How London's Thames Tideway super sewer was engineered

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.

We’re working on the Tideway East section of the 25km tunnel

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.

The Tideway tunnel takes shape

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.”

Our project team had to investigate archive drawings

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.

Examining the existing brickwork

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 Greenwich pumping station shaft

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)

A 3D model showing the below ground layout at King Edward Memorial Park (Credit: Tideway)

There is a significant weight of extra ground to add, and that has caused nearby settlement and movement. “The ground conditions there are a lot worse than we expected, and existing cracks in this Victorian chamber have opened up, very significantly,” Nick says, explaining that the front face of the chamber has moved about 50mm while the back face hasn’t moved at all. “It’s been stretched, and then cracked and settled. We’re putting in extra props and monitoring the movement and the cracking to keep that chamber safe while we design a solution to strengthen the walls, but it’s been challenging.”

The design uses the same strategy as applied to the Greenwich chamber of drilling new steel rods from the surface. “It’s a really good technique because you don’t need to go inside the chamber. You can strengthen it from the outside,” he says. Working in combined spaces used for sewage requires comprehensive health and safety including permanent rescue teams, so avoiding the hazards is always preferred.

Nick acknowledges that the easy solution would be to rebuild the chambers. But that’s not the preferred approach on this project. “The general principle with these chambers is that we try to leave as much as possible in place. It’s more sustainable to do that,” he says. “Prolonging the life of these assets and assessing damage requires careful thought. You need to bring in people with the right experience to be confident in your designs and solutions. It only works if you have real, in-depth skills and experience right across the team.”

Flow control

Each CSO chamber is connected to a shaft up to 60m deep, which houses a drop tube of up to 35m that causes the wastewater to drop in a vortex, or spiral, motion around the inside of the tube as it descends. Across the whole of the Tideway project, these are being used to control the water in the system’s shafts, the deepest of which are on the Tideway East contract. These stainless steel-lined drop tubes are typically 3m-4m in diameter, meaning each shaft must be incredibly large at 17m to 25m diameter and 48m to 65m deep, to accommodate them.

Controlling the flow of water in the system

“The design of the vortex structure inside the shaft, combined with the penstocks in the CSO chambers, controls how much flow is allowed through and at what velocity,” explains Tejal Shah, who led our mechanical and electrical design team for the project. Without this equipment controlling the flow of water into the tunnel, the upstream sewers would back-up and flood into streets and homes.

The King Edward Memorial Park shaft

The King Edward Memorial Park shaft

The King Edward Memorial Park shaft

The King Edward Memorial Park shaft

Hydraulic modelling

Her team used hydraulic modelling to help the durability designers assess the risk of abrasion in the CSO chambers and shafts – specifically how the flows could damage the concrete. This can be prevented by strengthening those areas, while at the same time avoiding over specifying concrete in places where there are lower flow speeds. These models are also integral when studying the vortex drops and learning how much air is drawn into the tunnel. This is vital because air in the tunnel traps in pockets that pressurise, potentially causing structural damage. It also reduces the hydraulic capacity of the tunnel.

As the water drops 60m down the vortex, it flows at a very high velocity and the stainless-steel lining takes the water down in a spiral fashion as it travels to the base of the shaft. “These vortex drops control the flow, but then they also allow the air to dissipate,” explains Tejal.

The vortex drops are mostly encased in concrete except for the bottom 6m, where the aim is to give the water as much space as possible to slow it down and release the air. “As a result, we’ve got very exposed portions of the tube that are buffeted by the water,” says Nick.

These are similar concepts to those considered when designing railway tracks and bridges. The modelling team calculated the wave frequency, and our dynamics specialists helped verify that the design of those 6m-long steel tube extensions was capable of accommodating vibration and fatigue cycles from water in the shaft.

In addition, Tejal and her team developed physical models of the CSO chambers and drop shafts; even on that smaller scale, it proved to be a very turbulent flow. “The structures have to be big and robust,” says Nick, “because the forces are enormous.”

When a tunnel fills to capacity and a shaft starts to backup with water – which happens a few times per year – the pressures can be intense. This can put the shaft’s secondary lining into tension and even cause cracking. To ensure a watertight shaft lining, the design must resist the pressure and control the cracking, which often calls for significant amounts of steel reinforcement. We faced a similar design challenge with the shafts on the nearby Lee Tunnel – a separate project that is part of the wider Tideway system; the same process employed there can be used for Tideway East.

Computational fluid dynamics modelling: red represents the high velocity of the water and the models demonstrate that water can flow steadily

Computational fluid dynamics modelling: red represents the high velocity of the water and the models demonstrate that water can flow steadily

Computational fluid dynamics modelling: red represents the high velocity of the water and the models demonstrate that water can flow steadily

Computational fluid dynamics modelling: red represents the high velocity of the water and the models demonstrate that water can flow steadily

Large-scale post-tensioning

“We used a system like very large-scale post-tensioning,” Nick explains. The contractor poured water into the void between the secondary lining and a diaphragm wall, which compressed the lining, pushing it inward. Then concrete was poured into that gap, displacing the water and locking the lining into place. “The water and concrete generate the compression in the lining, whereas post-tensioned cables are used to similar effect on a smaller scale, often to refine structures on buildings or in water storage tanks,” he says.

