With Channel Island ferry services already set to restart in time for the peak holiday season in July, Balfour Beatty is under pressure to complete harbour wall repairs at Weymouth. Claire Symes reports.
As the London 2012 Olympics’ sailing venue, the coastal Dorset town of Weymouth was placed under the media spotlight last year and the hosting of the Games has gained the area many accolades. However, not everything has been plain sailing for the town as subsidence problems on a quay wall in the harbour has meant that ferry services to the Channel Islands have been suspended since February last year.
Nonetheless, Condor Ferries is already taking bookings for services from 17 July, a clear demonstration of its confidence in Balfour Beatty delivering a £3.3M scheme to repair the damaged berth. Under the contract from Weymouth and Portland Borough Council, the contractor is replacing the damaged quay wall with a new combi wall and scour protection to reduce the risk of future subsidence problems.
“The problem with the quay wall was first noticed in February 2012 with some settlement and cracks observed in the structure,” says Balfour Beatty contracts manager Mark Stanyon. “The other three berths at Weymouth were surveyed following the problems at Berth 3 but the subsidence was found to be limited to the initial area.
“Emergency work was carried out by the council to try to stabilise the area but the ferry service had to be diverted to Poole until permanent repairs could be carried out.”
The quay at Weymouth has been extended a number of times but the structure at the front of the quay is believed to date from the 1950s.
“Initial investigations revealed that the subsidence was caused by scour from where the ferries were turning as they entered and exited the port.” Mark Stanyon, Balfour Beatty
The damaged quay wall was formed from an L-shaped cast insitu concrete wall formed on precast concrete piles that extended to 10 or 11m below the base of the wall. The structure was faced with concrete planks to form a 3.5m high quay wall from sea bed level to the side of the quay.
“Initial investigations revealed that the subsidence was caused by scour from where the ferries were turning as they entered and exited the port. Some of the holes were up to 6m deep and had destabilised the wall by causing it to move down and out into the harbour,” says Stanyon.
The repair work being undertaken by Balfour Beatty is creating a new berth over the failed section by demolishing the old 1950s structure, the L-shaped wall and removal of some of the existing piles that obstruct the new piles.
The combi wall that will replace the existing quay wall will be formed from 14m long, 1,422mm diameter steel tubular piles infilled with A218 sheet piles, plus a new capping beam.
The new combi wall will be protected by a scour mattress that will be constructed by a subcontractor later in the contract.
“Essentially we are building a cofferdam using a combi wall,” says Stanyon. “The rectangular structure will be formed by a combi wall to the seaward side and ends, and closed on the landward side by a straight run of sheet piles. The front seaward side will be tied to the rear sheet piles by two rows of tie rods.”
Tubular steel piles
The 44 tubular steel piles being used at Weymouth are 15.5m long, 1,425mm in diameter with a wall thickness of 25mm. On the sides being formed by the combi wall method, the tubular steel piles are being infilled with a pair of AZ18 sheet piles that are also 15.5m long.
The structure is 7.5m wide with 6m, centre to centre, between the tubular piles. A row of 105mm diameter Macalloy tie rods are being installed 0.5m above ordnance datum, while the upper level of tie rods, which have a 76mm diameter, will be located at 2.5m OD and will be tied into the new 2.5m thick, 2.2m wide capping beam that will be built across the front of the structure.
Working close to tidal water, particularly when the site is located alongside a live harbour, is never easy, but Balflour Beatty has also had to overcome some other unexpected challenges at the site. “The work was originally planned to minimise demolition work and pile through the existing quay,” says Stanyon.
“But I think the stability of the quay has been an ongoing issue as when we started work on site, we discovered that extensive grouting work had been carried out in the past. We were expecting to be working with fill material but in the end we had to excavate out the grout with hydraulic breakers.”
Even that was not straightforward as Stanyon’s team were trying to work around the existing piles to help make removal of the ones that impinge on the new structure easier. In total Balfour Beatty had to remove 12,000m3 of material before the main piling work could get underway at the end of February.
The excavation work revealed the original Portland stone quay wall - believed to date back to the 1850s - behind and somewhat lower than its 1950s replacement. “The same extent of subsidence that was seen in the 1950s structure was also seen in the original buried wall,” says Stanyon.
Stanyon says that piling - if all goes according to plan - should take seven weeks to complete and was on schedule to finish as this issue of GE went to press.
All the piles are being undertaken by Commercial & Marine Piling and are being installed from the quayside using a 180t crawler crane fitted with a vibrohammer to minimise noise. “We are driving the piles as far as possible using the vibrohammer and then using a 30t BSP 1146 impact hammer to take the piles to the design depth,” says Stanyon.
Once the piling element is complete, the site team will excavate down to 2m below OD to install the tie rods before backfilling the structure, forming the pile cap and forming a new slab over the structure.
Despite the initial delays on site caused by the grout, Stanyon expects the work to be completed early this summer. Condor Ferries will certainly be hoping this will be the case so it can resume its crossings to the Channel Islands from mid-July.