CASE HISTORY Ref: UK / CH / RF — Rev:03, February 17 DEBRIS FLOW BARRIERS - A83 TRUNK ROAD (Pt2) ARGYLL, , UK DEBRIS FLOW PROTECTION (Work phases 7, 8,& 9) Product: DF30 & DF50 Dynamic Debris Flow Barriers

Problem The A83 trunk road is a highly important arterial route through the picturesque but rugged scenery of the in the internationally famous National Park. The route represents a critical life line from the Glasgow area to the whole of the Argyll peninsular and the important commercial and tourist interests in that region.

The Rest and Be Thankful is a mountain pass at the head of a long climb where the road makes a traverse up the flanks of Ben Arthur () and crosses from Glen Croe to Glen Kinglas en-route to Inverary. View of the trunk road on mountain slopes near the saddle The unstable mountain slopes have a long history of debris flow activity. The silty and bouldery superficial soils overlie glacially smoothed rock-head terraces which combined with unfavourable pore water conditions and extensive basement-cover groundwater flows make debris flows a constant threat. The frequent inclement weather experienced by the west coast of Scotland add to the danger of slope failure.

Over the past 5 years the road has been blocked by debris flows on numerous occasions, sometimes for long periods. When the road is closed the standard diversion route adds 50 miles (80km) for journeys to Inverary. The length of the delay, the trunk road status of the A83 and a full engineering assessment of the View of a recent debris flow on mountain slopes above the A83 geotechnical options for the route led the client to issue contracts for the protection of the A83, around the Rest and Be Thankful, from the effects of Debris flow.

Client: TRANSPORT SCOTLAND Designer: WATERMAN (PERTH) Main Contractor: GEOROPE Products used: DF30 Type 1 and 3 barriers, DF50 Type 2 barrier Date of construction: View of trunk road facing down-glen towards the Water of Croe NOVEMBER-DECEMBER 2013 Solution The design solution included the installation of five separate high-capacity dynamic debris flow interception barriers. The barriers were to be installed on the slopes above the trunk roads the intercept threats identified during the geomorphological and geotechnical assessments of the mountain slopes.

The selected systems comprise a “cascade” of two Type 1 (“in-channel” type) barriers which span a very high risk channel, two Type 3 (free standing) barriers providing protection over less tightly flow paths and a single Type 2 (composite morphology) barrier used where a broad flow path is restricted by a large but Helicopter lifting the barrier components onto the steep slopes stable debris levee.

The Type 1 barriers used a single sacrificial post due to the large span of the levee-bounded channel in-which they are located. The pair of barriers act together to form a “cascade” effect with design-overtopping and higher permeability integral features of the uppermost barrier.

The Type 2 fence was used due to the topography of the site in the relevant area. The composite nature of the structure not only adapts to the awkward terrain but also allows debris flows exceeding the design capacity of the barrier to be redirected into a nearby catch-pit. In this way the barrier is designed primarily to intercept material and secondarily to change the flow paths on Helicopter lifting the barrier components onto the steep slopes the slope.

The Type 3 barriers had to be installed across undulating topography which was present along the length of the site. The adaptable system design and the experience of the contractor enabled an efficient installation to be performed.

Maccaferri tailored the specification of the system components in each of the barriers in order to meet the impact and engineering criteria that were defined for each of the sites. All of the barriers use high strength 4v ring nets as the primary interception mesh and the Patented aluminium energy dissipaters common to all dynamic impact protection fences offered by Installation in progress for the two Type 3 barriers Maccaferri . Overview of completed Type 3 barriers above the A83 Overview of completed Type 1 barriers above the A83

Detailed view of completed Type 1 barrier cascade within the levee-bounded channel above the A83. The upper of the two Ph7 barriers was installed with a remote monitoring system which included real-time monitoring of the compression of the dissipaters combined with day/night photogrammetry and remotely controllable alarm thresholds. The data is is stored on a bespoke website accessable via a secure portal. (See seperate case history document.)

View of completed Type 3 barrier on slopes above the A83 View of completed Type 3 barrier on slopes above the A83

View of completed Type 2 barrier showing compsite form

View of completed Type 3 barrier showing local slope form View of completed Type 2 barrier showing slight curvature © 2017 Maccaferri. All rights reserved. Maccaferri will enforce Copyright. enforce will Maccaferri reserved. rights All Maccaferri. 2017 © Maccaferri Ltd Area offices Perth: Building 168, Harwell Campus, T: 01738 621317 F: 01738 442283 E: [email protected] Didcot, OX11 0QT, Oxfordshire, UK Belfast: T: (+44) 1865 770 555 T: 028 9026 2830 F: 028 9026 2849 E: [email protected] Fax: (+44) 1865 774 550 Dublin: E: [email protected] T: 01 885 1662 F: 01 885 1601 E: [email protected] www.maccaferri.com/uk Web: www.maccaferri.ie