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Project Profile: Seven Bridges BP Service Station

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Back in 2016 Glanville was appointed to provide highway design services to support the redevelopment of a disused service station on the A419 near Cricklade, Swindon, to provide a new modern service station for BP.

For those of you who are familiar with that particular stretch of road, you’ll be aware that it connects the M4 with the M5 so there’s never any shortage of traffic!

From a highways perspective, the principal issue that needed addressing was access to the service station. The original diverge and merge tapers, or slip lanes, were too short by modern design standards and needed to be significantly extended in order to meet Highways England’s requirements.

Our design work to demonstrate how access to and from this very busy road could be upgraded was a key factor in successfully obtaining planning permission for BP.

Construction work to overhaul the disused service station and road access began in 2018. However, the highways element of the project soon hit a bump in the road.

Original plans obtained from Thames Water during the detailed design process had identified the presence of a 600mm diameter pressurised trunk main within the A419 carriageway. Although this is a critical water main, which acts as a back-up supply for most of the county of Wiltshire, we were not overly concerned as it was beyond the area where alterations were proposed and Thames Water was satisfied that it would not be affected by the works. Although we advised through our design work that site investigations should be carried out prior to construction starting, the presence of the water main within the merge taper carriageway was identified by the contractor shortly after the works had started.

Glanville’s involvement with the project was not due to continue post-design approval. However, following the discovery of this water main, we were asked by the client to help solve the problem and keep the project on track.

Due to the depth and alignment of the water main, the challenge that we faced was to design a road with a construction thickness no greater than 600mm to sit over the main, whereas the original approved design was based on a minimum road thickness of 880mm, with the added complication of the subgrade having very low CBR values, meaning its bearing capacity would require some form of enhancement.

The highway works were put on hold and we were tasked with quickly finding options to resolve the issue. Working alongside the contractor, client, Thames Water and Highways England we presented four possible solutions.

  • Move the water main:  Quickly discounted as too complex and costly
  • Use reinforced concrete slab:  Ruled out due to the increased difficulties this would present to Thames Water for future access to the main
  • Chemical stabilisation of the soil to improve CBR values:  Also ruled out as it would increase potential impact on the main and introduce a hazardous substance
  • Mechanically stabilise the subgrade:  This quickly emerged as the most straightforward, efficient and cost-effective solution as it allowed us to stick with the original flexible design of asphalt pavement and stone sub-base meaning access to the main for maintenance would be straightforward, if required. However, it did mean some very careful excavation work around the main would be needed to strengthen the subgrade.

In order to carry out the highways construction work, the water main needed to be de-pressurised so that excavation around it could take place. The challenge with that was that although the main was only a back-up supply, it was back-up supply to an old 900mm water main a few miles away that regularly burst and required frequent fixing. This meant that, at the drop of a hat, the team had to down tools so that Thames Water could switch mains supply to this pipe and avoid cutting-off the water supply to most of Wiltshire!

The delicate nature of the excavation works meant traditional excavation methods were simply too risky. Our solution comprised the use of a powerful vacuum excavator to clear the area without affecting the main.

This project is a great example of Glanville’s expertise to work with and find solutions for challenging project issues. Over the last 50 years our reputation for careful attention to detail, innovative problem solving and collaborative working with the wider professional team has cemented our position as one of the leading engineering and design consultancies.

We’re thrilled that the project is now nearing completion with the service station set to open before the end of the year. We shall definitely be calling in for a pit stop!

Contact our Highways Team on 01235 515550 or email sreid@glanvillegroup.com