Most of London's sewers were built over 150 years ago. Their builder, Sir Joseph Bazalgette, designed them for a population of 4 million. Today the British metropolis has over 8 million inhabitants. The accumulated sewage regularly exceeds the capacity of the sewage system and every year tens of millions of tonnes of sewage overflows untreated into the Thames.
To prevent this from happening in the future, a new “super sewer”’ is currently under construction in the UK’s capital: the 25 kilometer long Thames Tideway Tunnel. Tunnelling started in November last year, and the project is due for completion in 2024.
The super sewer is the third and final measure in a series of steps to improve the water quality of the Thames. Together with upgraded sewage treatment plants and the Lee Tunnel completed in 2016, the Thames Tideway Tunnel will ensure that 90 - 95 percent less sewage enters the Thames in the future. It will collect the sewage from 34 overflow points, retain it and carry it on into the Lee Tunnel, which was built in 2014, also using a Herrenknecht machine. From there, it will enter the Beckton Sewage Treatment Plant. In order not to additionally burden the already dense traffic in London with the project, client Tideway is aiming to transport more than 95 percent of materials by river, removing more than 366,000 truck journeys from the road. This also applies to the delivery of five of the tunnel boring machines (TBMs) being used for the large-scale project.
The tunnelling activities are divided into three sections: a 7 kilometer long western section, a 13 kilometer long section through the center, and a 5.5 kilometer long eastern stretch. Also part of the project are two, smaller connecting tunnels – one in Greenwich and one from Wandsworth to Fulham. The tunnel alignment for the Thames Tideway Tunnel begins about 30 meters below the surface in Acton in west London and continues via Hammersmith to the Thames. It then follows the course of the river to the Limehouse Cut in east London, before heading north-east to Abbey Mills Pumping Station, finishing at a depth of around 70m.
Herrenknecht is supplying a total of three tunnel boring machines for the major sewage project. An EPB Shield will excavate the western, 6,950 meter long section of the tunnel from Carnwath Road Riverside to Acton. A Mixshield will dig the eastern, deepest tunnel section from Chambers Wharf to Abbey Mills Pumping Station over a distance of 5,530 meters. Another Mixshield will tunnel through London's underground for the 4,600 meter long Greenwich connecting tunnel. It will connect most of London's southeast to the Thames Tideway Tunnel.
The EPB Shield was transported from the Herrenknecht plant in Schwanau via Kehl, Rotterdam, Denham Wharf and Gravesend to the unloading point on the Thames in just seven days in late 2017. Shipment of the two Mixshield machines is expected to take place in 2019 and 2020. One of them is transported in as few pieces as possible to make time-consuming re-assembly in the launch shaft unnecessary.