Thames Tideway Tunnel, London, UK

Keeping the River Thames clean in London

About

Challenge

London relied on a 150-year-old sewer system designed for a much smaller population. This led to struggles with flooding and a heavily polluted River Thames. When the existing sewer system became too full, the untreated overflow was released directly into the River Thames via a number of combined sewer overflows (CSOs). With increasing rainfall and population growth, the problem would only increase.

Solution

A 25-kilometre tunnel that intercepts, stores and ultimately transfers sewage waste away from the River Thames.

Value

The increased sewer system capacity not only keeps the Thames clear but also improves wildlife. In turn, this creates a healthier city that is robust enough to withstand the future climate and population growth.

Specifications

Tunnels

A 25 kilometre-long bored interception tunnel and associated shafts; connection tunnels and pumping stations

Location

London

Period

2015–2025

Country

United Kingdom

Partners

Tideway, Ferrovial Agroman – Laing O’Rourke joint venture, AECOM and COWI

Services

Detailed design of vertical shafts involving a combination of piled, precast concrete segmental and sprayed concrete primary linings with cast-in-situ secondary linings; design of connection tunnels and main tunnel junction; adoption of sprayed concrete primary linings with cast-in-situ concrete secondary linings.

The Thames Tideway Tunnel: protecting Londoners and wildlife from pollution

The River Thames is the United Kingdom's largest river and most important water supplier, as it runs west-east through the centre of London.

The Thames Tideway Tunnel is a new major sewer that helps tackle the problem of overflow from London's Victorian combined drainage and sewer system, and thereby protects the River Thames from pollution. Before the tunnel was constructed, every year, 40,000,000 tonnes of raw sewage was dumped from the British capital's sewers into the Thames, enough to fill the Wembley Stadium 40 times.

London's 150-year-old Victorian sewer system

The combined drainage and sewer system of London dated back 150 years and was designed for a far smaller population. With a population of nine million, the capacity of the city's sewer system had become inadequate.

The existing sewer system captured combined sewage and stormwater runoff. When the system became too full, millions of tonnes of untreated sewage spilled into the River Thames each year, affecting both the environment and the health of citizens, as well as the river's wildlife and ecosystems. As little as 2 mm of rain could overwhelm the combined drainage and sewer system, forcing sewage to flow into the Thames.

In addition, Great Britain had experienced a series of extreme weather events, particularly flooding, in recent years as a result of climate change. Heavy rainfall increased the risk of flash floods and overflows from the ageing sewer system. To address these challenges and help protect the River Thames for future generations, the Thames Tideway Tunnel was constructed to provide a long-term solution for the next hundred years.

"Thames Tideway is a strategically important scheme to minimise untreated sewage entering the River Thames and will protect the river for at least the next 100 years, delivering long-term environmental benefit. It is exciting to have played a role in this major infrastructure project by adding value to the central section of the scheme through specialist shaft and tunnel lining design input."

Chris Phillips, Vice President, Head of Projects, COWI in the UK

New super sewer protects the wildlife and ecosystems of the Thames River

The super sewer transports the untreated sewage to the treatment plant at Beckton, preventing millions of tonnes from flowing into the river. Therefore, the Thames Tideway Tunnel protects wildlife and ecosystems in and near the river, while also bringing the river closer to Londoners through a series of new riverside parks and public spaces.

The 7.2-metre-wide super sewer stretches 25 km from Acton Storm Tanks in West London to Beckton Sewage Treatment Works in the Borough of Newham in East London. The tunnel has a depth of 30 metres at Acton and has a steady fall of one metre every 790 metres to facilitate the flow of the sewage. It reaches a depth of 75 metres at its end destination at Beckton Sewage Treatment Works. Overall, the Thames Tideway Tunnel cost around GBP 5 billion and entered operation in 2025.

Design of two 45-metre-deep vertical shafts

COWI was involved in all seven shaft sites within the Tideway Central section, responsible for the design of ten connection tunnels and culverts, including the design of the junctions between connection tunnels and the main tunnel. The connection tunnels are formed of sprayed concrete primary linings and cast-in-situ concrete secondary linings.

Furthermore, COWI designed two 45-metre-deep vertical shafts, involving a combination of piled, precast concrete segmental and sprayed concrete primary linings together with cast-in-situ concrete secondary linings.

A project of this magnitude, and in the middle of a metropolitan area, always represents a series of complex challenges that require a high degree of expert knowledge. One of the most challenging tasks was creating the complex connections between the main TBM sewer tunnel and the connecting tunnels. COWI's experts used advanced geotechnical engineering and analysis software to design the solutions for the complex tunnel junctions.

COWI's involvement in the super sewer started in 2015 when our experts provided designs for the two combined sewer overflow (CSO) 45-metre-deep vertical shafts, six low-level connection tunnels, and four high-level connection culverts using segmental, sprayed concrete and piled primary linings, along with cast-in-situ secondary linings.

Construction of the first of these elements, the Albert connection culvert, began in 2018, and our final sprayed concrete lining (SCL) tunnel at Chelsea Embankment was completed in the summer of 2022. COWI supervised the SCL Engineers during the construction of all the sprayed concrete lining works as part of the self-delivery by the Ferrovial and Laing O'Rourke joint venture (FLO).