Coastal construction work near South Shields will not only create a larger beach at Little Haven but also protect the area from frequent tidal flooding
Replacing a sea wall to prevent coastal flooding is no easy task but it is one that Galliford Try has successfully undertaken near South Shields. Not only did the demolition and reconstruction place the area at higher risk of flooding but the construction work itself was also at risk from the flooding during the work.
Despite the site being flooded three times during the construction work, Galliford is now on the verge of completing the £3.5M Little Haven Sea Defence Wall. According to the company’s contracts manager Dave West the main focus of work is now on the finishing touches.
The company was awarded the project in January and work started on site in April this year. Most of the construction work is on course to be completed by the end of the year but West said the company would probably be on site completing a few final parts of the project in January next year.
In essence the project, which is being funded jointly by South Tyneside Council and the Environment Agency, involves demolition of the existing sea wall and construction of a new structure on a slightly different alignment to create a larger beach area.
The main sea wall dates back to the late 1930s but, according to the council, some parts of the structure are believed to be over 100 years old. Unsurprisingly the structure was showing signs of its age and had reached the end of its serviceable life. “The orientation of the old sea wall was also affecting the movement of beach material and causing it to deplete more quickly than it would naturally, so the line of the new wall has been designed to encourage deposition,” says West.
The age of the structure wasn’t the only issue – it was also no longer able to cope with high tide surges and flooding behind the wall was a frequent problem and something West’s team had to contend with during the work.
“We kept a close watch on the tides and generally knew when flooding may have been a problem but in order to protect the equipment we had to move it all out of the way ahead of the surge and often wait for another three days for the water to subside,” he says. “Even then we had to carry out clean up operations before construction could get underway again. We always moved the equipment to higher ground when the site was not operational, just in case.”
Due to the change in alignment the existing 650m wall has been replaced with a 550m long new structure. “The height of the new wall varies from 3m to 4m but, with the movement of beach material, only the top 1m or 2m will be visible most of the time,” says West.
The new structure is formed in two parts with a lower sea wall in front and a higher rear wall connected by a concrete slab foundation and a stepped concrete revetment built over the slab to effectively join the two walls.
“The project can be split into two parts – the southern 220m of wall construction is a direct replacement for the existing structure, whereas the northern 330m of the scheme involves constructing the wall with a sweeping curve that extends behind the existing wall, which projects out at a steeper angle. It is this projection which has caused the issues with beach material.”
There have been up to four concreting gangs working on the project at any one time and the volume of workforce is essential as all the work has to be planned around the tides which limits the construction time available.
“We kept a close watch on the tides but to protect the equipment we had to move it all out of the way ahead of the surge and often wait for three days for the water to subside.”
Dave West, Galliford Try
The concrete being used is a fibre reinforced mix which has been pigmented to give it a sandy coloured appearance. “The consultant for the scheme, Royal Haskoning, worked with Bardon Concrete to develop the mix design,” says West.
On the like-for-like construction section of the project, West’s team started on demolition first but with construction of the new wall chasing the demolition team to ensure the coastline was not left exposed for too long. “We built up a sand bund in front of the work to help add protection while the construction was underway,” he says.
“Demolition took a few weeks to complete but was not quite as straightforward as we expected – the mass concrete structure was supported on wooden piles that were not on any of the drawings. They were at 5m to 6m centres and extended to some depth but we cut them off at formation level rather than trying to excavate them out completely.”
Work started with the front wall before forming the slab and the rear wall ready for the stepped revetment construction.
The method for building the curved section of the new wall was different though and called for a 330m sheet pile was to be installed. Galliford worked with piling contractor DC Engineering to install the 3m section piles using an excavator-mounted vibrating hammer to embed the piles to a depth of 2m.
“This part of the wall is located in the mouth of the River Tyne estuary so there was a risk of unexploded ordnance so surveys had to be carried out before we started the piling work,” says West. “After we carried out an initial strip of the surface material Zetica came in and carried out a magnetometer scan of the area. Several suspect areas were located but excavation showed that the area was clear and we were able to get the piling underway.”
At the northern end of the wall, the structure has been tied into the sand dunes using sheet piles that extend to 9m below ground.
“One unusual aspect of this scheme was the need to dig out the grasses from the sand dunes where we were carrying out the piling work so that they could be transplanted elsewhere,” says West. “I don’t think I’ve ever had to oversee anything like that before.”
With the sheet pile wall in place, Galliford has been working on constructing the front and rear walls and the stepped revetment which means the finished northern section will look just like the south part when it is completed.
As GE went to press, West reported that around 90% of the work is now complete so the Little Haven area of South Shields should be well protected from the risk of flooding during any upcoming winter storms.