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GOING STRAIGHT

PILING & DEEP OUNDATIONS

Wide section sheet piles, anchors and tie bars are supporting the walls of a new canal link in Gloucester.

Gloucester and Sharpness canal is having a couple of its bends taken out to accommodate the approach road for a new bridge.

Originally built to link Gloucester to Sharpness Docks and the sea, the 26km long canal was designed to take sea-going vessels and underpinned the city's trade and wealth in the 19th century.

Just south of Gloucester the canal has a double bend, Two Mile Bend, which crosses the southern end of the new Gloucester South West Bypass where it joins the A430.

The £12M contract to complete this final 1.5km section of the £43M bypass was awarded to main contractor Norwest Holst by Gloucestershire County Council.

Known as the Netheridge section, work here involves removing the double bend and replacing it with a shorter straight section. The old canal will be filled in and some of the material excavated from the new canal will be used for road embankments.

This straightening will give an acceptable gradient for the approach road to a swing bridge across the new 600m long channel. The bridge must be high enough to allow a range of pleasure boats and sailing vessels to pass, and its 'air draught' of 4.75m means it can remain open most of the time.

Under a £700,000 contract, river and marine engineer and sheet piling contractor Dawson Wam installed about 900 steel sheet piles to form the walls for the new canal.

Sheet piling began in June last year. The channel was excavated down to about 3m to allow installation of the sheet piles, with 10m wide dams left at each end.

Dawson Wam used its high capacity 22m telescopic mast TM18/22 flexible format rig which is capable of counter rotating auger boring, pitching hammer installation or hydro press installation.

The sheet piles used were of a new 750mm wide section - AU23 - supplied by Arcelor. Dawson Wam claims it was the first time these had been used in the UK.

Ground conditions in the area consist of very stiff Lias clays and mudstones, and despite these becoming particularly hard over the summer, the rig installed the 900 piles in under four weeks.

Dawson Wam was back in January to carry out the last stage of wall construction - installation of a tie bar and anchor system to provide support to the sheet piles and meet criteria for the scheme's design life of 120 years.

The system, designed by the fim, required installation of about 180 tie bars spaced at 4.5m centres - one per group of three sheet piles - along the length of each of the two new walls.

The walls are anchored by 10.5m long, 50mm diameter threaded bars.

These pass horizontally through the walls and a waling beam behind and link to three sheet piles driven 4.5m below the towpath level. The tie bars are secured at either end with standard steel plates and locking nuts.

To prevent intrusion into the water side of the canal wall, which could damage passing vessels, the tie bar ends are mounted on the 'inpan' of the sheet pile rather than through the 'out-pan'.

Anchoring was finished in February. All the land-side works were at the depth of the canal bed and therefore some 6m below the new tow path level. The banked slopes and trenches to either side of the new canal will be filled and raised to the water top level.

Excavations to take the canal to its full depth of 5m were expected to finish this month. At the tie-ins with the new link Dawson Wam will remove the old steel sheet piles which formed the banks of the original canal section. It will also install new sheet piling for the cutoff walls to enable the diversion to take place. The 10m wide dams will be then removed and the canal opened to boats.

The new bypass is expected to open in April 2007. It aims to reduce local traffic, allowing Gloucestershire County Council to provide better bus, cycling and pedestrian facilities and to create a faster route for traffic travelling to and from the city centre.

It will also open up access to landlocked brownfield sites in the city and enable further development to the south.

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