When the shaft fills with water, the lining will go to a neutral state and can accommodate those immense pressures without needing as much reinforcement. In fact, the process has saved about one third of the reinforcement that was initially estimated for the shaft lining – around 1600t of carbon reduction on a single shaft.

Damage control through domed solution

A final concern for these extremely large and deep shafts is water pressure on their base slabs from groundwater. The force created by the external water is 650kPa, equivalent to over six times atmospheric pressure. Here, the team used the benefit of the stiff diaphragm walls embedded in chalk to create a domed base design to transfer that force via compression, rather than it bending a horizontal slab or plug. This measure reduced the required reinforcement by 50%, saving time, money and carbon.

“Really, our job on Tideway East has been to look at every element and find the best way we can make it work, understanding the safest and easiest way to build it, and to keep challenging ourselves,” Nick says.

​A lot of this has been facilitated through Building Information Modelling (BIM) sessions held with the contractor, using the models to work through any issues and challenges. “The main benefit was that we could do design integration far more quickly compared to if we were producing drawings,” Tejal says. “If we’re reviewing designs using drawings, we’d have to go back to make changes. Whereas, using BIM in collaboration with the contractor, we could get through design iterations a lot quicker.”

High-pressure tunnelling

Across all of Tideway, the tunnel slopes downwards to the east to reach its greatest depth of 62m. The eastern contract includes two tunnel drives that are entirely in chalk.

“Multiple tunnels have been excavated in chalk around the world, but it’s something quite rare in the UK,” says Ben Lafarga, tunnel engineer.  “The tunnel depths for Tideway East are not often encountered here.”
While London has significant underground infrastructure, tunnels are generally found at shallower depths excavated through the London clay, and one of the challenges for this project is that there are far fewer case studies for tunnelling in chalk.

“The geology is not well tested and there are big differences excavating in chalk compared to what we find in the clay,” he says. “London Clay is usually impermeable or undrained, and even though you may have some water pressure or water ingress, it’s really quite minor.”

For the deep tunnels on the eastern contract, the water pressures in the chalk are much higher, and so is the potential for water ingress. When the water does begin to build up pressure, it can happen quickly.

The tunnel slopes downwards to the east (Credit: Tideway)

Experience pays off

However, we brought our experience gained from designing the nearby Lee Tunnel, completed in 2017, that will connect with the Thames Tideway Tunnel at the Beckton Treatment Works. This required excavation to depths of 80m, and the design team had expertise when it came to ground treatment options and protective measures that would be needed at the face where the tunnel excavation meets the chalk. 

Our previous work with VINCI Grand Projects in similar conditions on the Lee Tunnel project helped the team to select a slurry tunnel boring machine (TBM). This uses slurry to pressurise the front of the machine to balance the ground and water pressures at the face of the rock. It was a crucial decision for the project because slurry TBMs provide a higher level of control over other TBM types. “The water flows freely through the chalk,” says Chris. “Using a slurry machine is much more appropriate for the high-water pressures.” 

Along the tunnel alignment, they expected pressures potentially as high as 5 bar - like standing at the bottom of a 50m-deep swimming pool. Despite the challenging conditions, the contractor successfully completed excavation for both tunnels in 2022.

Tunnel boring machine launch on the Lee Tunnel (Credit: Tideway)

Long-life lining

However, handling the chalk’s features and water pressures does not end with the TBM excavation. The tunnel has a 120-year design life, during which it must prevent ingress of water from outside of the tunnel, as well as preventing leakage of the foul water.

“The chalk is categorised as a principal aquifer,” says Dan Ridout, tunnel engineer. “That’s another challenge we must deal with; ensuring flows in the tunnel will not leak out and contaminate that aquifer.”

As part of the TBM excavation, contractor CVB installed a primary lining comprised of precast concrete segments that come together to form a ring. This holds back the ground and makes the tunnel watertight through gaskets in each segment. While a secondary lining is not technically necessary for the tunnel to function, agreements with the Environment Agency required the whole of the 25km-long Tideway tunnel to have one.

Once the second liner is installed, it must perform. One potential threat is the same as that for the shafts – if water pressure inside the tunnel is higher than the water and ground pressures outside, it puts the structure into tension, making it less stable. As Nick notes, “We’ve got some issues that would be rare to find in a road or rail tunnel.”

We launched a feasibility study, working with the contractor and Tideway’s technical team, to explore options for reducing the secondary lining. In doing this, we also looked at optimising the contractor’s methodology for installing the lining, which is cast-in-situ concrete.

CVB opted to use a full round shutter rather than two parts. This lining is installed via a mini factory that travels through the tunnel on rails to cast it all in one go. But the process is subject to hydrostatic pressures, which can impose loads against the primary lining; while these loads are only temporary, they can still cause damage.

Advanced analysis saves carbon

“In terms of optimising the lining, we needed to balance more than just structural behaviour and constructability,” Ben says. “We did a lot of advanced analysis that is not usually applied in tunnel design. It required a lot of peer review and consultation with experts from outside the immediate team. We needed to take a holistic approach so we could capture everything relevant - it made it an interesting piece of optimisation.”

