Rockfall protection barriers from specialist manufacturer Maccaferri have been installed along a section of the main Londonderry to Coleraine coastal road in Northern Ireland. The installation follows a series of dangerous landslides, one of which lead to the closure of the road and the imposition of speed restrictions on the main Londonderry to Belfast rail line which runs adjacent to the road.
The Seacoast Road and neighbouring rail line, hug the exposed coastal strip through an area of outstanding natural beauty and, at a point a few miles to the west of Coleraine, pass beneath a steep rocky out-crop at Downhill.
The Downhill site has a long history of rockfall, the most serious of which, in 2002, caused a derailment and lead to the installation of a concrete crash barrier next to the line to prevent further incursions.
Mindful of the potential dangers to both road and rail travellers, the Northern Ireland Department for Regional Development, Road Services, commissioned URS / Scott Wilson to undertake a detailed assessment of the site.
URS Principle Engineer, Adrian Koe explains: "The north facing cliffs at Downhill are made up of layers of heavily jointed basalt – a volcanic material similar to the rocks that make up the Giant’s Causeway. Weaker layers of rock have weathered more quickly which has made over lying layers unstable, causing significant rock-fall."
The lower slopes of the 60 metre high cliffs are made up of grass covered wind-blown sands which form a natural pathway along which break-away blocks of stone roll before they reach the road and rail line at the foot of the cliff.
"Individual blocks as big as 1.0m cube and weighing 2700kg, that’s roughly the equivalent to a VW Golf with four people and their luggage, have fallen down the slope," said Adrian Koe. "Potential velocities, before they hit the road, were up to approximately 15 metres per second."
Following their site assessment, URS / Scott Wilson proposed the installation of a network of high strength, dynamic rock-fall catch fences placed near the bottom of the slope to prevent debris spilling onto the road and rail line.
The catch fence system selected was a Maccaferri ‘CTR 05-07-B’ system, comprising continuous, steel-cable mesh panels and energy dissipaters, stretched between articulated vertical posts. The catch fence is the first of its type to be installed in Northern Ireland and is one of a wide range from the company, capable of withstanding 500kJ impacts for Maximum Energy Level [MEL] designs.
Adrian Koe continued: "We devised a catch fence 3.0m in height and 180m long, positioned 20m up from the toe of the slope to allow for deformation of the fence during impact."
According to Dr David Cheer, Rockfall Mitigation specialist for Belfast based Maccaferri, catch fence design is now a sophisticated, high tech process with the development of ever-more efficient systems, capable of absorbing huge amounts of kinetic energy possessed by falling debris.
Much of the development work is European lead and has resulted in the adoption of new European testing Guidelines, ETAG 027.
David Cheer explained: "ETAG 027 – the European Technical Approval Guideline 027, sets out the minimum standards for the manufacture, performance and on-going product conformance testing of rock-fall protection kits sold within the EU."
Maccaferri’s CTR fence systems exceed the requirements of ‘Category A’ as defined by ETAG 027 [The most stringent category] and are supplied in kits which are designed to provide rock-fall protection from 250kJ, up to a maximum impact energy of 5000kJ – the equivalent of stopping a 16.5 tonne lorry travelling at 57mph, within 5.6m displacement.
Maccaferri fence kits are supplied to site part-prefabricated for simple, safe and rapid on-site assembly. The kits come with the majority of connections made in the factory so installation variables are minimised and reliable long term performance is assured
For the Downhill project, key components were supplied with anti-corrosion coatings to provide additional protection against the extreme weather conditions experienced at the exposed site.
Working with locally based minor works contractor, White Mountain Quarries, the Maccaferri catch fence system was installed by specialists, Skye Rope Access of Portree, Isle of Skye. Work on the Downhill project was completed in April 2011.