National Grid completes record pour of cement-free concrete

The 736m3 of more sustainable, cement-free concrete was poured to fill the base of the 55m-deep tunnel drive shaft at National Grid’s Hurst Substation in south London.

The cement-free solution, developed by Wagners and supplied by Capital Concrete, uses a binder of ground granulated blast furnace slag and fly ash geopolymer concrete system chemically activated by the use of industrial waste products instead of cement. The concrete reduces carbon by around 64%, saving an estimated 111kg of CO2 per cubic metre poured in comparison to concrete that would have traditionally been used.

The pour took 11 hours and was completed on Earth Day.

The use of Earth Friendly Concrete was driven by a team of young engineers on the project and supported by HMJV’s engineering experts and Aecom, Mott MacDonald and WSP, following several trials at different London Power Tunnels sites.

The pour at the Hurst substation site was needed to infill the base of the 55m-deep tunnel shaft to its permanent level, following completion of 9.2km of tunnelling over two tunnel drives from Hurst to Eltham and Crayford.

The London Power Tunnels project is employing several measures to reduce the environmental impact of construction and keep National Grid on track to achieve net zero carbon construction by 2025/26. They include HMJV’s optimisation of tunnel and shaft designs and construction methodologies of the tunnel, saving approximately 50,000 tonnes of CO2, providing a 30% reduction against baseline; 99.98% of project waste being diverted from landfill, a 21% reduction against the carbon reduction pre-project baseline for shafts, tunnels and headhouses, equivalent to 25,250 tonnes of CO2; and, in a UK first, a new 400kV substation at the heart of the project will be built using SF6 free gas insulated switchgear technology.

“We are always looking for new ways to innovate and to now be using this carbon-saving cement-free alternative to conventional concrete at scale and on site is exciting,” said National Grid Project director Onur Aydemir. “This world record-breaking pour also offers the opportunity to evaluate the technology ahead of future possible rollout across our network in England and Wales, demonstrating our ambition to achieve net zero construction across all our projects by 2025/26.”

HMJV technical lead Raj Kundan said the pour showed how a client, contractor and the supply chain could work together to help take a major step in moving the industry forward in reducing carbon emissions.

London Power Tunnels is a £1bn, seven-year project to rewire south London via deep underground tunnels. In total, 32.5km of 3m-diameter tunnels are being built between Wimbledon in south-west London and Crayford in the south-east, which will carry high voltage electricity cables. Phase 1 of the project was completed between 2011 and 2018, and carried the cable circuits north of the River Thames. The new network will be operational in 2027.