The Thames Tideway Tunnel Project

The 45th Annual Paviors’ Lecture was given at Imperial College London on 11 February 2020 by Andy Mitchell CBE, CEO of Tideway. His subject was ‘The Thames Tideway Tunnel: Building Infrastructure for a 22nd Century London’. The lecture was attended by a record 350 people. ​

Andy Mitchell outlined the background to the Thames Tideway Tunnel project and charted the course of the project delivery. He talked about both the engineering challenges and also about the relationship between London and the River Thames. He explained how this relationship was expected to develop, and how delivery of this vital piece of infrastructure will help to prepare London for the 22nd Century.

The project builds on the legacy of London’s Victorian sewer network created by London’s City Engineer, Sir Joseph Bazalgette, over 150 years ago. Following a major cholera epidemic and concerns about foul-smelling air in the city, Bazalgette designed and built large interceptor sewers north and south of the river which transported the raw sewage down-river to the his treatment works near Beckton. Although, the system was over-designed for its time, population increases and more extremes of weather since then now means that the original system can no longer cope and that overflowing raw sewage is now discharged into the River Thames around 60 times each year.

The new sewer is 25km long and runs from Acton to Abbey Mills Pumping Station (Bazalgette’s ‘cathedral’) and then on to Beckton. The tunnel is 7.2m wide and is being constructed at depths between 35 and 65 metres below the surface through three different types of subsoil. The sewer falls at a rate of 1 in 790, which is twice the fall of normal sewers, so aims to be self-cleansing. Its capacity has also been designed to act as a reservoir, so that in times of very heavy rainfall it can store sewage and allow the treatment works to cope with the volumes. In exceptional conditions, there could still be some discharge into the Thames, but these will be infrequent and the sewage will be much diluted when this happens. It is not economic to provide for 100 per cent certainty. Major design issues have been how to get air out of the tunnel at the interceptors when sewage is entering in high volumes, plus design and specification requirements for primary and secondary tunnel linings that will last for a design-life of 120 years, but with an expected life of up to 300 years.

The project aims to provide a legacy in a number of areas. It should have a big environmental impact. Currently, 200-300 items of sewage per square metre are left on the foreshore of the river, and this volume should be dramatically reduced. Health and safety of the workforce has received much attention, starting from a lengthy induction process for employees, but carrying on through daily briefings. This major civil engineering project has demonstrated how the river can be used to transport all construction and excavated materials, keeping thousands of lorry-movements off the streets; each barge-load of spoil replaces 50 lorry loads, and barges are over 90 per cent less-polluting than trucks. The project thus provides a lesson for other transport activities carried out within London. The majority of employees on the main project are women. Project staffing emphasizes employing local people and ex-offenders, and one in 50 of staff are apprentices. The project has also provided opportunities to construct entirely new sites by the river in London, and a lot of thought has gone into the design of these. There has been considerable engagement with local communities through all aspects of the project, and staff have taken great pride in volunteering on community programmes.

Approximately half of the tunnel is now complete and most of the 24 shafts are now constructed. Project completion is aiming for 2024. Tideway’s website shows live where each tunnel-boring machine is currently working.

Andy Mitchell was formally thanked for his presentation by Neil Sandberg, the Renter Warden of the Worshipful Company of Paviors. Neil said that the Lecture did not just concentrate on traditional engineering, but emphasized engineering in its wider context: in addition to technical details and ‘numbers’, the presentation also covered economics, the environment, people, culture and art. He considered that the Lecture would have been inspirational to students and young engineers.