Bored cast-in-situ pile foundation may be used by suitable choice of installation methods; covering the method of soil stabilization, that is the use of casing and drilling mud; method of concreting that is tremie and choice of boring tools in order to permit a satisfactory installation of a foundation pile at a given project. Detailed knowledge about the subsoil conditions is vital to decide the details of the pile installation technique. In this section, we know more about the construction of Pile foundation by Direct Mud Circulation(DMC) method.
Pile Foundation by DMC Method:
Pile Foundation Survey:
The survey is carried out from main grid lines, and pile points are marked with iron pegs of 150.00mm long, 8.00mm dia rods, covered with concrete and the tip of the rod painted, before commencing the work. Tripod pulley is positioned with the help of bailer by confirming the fixed pile centre.
Guide casing placing Before Piling:
Say the average thickness of overburden is about 2.50 m. Excavate up-to cut-off level to call it as working platform level. With bailer excavate further over a depth of about 2.0m. Place guide casing of 6.00mm thickness and 3.0m in length in excavated pile location pit along-with casing cap. Drive the casing with driving monkey till casing leftover above working platform is about 0.30 m. Carry out excavation through already placed casing up-to 2 to 3 m depth below already placed casing bottom. Cut a square hole of 150 mm width in the casing from working platform top level to allow bentonite /overflow to get out from the pile borehole. While driving casing, measurements w.r.t reference shall be continuously taken and the casing alignment will be corrected if found necessary by hammering casing laterally.
Pile Boring by DMC Method:
The pile boring is of DMC (direct mud circulation) type using Bentonite drilling fluid, Mendola type pile rigs which consist of two front legs freestanding attached to a centre leg with a pulley at one end, and fixed to winch at the other end. The front legs of the rig are placed on both sides of the pile point, and the legs are moved till the pile point approximately in the centre of the legs by eye judgment. Then about 50mm sand cushion around the pile point more than the required foundation pile diameter is formed, and a bailer of outer diameter equal to required pile/ guide casing diameter hanging from the pulley is slowly dropped. The bailer leaves a circular impression on the sand cushion around the pile point, from which the exact centre of the bailer coinciding the pile centre can be judged and if not can be adjusted by the leg movement. Once this process is completed, three more points are to be marked radially around the pile point at a distance about 2m, to verify the true centre of guide casing at the time of boring.
The boring is done by dropping bailer about 1m to 2.0m fall into the borehole. Then guide casing of inner diameter equal to pile diameter is inserted into the borehole by tamping on the top of the casing with the help of bailer is rigidly fixed in the borehole up to 4m. Below ground level to resist the loose soil of surface falling into the borehole. The bailer is removed, and boring is proceeded by percussion hammering with a chisel by pumping Bentonite slurry (specific gravity 1.03 to 1.08) through DMC rods, at the same time the slurry whichever dislodged in the bore flows out from the casing into the settling chamber through the trenches made. This process is continued until the boring is completed. Then the DMC pipes and chisel are removed. After completion of boring, bore depth should be measured by the sounding chain.
Pile Reinforcement cage lowering:
A prefabricated reinforcement cage is cleaned out of loose material and maintaining the proper cover throughout the length of the cage. Cage may be made in two or three parts. Cage lowering to be done from bottom to top (one after another) by welding the main vertical reinforcement. The cage should be welded in between the main vertical rod and stiffeners to maintain rigidity.
Tremie pipe lowering for Pile Foundation:
After completion of cage lowering, Tremie pipe of about 260.00mm dia to be provided throughout the pile shaft. Tremie pipe to be fixed one after another by threaded & socket joint. Tremie length to be provided up to the required depth of pile with minimum clearance to flow the concrete through the tremie pipe.
Flushing of pile Borehole:
As piles are in sandy/silty strata (friction piles), it is always necessary to carry out flushing of piles at the end of completion of pile borehole with fresh bentonite with a liquid limit of 400 %. By recycling of bentonite from vertical pump tank, it gets contaminated and contains a lot of silt. Flushing will be done through already placed tremie. Air flushing will attract side collapses, hence not to be used for flushing & even flushing with fresh bentonite slurry it is to carry out after lowering tremie for concreting and before concreting. For flushing, bentonite slurry will be directly pumped from the swelling tank where the slurry is ready for use after swelling. The bentonite coming out of borehole will join the regular “vertical pump tank”.Keep on checking the Sp. Gr. of bentonite slurry being fed v/s coming out of the borehole. When both the Sp. Gravities are matching then the flushing is declared over.
Pile Foundation Concreting Method:
- After flushing through tremie, funnel shall be attached to the tremie and 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.
- Concrete will be mixed either in mini batching/regular central batching plant/s. The concrete will be transported through transit mixers if it is from a regular central batching plant. Concrete will be poured into concrete funnel directly from mini batching plant/transit mixer. Concreting must be completed in a continuous operation.
- Remove bentonite-flushing arrangement from tremie. Place-concrete funnel and snap-out plug (MS circular plate) in funnel opening with one end of the wire rope connected with a plate and another end with funnel top edge. The quantity of the first charge of concrete shall be such that the weight of the column of green concrete formed is more than the hydrostatic head of bentonite standing inside the borehole. In the case of the first charge, the capacity of the funnel should be calculated accordingly; in this case, it shall be at least 1.0 cum. After placing first charge continue concreting and withdrawal of tremie pieces in between, so that at no point of time the tremie embedment is less than 1.2 m inside concrete to avoid mixing of fresh concrete with bentonite slurry.
- To ensure compaction by the hydraulic static head, rate of placing concrete in the pile shaft shall not be less than six meters (length of the pile) per Hour.
- The slump of concrete shall be checked for each batch and recorded in Concrete Pour report. (Slump of concrete shall be checked at Batching Plant as well as at pouring location.)
- Concreting of the pile should be uninterrupted. In the unusual case of interruption of concreting; but which can be resumed within one or two hours, the tremie shall not be taken out of the concrete. Instead, it shall be raised and lowered gradually, from time to time to prevent the concrete throughout the tremie from setting. Concreting should be resumed by adding a little richer concrete with a slump of about 150mm-200 mm for the smooth displacement of the partly set concrete.
- Theoretical and Actual Concrete built-up depth shall be checked with sounding chains in between the pile concreting operation and recorded in the format.
- Concrete in the pile is to be filled approximately 0.6 m (as per site conditions) above the cut-off level to permit removal of all laitance and weak concrete before capping and to ensure good concrete at the cut-off level for proper embedment into the pile cap.
- After concreting up to the required level is done, the casing pipe is withdrawn.
- Concrete pile up-to cut-off level, allow concrete to over-flow equivalent to overflow height as per specification. In this case, the working platform shall be at the cut-off level. The overflow concrete will flow through the guide casing square hole.
- Pile concreting data shall be entered in the relevant formats for future reference.
- On completion of the piling, all tremie pieces, funnel and fork be cleaned and stored properly. At the end of the concreting operation, the pile top will be green to at design cut-off level.
Guide casing withdrawal after piling:
After completion of the concreting operation, the guide casing, installed initially before pile driving, shall be extracted by using Vibro hammer and service crane.
After Piling Shifting Rig, next location:
As soon as pile guide casing is withdrawn rig will be shifted to the next pile location as per pre-decided sequence by towing the assembly with the help of crowbars and by taking reaction from piling winch. In case of shifting rig to the next foundation/bridge location, the rig will be dissembled and shifted & re-erected/re-commissioned.