Dense ground conditions and stringent noise limitations have resulted in some innovative solutions for construction of new facilities for the latest generation of container ships at the Port of Southampton. Claire Symes reports.
Much of Southampton’s history is founded on its thriving commercial port. Work is currently underway to ensure it remains one of the UK’s leading harbours by welcoming the latest generation of container ships. These massive Post-Panamax vessels can carry up to 12,000TEUs (20 foot equivalent units) and having berths capable of handling them is seen as the way forward in the world of shipping.
“The overall project has been valued at £30M, but the ground engineering alone makes up £6.5M of the work”
Chris Thomas, Volker Ground Engineering
But these 366m long, 49m wide and 15.2m deep ships require deeper draughts, longer berths and higher capacity cranes to accommodate them so major work was needed to adapt Southampton’s Berth 201 and 202. Piling work on the upgrade for Associated British Ports (ABP) is now nearing completion allowing main contractor Volker Stevin to start work on the quayside facilities that will ensure Southampton is ready to accept its first Post-Panamax vessel by the end of this year.
“The overall project has been valued at £30M but the ground engineering alone makes up £6.5M of the work,” said Volker Ground Engineering director Chris Thomas, who is leading the team undertaking the piling element of the project for parent company Volker Stevin. “Around £5M of the ground engineering is formed by the cost of the steel piles but it is still the largest contract that we have taken on so far.”
Essentially the piling work involves construction of a new quay wall from a combi wall formed in front of the existing quay, which was built in 1968 and is starting to show its age. The new wall has been built from steel tubular piles with sheet pile infills positioned by a combination of vibration and hammering. They will be tied into a sheet piled wall positioned 40m back from the new quay wall.
The scheme also involves installing a series of continuous flight auger (CFA) piles in pairs just in front of the rear sheet piled wall to support one of the rails for the new dock cranes. The front rail will be constructed as part of the pile capping beam that will be built by Volker Stevin on top of the combi wall.
Although Thomas has plans to take Volker into the world of concrete piling, the CFA work has been subcontracted to Keltbray which will move onto site in May. “We want to expand the techniques we offer but only when the time is right,” explains Thomas. The 750mm diameter CFA piles will be constructed to 23m.
While the site may not have the space constraints of many piling jobs, Thomas’s team has had plenty of other challenges to deal with.
First there were some delays to the main contract being let but, although the start date may have slipped, ABP’s end date for handover of the completed berths has not, so a lot of work has been fast-tracked and sequenced to avoid delays.
“The space on the site has meant that we have been able to have several elements of the work underway at the same time,” says Thomas. “We were also able to get the steel piles delivered straight from Arcelor Mittal by ship.”
This delivery strategy was necessary too as the steel piles are 1.8m in diameter and, although the 40m length of each pile is split into three sections for installation, they are still substantial - and there were 149 piles to install too.
The piles have been driven at 3.6m centre to centre spacings to leave 3m exposed above the high tide level. They are founded 33m below sea level. “The sea floor is currently at 10m to 12m below sea level but the area will be dredged by Boskalis Westminster later this summer to ensure the minimum draft is 16.5m with a maximum of 20m,” says Thomas.
Getting the piles into position was not a simple task. Volker worked with consultant Tony Gee to develop a temporary works access platform from which the piles could be driven safely 2.18m in front of the existing quay wall. “The platform allowed six piles to be driven in each phase of work and allows site staff access to connect the sections safely, but also helps to guide the piles into position,” says Thomas.
Each of the piles had to be positioned within 75mm of the design position and verticality has to be within 5%.
But driving the piles was also something of a challenge on more than one count. “The ground conditions at Southampton were dense sand and clays of the Earnley Sands and Wittering Formations, with SPTs of up to 95 in places and values in the 50s common,” says Thomas. “These ground conditions present most driven piles with a challenge, but with residential developments nearby we also had to work under strict noise limits that complicated things further.
“Steel driven piles were the only option because the rest of the port was still in use, so we had to find a solution to meet the noise and geotechnical limits.”
The noise limit was set at 71d(B)A and then this noise level was only permitted between 8am and 6pm Monday to Friday and 9am to 1pm on Saturdays, so using a larger hammer was not possible. The result was the use of an 55t S-280 IHC Hydrohammer fitted with a noise-reducing shroud to drive the piles to full depth once vibro techniques have failed.
“Mostly we only needed to use the IHC hammer for the last 17m,” says Thomas. “The shroud has reduced the noise levels to the mid-60s, whereas it would be nearer mid-70s without.”
“These ground conditions present most driven piles with a challenge but with residential developments nearby we also had to work under strict noise limits”
Chris Thomas, Volker Ground Engineering
The piles were also fitted with a cutting shoe following some in-depth pile driving analysis by Volker. Each set of six piles, supported by the temporary structure, are driven in a set order to avoid over-densifying the sands during installation. “It took around three days to drive each set of six tubular piles,” says Thomas. “But much of that time was taken up by positioning the temporary platform and ensuring pile verticality.”
Now Volker Ground Engineering has completed its piling work, Volker Stevin will soon be starting work on installing the tie rods. Pairs of 250mm diameter, 40m long tie rods will connect each tubular pile to the rear sheet piled wall.
“We need to demolish the upper part of the existing quay wall and remove the concrete slabs before we can construct the new concrete capping beam in order to tie the rods into the quay wall,” says Volker Stevin project manager Steve Garrigan.
In total 100,000m3 of bulk excavation will also be needed below the existing 140mm thick concrete slabs to enable drainage and ducts for lighting to be built but most of the excavated material will be reprocessed onsite for reuse. The recompacted sub base will be built to highway specifications before a 400mm thick concrete ground bearing slab paving is constructed on top to cope with the higher capacity cranes.
“One of the main concerns was the piling work, so I am relieved that everything has gone smoothly,” says Garrigan. “However, the next couple of months are also key to successful completion of the project. ABP’s new cranes are due to be delivered to site in October and space is needed to store and commission them, so there are constant reminders that the end date for the work is a fixed date on the calendar.”