Bored cast in-situ piles is built by digging a hole within the ground by suitable means like percussive or rotary method with the utilization of temporary or permanent casing or drilling fluid. then, the development is finalized by filling the opening with reinforced concrete .
In this article, the various steps involved to be carried out to execute the Pile construction works (Bored Cast- in- situ type), at the designated location to meet the requirements of specifications, from designed depth to the designed cut-off level to serve as a foundation is discussed .
References and Codes of Practice :
All piling works will be executed in compliance with the requirements of the respective reference and codes of practice.
List of Reference & Code of practice –
IS 456: 2000 code of practice for Plain and Reinforced Concrete.
IS 2911-1979 (Part-1, Section -2) Design and construction of Bored Cast In-Situ Piles.
IRC:78-2000 Standard Specification for road and bridge works- Foundations
Detailed Construction Sequence:
Formation of borehole –
Set out pile Position: Pile positions set by the surveyor, and checked by site in-charge, before commencement of piling. The existing ground level at the pile position will be measured and recorded.
Set boring unit: Prior to boring, the following steps will be carried out,
• Offset surveyed pile position in two perpendicular directions by placing 2 offset markers at appropriate distances from the pile position, to facilitate the installation of temporary steel casing. The offset distance will be such that these markers will not be disturbed during installation process.
1. Level the platform for the boring rig by placing steel plates, if there is no working platform.
2. Check the verticality of the Kelly Bar of a boring rig, by banks-man prior commencement of boring.
3. Position the boring rig at the designated location to commence boring works.
Commence boring with auger –
1. Install temporary casing into ground.
2. Boring shall continue till that it has reached the design pile cutoff level.
3. Stabilize borehole by polymer fluid.
4. Cease boring works when the designated level has been achieved.
5. The designated level is usually the design pile toe level shown in the shop drawing. The competent stratum is therefore the governing factor to determine the designated level for boring works.
Verticality Check during Temporary Casing Installation and Boring Works –
During the course of boring, the following will be performed:
- Check the casing position during and after placing of the casing, prior to boring work.
- Check the verticality of casing during installation, using plumb-lines method from two perpendicular directions.
- Adjust or extract the casing if in any direction is deviated, and re-installed until the plumb-line is in verticality with the side of the casing.
- Take and record the top level of temporary steel casing to enable measurement of pile depth. Exact length of casing will be decided prior to commencement of work based on soil condition.
Cleaning of Base –
• Clean the base of the borehole to remove any loose, disturbed or remoulded soil with a cleaning bucket. Polymer solution is free of sand and hence keeps the pile base cleaner. No flushing is being required while using polymer.
• Measure the depth from the base of the pile bore to the top of the temporary steel casing.
• Confirm the depth of the Pile bore .
Installation of Reinforcement Cage –
- The cut off level of the pile would be as per the relevant drawings.
- Weld main bars with links at the top of the reinforcement cage and use U-Clips to hold the main bars and main links together for lifting purposes during installation.
- Hook the wire sling with shackles on to the main links.
- Use the spreader beams (With wire sling) to hoist the reinforcement cage and lower into the borehole.
- Use concrete cover blocks of similar concrete grades as of the bored pile to ensure the reinforcement cage is in positioning and to achieve required concrete cover.
- Hold the reinforcement cage at the temporary steel at casing top for further lapping process.
- Dislodge the wire sling with shackles from, main links.
- Hoist the subsequent reinforcement cage and lap the reinforcement cages using Couplings/u-Clips/Welding at alternate main bars.
- Place additional Couplings/U-Clips/Welding at the section of the reinforcement where the main lifting point is located, if necessary.
- Repeat the above procedure for other reinforcement cages.
Tremie Pipe lowering –
- Use a tremie pipe of appropriate diameter for concreting bored pile.
- Join the units of the tremie pipe prior to lowering it into the borehole.
- Lower the tremie pipe into the borehole until 300 mm above toe level to facilitate flow of concrete.
• Pour designed concrete mix into the borehole via a hopper attached to the top section of the tremie pipe.
• Withdraw the tremie pipe from the borehole as the concrete level rises. This is to facilitate the flow of concrete .However, it is important to note that the tremie pipe is to be embedded not less than 1.5 to 2.0 meters below the concrete level.
- Dismantle the tremie pipe from the top, section by section.
- The level of stabilizing fluid shall be maintained at an appropriate level to prevent collapse of the borehole during concreting works.
- Measure and record the concreting level in the borehole when discharging concrete from each transit mixer.
- The concrete shall be poured into the pile till there is clear displacement of all polymer fluid/lean concrete(laitance)
Extraction of Casing and Backfill –
- Backfill the empty borehole above the pile cut-off level with suitable material after appropriate period.
- Remove the base frame and the suspension beam after backfilling.
- Carry out the extraction of temporary steel casing using a Piling Rig/Crane after backfilling.
- The overcast of the concrete volume will be included in the record, which is as per technical specifications.
Quality Assurance and Quality control –
• All the necessary concrete pour cards and checklists will be filled up during pre- pouring, during pouring and after-pouring. Typical checklists are enclosed.
• The concrete cubes will be cast in frequencies based on the concrete quantity, as per the relevant clauses of contract by client.
• The acceptability criteria of concrete cube compressive strengths in 28 days of curing shall also as per the specifications.