The UK’s first flexible debris flow barrier is being installed at the landslide-prone Rest and be Thankful site in Scotland. David Gibson reports.
Rest and be Thankful on the A83 in northern Scotland has a history of landslides.
In 2007 the site was closed for several weeks after a debris flow, a form of shallow landslide, deposited 400t of material on the road.
In early September 2009 a further event resulted in 1,070t of material slipping onto the road at the same place, forcing its closure for 48 hours.
No one was hurt in either incident, but these debris flows pose a serious threat to the country’s main rural routes.
In a move to minimise disruption caused by such events to communities, tourists and businesses, Transport Scotland carried out the Scottish Road Network Landslides Study, the implementation report for which was published in 2008.
This identified areas of potential events and mitigation measures.
The Rest and be Thankful site was identified as one of the highest risk sites for debris flow.
Following the 2007 event, it had been decided to enlarge a culvert at the site to reduce the risk of blockage.
Ground investigation was being procured when the 2009 event occurred and this, plus the availability of shallow landslide barriers from Switzerland, led to a review of the specification and installation of protection measures up slope of this key public road.
Working with its operating company Scotland TranServ, and consultant Transport Research Laboratory (TRL), Transport Scotland developed a package of proactive measures aimed at addressing the disruption caused by debris flow.
“It is very pleasing to see the research and development that I have been so closely involved in Switzerland now being used to the benefit of the Scottish road user,”
“While we can’t prevent future events, we are working closely with Scotland Transerv to keep the A83 open as safely as possible,” says Transport Scotland head of network maintenance Graham Edmond.
“We are investing £760,000 in this area, helping us to develop early prediction and warning tools.”
Most of the funding is being used to install shallow landslide and debris flow barriers and a bespoke remote monitoring system.
While rock falls tend to have discrete blocks falling at high velocities, and debris flows tend to be associated with significant water flows that erode and entrain soil, shallow landslides generally involve lower volumes of water than debris flows and are often triggered by elevated water pressures at the rock/soil interface.
Understanding the difference between these event types is the key to the provision of suitable tailored catch fences or barriers.
The development of flexible debris flow barriers has reached a point where they may be designed, specified and installed with confidence.
Indeed, installations are now common in European alpine areas, California, Japan and Korea.
Manufacturers now have a good understanding of the behaviour of both the landslide material and velocities (and hence energies) and of the barriers themselves.
At a landslide test site in Veltheim, Switzerland, an 8m-wide, 41m-long slope with an average inclination of 30° is used to test barriers at full scale by releasing material down a channel.
A variety of measurements and videos are taken and assessed to validate and improve on the basic design created by computer and finite element analysis.
Following detailed assessments of previous shallow landslide events at Rest and be Thankful, the potential event size has been determined.
Following consultation with barrier supplier Geobrugg and installer Bam Ritchies, a 4m-high by 80m-long shallow landslide barrier has been specified.
In a nearby stream gulley a 4m high by 15m wide debris flow barrier is required to eliminate the possibility of the culvert under the road becoming choked with debris.
Due to the fine material mobilised in 2009, and based on the experience of field tests, a Rocco ring mesh with two secondary 50mm aperture mesh layers was recommended for the debris flow barrier in the gulley.
The supporting ropes consist of two lower ropes of 22mm diameter with brake rings, along with two upper ropes, also of 22mm diameter, complete with brake rings and abrasion control protection.
The debris flow barrier is a standard Geobrugg debris flow barrier designed using in-house Debflow online software.
Based on research and development carried out in Switzerland, the shallow landslide barrier has been designed and manufactured to match the assessed risk and detailed requirements.
Geobrugg’s Spider mesh is used with secondary 50mm aperture mesh to retain the finer material.
“This project will be the first in the British Isles to use a dedicated flexible debris flow barrier using this system”
These meshes are held top and bottom by ropes on posts secured by upslope and lateral rope anchors.
“It is very pleasing to see the research and development that I have been so closely involved in Switzerland now being used to the benefit of the Scottish road user,” says Geobrugg engineer Corinna Wendeler, who visited the site after the 2009 event.
“This project will be the first in the British Isles to use a dedicated flexible debris flow barrier using this system and is particularly interesting in that it will also have the latest flexible landslide barrier - a world first to my knowledge that the two types have been installed on the same site.”
The barriers are being installed by Bam Ritchies, which, with its locally based workforce, has experience of the installation of these new generation flexible barriers.
Work began in January and was completed in mid-March.
With traffic restricted to one lane only by traffic lights, the team have been working in one of the worst winters in recent years.
Ground conditions on the 35 degree slope are also challenging, with overburden thickness in places being deeper than expected, leading to increases in pile and anchor sizes, depths and drilling times.
The landslide barrier consists of 11 posts at 8m centres located by upslope restraining ropes and top and bottom longitudinal ropes, which carry the Spider and secondary mesh facing.
These upslope and longitudinal ropes are fitted with brake rings which will absorb energy in the event that another landslide occurs.
“The weather and the ground conditions on a steep slope have proved difficult but I am really pleased the way the whole project has come together”
Posts are hinged onto spheroidal cast iron base plates at ground level, secured into the rock by 3m-long, 35mm GEWI Plus bars.
Where there is overburden present the base plates sit on cast insitu concrete blocks secured by a vertical 40mm galvanised GEWI bar and an inclined 50mm galvanised GEWI bar acting in tension.
The upslope ropes and the ends of the longitudinal ropes are secured by 22.5mm Geobrugg rope anchors grouted into rock.
The advantage of these rope anchors is that they have flexibility to adjust to the changing angles that occur if and when the barrier is loaded.
All the holes for the bars and rope anchors have been drilled using a rope-supported Ripamonte drill rig with compressed air rotary head and down-the-hole hammers.
Bam Ritchies site engineer Rachel Long has been working closely with Scotland Transerv to realise the project on the ground.
“The weather and the ground conditions on a steep slope have proved difficult but I am really pleased the way the whole project has come together as a result of excellent teamwork from everyone involved both in Scotland and Switzerland,” she says.
Remote sensing equipment will be installed at specific locations on the slope above the road as part of ongoing monitoring.
This monitoring also includes laser scanning to identify any movements or changes of concern.
The monitoring system developed in conjunction with Datum Monitoring comprises overburden movement detection, measurement of water pressure at the rock/soil interface and an automated weather station.
When the system records movement an alert is automatically sent to the operating company allowing it to instigate road user protection measures.
The route is also set to trail Wig Wag road signs, a first in the UK, which will warn the public that there is an increased risk of landslides and to take extra care when travelling the route.
Following a period of operation, it is hoped that the logged data can be used to correlate shallow landslide events with combinations of weather events and pore pressure behaviour, allowing an element of predictability and thus greater road user protection and road maintenance efficiency.
After the end of the sign trial in 2012 Transport Scotland and Scotland TranServ will also undertake consultations with local authorities, businesses, communities and other stakeholder groups and organisations and review the collated landslide data.
This will enable Transport Scotland to assess the effectiveness and impact of the signs in promoting road safety and providing reassurance to the road user.
In addition, the value the various measures in reducing the risk along this section of the A83 will be determined, before analysing the possibility of introducing these measures elsewhere in the country.
- David Gibson is business development manager at Bam Ritchies