Poor Orwell Bridge has suffered problems with its bridge bearings for nearly a decade now. The structure is located south of Ipswich and is a 1.3km-long 18-span, continuous twin post-tensioned concrete box-section structure. Built in 1982 to carry the A14 over the river Orwell to and from the port at Felixstowe, 22 weeks of treatment at three pier locations is currently nearing completion.
Defects were initially observed on roller bearings during an inspection in 1992, resulting in them being replaced on one pier location. But by 2000, bearings at five more pier locations had to be refurbished. To keep a closer eye on the bridge's deterioration a remote monitoring system was set up in 2003 by specialist Strainstall. Soon, it detected excessive movement at piers 12, 13 and 14 on the southern carriageway. Consultant Atkins with main contractor Jackson has been investigating the cause of the bridge's bearing deterioration and carrying out the work on these piers.
"When the bearings were replaced in 1999/2000, the original bearing top 'sub plate' and bottom plinths were retained and not broken out as done in the current contract," explains Atkins assistant supervisor Chris Blackburn."This limited the scope for positioning the top and bottom bearing plates of the new bearings, which ultimately limited the movement of the new bearings [when they were replaced in 2000]."
Current bearing replacement involves jacking up nearly 2,000t to lift the deck by just 10mm to remove each bearing. Jacking specialist Heavylift is carrying out this work at night with subcontractor Martins Traffic Management.
After the worn-out bearings have been removed, they are replaced with temporary ones while each plinth and soffit is cleaned up ready for bearing fabricator Freyssinet to install the permanent component.
While carrying out this operation at piers 12 and 13, more problems were discovered."On removal of the top 'sub-plate' of the bearing, voids and poorly compacted concrete were detected. Also, existing reinforcement was found to be exposed," recalls Blackburn.
Worse still, when the honeycombed concrete was broken out, it revealed that the four layers of transverse and longitudinal layers of reinforcement were packed together so tightly that concrete could not flow between them, causing further structural problems. All of this would have to be removed until sound concrete was reached.
The solution was to stabilise the area where concrete had been broken using grout injection and then fill the void with Thoro Structurite mortar. After 24 hours, the Structurite had cured and any lingering voids were injected with BASF Masterflow 101, a high early strength epoxy compound.
The work is now nearing completion with a £150,000 cost overrun bringing the total cost up to £1.95M. But the project has shed much light on some of the problems with Orwell.
Blackburn adds that having brought so many specialist subcontractors together and really getting to the bottom of Orwell's problems has meant that any future work will be much more straightforward to programme and manage.
"If we have to return to replace more bearings, we know the team we need to put together," says Blackburn."
ORWELL BRIDGE HISTORY
This bridge was originally conceived in 1965 but construction did not begin until 1979. Consultant Halcrow and contractor Stevin Construction Civil Engineering BV, Rotterdam, worked on the original structure. The structure is made up of 19 piers with the longest and navigable span at 190m between piers 9 and 10. It is founded on 1,138 piles, 1m diameter and 40m deep. The bridge was completed in 1982, at a cost of £24M (1982 prices) and is currently used by 50,000 vehicles per day.
Delivery of plant and materials demanded some creative planning. Lowering plant and steel bearings over the side of the bridge would have involved a lot of heavy-lifting gear on the westbound carriageway and required the road to be closed. The solution was to ferry plant and materials up the river from Manningtree by maritime hauler Mistley Marine. But since the Orwell is tidal, deliveries had to take place when the water level was at its highest Đ even if that was 4.30am.