CASE HISTORY
Ref: UK / CH / EP041 — Rev:02, Jul 14
DEBRIS FLOW BARRIERS - A83 TRUNK ROAD (Pt2)
ARGYLL, SCOTLAND, UK
DEBRIS FLOW PROTECTION (Work phases 7, 8,& 9)
Product: DF30 & DF50 Dynamic Debris Flow Barriers
Problem
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The A83 trunk road is a highly important arterial route
through the picturesque but rugged scenery of the
Arrochar Alps in the internationally famous Loch
Lomond 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 (The Cobbler) 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
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:
NOVEMBER-DECEMBER 2013
View of a recent debris flow on mountain slopes above the A83
View of trunk road facing down-glen towards the Water of Croe
Solution
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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
stable debris levee.
Helicopter lifting the barrier components onto the steep slopes
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
the slope.
Helicopter lifting the barrier components onto the steep slopes
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
Maccaferri .
Installation in progress for the two Type 3 barriers
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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.)
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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
Maccaferri Ltd - Head Office
7600 The Quorum,
Oxford Business Park, North,
Garsington Road, Oxford, OX4 2JZ
Tel: 01865 770555
Fax: 01865 774550
Email: [email protected]
Web: www.maccaferri.co.uk
View of completed Type 2 barrier showing slight curvature
Area offices
Perth:
T: 01738 621317 F: 01738 442283
Belfast:
T: 028 9026 2830 F: 028 9026 2849
Dublin:
T: 01 885 1662
F: 01 885 1601
Web: www.maccaferri.ie
E: [email protected]
E: [email protected]
E: [email protected]
Oxford, Perth, Belfast