Underwater Concreting is a specialised way of placing concrete where water surrounds the work — think bridge piers, cofferdams, underwater pile caps and sea-bed foundations. The tremie method is the most common approach because it keeps the fresh concrete separate from water while it is being placed, giving a dense, durable result that resists washout and segregation.
What is the tremie method?
A tremie is a watertight vertical pipe with a funnel (hopper) at the top and a smooth outlet at the bottom. Concrete is poured into the hopper and flows down the pipe; the lower end of the tremie stays embedded in the freshly placed concrete so water is pushed away rather than mixing with the mix. This simple principle — keep the concrete head under the concrete surface — is what makes this form of Underwater Concreting reliable.
When is tremie Underwater Concreting used?
Use it when you need to place a continuous bulk of concrete underwater without dewatering, for example:
- Filling bored piles and pile caps.
- Casting base slabs for piers, caissons and cofferdams.
- Repairing or grouting subsea structures and scour protection.
The method suits situations where other means (diver-placed bags, pumped mixes) would risk segregation or washout.
Materials and mix design
Underwater Concreting uses mixes designed to resist washout and stay cohesive. Key features:
- High slump (flowable) but controlled so aggregates don’t segregate.
- Use of anti-washout admixtures and viscosity-modifying agents.
- Well-graded aggregates and often a cement-rich paste to improve impermeability.
Quality control of batching and delivery is critical because you can’t easily rework placed concrete underwater. Technical guidance for mix design and placement is available from industry bodies and specialist contractors.
Equipment and basic procedure
Typical tremie equipment includes the pipe (rigid or flexible), a hopper and funnel, a plug or “pig” to start the pour (sometimes foam or a soft plug), and pumps or buckets to supply concrete. A simple sequence:
- Lower the tremie to the placement location and ensure the outlet rests on the base or previously placed concrete.
- Fill the pipe and hopper so it stays full of concrete (no air or water pockets).
- Remove the plug carefully so concrete flows from the bottom; keep the outlet buried in at least 1–2 metres of placed concrete where possible.
- As the level rises, lift the tremie slowly — never lift so high that the outlet emerges into water.
- Stop when the required level is reached; seal or withdraw the pipe as directed.
Advantages and common problems
Advantages of tremie Underwater Concreting:
- Minimal washout and segregation — better strength and durability.
- Simpler and often cheaper than full cofferdams or dewatering.
- Effective for deep pours and congested reinforcement.
Typical issues to watch for:
- Air pockets in the tremie causing contaminated concrete.
- Lifting the pipe too quickly and allowing water ingress.
- Improper mix leading to washout, honeycombing or voids.
Good supervision, correct mix design and experienced operators reduce risks substantially.
Safety and quality controls
Underwater Concreting requires checks on concrete temperature, slump, admixture dosing, and continuous monitoring of the tremie head. Where divers work, ensure diving protocols and life-support measures are in place. Inspection and non-destructive testing (e.g. core sampling after curing) can confirm the integrity of the placed concrete.
Real companies that do this work
If you need contractors, look to marine and geotechnical specialists. Examples of established firms that carry out tremie and underwater concrete work include Keller (ground and marine specialists), Van Oord and Boskalis (marine construction and seabed works), and Subsea7 (offshore construction services). There are also regional specialists such as Ven-Tech Subsea and companies that provide underwater construction and diving services. These firms operate vessels, specialist pumps and experienced teams for complex Underwater Concreting contracts.
Practical tips for a successful tremie pour
- Keep the tremie pipe continuously full of concrete — never let it run dry.
- Use an anti-washout mix and test it first in small trials.
- Maintain clear communication between the batching plant, pump crew and divers/operators.
- Plan for contingencies: plugs, spare pipe sections and trained divers.
- Document the pour (video where possible) and keep records of mix batches and timings.
FAQs — quick answers
Q: Is tremie the only method for underwater concreting?
A: No. Other methods include pre-placed aggregate and grouted systems, pumped mixes, and use of bags or tremie variants. Tremie is preferred for large continuous pours.
Q: What slump is typical for tremie concrete?
A: Slumps are higher than normal (to improve flow) but controlled with anti-washout admixtures; the exact slump depends on mix design and project specs.
Q: Can I use tremie in tidal or fast-flowing waters?
A: Yes, but flows increase risk of washout. More robust mixes, larger tremie heads, and careful scheduling (slack water) are often needed.
Q: How do I inspect underwater concrete?
A: Visual diver/ROV inspection, coring and compressive-strength testing after curing, plus sonar/ultrasonic techniques for certain defects.