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Northern line extension goes from strength to strength

Mott MacDonald’s work on Battersea’s new tube line has produced a concrete composition that both fits the bill and reduces environmental impact.

Construction contributes about £90bn to the UK economy each year.

In 2013, the coalition government set out its aspirations for the industry in 2025. Goals included halving carbon emissions from the built environment and for the UK to become a world leader in low-carbon and green construction exports.

Major infrastructure projects are an opportunity to push the boundaries of what is possible to provide innovative, cost-effective, low-carbon solutions that can filter down through the industry.

Mott MacDonald was appointed lead designer for FLO, the joint venture between Ferrovial Agroman and Laing O’Rourke that is delivering the Northern line extension – a 3.2 km addition to the tube network set for completion in 2020.

The project consists of two new subterranean stations: one at Battersea’s iconic former power station, and another to the east at Nine Elms. There are also two ventilation shafts in Kennington, bored and sprayed concrete lining tunnels, and turnouts formed using spheroidal graphite iron.

Overall, about 50,000 cu m of concrete will be poured to build the line and stations. With cement manufacture a major source of emissions, Mott MacDonald’s work focused on finding low-carbon alternatives to concrete mix designs that traditionally use Portland cement.

Putting steel waste to use

It is the cementitious product in concrete that contributes its embodied carbon and can account for as much as 50 per cent of total emissions over the lifetime of a building, bridge or tunnel.

Adding industrial waste products such as fly ash and ground-granulated blast-furnace slag is a tried and trusted way of significantly reducing the carbon footprint of concrete. This is because they are by-products from coal-fuelled electricity power stations and steel manufacture respectively, and the main carbon emitted is from grinding the waste into a powder and hauling it to a mixer.

Mott MacDonald Northern Line Extension Battersea 2

Mott MacDonald Northern Line Extension Battersea 2

The 3.2 km tube extension is set for completion in 2020

To put the potential savings into perspective, Portland cement (CEM I) typically generates 913 kg of CO2 for each tonne of finished product. In contrast, cementitious materials made using limestone, fly ash or GGBS (CEM II) generate 615-859 kg of CO2, depending on the amount added.

According to the Cementitious Slag Makers Association, GGBS typically replaces about 50 per cent of the Portland cement component in concrete, and sometimes up to 70 per cent. Mott MacDonald went further on the NLE, raising the proportion to 95 per cent for the secant piling concrete, reducing the CO2 equivalent content compared with the CEM II mix by more than 80 per cent.

Tackling unnecessary restrictions

Most specifications limit the amount of GGBS that can be added to the concrete and this was the case on the NLE, with the client allowing only CEM I and CEM II cements.

“NLE shows that engineers free of unnecessary specification restrictions can design solutions that minimise environmental impact”

Not only was this stipulation detrimental from a sustainability perspective, but it unnecessarily limited opportunities to enhance the durability of the concrete. This may have prevented the contractors from using secant piling, which, given the short timescale available to complete the works, was their preferred construction method for the station boxes and shafts.

Female piles in a secant system require low early-age strength to allow male piles to drill through them after a few days to form a wall of interlocking alternate male and female piles. The piling contractor on the NLE required low strength in the female pile at ideally less than 10 MPa at seven days.

Mott MacDonald Northern Line Extension Battersea 3

Mott MacDonald Northern Line Extension Battersea 3

The proportion of GGBS was raised to 95 per cent for the secant piling concrete

CEM II limits the amount of GBBS to 35 per cent, which was insufficient to reduce the early strength and provide an optimised programme for construction of the hard piles. However, the concretes also had to be durable because some of the secant walls form part of the permanent works, designed to last up to 120 years. Also, both the male and female piles must be able to resist sulfate attack.

To achieve the required sulfate resistance, BS 8500 requires concrete with up to 80 per cent GGBS content to contain a minimum 360 kg/cu m cement content and a 0.5 water-cement ratio. This level of cementitious content would produce concrete with an excessively high early strength, however.

A constructible, durable solution required a higher GGBS content; trial concretes onsite demonstrated that a blend containing 95 per cent would indeed work.

It is now being used on the NLE and shows that engineers free of unnecessary specification restrictions can design solutions that minimise the environmental impact of a project.

Ian Gibb is a principal materials engineer at Mott MacDonald

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