Tideway, the company responsible for construction of the Thames Tideway Tunnel, has revised its estimate of overall costs based on work left to do – with five years left to go, the revised estimate is now £3.8 billion compared to £3.52 billion.

As the 2018/19 financial year closes and with the first and most unpredictable phase of the project now complete, Tideway said 40% of construction work has been completed and it remains on track to deliver the super sewer on time in 2024.

Complex engineering work on the project includes building seven new pieces of land in the Thames, including at Chelsea, Blackfriars and Putney, and digging five giant shafts up to 50m deep below London.

A 2,000- plus workforce are now tunnelling underground to create the 25km-long tunnel. The first kilometre from Battersea to Chelsea was completed at the end of March and two other huge tunnelling machines have also started their journeys.

Despite the projected increase, Tideway said there would be no change to the estimated £20-25 annual cost for Thames Water bill-payers and that costs remained well within the original projection for customer charges. The company has already raised enough money to complete the project and requires no further funding as a result of the cost revision.

Andy Mitchell, Tideway’s CEO commented:

“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.”

Construction is taking place at 23 sites across the city and Tideway is using the river instead of roads as far as possible to build the project, cutting the impact on road congestion and air quality. By the end of April 2019, one million tonnes of material will have been moved by river – keeping 60,000 HGV movements off London’s roads so far.

Earlier this year, Tideway announced it had avoided the need to divert a Victorian gas main at Blackfriars, one of its most complex sites. The diversion would have meant a closure of the Embankment to traffic for up to six months. To avoid the diversion, Tideway had to make fundamental changes to the construction methodology for the site, adding costs to the original estimate.

At Tideway’s King Edward Memorial Park site in Wapping the team found difficult and unexpected ground conditions which meant they have had to do significant work to reinforce the ground to make it stable for construction, also adding cost.

Tideway also took the decision to reduce HGV movements, which required additional investment in the infrastructure needed to move materials by river, as well as investment in skills and jobs through the new Thames Skills Academy. This will cut the number of lorry movements by 360,000, reducing impacts on congestion and air quality.

Tideway will publish its full annual report and accounts in June.