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Fixing the mix

Cardiff Bay

Some 107,000m3 of concrete was required within the sand bund working area for the major barrage structures, says deputy project director Rick Randall. Rather than try ferrying such quantities, the JV decided on a site batching plant, located on a widened out platform in the bund wall.

The Elba 105 plant proved very reliable throughout the project, feeding most of its output straight into a static Schwing pump and a set of fixed delivery lines taking around 60m3/h of mix to very large pours on the 18m high structures. Many pours of 800m3 and a few of up to 1,500m3 were needed for sluices, locks and fish pass.

Mobile Schwing pumps and four truckmixers supplemented the main lines. Three tower cranes, two Potains and a Liebherr moved the formwork and reinforcement into place.

The largest quantities were needed for massive concrete gate supports at either end of the three locks, lock cills and for sluice headworks, cills and slabs for big double stepped stilling basins. These were needed to kill the energy from a rush of water which at maximum can reach 2,300m3/s through the five main sluice gates.

'The sluice area is crucial for control in the barrage,' says project director Paul Neal. Its 9m wide steel gate sets must be shut against spring tides which can rise 3m higher than the 4.5m impounded bay water level. Opened on the ebb they may have to discharge accumulated flood water if the rivers were high during several hours of tidelock.

'That means flows more than twice the 950m3/sec maximum river flow are needed,' explains Peter Hunter, designer Gibb's project manager at the site. Discharging is through double leaf gates, each 7.5m high and 30t in weight, supported on rollers running in channels in the massive concrete headworks.

Fitting these using large telescopic cranes from within the cofferdam to lift them and the equally massive curved sector lock gates, came later however. First was foundations.

'These were relatively straightforward,' says Randall. 'The sands and gravels were taken out and mixed with cement to form a 2m thick layer of cement bound material.' Usually this was 10m below mean sea level but in places up to 13m deep.

Sheet piles driven to refusal in the Mercia mudstone enclosed everything to foundation height to cut off any piping. Water head differences can be up to 13m. A line of sheet piles also went in for a cutoff under the gate cills. There was also a 'box' of longer sheet piles reaching to above high tide level which had been driven from a jack up barge before the sand bund was sealed. These were for the sand filled 'control island'.

Above the foundations came the lock gate supports, essentially huge 16.7m high blocks of concrete at the nose of the lock structure. The barrel of the locks between these massive gate supports was made up from a sheet pile wall enclosing sand and held together with 96mm Macalloy tie bars. A 3m thick concrete cap sits over the top.

Reinforcement in each gate block runs right through the 3m thick concrete cill slabs in between and into the next gate block, locking the structure together against future differential settlement which might throw the gates out of true. The blocks have large recesses which both accept the gate during opening and provide a stilling basin for the water which flows in around the gate edge when it first opens.

'The kind of pours we were doing were like dam pours, and vulnerable to cracking from high heat of hydration,' says Randall . Temperature control was obviously a big factor in mix design and operations. The designer had wanted a 75mm aggregate mix 'like that used in hearting mix in dams', says Gibb's Hunter The eventual compromise was a 40mm aggregate that allowed easier pumping.

Even with a 30% PFA substitute there were still going to be heat problems however, because the overall cement level had to be high for durability to achieve a 120 year design life. Pours were limited to 2m lifts - Gibb had originally wanted just 1m to minimise risk of clearing - and concrete had to be placed at no more than 18degreesC and then insulated with blankets while it went off to reduce the temperature gradient.

Horizontal lift joints were green cut - water jet blasted to reveal the aggregate surface - before the next pour. 'It is much better than water stops, though there were some of those too,' says Randall. 'We got through quite a quantity of retarder,' he adds.

To cool the mix in the summer liquid nitrogen was injected directly into the mixing drum. This was found to have limited effectiveness and water chillers were used to supplement cooling.

Concrete finishes were to be high grade both for the high velocity water flow zones where irregularities can lead to erosion, says Hunter, and around the upper levels where architectural finishes were specified. Architect Alsop & Stormer contributed to much of the external appearance of the barrage including the distinctive control building, the bascule bridges that allow road access across the locks and the sluice control housings.

Achieving the close tolerances needed for the interface with complex mechanical and electrical features was difficult. Sluice gates in particular required very precise setting in the vertical running channels. The leaf gates run on rollers sealed front and back and atthe edge because they have to be completely watertight. 'Flood barriers can leak a little but here we have to prevent virtually any salt water leakage into the bay at all,' explains Hunter. Gates must also resist a head in two directions.

For these and the lock gate pivots a primary steel plate was set into recesses in the main concrete and then hydromechanical subcontractor Preussag Noell of Germany would attach the mechanisms to exceptionally tight 1mm tolerances. These were then sealed in with a secondary concrete pour. Noell made and installed the leaf sluices, the three bascule bridges, the gantry crane and the sector radial gates for the locks. Sea side lock gates are substantial since for the two deeper locks the base cill is at -7m. Gates must be 16m high.

Fabrication and fitting of M&E elements took longer than hoped and was one of the factors involved in the granting of an extension to the contract period from its original date.

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