Purpose
The 2012 Crag End landslip near Rothbury in Northumberland, caused part of an existing 90m-long sheet pile retaining wall to fail, leading to significant cracking along the B6344 carriageway and distortion of the wall capping beam. Other sections of the road collapsed and slipped down a slope, which was over a 300m stretch. We were appointed to design and build a robust repair and long-term improved solution, that could quickly reopen the road and reconnect remote rural communities, whose residents were majorly inconvenienced by a long detour.
What we did
We made safe and reopened the crucial B6344 road that connects remote communities in rural Northumberland. Through ingenuity and engineering excellence, we overcame geotechnical challenges, unstable ground conditions, artesian water, difficult access, and environmental constraints.
Extensive fieldwork during the early contractor involvement phase, identified groundwater as the major cause of instability. We used hydrogeological modelling to understand the groundwater regime and how it might react to dewatering wells. Results from pumping trials were used for calibration. A 3D model informed our design.
We installed an anchored bored piled retaining wall, to support the road and an innovative passive dewatering system to reduce groundwater pressure and prevent future landslips. This robust, low-maintenance solution ends decades of instability, leaving an engineering legacy for the people of Northumberland.
We adapted construction techniques to suit the location and designed our solution around plant and equipment, that the unstable ground could support. Through careful planning and supervision, we deployed a number of 14t piling rigs across the constrained site footprint. We installed 436 small diameter piles, reinforced with high strength circular hollow tubes, that provided the required moment resistance.
The 31 dewatering wells we installed required less material import, compared to other drainage solutions. Our passive solution is easy for the client to maintain, with the wells working under gravity so they don’t require maintenance-intensive pumps.
The road runs adjacent to a Site of Special Scientific Interest. We brought in ecologists to carry out field surveys and we moved slow worms and lizards to suitable nearby habitats. During construction, our observant team continued to spot otters, badgers and rare birds and a pair of grey wagtails nested in a cavity within the collapsed wall. By carefully directing activities away from this location, we allowed the pair to fledge young. The ecologists praised our approach. Vicki Mordue from EcoNorth consultants, said: “It was refreshing to have ecology at the project’s heart rather than it be seen as a problem.”
Project successes
Our best practice was recognised with the prestigious ICE Northeast Robert Stephenson Award and British Geotechnical Association’s Fleming Award. Chair of the ICE Robert Stephenson Awards Judging panel, Derek Smith said: “We awarded Crag End Landslip as the winning project because VBA was highly technical and brought together further specialist teams to provide innovative solutions to overcome complex geotechnical challenges. Despite the restricted site, the continuous design review and bespoke access arrangements led to a safe and successful project and a close working relationship with the local community who welcomed the benefits of the scheme.”
Northumberland County Council’s senior engineer, Peter Brewis said: “We believe the project exemplifies fully integrated end-to-end project delivery at its best. We are delighted. We scored VBA 10 out of 10 in their customer experience survey.”
We’ve shared lessons with the engineering community via presentations at Leeds University and the Ground Engineering Slope Conference, and written papers.