Civil engineering and infrastructure specialist Barhale has completed the first shaft secondary lining in the central section of the Thames Tideway Tunnel.

The Ferrovial Construction and Laing O'Rourke (FLO) joint venture, the principal contractor for the 12.7km central section of the 25km Thames Tideway Tunnel, appointed Barhale to carry out the works at the Victoria Embankment Foreshore.

Passing the important milestone at the Foreshore site clears the way for the internal shaft work to begin. The next steps will include the installation of the baffle/divider wall and vortex surround – the two elements that facilitate the flow transfer from the top to the bottom of the shaft where a 17m tunnel connects to the new super sewer. The secondary lining of the connection tunnel has also just been completed.

When completed the site will be used to control the existing local CSO, known as the Regent Street CSO, through the connection of the northern Low Level Sewer No.1 to the main tunnel.

The lining is a key structural element of the 48m shaft. It will protect against the chemical and physical impact of the sewer discharges and ensure its 120-year design life.

Barhale used a custom-specified concrete mix for the lining. Using more than 70% Ground Granulated Blast furnace Slag (GGBS) and admixtures, it generates very low CO2 emissions. GGBS is a by-product of iron-making so requires no quarrying or mineral extraction and reduces the level of embodied carbon in the concrete by around 900kg compared with one tonne of Portland cement.

The concrete was poured in situ using a 160Te crawler crane. Each pour exceeded 100m3 of volume.

Contracts manager Filipe Mello said although approximately 1900m3 of concrete has been used in total the team had been able to maintain good progress throughout.

“We employed thermocouplers, maturity curves and the on-site testing of cubes to confirm the in situ concrete strength,” he said. “That allowed us to jump the formwork in the next pour position to maximise progress.”

Barhale regional director Phil Cull highlighted the importance of reaching the milestone.

“Tideway is a huge project which will upgrade London’s infrastructure not only to meet existing demand but also for generations to come,” he said.

“At Victoria Embankment we are working on a key site within the Whitehall conservation area and adjacent to one of the capital’s principal arterial routes. It means completing major infrastructure works to a very tight schedule so it is very pleasing to get the shaft lining completed.”