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Going round the bend

Cover story - New infrastructure clashes with old on a bypass project in south west England. Jon Young reports.

Gloucester city centre has a medieval street plan that is ravaged by a plague of congestion.

The cure was unveiled in 1988 when Gloucester County Council announced plans for the South West bypass. But the new infrastructure interfaces with the old at Two Mile Bend, some 3km along the Gloucester & Sharpness Canal from the docks at Gloucester. To smooth the bypass's passage the UK's longest ever canal diversion is taking place.

On its completion in 1827 the Gloucester & Sharpness Canal was the largest and deepest ship canal in the world. Its purpose was to bypass a narrow treacherous stretch of the River Severn that had claimed several lives. Thomas Telford and Thomas Fletcher built the canal to provide a link to South Dock, the largest inland dock in the country.

Construction of the bypass began in April 2003. Most of the project is straightforward dual carriageway but the 1.4km Netheridge section, in the middle of the bypass, has thrown up all sorts of complexities accounting for £13M of the total £43M target cost.

Main contractor Norwest Holst project manager Richard Hogg says: 'The route is complex because it crosses the River Severn flood plain and two local water courses as well as the canal at Two Mile Bend. Along with a diversion we need to build a swing bridge to take the bypass over the canal.'

At Two Mile Bend the canal changes direction from south to west and then back to south, in a mirrored lightening bolt shape. It is at the first direction change that the bypass crosses the canal. At this point, in close proximity to a major road junction, the most complex engineering works of the entire project are taking place.

'At the moment the canal is too close to the road junction.

Given the head room required for the boats to pass under the bridge, the level difference between the bridge and the junction would be significant, ' says Hogg.

'The approach gradient would be too steep and cars could end up stacking back to the junction when the bridge opens for boats.' Stacking is not the only problem to come from a steep gradient. On a quiet night the bypass may be clear and cars could enter the junction at speed.

If the ramp to the bridge is steep then it may not be clear that the bridge is closed to traffic and the car could end up through the barrier and in the canal.

To solve the problem the canal had to be moved away from the road junction. The obvious route was to cut the corner at Two Mile Bend, thus increasing the distance from the road junction enough to allow for a gentle approach without altering the height of the bridge.

'Keeping enough head room is important. If it's too low then the bridge will have to open for every boat, causing congestion on the bypass, ' says Hogg. Halcrow has designed a £4.5M swing bridge that provides 4.75m clearance above water level, allowing 90% of craft to pass without the bridge having to open.

The 40m long bridge is an ingenious twin steel box design weighing approximately 350t.

It sits on concrete abutments founded on 750mm diameter, 13.5m deep bored piles.

When the bridge needs to open it is jacked off the permanent supports and turned on the 4m diameter pivot fitted with a slew ring bearing - the system used by cranes and excavators.

This allows the bridge to swing horizontally on a single point, turning just enough to clear the canal for the largest craft.

Few bridges in the world use this system, but it has two major advantages. First, the space required for the turn is greatly reduced, and secondly the time to perform the opening and closing sequence is shortened.

In this case it is just five minutes excluding ship-passing time.

A manned operations tower overlooks the bridge. 'The controller is able to perform the whole opening and closing sequence without leaving his seat. The long term plan is to control all bridges from a central control centre, but that's some way off, ' notes Hogg.

Even now the canal remains active, supplying the materials for the redevelopment of Gloucester and the South Dock. New 50m long barges, capable of carrying 500t of aggregate, are in use on the canal. Each barge is doing the work of 20 trucks and travels almost silently, unnoticed through the city centre. Because of its continued use the canal must remain fully operational throughout the diversion.

To achieve this Norwest Holst has come up with a construction sequence that completes the new section of canal without disturbing the old and in a single moment the swap will take place.

The route of the new canal is being excavated but large mounds of spoil are being left at the northwest and southeast ends. The earth maintains the wall of the old stretch of canal, allowing it to function as usual.

The new stretch is built using 600m of sheet and 75m of bored secant pile walls. The secant pile walls are being used around a 2.4m sewer that passes under the canal to lower the risk of damage to it.

If the sewer were to leak the canal and River Severn would become contaminated, threatening Bristol's drinking water, which is downstream of the sewer. Silt from the old stretch of canal will be dredged and used to line the new.

Approximately 60m of canal under the swing bridge is narrowed from 30m to 12m wide, allowing just enough space for the passing of two barges.

This section has sheet piled walls but uniquely also has a reinforced concrete slab on the base to prevent heave.

Vibration piling is being used to form the 4.5 to 6.5m sheet piled wall. The length of the pile varies according to the driving conditions so as not to damage the top of the pile. When the ground is too hard the piles have been cut to the correct level.

Once piling has finished and the swing bridge is complete hydraulic barriers will be placed in front of the earth mounds on the old canal side. The new section can then be flooded.

After the new stretch is flooded the earth mounds separating the two sections will be removed and the hydraulic barriers swung round to block the old route of the canal. From this point on all traffic will use the new stretch.

With the barriers in place the old stretch will be backfilled and sheet piles sunk to seal off the new route. The hydraulic barriers will then be removed leaving a short section of the old canal at either end to allow for turning.

The project is due to be completed in April 2007.

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