Installation of an extensive network of debris flow barriers is under way at the Rest and Be Thankful pass in Argyll & Bute.
The Rest and Be Thankful carries the busy A83 trunk road between Arrochar and Inveraray through mountainous terrain in the west of Scotland. The pass has suffered frequent debris flows, especially over the last seven years, with the road closed four times in the last six months.
When the road is closed, a diversion of roughly 80km is required.
As the A38 trunk road crosses an area of outstanding natural beauty around the Rest and Be Thankful, which is part of the Loch Lomond and Trossachs National Park, Transport Scotland required that any debris flow prevention measures had to have a minimal visual impact on the surrounding landscape.
The area suffers high levels of rainfall, which act as the trigger for failure. Movement of the surficial soils usually starts high up on the hillside and is progressively channelled into natural gullies above the road, forming a debris flow.
Because of the remote location on steep and unstable ground, the measures had to be easily installed with the minimum of heavy plant and machinery
By the time a debris flow has reached the road it has gathered sufficient material, including large boulders, to completely inundate the carriageway. It has also developed sufficient translational kinetic energy to cause significant damage to any vulnerable structures, vehicles or people that it may strike. Consequently, debris flows represent a considerable risk that must be mitigated wherever possible.
“Because of the remote location on steep and unstable ground, the measures had to be easily installed with the minimum of heavy plant and machinery,” explains Waterman senior geotechnical engineer Chris Gell, who is leading the design of the current phase of work in association with rockfall protection specialist Maccaferri.
“In addition, variable depth to rock head required a range of barrier foundation and anchor designs to account for differing ground conditions.”
The barrier installation work builds on detailed geomorphological assessments by Geomorph Consulting and additional surveys by the project’s main contractor Geo-Rope.
Due to the scale of the site, it was felt to be impractical to erect barriers along the whole length of the roadside.
Costs would have been prohibitive and the barriers would also have created a hazardous physical obstacle to local wildlife - potentially trapping animals on the carriageway.
Debris flow barriers
For this reason, Maccaferri proposed a network of debris flow barriers totalling around 1,500m2. The barriers are positioned in such a way as to conform to the site topography and offer the best balance of engineering performance and minimum cost/materials requirement. At the same time, they exceed the client’s requirement for volumetric impact capacity while creating a relatively small and discrete visual profile on the slopes.
The barriers are loosely based on Maccaferri’s high capacity 3,000kJ and 5,000kJ RMC rockfall catch fence systems. They comprise hinged steel supporting posts from which heavy duty interceptor panels of steel wire ring mesh are suspended. Bracing cables, anchored laterally and up-slope, are equipped with patented aluminium energy dissipaters.
Debris flow is a serious problem, but one that can be successfully addressed using the right technology and the correct technical approach
These act to minimise impact-induced shock loadings on the system in a controlled way, and are specified on a rope by rope basis in debris flow systems. The dissipaters are also highly resistant to corrosion and, according to Maccaferri, are simple to install as they are fitted into the relevant ropes in the factory, thus requiring no on-site assembly.
Maccaferri rockfall mitigation specialist David Cheer explains some of the issues faced by the design and installation team: “Compared to rockfall barrier projects, debris flow situations constitute a much more complex scenario. There are many more variables to consider, and all the time we have to keep functionality and practicality of installation in mind. Debris flow is a serious problem, but one that can be successfully addressed using the right technology and the correct technical approach.”
He continues: “In this project the geometric configuration and positions chosen for the barriers allowed us to reduce both the impacting forces and, consequently, the size and costs of the foundations and barrier anchorages - especially important, bearing in mind the highly challenging ground conditions on site.”
Cheer concludes: “By carefully selecting the barrier locations it was possible to minimise the total requirement for barriers. This approach is more design-intensive but it offers much better value to the client compared to the “spread-shot” approach of protecting large lengths of road in a blanket fashion, hoping to intercept a flow.”
Specially adapted helicopter
In this latest phases of work, 10 bespoke debris flow barriers are being installed by roped access geotechnical contractor Geo-Rope. Because of the difficult terrain, the company has brought in a specially adapted helicopter to lift and place components of the 2m to 6m high barriers into position.
Performance of the barriers will be constantly monitored using a web-hosted asset monitoring and data storage system developed by itmsoil. The instrumentation system will integrate real time remote monitoring and logging of barrier conditions and automatically triggered day/night photogrammetry.
This will provide quantitative data on barrier performance and condition while helping with network management and scheduling of unplanned maintenance events in response to debris flows.
The system is remotely controlled and adjusted, with the ability to monitor slope and barrier conditions and take photographs of the area around the barriers either in response to a previously defined alarm condition or on request by a web-portal user.
When the current works are completed, a total of 18 Maccaferri debris flow barriers will have been installed over nine phases at the Rest & Be Thankful.