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Dock delivery

Pouring high quality concrete underwater is never easy. New Civil Engineer reports on one project which pushed existing concrete technology to the limits.

A high strength homogeneous concrete foundation 10m thick was what client Devonport Royal Dockyard wanted for its new low level refuelling facility (LLRF). Of course, since the vessels which were going to be refuelled were Royal Navy nuclear submarines, the foundation would also have to comply with nuclear safety standards - including seismic resistance.

The problem was that the required 11,000m3 of unreinforced mass concrete would have to be placed on top of heavily weathered rock and under 16m of salt water. And this after cleaning up to 400mm of silt off the top of the rock.

Early in the tender process, the successful contractor Taylor Woodrow Construction identified the underwater pour as being critical to the success of the £12M LLRF project, and began to consider the options.

Taylor Woodrow deputy project manager Rob Williams says the first challenge was preparing the rockhead within the existing cofferdam around the dock site. 'We wanted to use an airlift and divers, but for this we needed permission to remove silt and rock rubble together. Then the rock could be washed, and the silt returned to the sea outside the cofferdam.' This was eventually granted.

The client specified a concrete with a characteristic strength of 40N/mm2 at 56 days and a maximum water/cement ratio of 0.45. Other requirements included minimal washout - to minimise the weak layer of laitence on top of each pour and minimise turbidity in the surrounding dock, very high workability, slow evolution of heat from cement hydration, and a design life of 50 years.

Minimal wash-out, and the low turbidity so vital in an operating nuclear environment, could be achieved with a cellulose-based underwater concreting admixture. Superplasticisers offered high workability. Replacing a large percentage of the Portland cement content with ground granulated blast furnace slag (GGBFS) was an obvious way of keeping the heat of hydration under control and minimising the risk of early age thermal cracking, as well as enhancing long term durability.

But high workability, essentially self-levelling concrete is particularly sensitive to aggregate properties, not least particle size distribution. And preliminary trials by preferred concrete supplier Readymix UK on behalf of the client cast severe doubts on the plastic properties of concrete made with the locally available crushed limestone aggregates.

One option was to import quartzite aggregate, a move Taylor Woodrow estimated would add £100,000 to raw material costs alone. And quartzite aggregates would significantly increase the risk of early age cracking, which could only be brought down to acceptable levels by thinner pours. This in turn could have meant taking three times longer to complete the foundation.

So the contractor turned to its specialist consultancy arm, Taywood Engineering to develop a mix using the local aggregates that could be pumped over long distances and placed under water without segregation, bleeding or washout.

Luckily, the local limestone quarry could offer aggregate in 28mm, 20mm, 14mm, 10mm and 6mm fractions, plus fines and dust. TEL eventually discovered that by combining 20mm, 14mm and 10mm fractions with a small proportion of 6mm a smooth grading curve was produced which optimised plastic properties.

With 75% of its 450kg/m3 cementitious content made up of GGBFS, the mix achieved 40N/mm2 in 14 days rather than 56 , without excessive heat build-up during hydration. A carefully balanced dosage of superplasticiser and underwater admixture, both supplied by Sika, completed the mix design. Further trials on site confirmed the excellent plastic properties of the mix and significantly, the minimal laitence on the surface of the concrete placed underwater.

Williams explains: 'We saw the key to success as minimising time between pours and using divers as little as possible. A concrete that could be poured without aggregate segregation and which generated very little laitence cut down the time needed to prepare the surface of the previous pour and enabled us to meet the two week pour cycle.'

Pouring as fast as possible in as thick a layer as possible was also vital. Modelling of laboratory and site trials had indicated that pours of up to 2.75m deep and 3,500m3 would be free from the risk of early age thermal cracking.

Originally, Taylor Woodrow planned to use 40 tremmies of the lay-flat type to cover the large 46m by 27m plan area of the pour, but the increased workability and cohesion of the final mix design allowed this number to be cut to 24. There was, however, one unwelcome discovery during the final trials.

Two mobile concrete pumps with 40m booms had been selected to transport the concrete from the truckmixers to each group of 12 tremmie pipes. 'But we found the highly cohesive mix didn't like flowing through all the bends and joints in the mobiles' booms,' Williams reports.

'Pump pressures were as high as 200 bar or more, which we considered would put the reliability of the pumps at risk during long pours lasting up to 90 hours. Pump breakdowns and cold joints were unthinkable. We had to come up with something that worked at lower pressures.'

Working closely with concrete pumping specialist Ascus, Taylor Woodrow selected a rotating horizontal placement unit fed by a fixed line from a static concrete pump. With fewer joints and bends in the system, pressures were reduced to little more than 100bar, a more acceptable level. Two units were set up, each feeding one set of tremmies, capable of placing 40m3 of concrete an hour between them. In the event, Taywood had less than two weeks to tune the mix design before placing began.

Work began in October 1998, with subcontractor Brantford Civil & Marine responsible for the airlift cleansing of the rockhead. The first concrete was poured in February. With strict quality assurance procedures in place, TEL providing constant site supervision and two local RMC plants dedicated to the project, Williams is happy to report that only seven out of 1,700 truckmixer loads delivered to site were rejected on the grounds of workability.

And he points out that all through this part of the project all Taylor Woodrow's proposals had to be independently vetted and approved by a peer review group which included representatives of the client, BNFL Engineering, the Ministry of Defence's nuclear works advisors and independent concrete technologist Halcrow. The foundation was completed, on schedule, by Easter this year.

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