Annual review 2022
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Chris Lile
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Flow control
Connecting to the past
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In November 2022, particularly heavy and persistent rain caused the combined sewers across the City of London to overflow into the river Thames every day of the month. To prevent this from happening in the future, Tideway is spending more than £4bn to build a 25km tunnel that will divert flows away from the river and convey them to a treatment works. This can only happen safely and efficiently through a series of underground chambers and shafts designed to control the huge volumes of water. We are designing all the permanent structures and tunnels for joint-venture contractor Costain/Vinci/Bachy, which is delivering one of three Tideway packages.
It’s been more than a century and a half since the Victorians designed London’s sewers. In that time, the city’s population has grown rapidly. Now, when the dated infrastructure reaches capacity, which it often does, polluted overflows are released into the river Thames.
To prevent this, Tideway is building the super sewer to end this source of river pollution. Tideway is a £4.4bn programme designed to intercept the polluted overflows before they reach the river, diverting them to a treatment plant through a 25km tunnel.
“The whole scheme is effectively a huge drainage pipe beneath the Thames, from shallower depths in the west, using gravity as the tunnel goes deeper to the east,” explains Chris Lile, our project manager for Tideway.
For the tunnel to collect the flows and safely convey them for treatment requires a complex series of new structures. Design and construction for this infrastructure presents a series of challenges including interaction with ageing sewers, risk mitigation from water travelling down a 60m drop, as well as tunnelling through the chalk below the river Thames.
Tideway – the company delivering the project – tendered the programme in three main contract packages. As the designer of the main permanent structures for the design and build contractor – a Costain/Vinci/Bachy joint venture (CVB) – we deployed numerous teams on Tideway East and drew on a wide range of technical experts to support the project.
Joining the client CVB in 2014 to prepare the tender, the teams have designed five shafts with depths ranging from 50m to 60m and internal diameters ranging from 17m and 25m. This work includes four combined sewer overflow (CSO) structures that each comprise several chambers to connect the new shafts to existing sewers. There are two tunnels in the Tideway East contract, comprising 5.5km of the main Thames Tideway Tunnel, and a shorter 4.5km tunnel extending south-east beneath the London neighbourhoods of Deptford and Greenwich.
Connecting to the past
The existing sewers date back to the mid-19th century, and we have investigated archive drawings for many of these historic structures, some signed by original chief engineer Joseph Bazalgette himself. However, they aren’t always reliable to verify building materials, conditions or even exact locations. What’s more, these structures contain live discharges of water, notes Nick Dobson, our design lead for structures.
“Some have a continuous valve flow and others are only operational in storm conditions,” he explains. “That can be with very little notice – suddenly there’s a sewer full of water coming through at speed.”
At Greenwich, the contractor is connecting into the side of a 10m-deep chamber that was built more than a century ago. “We’re excavating a larger chamber immediately next to it, so we’ve got to provide support and make a watertight connection, and then demolish the side walls to open it up to become a single chamber,” he says.
The existing walls are more than 1m thick and rely on ground pressure for stability. Because there is compression on each side, removing one wall to create the new chamber would destabilise the existing one.
The team needed to design a solution that would strengthen the existing chamber while also providing a fixture and support into the new chamber. On top of that, the design needed to accommodate the ongoing operation of the existing sewer chamber.
Adopting an above ground solution
Nick gathered a team of multidisciplinary experts to assess the chamber, who selected a masonry-strengthening technique. “I’ve not heard of it being used on underground structures before,” he says. “It’s normally used above ground to strengthen masonry walls.”
The technique involves drilling new steel rods from the surface to 12m below ground into the existing masonry walls to add strength and provide new, safe load paths that prevent the side walls from collapse. This is important because the expanded chamber will have large openings measuring as much as 5m wide and between 6m and 7m high. Our instrumentation and monitoring team designed systems to monitor the old brickwork for movement and cracking, while the structures team worked closely with the contractor on how to stage construction.
The more standard approach for the large adjacent excavation would be to use temporary props and then build the permanent structure. That approach would have created more ground movement, Nick explains, which could have damaged the Grade 2 listed pumping station. The method used at Greenwich provides a rigid top-down construction: propping at a high level, and alternating between excavation and propping until the appropriate depth is reached. The structure is then built between the props, which stay as part of the permanent works. While not an unusual approach, it’s one that required much collaboration.
The digital key
The key to success here was digital co-ordination. “The archive records for that chamber are patchy,” says Nick. “We relied on 3D scans, as well as a point cloud that we could convert into a digital model.”
Through a series of meetings with the contractor, the design team devised the building strategy for the chamber, as well as deciding how to install the various hydraulic systems linking into the new works. “It was a lot of digital coordination,” Nick recalls. “We were basically sharing the model back and forth until we were happy with it, and this meant there would be fewer surprises once on site.”
Green space creation
At King Edward Memorial Park, the contractor is building out the foreshore into the river, creating a new peninsula approximately 10m above riverbed level, 50m long and 30m wide. This will camouflage and cover Tideway’s newly built underground infrastructure – including the CSO chamber connecting into the existing sewer – while providing new green space for the local community. The river walls include terraces that are submerged by each tide, integrating the new foreshore with the river and providing new habitat for estuary wildlife.
The construction site at King Edward Memorial Park [Credit: Tideway]