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Seven Bridges, Wiltshire

Glanville was appointed by BP to provide highway design services for the redevelopment of a disused service station to provide new modern facilities on the A419.

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. The original diverge and merge tapers (slip lanes) were substandard and needed to be significantly extended in order to meet Highways England’s (HE) requirements.

Although we were not due to be involved with the project post-design approval, the discovery of a trunk water main supplying a large catchment of Wiltshire within the merge taper (not where records placed it) meant 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 main the challenge that we faced was to design a road with a construction thickness no greater than 600mm where the original, approved design was based on a minimum road thickness of 880mm, with the added complication of the subgrade having very low bearing capacity.

Our efficient and cost-effective design solution to mechanically stabilise the sub-grade, coupled with extensive negotiation with various specialists and HE, allowed us to reach a design solution acceptable to all parties and HE’s agreement to adopt. However, it did require very careful excavation work around the main. Traditional excavation methods were too risky so our proposals employed the use of a powerful vacuum excavator to clear the area without affecting the main.