodolite/Total Station, and reference grid pillars shall be established. Reference points shall be marked/fixed at a suitable distance from pile points to cross-check the centre of the pile during the drilling operation.
The setting out of pile positions shall be carried out by a qualified surveyor for every location. The piling in charge shall be informed of the setting out and shall be requested to check the pile locations in advance of any piling work being carried out. The setting out pegs will consist of steel reinforcing bars or the nails as required shall be driven slightly below the working platform level of the piling rig so that they will not be disturbed by the pile construction activities. These pegs shall be marked so that they can be easily located.
Guide casing Driving Before Start of Pile Boring:
After completion of the pile point survey, the piling rig moves to the position of the pile to be installed, and the pile point shall be made visible by hand excavation. From the centre point of the pile, two reference pegs are set at 90 degrees to each other at a distance equal to 1m from the outside of the temporary casing to be installed.
The temporary steel casing of outer diameter equivalent to the nominal pile diameter and minimum 6mm thick with necessary guide collar shall be provided down to 4.0m below Ground Level (or specific as per GFC drawing issued) depending on the soil, and other conditions are pitched and centralized over the centre point of the pile as a guide for the auger. Temporary casing shall be installed using a vibro hammer. The verticality shall be controlled by means of long base level alternatively positioned in two mutually perpendicular directions on the casing or with two plumb lines located on the perpendicular axis. The casing will extend about 250 to 300 mm above working platform.
Pile boring by Hydraulic Rig:
Once the temporary casing has been installed, excavation for foundation pile shall be carried out using Rotary Hydraulic piling rigs. The Boring bucket with the cutting tool will have an external diameter equal to the diameter of the pile less 75 mm. For example, for pile diameter of 1200 mm, the tool diameter shall be 1125 mm.
As the boring is carried out by the bucket, the pile borehole is then filled with Bentonite slurry, fed from Bentonite storage tank. Bentonite circulation channel shall be made from borehole to Bentonite tank, and fresh Bentonite slurry shall be pumped to borehole via flexible hose installed. Bentonite powder as earlier mentioned shall be mixed in the freshwater tank at least 24 hours prior to start of pile boring to ensure that bentonite is completely dispersed in the water and attain required density to stabilize the sides of the borehole during drilling. It is essential that the properties of bentonite material shall be carefully controlled at stages of mixing, supply to the borehole and immediately before concrete is placed. It is usual to limit:
- The density of Bentonite suspension to 1.05 gm/cc to 1.1 gm/cc
- The marsh cone viscosity between 30 to 40
- The pH value between 9.5 to 12
- The silt content less than 1%
- The liquid limit not less than 400%
Bentonite slurry is pumped by high-pressure by 15 to 20 HP reciprocating pumps/vertical pump into the borehole through the cutting chisel, and the same is allowed to overflow the borehole. The overflow bentonite slurry with bored mud, comes out along with the slurry is passed through the earthen drain and is collected in sediment tanks where the sediments settle, and bentonite can be re-used. Otherwise, the bentonite may be routed through Desanders
before the storage tank to minimize the sand content below 2 % as earlier mentioned. Boring is continuously done by drilling tools boring bucket/auger controlled by piling rig.
Each time when the drilling tool is filled by boring muck, it is taken out and unloaded on the ground at a specified location. Then the tool is again lowered into borehole and boring is continued. The telescopic arrangement of piling rig lowers the bucket further deep.
Though verticality of the pile being bored mainly depends upon the skill of the operator, it
shall be checked periodically by checking the verticality of the rig mast. The verticality of the pile can also be ensured by incorporating the changes in the design of Kelly, like increasing the overlap of the Kelly assembly pieces, hence reducing the play in joints thereby reducing the possibility of deviation in verticality. Some advanced tests like Ultrasonic Echo Sounding Test may ensure verticality of the pile being bored. Specialized equipment such as ‘High Precision Bore-hole Inclinometer’ or ‘Sonicaliper’ a device which gives 3D images of bore area profile may also be used to control and ensure verticality of the pile being bored but shall be an expensive option. Boring is done up to the founding strata as per drawings/ specifications etc.
