This is the smallest site I have ever worked on for this type of operation,” says Ivor Thomas, referring to the challenges of working at the ‘cosy’ Carnwath road site on the Thames Tideway project, a sewer scheme designed to catch overflows that would otherwise be bound for the River Thames.
Situated in West London, the Carnwath road site is 250m long, 86m wide at the eastern end and 48m wide at the western end. It is the launch site for the TBM that will excavate the western section of the new sewer.
This site is split into two parts; a main site, which includes Whiffin Wharf, Hurlingham Wharf, Carnwath Road Industrial Estate and an area of the River Thames foreshore.
SITE SETUP
Construction of the west section is being carried out by a joint venture of Bam Nuttall, Morgan Sindall and Balfour Beatty (the BMB joint venture). This contract, known informally as Tideway West, covers the work taking place from Acton in west London to Wandsworth in southwest London .
“We had to sink a 24m shaft and to build an acoustic shed 18m by 30m and to reconstruct a river wall and to sink 560 piles,” Thomas says. “The whole process required careful choreography on site.”
The site at Carnwath road required the sinking of more than 160 piles, some permanent while others are just for temporary works. “For example it was necessary to reinforce 250m of the river wall to allow the construction of the acoustic shed,” says Thomas.
While there is no combined sewer overflow being intercepted at this site, it will be used to drive the main tunnel to Acton Storm Tanks, and receive the main tunnel from Kirtling Street. It will also receive the Frogmore connection tunnel driven from Dormay Street.
SWITCHING SITES AND ALLAYING FEARS
Ben Green, Tideway’s delivery manager for this section of the project explains that the original TBM drive site was to be in Barn Elms, which is public land used by schools, colleges and residents of South West London.
“At the initial consultation, we evaluated whether the use of this greenfield might be really essential for the project,” Green says. “We also considered a brownfield site as a more appropriate option, and given the success of Crossrail and Jubilee Line extension, that criteria looked more suitable. Thus, we explored further options on even smaller sites to successfully deliver the construction tunnel.
“We set up this site in Carnwath road. Some residents said that Barn Elms would be a more appropriate site for us,” he says. “Of course they had to understand facts rather than fears. So we invested in collaborating with teachers and local schools to involve them in the project.” Site deliveries have been scheduled so as to avoid peak traffic times and minimise disruption. All data on noise, dust and so on is being released to the community.
Green adds: “We have had to do a decent amount of preparatory work, such as clearing the site, demolishing buildings, strengthening the river walls, creating foundations for offices, site logistics and for the acoustic shed. Then, we had to construct the acoustic shed, and then the shaft after the acoustic shed was complete.”
Over the last 12 months, demolition works have been undertaken to prepare for the start of main tunnelling works. Thomas says that this is the first major piece of infrastructure in the UK enabled by the Planning Act in 2008, which consents all the process. “The construction team onsite are left with challenge to proceed the work”, Thomas says. “This is the first piece of major infrastructure in UK to be constructed using the Development Consent Order Process (DCO). This is a relatively new system for providing a finite timescale for the planning consent for infrastructure.
“That is very different from the Hybrid Act system that we have seen for projects like Crossrail, Jubilee line and Channel Tunnel. This is the first project with a DCO process but many other projects are coming with this process, as well.
“To overcome any challenges, we focused on coordination between designers and the engineering team, but we had also to deal with technical specialists for noise, air quality and vibration.”
EXCAVATION METHOD
An 8m-diameter Herrenknecht EPBM was chosen for the drives in this section, roughly 7km. “We decided to use an EPBM as we have mainly London Clay at the west section and we wanted to minimise settlement,” Thomas says.
“Other types of machine would not be able to deal with the ground and provide the settlement control that we required. EPB is generally the most suitable machine for working with London Clay.”
Green adds: “A huge programme of geological investigations have been undertaken by Tideway. The results show the presence of a mix of London Clay, sand and gravels at the central section; predominately chalk at the East section and London clay at the West section.
LINING AND SUPPORT REQUIREMENTS
Thomas explains that there is a primary lining of a precast segmental concrete lining and a secondary lining of concrete within the primary lining.
