Construction of a tunnel for a new gas pipeline below the Humber will safeguard around 20% of the UK’s gas supply for the next 40 years.
Much of the focus for a new tunnel project beneath the Humber has been on keeping water out of the site but ultimately flooding the bore will enable the new gas pipeline to be completed. But before the project reaches that stage, there is the small matter of a 4m diameter, 5.4km long tunnel to be constructed below the estuary on the £100M scheme.
humber tbm mary
The new pipeline will replace one that runs in the river bed itself which is at risk from shifting tidal deposits – this is not ideal given that the pipe carries up to 20% of the UK gas demand in winter.
A joint venture of Skanska, Porr and A Hak is about to launch the tunnel boring machine (TBM) to drive the tunnel for National Grid and completion in 2020 will ensure the security of this gas infrastructure for another 40 years.
“The existing pipeline was built in the early 1980s and we currently use foam mattresses and gravel bags to provide protection to the pipeline but this requires continuous monitoring,” says National Grid project manager Steve Ellison.
“Placing the new pipeline in a tunnel will remove the risk.”
Once the tunnel bore is completed it will be flooded to allow the new 1,050mm diameter pipeline to be floated through the tunnel and the water will remain in situ throughout the lifetime of the pipe.
“There are only five similar projects to this one worldwide and this is believed to be the longest,” says National Grid senior project manager Phil Croft.
“The concept has been done before but we are stretching the limit with both the tunnel and the pipeline.”
Ready to tunnel
Work on the new tunnel is due to get underway in early March with the launch of a Herrenknecht slurry tunnel boring machine (TBM) on the southside of the Humber at Goxhill. From there it will drive under the Humber at depths of up to 31m below the river level and emerge on the north side at Paull.
Cementation Skanska is set to start work on the secant pile wall for the reception shaft at Paull in March but much of the construction activity has been at the launch site so far.
The work site at Goxhill is no stranger to gas pipeline work as the existing pipe was strung together and launched from the same location on 1984. With the use of tunnelled techniques this time, the preparation work is more involved.
“The site will be returned to fields at the end of the project,” says joint venture project manager Craig Sewell.
The TBM launch is being carried out from a sloped chamber that is 209m long, 8m wide and up to 11m deep.
The launch chamber is formed of steel sheet piles – installed by Sheet Piling UK – and a secant pile wall at the headwall end which was constructed by Cementation Skanska with soft piles for the TBM launch.
“The combination of steel sheet piles and secant piles was selected to the launch chamber to provide a cost-effective solution,” says joint venture construction manager Steve McNiccoll.
“The secant piles are 21m deep and steel ones would have needed to go deeper and have a second layer of props at the deepest point to provide enough support but this would have been an obstruction for the TBM, plus you can’t drive the TBM through steel.”
During construction two levels of props, supplied by MGF, were used until the base slab was cast, enabling the lower level to be removed.
“The secant piles are 21m deep and have a diameter of 1,080mm,” adds McNiccoll. “In total there are 59 piles in the chamber with 12 2m diameter piles in the headwall with glass fibre reinforcement to form a soft eye for the TBM to break through. These soft piles have concrete with a maximum strength of 5N, while the remainder of the wall is formed by piles with 28 or 35N concrete with reinforcement only in the male piles.
“There were some issues with flints when it came to driving the steel sheet piles. They extend to 9m below the base slab to create a cut off.”
Before the chamber could be excavated and base slab cast, a major programme of dewatering had to be carried out by WJ Groundwater.
WJ Groundwater carried out pump flow tests to design the well point extraction system to dewater the site for the excavation and base slab construction phase. There were eight extraction wells within the launch chamber box and 10 outside where flows of up to 8litres per second were recorded.
“The site lies within a principal aquifer so careful monitoring was required to ensure the recharge was solely from the aquifer and did not result in saline intrusion from the nearby Humber,” explains Sewell.
“We used real time monitoring to show the recharge of the aquifer and I believe this is the first time it has been done.”
With the launch chamber completed, the TBM was brought in by road from nearby Immingham docks.
The slurry machine was selected for the job after careful consideration of earth pressure balance tunnelling techniques. While either would have suited the Chalk, it was considered that the slurry system was better suited to dealing with the glacial deposits at the start and end of the drive.
The Herrenknecht TBM has 16 gantries and will be serviced by a wheeled multi service vehicle that will take materials to the construction face and transport workers too. The MSV feature a collapsible cab so the segments can off-load straight over the top.
“There are six segments per ring and each ring in 1.2m wide,” says Sewell.
The first 125m of the tunnel will have segments with steel reinforcement until the drive has reached the Chalk where the segments will be switched to fibre reinforced and then back again to steel reinforcement as the drive emerges out of the Chalk as it nears Paull.
“The TBM will be launched using bentonite face support through the glacial tills until the bore passes into the Chalk,” says McNiccoll.
The tunnel will have a cover of around 15m during the drive. The TBM will dive at a 4% gradient for the first 420m of the bore and will then level out for 3.7km, before returning to a 4% upward gradient for the final 720m to emerge within the shaft at Paull.
McNiccoll is not expecting issues with groundwater flows within the Chalk during the drive and varying the slurry support pressure is expected to be sufficient. Groundwater pressures of up to 3.5bar are anticipated.
The TBM cutter head is equipped with different types of cutter tools including disc cutters for boulders or flints. Wear indicators will help to detect wear on the tools. National Grid has said that additional interventions will be undertaken at 600m to 1,000m intervals depending on the ground conditions.
According to National Grid, the key challenge on this project is driving such a long tunnel with small internal diameter without any intermediate shafts.
At Paull, the secant piled shaft will be excavated to allow the base slab to be cast before being backfilled to receive the TBM. Once in the reception shaft, the TBM cutter head will be excavated and removed followed by the gantries.
At the end of the project, some of the steel piles forming the launch chamber will be pulled out but most will be cut off as they care cast into the base slab to create a water-tight cut off. The steel piles have been installed with an upstand to create a flood barrier and there is a gate to enclose the end of the chamber.
The slurry treatment plant and filter press close to the chamber has also been raised up to protect the equipment from flooding. This facility will be used to process more than 40,000t of tunnel waste during the life of the scheme but the end destination for this material has yet to be determined.
The pipeline itself will be supplied from Germany and will be brought to Goxhill by road from Leith in Scotland where trials are currently underway on the concrete weighting coating.
“The coating is designed to be just negatively buoyant to allow it to be floated through the tunnel using a pipe thruster,” explains Sewell.
“It will be a five-week process to install the pipe.”
PU collars will be fitted around the pipe to protect the pipeline and concrete lining of the tunnel during the installation.
The new pipeline has a design life of 40 years, while the tunnel has a design life of 120 years. “In theory, the pipeline could be replaced but that will depend on what the energy demand is in the UK in the 2060s,” says Croft.
The future of the existing pipeline is not certain yet either.
“No decisions have yet been made about what happens to the existing pipeline once the new one is commissioned,” says Croft. “Removing the pipe could be more environmentally damaging than leaving it in situ but that is a project for the 2020s, rather than now.”
Whatever the future for the existing pipeline or the new one in 40 years’ time, it is clear that the current investment will not leave the UK out in the cold with a shortage of gas.