On reaching foundation level, depth of borehole shall be measured by taking sounding, and exact depth shall be measured and recorded. Measurement for pile length shall be got certified from the Engineer and secured clearance for cage lowering and concreting.
Flushing of the pile bore for Construction of pile foundation:
On approval of the founding level, the bore is flushed by bentonite slurry with direct mud circulation. The pumping for the flushing is done by use of mud circulation pump. During flushing, the chisel is kept resting on the founding strata to remove all the loose mud/soil which might have accumulated on the founding level. Further, the flushing operation shall be continued until the consistency of inflowing and out-flowing slurry is similar.
After flushing is over, the piling rig shall be removed, and the borehole is ready for lowering of reinforcement cage.
In the event of pile boring being completed and the time lag between completion of boring and the start of concrete is more than 12 hours, the bore shall be re-cleaned by Air flushing before the start of pile concreting.
Pile Reinforcement cage lowering:
Pre-fabricated,pre-certified and pre-recorded reinforcement cage shall be lowered in the borehole by the rig/service crane in one piece. If two or more cages are lapped with the lap length as per relevantdrawings and specifications, spirals shall be placed around the lapped length and tack-welded to main bars. Also, cover blocks of required numbers shall be provided for the lap length portion. Lapping /Staggering etc. as per drawing/specifications shall be provided. Spiral rings shall be tack-welded to main bars of the pile to ensure rigidity/stability during cage lowering.
Whenever reinforcement cage top is below the operating floor level of piling equipment, provide temporary suspenders in the form of 4 Nos of 16 mm dia rods at the top of reinforcement cage by welding. Maintain cage top at design level only. Measurements shall be written for reinforcement in approved bar consumption schedule & got certified from the client.
Tremie pipe lowering:
The tremie pipe with funnel-shaped hopper shall be placed in position at the top of the casing to transport the concrete to the base of the pile. Pre-calculated, pre-assembled and pre-recorded in sequential order, tremie shall be lowered through reinforcement cage with the help of rig/service crane, keeping 300 mm clearance from bottom of pile borehole & supported against guide casing through the fork. If required adjustable tremie pieces shall have to be replaced in place of full pieces. The funnel should always be hanging & supported from fork only. Taking support of rods/guide casing/pile bore base will harm the quality of the pile so constructed.
The tremie pipes shall be clean, watertight and of adequate diameter (usually 200mm dia) to allow the free flow of concrete. Before lowering, all the tremie pipes shall be thoroughly checked, and form oil shall be applied inside the pipes for the free flow of concrete inside the tremie.
The length of tremie pipe (total of individual joined pipes) shall be more than the length of the pile and base of the tremie pipe shall be lowered to the bottom of the borehole allowing water or drilling mud to rise inside it before pouring of concrete.
The top of the tremie pipe shall be fitted with a large diameter Funnel to receive the concrete with steel sealing plate arrangement at the bottom of the funnel.
Before the start of concreting operation, the borehole shall be flushed with fresh Bentonite slurry to wash the borehole and flush out all loose particles, deleterious materials, if any, present in the bore.
Before pouring the concrete through tremie, the bottom of the concreting funnel should be closed with steel plate fitted with the funnel. After the funnel is filled with concrete, the plate is removed, and concrete is discharged. Thereafter, concreting is done in a continuous manner, up to the required level.
Concreting operation is similar to the construction of pile foundation by “Direct Mud Circulation(DMC) method.”
Guide Casing Extraction after Construction of pile foundation:
After completion of the concreting operation, the guide casing, installed initially before pile driving, shall be extracted by using Vibro hammer and service crane.
Tolerances for Construction of pile foundation:
Permissible tolerances for Bored Piles shall be as follows (Ref Cl.1116.1 (iii) of MORT &H).
a) Variation in cross-sectional dimensions: +50 mm, – 10mm
b) Variation from vertical: 1 in 50
c) Variation in the final position of the head in plan: 50 mm
d) Variation of the top level of piles: +/-25 mm