How Wills Bros cut carbon by 87% on a high-voltage cable crossing

Wills Bros was awarded the design and build contract for the Glasgow Airport Investment Area (GAIA) project by Renfrewshire Council. RPS was appointed by Wills Bros as design consultant and the client’s representative on the job was Sweco.

One of the key stakeholders was Scottish Power Energy Networks (SPEN), the asset owner of a series of high-voltage cables crossing the site.

The funding for the initiative was provided by the Glasgow City Region City Deal. The scheme provides the infrastructure required to support the development of a new high-tech business district, to be known as the Advanced Manufacturing Innovation District Scotland, which is hoped will deliver high-skilled jobs to the region.

One striking element of the GAIA project was that it was one of the first in the world to be developed using the PAS 2080 specification for carbon management in infrastructure.

As a result, a set of carbon baseline values had been developed for every element of the project.

Low-carbon alternatives

From the baseline information, it was clear that some of the biggest carbon emissions were associated with the structures on the project. This is typically because structures traditionally use lots of concrete and steel – two materials with a reputation for large carbon footprints.

On this project there were three main structures, including a road bridge and a cycle bridge. The third was a protection structure for a series of high-voltage cables – this is where the biggest opportunity for carbon savings was identified by Wills Bros.

The cables had to be crossed by an embankment supporting a section of the newly realigned road. The cables had originally been installed in the 1940s, had fragile oil-filled cores, and now serve well over 20,000 homes and businesses, including Glasgow International airport.

The specimen design proposed a structure to facilitate crossing the cables supported on a total of 22 bored concrete piles – each 30m in length – with precast concrete beams spanning between them. The carbon footprint for this structure was 1,546 tCO₂e and this represented about 13% of the whole project’s baseline footprint.

A big problem with the specimen design was the huge amount of concrete involved. As concrete is responsible for about 8% of all global carbon emissions, reducing the amount used in construction is a key priority for environmentally responsible contractors. As well as the concrete, the proposals required significant quantities of steel reinforcement.

Wills Bros collaborated with its design consultant, RPS, to explore low-carbon construction alternatives. Drawing on extensive experience on road projects dealing with soft ground, a lightweight embankment solution to cross the cables was developed.

The idea was to build the embankment with plastic drainage attenuation crates which are 95% voids and only 5% structure. As a result, they were ideal for reducing the weight of the structure while comfortably accommodating the required traffic loadings.

Wssentially, the proposal was to swap concrete and steel for air and a small amount of recycled plastic. It was clear that this solution could deliver significant reductions in the carbon footprint while reducing some of the construction risks associated with the specimen design.

Eco-friendly materials

After consulting various storm crate manufacturers, Wavin’s Aquacell range was selected. Benefits included the fact that the crates were formulated from 100% recycled plastic materials and manufactured in the UK.

The crates were combined with a specialist lightweight fill – ‘LECA’ – and a very small quantity of lightweight concrete for a slab directly beneath the road. Crucially, the solution allowed steel to be eliminated from the structure.

While acknowledging that the proposed solution was unorthodox, SPEN – the asset owner for the cables – retained an open mind and was ultimately able to sign off on the design proposals. As part of the consultations, concerns about the potential for overheating had to be resolved.

A thermal dynamics specialist was engaged to provide a detailed assessment of the effects of the proposals on heat dissipation around the cables. This concluded that there would be negligible impact on the operation of the cables.

Attenuation crates have been around as a product for a while and are typically used in stormwater storage solutions where a design life of 50 years is sufficient.

However, as they were being used here to construct an embankment, additional testing data had to be obtained to demonstrate effective performance for at least 100 years.

Taking shape

After the extended efforts that were put into the design development and consultation process, the construction was perhaps the simplest part of the whole project. Works commenced in summer 2020 and lasted about 12 weeks. The new road began to take live traffic in September 2021.

To ensure the successful performance of the structure, regular monitoring for settlement has been ongoing since construction commenced. There has been very little change in the observed levels over the 15-month period, with cumulative settlement within the expected limits of 5mm.

A hydrostatic gauge within a duct adjacent to the cables was also used to monitor for any potential movements and this also recorded negligible changes in level.

For Wills Bros, the proudest part of this solution was the 87% reduction in carbon emissions it facilitated when compared with the original baseline.

This equates to a figure of 1,350 saved tons of carbon dioxide equivalent – equivalent to the annual emissions of about 800 cars.

The project was featured in New Civil Engineer (NCE)’s February 2021 edition and Wills Bros has sought to share the success of its lightweight embankment solution in other forums across the construction industry.

The solution was included in a post on the Considerate Constructor’s Schemes Best Practice Hub and was a key part of the company’s nominations for the Scottish Civil Engineering Awards and NCE’s Techfest Awards last year.

Will Bros was delighted for the project to be recognised by ICE’s inaugural Carbon Champions initiative, with eight members of the project team being given this title.

Name of Project: GAIA - 132kv Cable Crossing

ICE Carbon Champions involved in this project:

Contractor and Designer:

• Graham Lloyd - Wills Bros Civil Engineering Ltd;
• Cian McGuiness - RPS Group

Project Management:

• Douglas Halliday - Wills Bros Civil Engineering Ltd

Utility/Asset Owner:

• Adele Ramsden - Scottish Power Energy Networks;
• Steven Bisset - Scottish Power Energy Networks

Carbon Measurement and Management:

• Keith Fernandes - Wills Bros Civil Engineering Ltd
• Chris Landsburgh - Wills Bros Civil Engineering Ltd (now AECOM)