A major project is currently under way to improve road access into the Port of Immingham in Lincolnshire. The £88 million A180/A160 Road Improvement scheme is designed to relieve congestion on this key strategic route, as well as stimulating growth and unlocking economic benefits in the area.
The Highways England scheme involves widening and improving approximately 5km of road, and includes upgrading a section of single carriageway to dual carriageway standard and improving existing interchanges.
Highways England appointed Costain as main contractor for the project, and work started in March 2015.
As part of the scheme, Costain appointed local contractor CR Reynolds Ltd to construct and install two surface water attenuation tanks within a piled cofferdam measuring 32m long, 7.5m wide and 8.5m deep. When the subcontractor arrived on site, the piles had already been driven, and a dewatering system installed to lower the existing ground water level and relieve the ground pressure from an underground aquifer.
CR Reynolds Ltd was responsible for the rest of the work, including the staged excavation of the cofferdam and installation of the temporary works needed to support it while the tanks were lifted into position.
Costain commissioned the temporary support directly, working with ground support specialist Groundforce Shorco to design a system of frames and struts that would provide the necessary support, while enabling the two 14m long tanks to be installed. That design process took around six months, as the two companies worked together to find a workable solution.
Costain Temporary Works Coordinator Faz Neysari says: “We started with the initial discussion on site with the Groundforce Design Manager Emma McKeen around January 2016, and then went through a very thorough process on design and coordination between the Costain site team and engineering services and the Groundforce design teams"
The resulting design consists of three levels of frames formed using Groundforce’s Super Mega Brace sections, supported by MP250 cross struts. The cross strut layout was designed so that struts could be removed and re-installed in a specific sequence to allow the two 14m long tanks to be installed.
One of the challenges in designing the temporary support was ensuring that CR Reynolds had enough clearance to pour the pumping station tanks concrete surround which has been designed to counteract the possibility of uplift caused by the presence of the aquifer and ground pressure. The design also includes a particularly deep concrete base.
“The depth of the base meant a large clearance was required at the first phase of excavation before the lower frames were removed to allow the tank installation,” explains Groundforce senior technical sales representative, Trevor Kendrick.
Ground conditions are good, with all the cofferdam excavation being in clay, but the presence of the underground aquifer meant dewatering had to be installed and maintained to run 24 hours a day, seven days a week throughout the construction process. The dewatering system pumps 10l of water per second from around the perimeter of the excavation.
Due to the water pressures, a passive relief well was installed within the cofferdam to allow the water to flood the excavation rather than blowing the base of the excavation in the event of a complete failure on the dewatering system. This was at its most critical when the excavation was at formation level.
CR Reynolds used a 40t excavator to remove most of the clay within the cofferdam, but with the base at 8.5m below ground level, an alternative method had to be found for the lowest section.
“Our 40t excavator was unable to excavate the last 2m of spoil from within the piles, so an 8t excavator was lowered into the excavation – with associated gas detecting monitors – and excavated the remaining spoil to formation level,” explains CR Reynolds Site Manager Ross Benson.
The excavated material was placed in a 2,000l boat skip and lifted out of the excavation using the 40t machine.
The three levels of frames where installed along with ladder access into the excavation. “At such a depth, and with space tight, a safe means of entry and egress from the excavation was essential,” says Benson. Groundforce supplied the contractor with a combination of its Mega LadderSafe and Premier LadderSafe systems.
The top support frame was hung from the sheet piles, and the two lower frames where installed on gallows brackets welded onto the sheet piles. CR Reynolds lifted the frames into position using its 40t excavator and the company’s own labour force.
Once the base slab had been cast, the lower frame and struts were removed, leaving two sets of frames to support the excavation.
“Each frame has three support struts running across the width of the excavation and, due to the size of the tanks, a conflicting strut from both frames had to be removed before the tank could be installed,” explains Benson. “The strut then had to be reinstalled and pressurised before we could proceed with the second tank.”
He adds: “We used a 100t crane fitted with a man basket to gain access to the struts, and then lifted them out using our 40t excavator. This process was repeated to reinstall the struts after the tank had been installed.”
Each tank measures 14.5m in length and 4.2m in diameter, and weighs approximately 8t. They were both installed in one shift using the 100t crane, and w will eventually be surrounded by 1,000m3 of concrete, enabling the supports to be removed.
Kendrick says: “The designers really had to think “outside the box” on numerous occasions to find a workable solution. They showed a huge amount of determination to achieve a solution, and the adaptability of our product range was demonstrated in this pursuit.”
Neysari adds: “The pump station at A160 Port of Immingham is a complex scheme and due to the location and the sensitivity, it did require close coordination and good execution. I approached Groundforce for an initial consultation, and the way they took off with it and developed the idea rapidly to a workable scheme was great. Their design team, as well as their site technical representatives, were very well coordinated – from organising a workshop on site for the whole of the operation team and subcontractors to the onsite tool box talk, all activities were well executed.