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Bored piles are used when loads are heavy, the bearing layer is deep, or driving and jacking piles would affect neighbouring buildings. Every method follows one of two principles: drilling with a casing, or drilling without a casing (under supporting fluid, or reverse circulation). This article summarises the procedure World Steel applies to bored pile foundations for factories and office blocks.
Equipment and materials
| Equipment, material | Use |
|---|---|
| Reverse circulation rig or bucket drilling rig | Drilling the hole |
| Ultrasonic borehole logger | Measuring hole depth and verticality |
| Grab excavator | Loading spoil onto trucks, breaking the existing slab |
| Air-lift cleaning unit | Removing sediment from the base of the hole |
| Generator, welding sets, bar cutters and benders | Fabricating and welding cages |
| 8"–10" tremie pipes and hopper | Base cleaning and underwater concreting |
| Temporary casing | Stabilising the top of the hole |
| Steel plates and sections | Covering fluid pits and holes, cage support frames |
| Polymer or bentonite powder, soda | Mixing the supporting fluid |
| Crawler crane | Handling casing, cages and heavy equipment |
Work sequence
| Step | Work | Control |
|---|---|---|
| 1. Setting out, casing | Set out the pile centre; lower the casing with the crane and push it down to existing ground level with an excavator. | Casing coordinates checked with a total station. |
| 2. Fluid check | Test the fluid before drilling and before concreting. | Indicative values for polymer at 21 °C: viscosity 32–50 s (Marsh funnel), pH 8.5–11.5, density 1.0–1.1, sand content below 2%. |
| 3. Drilling | Centre the tool on the casing; level the rotary table; keep the kelly vertical throughout drilling. | Verticality with a spirit level; hole depth and diameter. |
| 4. Cage installation | Lower the cage section by section with the design lap length and link spacing at the splices. For top-down basements, insert the kingpost and weld it to the cage. | Lap length, spacers, cage top level; the kingpost welded to the cage at three evenly spaced points. |
| 5. Base cleaning | Clean the base immediately before concreting. | Base level after cleaning measured with a weighted tape. |
| 6. Concreting, extraction | Pour continuously, logging concrete level against pipe depth; shorten the tremie as it rises; extract the casing after pouring. | The tremie stays at least 3 m inside the concrete to prevent segregation. |
Typical problems and remedies
| Problem | Cause | Remedy |
|---|---|---|
| Hole diameter too small | Drilling too fast, the tool not completing full turns at a level | Redrill where many sections are undersized; where few, add concrete spacers and rotate and raise the cage to scrape the wall |
| Hole collapse | Poor fluid, heavy loads round the hole, drilling too fast | Control the fluid closely; adjust drilling speed to the ground; keep vehicles and heavy loads away from the hole |
| Tremie blocked at the start of the pour | Pipe tip pushed into the soil at the base | Pull the whole tremie, clear it, reinstall and clean the base again before pouring |
| Tremie blocked during the pour | Slump out of range, long wait for concrete | Pull the tremie, redrill, remove the concrete placed and start again; if the concrete has hardened, abandon the pile and replace it as instructed by the designer |
Acceptance
- A record for each pile: coordinates, levels, depth, fluid and hole checks, cage inspection and the concrete pour log.
- Pile integrity and load tests as specified by the designer.
Reference standards
The edition in force when the contract is signed applies.
- TCVN 9395:2012 Bored piles – Construction and acceptance
- TCVN 9393:2012 Piles – Standard test method in situ for piles under axial compressive load