“We have a secondary lining to give a standard design life of 120 years to protect the aquifer in some areas where the tunnel runs through it. The contractor supplied the lining and we subcontracted the segmental lining to Morgan Sindall, which produced over 4,000 rings at the Ridham precast facility.
“The contractor was responsible for the design of the primary lining and we selected a boltless grouted ring system, which is the first in the UK,” Thomas says. “We are not using bolts but guide rods on the seat joint or longitudinal joint and dowels on the circumferential joint.
“We sought advice from colleagues who have used the guide rod system overseas before committing to their use on this project. The hope is that the boltless system will go a long way in reducing worker exposure to hand-arm vibration while building rings.”
WATERPROOFING NEEDS
To ensure watertightness of the tunnel, EPDM gaskets are being used between segments. These gaskets come through Morgan Sindall and are cast into the segments. Thomas explains that they opted for cast-in rather than precast as Morgan Sindall has had good experience with cast in situ linings for the Lee tunnel, which forms part of the same overall scheme to improve water quality in the River Thames. “On the shaft we are using zypex additive to provide additional waterproofing,” Thomas adds.
RIVER VALUE
The TBM was shipped before Christmas 2017 and the client was eager to bring its components in by water from Germany straight to Carnwath Road. It was then assembled at the surface on site ready for lowering into the shaft. Thomas explains that they weren’t sure of that because it was the first time in London that a TBM was shipped to site entirely by water.
“My personal commitment to road safety comes from witnessing the death of a cyclist at Chelsea Bridge, who was crushed beneath a muck lorry,” says Thomas. “Anyone witnessing a similar accident would try to reduce the number of lorries working in London.
“The 700t shield successfully arrived on site. That was a fantastic piece of hard work carried out by our team. The machine was built inland, some 20km from the River Rhine; it was then transported by road to Kehl to the river, and then transported up the Rhine by barge. It took approximately three days to get to Rotterdam, where it was stored in a yard, and then the components were transferred to a barge to cross the Channel. The barge travelled from Denham Wharf to Gravesend where it stayed for three days to wait for the right weather and tide to bring it up to Carnwath road.
“The whole operation took seven days. The last leg of the journey saw the two tugs procured by the joint venture bring the TBM up the River Thames. These tugs are specially designed to work in this type of environment. The two tugs had an interesting design feature: bridges that could be hydraulically elevated so that the skipper was able to see over the load to the water in front to the barge.”
Thomas adds that the entirety of muck removed is being transported by water. “BMB invested with their site supply chain (AC Bennett) not just in tug boats but also in a new fleet of barges specifically designed to operate in conjunction with those units, which push rather pull,” Thomas says.
“Historically, barges placed on the Thames for waste operations had to be pulled on ropes, whereas the ‘tugs’ were designed to push, so it’s more efficient in terms of safe operation,” Green says.
Green adds that not only is the muck going up river by barge, but tunnel segments are being transported to site by barge by water from the Ridham factory.
Thomas adds that another issue for the project was the selection of craneage. “As we are operating in a port, we wanted to find the best solution for off-loading segments,” he says. “We would usually use a crawler crane, but not in this case.
“We visited the port of Cork in Ireland, which has a similar riverfront to the one at Carnwath road and deals with a similar amount of material. We consulted with them to adopt the best logistical solution. They suggested an electrically operated port crane and we decided to go with a Liebherr port crane.”
Green explains it was another investment designed to increase the facility of the river as an asset. “The port crane ensures good visibility for the operator down to the barge, allowing us to reduce the number of people on the barge and the amount of handling required on it.”
HEALTH AND SAFETY
Green explains that as part of the transformation programme, the client team has developed a training programme, called Employer Induction Centre (EPIC). “Everybody who comes into the project, more than two days, has to go through EPIC, which is a daylong process where you are exposed to a construction accident acted by professional actors in a theatre near Vauxhall bridge,” he says. “Actors act how the accident arose, showing consequences on the family members of the person involved. There is a dialogue with participants to encourage them to take part in it. At the end of the discussion, the project manager comes to reinforce the message of health and safety.
“It was a big investment to make a profound awareness in people before going to work on site. This is a good innovation as we bring people from all over the world to work on different tunnelling projects in different locations. We also look after wellbeing of workers providing mental health sessions.”