The study reduced the thickness of secondary lining in the main tunnel by 60mm, cutting the amount of concrete needed by 27%. It also reduced the shorter Greenwich connection tunnel lining thickness by 70mm, bringing the concrete volume down by 19%. We estimate this saved around 4500t of embodied carbon and contributed to CVB nominating the design team for a Tideway ‘Rightway Award’ for carbon saving in autumn 2022. These awards recognise individuals and teams that go above and beyond when working on the megaproject.

The benefits extend further. “The optimisation exercise reduced the number of material deliveries,” Dan explains, “which was a positive outcome for the project’s neighbours in terms of noise, safety and air quality.” As well as saving around 1,600 lorry movements through the local community, the reduced thickness also increased the tunnel’s capacity.

Eye for detail reduces rebar

We foresaw another carbon-reduction opportunity via the introduction of updated codes for reinforcement. Partway through the project, the Construction Industry Research and Information Association (CIRIA) introduced an updated guideline that moved CIRIA 660 to CIRIA 766, an update that Mott MacDonald contributed to as part of the guideline steering group. This eased the crack-width limit, allowing the team to design to 0.1mm rather than 0.05mm. It might sound miniscule, but it translated to a 30% saving in reinforcement on the contractor’s shafts, which in turn saved about 2000t of carbon. The tunnel-design team was also able to apply the new code at selected sections of the secondary lining.

“Tideway has been focused on sustainability from the very beginning of the programme,” says Ben. “That doesn’t suddenly appear – it comes from a plan. We are always engaging with the contractor and the client and providing designs in a way that contributes to Tideway’s goals.”

The grand finale

CVB has a limited window to access one location which presents another challenge. This is at Abbey Mills Pumping Station, an already operational Thames Water site in East London that forms part of the Tideway project.

All shafts on the project will have covers that are typically built with a beam and slab system, whereby beams weighing hundreds of tonnes are cast on site and craned into place. It’s a time-consuming operation, assembling as many as a hundred beams before pouring the top, which links it all together.

At Abbey Mills, where the slurry TBM completes its journeys, CVB asked us to design a lid for the shaft that could be installed in a single lift.

For the feasibility study we brought in our bridge specialists to conduct a peer review. “It’s effectively a highway bridge structure in terms of its scale,” Nick explains. “It’s quite different from the underground structures and building works we do on the structural design team. We’ve brought in a lot of advanced analysis as early as possible to solve this problem and give the client confidence in the solution.”

Gigantic concrete lid

That solution is a single-piece concrete lid with a temporary steel frame attached to the top. The contractor can cast the lid on site next to the shaft in preparation for the moment it can be lifted into place–finalising not only its contract, but also the entire Tideway programme.

Detailed design is now finished for the 25m-diameter lid, which will weigh just under 1200t and is more than almost any crane can lift. However, the team has designed a solution for that, too. The temporary steel frame will have eight strand jacks that will be carefully tensioned in tandem to raise the concrete lid several metres into the air. Speciality trailers called self-propelled modular transporters, typically used for bridge construction, will then slowly and carefully drive it into place, ready for the lid to be lowered.

“The structure is very sensitive to how you lift it,” says Nick. “We had to check multiple cases for variations in stress during the lift and create practical limits with the contractor to avoid cracking.”

Specialist subcontractors are expected to perform the single lift next year. A meticulous task, it will save six whole weeks for the programme.

Leaving a legacy

Tideway is the largest project underway in the UK water industry, with few others worldwide that match its size and complexity.

“These are enormous pieces of engineering, and the scale and volume of the water that will move through this system is that of a decent sized river,” says Nick. “And it’s all controlled. We know how that flow will behave and how to control it down into the shafts. We’ve brought it all together in a way that the client is confident will be watertight, effective, buildable and safe to maintain.”

The project will integrate the new foreshore with the river (Credit: Tideway)

Most of this infrastructure will be forever hidden underground, but there will also be drastic improvements to the public realm. For example, the new shaft and chambers are located within the foreshore extension to keep future maintenance away from the existing park. But with this comes the opportunity to introduce green spaces and play areas at the King Edward Memorial Park site.

Tideway will also have a once-in-a-generation impact for many of the young engineers who joined the industry during its design and construction, creating a legacy by developing skills and lessons learned that will inform future tunnel projects.

“It’s a big privilege to have the opportunity to work on a project that is going to improve the health and safety, the quality of life and the environment in London for generations,” says Dan, “as well as contributing to sustainability through things like the secondary lining optimisation that’s saving carbon. Having that exposure so early in my career sets a good precedent.”

However, it’s not over yet. We have a dedicated construction design support team helping CVB to deliver our design and we continue to collaborate and innovate with our client.  “Being on site allows us to tailor our services and expertise in line with the construction programme and activities currently ongoing and help to manage risk.” says Chris. “It’s about being able to respond dynamically,” Tejal says. “When the contractor communicates something to us, we are there to quickly interpret that and develop a solution.”

Construction of the main elements continues until mid-2024 and Tideway is due to open in mid-2025.