Work to restrain a 300t boulder perched high above the landslip-prone Rest and be Thankful Pass in Scotland has been successfully completed.
Debris flows are the usual cause of closure of the A83 route between Arrochar and Inveraray but ground investigations following one debris incident identified a vulnerable 300t boulder.
Recent investment in catch fences has reduced the frequency of landslides at the site but Transport Scotland and its trunk roads operating company Bear Scotland initiated the investigation after a debris flow that start 350m up the slope above the road.
The near under-mining of the boulder by the debris flow highlighted the need for protective action and a plan was put in place to prevent a potentially catastrophic failure.
Specialist geotechnical contractor Geo-Rope worked with consultant Waterman Infrastructure and rockfall mitigation specialists Maccaferri to develop a solution.
“Site assessments by Geo-Rope suggest that the boulder is currently acting as a ‘choke’ to material behind and above,” said Waterman principal geotechnical engineer Chris Gell. “Controlled removal of the boulder was discounted at an early stage due to concerns that this may result in a subsequent debris flow. Also the safety and cost of disposal of the large quantity of blasted material that would be produced could not be assured at reasonable cost.
“The installation of some form of restraint system was proposed. The design called for a bespoke approach in order to restrain the boulder, while also being practical to install 350m up a hillside and achieve Transport Scotland’s requirement for a 60 year design life for the system.”
A series of ground anchors were installed into stable areas of the hillside located 20m to 30m above the boulder and were connected to the boulder by means of high strength Kevlar tendons from Maccaferri subsidiary Linear Composites – effectively tethering it in place.
Following temporary stabilisation and monitoring of the boulder by Geo-Rope, anchors fitted with bespoke high-strength termination plates were installed into the slopes above and 90mm diameter holes were then cored through the boulder to accept the tendons. According to Maccaferri, it was critical to achieve an accurate alignment of the drill holes in order that tendons would correctly align with anchor positions higher up the hill.
Each of the seven tendons were connected to the bespoke, high-strength steel anchor plates using proprietary Crosby shackles before being fed through the boulder drill-holes and tensioned against the down-hill face of the boulder.
Ground anchors were high-yield hollow bar type, installed with sacrificial drill heads as have been used successfully in designs by Waterman on previous works at the Rest and be Thankful.
Maccaferri rockfall mitigation specialist David Cheer said: “Each Parafil tendon has a capacity of 45t and, being made from sheathed polymer fibres, has near unlimited environmental durability, while the Kevlar fibres exhibit virtually no long-term loss of mechanical performance.”
According to Maccaferri, the extremely low weight (less than 0.6 kg/m) of the Parafil makes them easy to handle.
The tendons are positioned in a fanning arrangement which will maintain the stability of the boulder in both the along-strike and down-dip directions on the slope as exact failure mode of the boulder cannot be predicted with any precision. The terminations positioned on the downhill face of the boulder are adjustable allowing the facility to add tension to the tendons after installation. This also allows direct and full transmission of the forces from the boulder through the tendons and the upslope non-adjustable terminals then on through the shackles and into the anchored steel plates.