Secondary lining on a 1.1km connection tunnel linking Wandsworth to the main super sewer in West London is now complete, marking a major milestone for the Tideway project.

The Frogmore Connection Tunnel (FCT) runs from the northern corner of King George’s Park, through Tideway’s site at Dormay Street, beneath the Thames and finishes deep beneath the northern bank of the Thames at Carnwath Road in Fulham.

Working with specialist tunnelling firm Kern, the team used a system of four individual ‘shutters’ to cast the concrete in situ using a ‘hit and miss’ sequence allowing for faster production.

In March 2020, just a month into the secondary lining production, the team halted all work after the UK was placed under lockdown.

The 2.5m internal diameter of the FCT meant that social distancing was a huge challenge, but thanks to the efforts of Tideway’s main works contractors (Bam Nuttall, Morgan Sindall and Balfour Beatty), along with the efforts of Kern, the team later resumed production in accordance with the latest guidance.

A tunnel boring machine called Charlotte had previously excavated the full FCT in two separate drives – first mining south from Dormay Street to King George’s Park, then (after a short journey by road back to Dormay Street) north to Carnwath Road (see graphic below).

Project Director Sally Cox said: “Well done to the entire team for completing the secondary lining of the Frogmore Tunnel. It has not been an easy one with the disruption of Covid in the middle of the works, but the team have worked brilliantly in making it a success.”

The team used a total of 1,420m³ of concrete for the secondary lining and were able to cast an average of 60m each week – with a peak production rate of 72m.

Attention now turns to cleaning both the northern and southern tunnels, completing the contact grouting and fixing any outstanding snagging items.