Supervision Lapse and Failed Cube Results on Bored Piles
A nine-storey residential block with a two-level basement is under construction on a 20 m × 30 m site in Kennedy Town. The scheme uses 600 mm diameter bored piles socketed into rock at roughly 45 m depth, with a 3 m thick pile cap over the basement raft. Under the Building Authority's site supervision regime, the supervision plan prepared by the Registered Structural Engineer assigns continuous, full-time supervision to pile concreting, with a Technical Competent Person who must be present throughout each pour. Site records show that during a 90 m³ pour, the assigned TCP left site for about two hours to attend another project and the pour continued; the tremie concrete level records for that pile are incomplete. Three days later, two 100 mm cubes from that batch failed at 28 days, returning 32 and 34 MPa against the specified 45 MPa grade. The contractor proposes to accept the pile on the basis of the remaining cube results and to proceed to pile cap construction next week.
1. Assess the supervision lapse and the cube failures, and state what must happen before the pile cap is cast.
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Supervision. Continuous supervision was correctly assigned: pile concreting is irreversible, hidden and safety-critical. Two hours without the TCP means roughly two hours of the pour were unsupervised, and the supervision plan does not permit a substitute to be absent without a nominated replacement. That is a breach of the plan, not a minor administrative slip. The Registered Structural Engineer must investigate, record findings, review whether the plan was followed and whether it needs amendment, and report to the Building Authority where works may not have been carried out in accordance with the approved plans or may be defective. Proceeding to the pile cap would bury the problem permanently.
Concrete. 32 and 34 MPa against a 45 MPa grade is roughly 70–75% of the specified strength - below any acceptance criterion in common use. The results cannot simply be averaged away. First establish whether the cubes genuinely represent that pile: check batch records, delivery tickets, sampling location, who took the cubes, curing and the testing laboratory's certificate. A sampling error is a real possibility and is worth ruling out.
The pile is, at present, of unverified strength and unverified integrity. The incomplete tremie records mean you cannot confirm the concrete was placed continuously under the slurry, so necking, soil inclusion or a cold joint cannot be excluded.
Before the pile cap: stop the pile cap; complete the records audit; carry out integrity testing appropriate to a 45 m socketed pile (cross-hole sonic logging is far more informative than low-strain PIT at this length); recover cores through the pile to verify strength and continuity; and reassess the structure on the verified strength. Only then decide between acceptance, capacity reduction, augmentation or replacement.
2. The contractor argues the remaining cubes passed and that cores are unnecessary because a working pile load test is required in any case. How do you respond?
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The two tests answer different questions. A working pile load test verifies the pile–soil system at a proof load with a factor of safety. It does not verify the concrete strength in the shaft, and it can easily be passed by a rock-socketed pile whose capacity comes largely from the socket - a low-strength or locally defective shaft can be masked by the socket and the surrounding ground. A pile can therefore pass the load test and still be unacceptable in terms of durability, structural capacity and the assumptions in the design.
Further points:
- The remaining cubes are only evidence about the batch they came from, and the sampling is precisely what is in doubt. 'The other cubes passed' is not a finding; it is an observation that needs the same audit.
- Core results give direct evidence of in-situ strength at depth and, with logging, of continuity. They are the appropriate response to a failed batch and incomplete placement records.
- The load test itself may be inconclusive if the pile fails prematurely in a way attributable to the shaft; you would then be arguing about the cause with the pile cap already cast.
The professional position: as Registered Structural Engineer you cannot certify a pile whose strength and integrity are unverified because the contractor finds the evidence inconvenient. Say so in writing, put the hold on the pile cap, and set out the verification programme with the contractor's and client's agreement. If the verification shows the pile is deficient, the design options are re-analysis of the pile group with the reduced capacity, augmentation with additional piles, or replacement - and each of those is far cheaper to decide now than after the cap is cast.
3. What should be recorded and reported, and to whom, arising from these events?
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Records to create now:
- A dated factual account of the supervision lapse: who was on site, when the TCP left and returned, who supervised in the interim, and what work was placed during the gap. Interview the site staff and the TCP; do not reconstruct it from memory later.
- The concreting records for the pile, including delivery tickets, pour sequence, tremie depths and the reason for the gaps.
- The cube results, the sampling chain, the laboratory certificates, and the audit outcome if a sampling error is found.
- Instructions given: the stop on pile cap construction, the verification programme, and the contractor's response.
Reports:
The Registered Structural Engineer has a duty to report to the Building Authority where works are not carried out in accordance with the approved plans, or where there is doubt about their adequacy. That report should be factual and non-accusatory: the facts, the uncertainty, the proposed verification, and the hold on subsequent works. Silence while the cap is cast is far worse than an early, measured notification.
The client should be informed in writing because the decision affects cost, programme and their risk. The contractor should receive formal instructions and be told that the supervision plan will be enforced, with a nominated deputy named so that a repeat absence cannot occur.
File discipline: keep everything, including the drafts of reports and the reasoning. If the pile later performs badly, or if the Building Authority audits the site supervision records, this file is the evidence of whether you acted promptly and proportionately.