The Thames Tideway project is now estimated to cost £3.8bn as opposed to the previous estimate of £3.52bn.
Tideway has updated its cost estimate based on the work left to do, with 40% of construction completed. The first and most unpredictable phase of the project now complete, it said, and the project remains on track for completion in 2024.
Engineering feats accomplished to date include building seven new pieces of land in the Thames, including at Chelsea, Blackfriars and Putney, and digging five giant shafts up to 50-metres deep below London.
A workforce of more than 2,000, including miners from across the world, is now tunnelling underground to create the 25km tunnel. The first kilometre from Battersea to Chelsea was completed at the end of March 2019 and two other tunnel boring machines have also started their journeys.
The cost increase is not enough to jeopardise the economics of the project. The company has already raised enough money to complete the project and requires no further funding as a result of the cost revision. There will be no change to the estimated £20-25 annual cost for Thames Water bill-payers, as costs remain well within the original projection for customer charges.
The tunnel will run from Acton in west London to Stratford in east London, stopping millions of tonnes of raw sewage polluting the Thames.
Part of the cost increase has come from changes to reduce traffic disruption. Tideway previously planned to divert a Victorian gas main at Blackfriars, which would have meant a closure of the Embankment to traffic for up to six months. Instead, there construction methodology has been changed to avoid this disruption but at extra cost.
Tideway also decided to reduce HGV movements, which required additional investment in infrastructure needed to move materials by river.
Other increases have come from unexpected ground conditions. At Tideway's King Edward Memorial Park site in Wapping the team met ground that needed reinforcing to make it stable for construction, also adding cost.
Tideway chief executive Andy Mitchell said: "To build the super sewer we are working in an unforgiving tidal river in the centre of one of the busiest cities in the world. We have got train lines and bridges above our heads, and the tube and utility pipes below our feet.
"As we approach the half way point of construction, the time is right to update our cost estimate. The most important thing to say is that there will be no impact on the estimated cost to Thames Water bill payers and to recognise that our teams have done a great job in getting us this far and keeping us on schedule. Every day we get closer to our aim of giving London a cleaner river."