When Dumfries & Galloway Council decided to widen part of the busy A7, slope reinforcement was crucial to the project. Natalie Hardwick reports
The busy A7 trunk road through the rolling lowlands of east Scotland is an important connection to Edinburgh.
But when growing demand began to clog this route, Dumfries & Galloway Council commissioned a £7.75M project to upgrade the road.
The main priority was a 3.25km stretch of single-lane road around Auchenrivock which needed widening to improve overtaking opportunities in both directions.
Work called for extensive earthworks, with 750,000t of material being excavated in the initial construction phase.
This then made way for a completely new, widened carriageway, 690m of farm track, an underpass carrying a minor road, a new cycle lane and two ghost island road junctions at either end of the new overtaking section.
Engineer Willie Porteous worked with Dumfries & Galloway Council’s design team and later as an assistant resident engineer during the construction phase.
He says the initial design difficulties revolved around the steep terrain of the area.
“The area had deep gorges of up to 20m,” he says. “So we needed to accommodate design standards for the vertical and horizontal profile of the new road alignment and at the same time minimise the impact on the surrounding environment.
“Traditionally, a design approach would be to over-bridge the deep gorges with high-level structures,” adds Porteous. “Alternatively or as an addition, you could build low level concrete or corrugated steel structures with conventional 1:2 side slopes. We decided against these solutions.”
Instead, it was decided to remodel the slopes at a steeper incline, working with reinforced earth structures.
The decision was reinforced by proximity of the road to the Border Esk river, which meant designers had to consider the environmental impact of the work.
“We used our Natural Green system. It is an economical solution as it not only uses recycled material, but it also creates a cost-effective and versatile green looking slope, meaning it has an aesthetically pleasing end result”
The river and its culverted feeder streams contain important spawning gravels for migratory fish which could not be disturbed by construction work.
To fit in the road extension, adjacent slopes needed to be built and reinforced to maintain acceptable road gradients.
This process again needed to take into account the river and its existing culverts.
Conventional shallow slope designs for this would have required long culvert lengths to convey the flow of water down the streams and channels without overflowing the embankment.
But long culverts would have deterred migratory fish from entering the stream in search of the upstream spawning gravels.
On the way downstream, small fry would have been discouraged to enter and feed on their way to the sea.
With culverts posing a threat to river life, an alternative solution was required.
Contractor Morrison Construction began work on the road improvement in January 2008.
This part of the work was completed in June 2009.
In a separate but related contract, Dumfries & Galloway then called in the services of Tensar as for the subsequent slope development work that began in early 2010.
Tensar engineer Mike Horton says: “We were dealing with some very high slopes. Also, some of the site was located at the bottom of a deep valley, meaning there were additional access issues. We also had to consider the adjacent river.”
The solution was a multi-material approach to reduce culvert lengths by around 30m.
As well as environmental benefit to the river, the design would use recycled material - locally sourced sands, gravels and glacial till - rather than high-grade externally bought aggregates.
The differing nature of the slopes requiring reinforcement meant that Tensar used several different techniques.
“Where we were reinforcing slopes with a gradient less than 1:1 in ratio, we wanted to have a finish that was sensitive to the surrounding natural environment,” explains Horton.
“We used our Natural Green system. It is an economical solution as it not only uses recycled material, but it also creates a cost-effective and versatile green looking slope, meaning it has an aesthetically pleasing end result.”
This system uses uniaxial geogrids to reinforce the soil for internal and global stability.
Anti-erosion matting is then installed on the topsoil surface to provide protection and assist the establishment of natural vegetation.
In another area of the road, concrete bridge abutments resulted in small areas of slope needing a gradient above 1:1 ratio.
Here, a TR2 wall system was used which installs a steel mesh facing attached to the uniaxial soil reinforcement.
This system favours practicality and economy over aesthetics and produces a straightforward structure.
A third technique was used to accommodate where the road had been widened.
Here, a TW3 modular concrete block facing wall was built to place limitations on the surrounding slopes.
“With the TW3 technology, we worked with the existing slope wall,” says Horton. “We placed the new wall, made of precast concrete modular blocks, around 7-8m in front of it.”
Uniaxial geogrids were then fixed onto both the existing wall and the new facing wall to create a reinforced soil mass anchored into the original construction.
“Our design had to be flexible enough to allow for sudden changes in application depending on what was going on on-site”
The Natural Green system was then used to create a green-looking vegetated slope on the surface.
Horton says that once the design team overcame the challenge of the adjacent river and the general topography of the area, they were still required to closely monitor work as it progressed on site using feedback from Morrison Construction.
“We had to take into consideration the fact that while Morrison Construction were excavating they were finding different things within the geography,” he says. “Our design had to be flexible enough to allow for sudden changes in application depending on what was going on on-site. We are used to working with this design so it was easy for us to be able to adapt it continually.”
The slope reinforcement work was completed in spring this year, meaning all work on the project is now complete, leaving only monitoring work outstanding.
Dumfries and Galloway Council are satisfied with the end result.
“Work on the 1:1 slopes reduced culvert lengths by around 40m, benefiting fish and mammal movements,” says Porteous. “The earth embankment is less obtrusive and is more in keeping with the adjacent landform.
The reduction of the pipe length also provided cost savings from using a 1:2 slope and minimised disturbance to the watercourse.”
During the monitoring stage, one area of continuing improvement is the growth of grass on the slopes. The method of seeding used by Tensar has taken longer than expected to establish new grass.
This will be reviewed and improved throughout the maintenance period.