The firm delivering London’s super sewer, better known as the Thames Tideway tunnel, is not an organisition to shy away from ambitious targets.
It is trying to deliver the £4.2bn tunnel two years ahead of target, while marrying state-of-the art engineering with a complex system of Victorian sewers.
So it is no surprise that the project has its fair share of technical challenges. The tunnelling that was expected to begin this year has now been pushed back until March 2018.
“We’re a little bit behind that [target date] but again, we’ve had to do a lot more river wall strengthening here than we had originally anticipated,” said Tideway delivery manager for the West section Ben Green.
The West section of Tideway covers 7km from Acton to Hammersmith, and seven construction sites. BMB, a joint venture between Bam Nuttall, Morgan Sindall and Balfour Beatty, is delivering this section.
The £4.2bn Thames Tideway Tunnel is needed to cope with an increase in London’s population and reduce untreated sewage discharges into the River Thames from combined sewage overflows (CSOs): 39M.t of sewage enters the river each year. At present the 25km tunnel is expected to open in 2023. It will run from Acton in the west to Abbey Mills Pumping Station in the east.
The Tideway West team has encountered major engineering challenges at several of the construction sites.
Hammersmith Pumping Station
“The main focus of activity over the last 18 months has been the early start of Hammersmith Pumping Station,” said Green.
Hammersmith Pumping Station, a complex structure where flows can be as high as 40m³ per second, is one of Thames Water’s major assets and protects Hammersmith and Fulham from flooding. As a result it must remain operational while work takes place. To do this half of the station is being closed for works while the other remains open, before switching over.
“Effectively we’ve got to do some quite radical structural surgery to the Hammersmith Pumping Station,” said Green.
The large central wall, which is 15m long, 2m wide and 4m high, will have to be removed so sewage flows can be diverted into a new tunnel. The work will take place 25m underground. The temporary works design team and the site construction team are working on how best to dismantle the wall, which is structurally important.
To ensure the stability of the pumping station a temporary propping system is being developed. While work is ongoing sewage flows will continue through a temporary flume.
The Carnwath Road site has had many uses over the years, from an ambulance station to a cement works in the 1950s. Old piles at the site were broken out before fresh piling started to avoid clashes.
Piling at depths of 15m to 20m for the top of a shaft is now being carried out until the end of May. Excavations for the shaft, which will be 45m deep and 25m in diameter, will start in October and last four months.
The shaft will intercept the Frogmore connection tunnel, which will connect CSOs from Dormay Street and Kind George’s Park to the main tunnel. As a result sewage will be carried away through the main tunnel rather than spilling out into the Thames.
When the shaft is complete, a tunnel boring machine (TBM) will be lowered down for tunnelling towards Acton. The Central section’s TBM will be lifted out of it once it has completed boring.
To minimise noise pollution, a steel framed, three storey shed will be erected over the top of the shaft in June so that work can continue 24/7. But the shed will have another use too, as it will house a crane.
“That shed is also going to be designed to lift and lower our TBM and also then pick Central’s out,” said Green.
The rest of the West
Acton Storm tank
Around 312,000t of untreated sewage is released into the Thames from the Acton Storm Tank area each year. After Tideway opens there will be no discharges at the site, which is at the start of the super sewer’s 25km route across London.
Construction is not expected to start until 2018, later than other parts of the West section. However work on setting up the site will begin at the end of this year.
King George’s Park
The Wandsworth location will be used to connect the Frogmore Storm Relief and Buckhold Road CSO to the Frogmore connection tunnel, which will join the main tunnel at Carnwath Road. It will drastically reduce the 21 untreated sewage releases into the Thames each year, to just one. At present untreated sewage discharges from this location amount to 86,000t.
Construction of the shaft is expected to start later this year.
The second location on the Frogmore connection tunnel after King George’s Park is the Dormay Street site. This will control discharge from the Frogmore Storm Relief and Bell Lane Creek CSO.
Investigation works were carried out in January and February. Later this year a shaft will be constructed and the river wall strengthened.
Putney Embankment Foreshore
Putney Embankment Foreshore is home to the famous university Boat Race. It comprises two areas: a main construction site with work due to start this year and a temporary slipway for public access, which will be finished this month.
With the temporary slipway complete, piling work will start later this year for a shaft.
At the Barn Elms site in Richmond, the West Putney storm relief CSO will join the main tunnel. Part of the site sits on a school sports centre, and so BMB will replace an existing changing room building as part of the work.
Ground investigation work started at the sports centre on 27 March and lasted two weeks. Work on a shaft and connection tunnel at Barn Elms is not due to start until 2018.