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Behind bars


Dock 9 will use more than the normal UK supply of 50mm diameter reinforcing bars.

Working at Devonport is 'like building a dock at the end of a cul-de- sac in central London', says DML construction logistics manager Alistair Harpur.

In addition to co-ordinating more than 10,000 personnel from over 100 subcontractors, the DART alliance has to manage plant and materials for some mammoth construction. At its peak, when construction of 9 Dock is going full tilt, between 30 and 35 trucks a week will be delivering reinforcing steel from supplier BRC to the cramped and labyrinthine site.

Dock 9, built at the turn of last century, will consume 15,500t of reinforcement and 90,000m3 concrete. The project will outstrip normal UK supply of 50mm diameter rebar.

At the moment subcontractor Christiani & Nielsen is completing installation of a cofferdam isolating Dock 9 from the adjoining basin. The walls have been temporarily anchored for stability and excavation of 70,000m3 of Victorian concrete and shillate - decomposed sedimentary material common across the site - is under way.

To take crane loads and provide stability to seismic standards, the dock's gravity walls are to be reinforced with tubular piles by specialist contractor Expanded Piling. The 700mm-800mm diameter piles, 36 per wall, will be socketed into rock to a depth of 12m and act as non-post-tensioned anchors. Depending on rock depth, the piles will be between 35m and 40m long. Two test piles have been installed.

Meanwhile, main contractor Tarmac and concreting sub-contractor RMC have started pours on the base slab. The slab is being founded on 1.5m deep downstands to facilitate drainage and prevent uplift.

Ground granulated blast furnace slag replaces 70% of the Portland cement in the mix to reduce heat generated as the giant pours cure. Project manager Loftus Buhagair says the relatively slow development of strength in GGBFS concrete is not critical in dock structures since the 50N strength concrete acts in the main under gravity only. He says GGBFS gives more durable concrete and, significantly on such a large job, is cheaper than ordinary Portland cement.

Though superplasticisers have been used to attain flow in the tight reinforcing matrix, ready-mixed concrete supplier RMC has minimised other additives. Buhagair says vibrating poker compaction is crucial to achieving good results.

Reinforcing and concreting to be carried out on 9 Dock will closely resemble that in the upgrade of 15 Dock, completed by Kier Construction last month. On 15 Dock, however, the existing 2m thick concrete lining, built in the 1970s, has been left in situ and overclad with a 2.5m reinforced concrete layer.

The new lining has been rock-anchored by subcontractor PSC through the existing concrete and into bedrock. 206 Macleoid bar rock anchors, 30m long with a 200t capacity at 1.5m centres, were used. Anchoring means the structure works compositely. Performance was analysed with a combination of finite element and hand calculation.

The adjoining walls of 15 and 14 Dock have been tied together with steel dowel and the space between them - a voided 'cellar' - filled with reinforced concrete, again from RMC. Steel fixing was by local subcontractor Richard Medlin.

Construction on 15 Dock started in February 1998. Work was completed with the bolting in place of the seismically designed submarine cradle, manufactured by Strachan & Henshaw.

All three docks are sealed by a cellular reinforced concrete caisson that can be floated into position and then 'sunk'. Each of these has a displacement equivalent to that of a Trident submarine.

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