Thames Tideway Tunnel

Designing the western section of London’s 25km super sewer- the largest upgrade to the city’s sewage infrastructure in over 150 years. 

The project 

London’s sewer network was built in the 1860s for a population of around four million. With the city now home to nearly nine million people, the system needed a fundamental upgrade to manage the volume of wastewater flowing through it. 

The Thames Tideway Tunnel is a 25km sewer running beneath the River Thames from west to east London, connecting to 34 combined sewer overflows and transferring flows to Beckton sewage treatment works. The tunnel became fully operational in February 2025 and was officially opened in May 2025, making it one of the largest combined sewer projects ever delivered.

Our role

AtkinsRéalis, in a joint venture with Arup, was appointed as designer for the western section of the tunnel under a Design and Build contract. The team provided detailed construction designs and on-site engineering support from 2015 through to completion.

The western section covers a 7km main tunnel with a 7.2m internal diameter, running from Acton Storm Tanks to Carnwath Road, plus a 1.5km connection tunnel. Across seven worksites, the design had to intercept combined sewer overflows, with each site requiring a drop shaft, tunnel connection and underground chambers- all constructed next to active pumping stations and live sewers.

The engineering challenge

Designing deep, complex structures alongside operational infrastructure required close collaboration between the design team, the contractor (BAM Morgan Sindall Balfour Beatty Joint Venture) and the client. The engineering challenges included:

Designing for a 120-year operational lifespan- every structural decision had to account for long-term durability, ground conditions and future maintenance access

Minimising impact on adjacent buildings, infrastructure and utilities across seven central and west London worksites

Working within strict planning constraints under the Development Consent Order, balancing programme delivery with community and environmental commitments

Integrating landscape architecture at each site to create new public spaces that reconnect communities with the River Thames

How the team worked 

The project demanded a genuinely collaborative approach across multiple disciplines. Structural engineers, geotechnical specialists, tunnel designers, environmental consultants and landscape architects all worked together through a structured design assurance process, from early planning through to final construction.

A shared 3D BIM platform was central to how the team operated- enabling real-time coordination across design partners, the contractor and third-party stakeholders. Value engineering was embedded throughout, with the team continuously identifying ways to improve efficiency and cost-effectiveness without compromising the 120-year design life.

The outcome 

25 km

Total tunnel length

95%

Reduction in sewage overflows entering the River Thames

1.6 million m³

Storage capacity- equivalent to around 640 Olympic swimming pools

33–66 m

Tunnel depth beneath London

120 years

Design lifespan

Beyond the engineering, the project delivered new public spaces at each worksite- creating areas designed to reconnect Londoners with the river. The landscape architecture was treated as integral to the design, not an afterthought, with welcoming spaces that will serve communities for generations.

What this project involved

Working on the Thames Tideway Tunnel meant working across tunnel engineering, structural design, geotechnical engineering, environmental assessment, 3D BIM coordination, landscape architecture, construction support and stakeholder engagement- often simultaneously and at scale. It’s the kind of project that gives engineers and designers the chance to work across disciplines on infrastructure that will serve London for the next century.

Explore water careers at AtkinsRéalis

If you’re a tunnel engineer, structural designer, geotechnical engineer, environmental scientist or water infrastructure specialist looking for projects at this scale- take a look at our current water roles or join our talent network to hear about opportunities as they come up.