Motorists Thankful for action at landslide site
In 2007 the A83 trunk road was dosed at the famous Rest and Be Thankful section for several weeks after a debris flow dumped some 400 tonnes of material on the road. In 2009 a further 1,000 tonnes slipped on to the road at the same place, dosing it for 48 hours. David Gibson, business development manager at BAIA Ritchies, reveals how the problem was tackled by combining Swiss technology and Scottish knowledge.

NO one was hurt in either incident but in recent years some of the country's rural lifeline routes have been seriously affected by debris flows, a form of shallow landslide. In a move to minimise disruption caused by these 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 has been the subject of intensive investigation as it was identified as one of the highest risk sites for debris flow, an assessment fact confirmed by the events that have occurred. Following the 2007 event, it had been decided to enlarge the culvert at the site to reduce the risk of blockage and the necessary ground investigation was already in the course of procurement when the 2009 event occurred.

However, the latest debris flow, together with the new availability of shallow landslide barriers from Switzerland based upon very recent research and development, 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 TranSery and consultants Transport Research Laboratory (TRL), Transport Scotland developed a package of proactive measures aimed at addressing the disruption caused by debris flow events.

As Transport Scotland's head of network maintenance Graham Edmond said: "While we can't prevent future events, we are working closely [with operating company Scotland Transerv] to keep the A83 open as safely as possible. 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 that work to deal with the type of hazard effectively and in a cost effective manner.

The development of flexible debris flow barriers is more recent but has reached a point where they may be designed, specified and in­stalled with confidence. Indeed, installations are now quite common, in particular in European alpine areas, California, Japan and Korea.

The research and development of flexible landslide harriers is now well advanced with 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 community in the Aargau canton of Switzerland, a test slope channel of eight metres wide and 41 long with an average inclination of 30° is used to test barriers at full scale by releasing the material down the 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 the circumstances that occurred at the Rest and Be Thankful, the potential event size has been determined. Following consultation with both barrier supplier and installer, a four metre high by 80-metre long shallow landslide barrier has been specified. In a nearby stream gullet', a debris flow barrier, also four metres high by 15 metres wide is required to eliminate the chance of the culvert under the road becoming choked with debris.

Because of the fine material mobilised in 2009 and based on experiences from 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 support ropes of 22mm diameter with brake rings, along with two upper support ropes also of 22mm diameter complete with brake rings and abrasion control protection. The barrier is a standard Geobrugg debris flow harrier designed using in-house DEBFLOW online software.

Geobrugg, a Swiss company specialising in debris flows, has been closely involved in the research and development of both debris flow barriers and more recently landslide barriers. Based upon the research and development carried out in Switzerland, the shallow landslide barrier was designed and manufactured to match the assessed risk and detailed requirements. The design has been carried out by Geobrugg's technical department in Switzerland and its SPIDER mesh is used with secondary 50mm aperture mesh to retain the finer material. These meshes are held by top and bottom ropes on posts secured by both upslope and lateral rope anchors.

Corinna Wen deler, an engineer in Geobrugg's technical department who visited the site after the 2009 event, commented: "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. 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 were installed by BAM Ritchies which has experience of the new generation flexible barriers. With traffic restricted to one lane by traffic lights, the team worked during one of the worst winters in recent years. Ground conditions on the 35° slope were 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 eight-metre 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 posts are hinged on to spheroidal cast iron base plates at ground level secured into the rock by three-metre long 35mna GEWI Plus bars. Where there is overburden present the base plates sit on cast in situ concrete blocks secured by a vertical 40mm galvanised GEIVI 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 were drilled using a rope supported Ripamonte drill rig with compressed air rotary head and down-the-hole hammer.

Rachel Long, BAM Ritchie's site engineer, worked with Scotland Transerv's engineering geologist Sarah Walker to realise the project on the ground. She said: "The weather and the ground conditions on a steep slope have proved difficult but 1 am really pleased the way the whole project has come together as a result of excellent team work from everyone involved both in Scotland and Switzerland."

Article courtesy of Ground Engineering - July 2010