Many sections along the A83 are high-risk areas for potential landslides. Alongside significant work at Rest and Be Thankful, the Route Study identified three key areas for landslide protection work. In Autumn 2013 Transport Scotland instructed BEAR Scotland to produce detailed report into each of the following known landslide areas.
In the mid-70s, a major landslide severely impacted the trunk road passing by Loch Shira. Since then, the slopes above the road on the eastern shore have been a source of concern.
In 1994, contracts were awarded to cut back the slope, install cut-off drains and the rockfill blanket. Ongoing slope movements, however, have led to signs of distress in these installations. The rock blanket and cut-off drains are now at the end of their useable life.
A number of options to replace them have been investigated. The preferred options are:
- Phase 1 - Installation of a new flexible, on-slope drainage system.
- Phase 2 - Construction of a new, improved rock blanket that includes deep, counterfort drains and carriageway drainage improvements.
The Glen Kinglas study area is located between Butterbridge in the east and the settlement of Cairndow in the west. It is approximately 5.5km long and contains 32 channels that flow perpendicular to the Trunk Road. Typically events in Glen Kinglas are less dramatic that those experienced at the Rest and Be Thankful.
A number of potential risk reduction methods were considered during the assessment process, including:
- forestry planting
- enlarging and re-aligning existing culverts and associated ditches
- flexible (steel) debris barrier
- catch pits
- linear catch ditches with associated bunds
The preferred option is a combination of linear catch ditches and bunds. This could take the form of a an earthworks ditch and associated bund, which will be in t he region of 1.5m to 2.5m high.
The Cairndow study area is situated immediately north of the hamlet of Cairndow. A comprehensive desk study and site walkover survey were conducted to identify potential hazards and assess factors that influence the likelihood and severity of a potential landslide. These factors include, but are not limited to:
- slope geomorphology
- land use
- probable ground model
- history of instability
Additional elements contributing to road user exposure were also considered, including parking spaces and the suitability of existing infrastructure.
A review of the site observation and desk study information determined the primary hazard to be from shallow landslide failures and channelised debris flows, the likelihood and severity of which vary throughout the route.
The study of the A83 concluded that the Cairndow study area is the lowest risk of the three highlighted landslide areas.