An on-going maintenance and structural stability plan by contractors, Skyline Rope Access Ltd, identified an 8m long tension-crack behind a large sandstone block 2/3^rd of the way up the 23m deep cutting. The block, which was estimated to weigh in at a massive 300 tonnes, sat on weak, shaley mudstone that had suffered progressive weathering over the years. Collapse of the block on to the 100mph rail line beneath could have had catastrophic results.

 

Consultants White Young Green considered a number of options but, as the line had to stay operational at all times and overhead pylons and power cables prevented crane access to lift out the block, the decision was taken to stabilise and secure the block using a combination of innovative rock-fall containment systems from Maccaferri.

 

Firstly, six vertical and two horizontal wire straps were installed to hold the rock in place. Mass shuttered concrete was used to stabilise the undercut weathered shale directly beneath the rock and sprayed concrete was applied to areas of similar material on each side to prevent future degradation through weathering. 

 

The next phase involved the installation of a high tensile steel catch fence to prevent smaller, yet still significant debris falling on to the line. White Young Green specified a Maccaferri OM250 system, which had independent certification to provide impact energy resistance of 250kJ. The catch fence was to be 2m high and 35m long with support stanchions at 5m centres. Although not used before in the UK, the system was part of a wider range of rockfall mitigation systems including catch fences, high strength rockfall netting and active restraint nets, which had been used extensively throughout Europe, particulary in the Alps. Energy absorption capacities up to 3000kJ are claimed for the system.

 

The Maccaferri system consists of high energy absorption panels strung between “I” section galvanised steel stanchions. The panels are made from high tensile steel cables with all nodes within the mesh locked by a patented ‘knot’ connection formed by 4 lengths of 3mm diameter Galfan galvanised wire. According to the manufacturers this ‘knot’ offers greater pull-apart resistance than any cable connection system currently on the market, with the reassurance that moving boulders and debris will not ‘burst’ through the mesh.

 

All components for the barrier, including grips, cable, shackles, friction brakes and high strength mesh are included within a system kit supplied to the site. The only other components the installing contractor needed to purchase were ground anchors for the barrier, which were bespoke for the St Anne’s Cutting project.

  

The whole catch fence system was installed within 2 working days – an excellent achievement considering the contractor had not used the system before and they were working on a near vertical slope whilst the rail line remained open to 100mph trains

 

Graham Mann of Skyline was pleased with the results of the project.” This was a difficult site with poor access and a live rail line to consider. The co-operation and technical support we got from Maccaferri made the job more straightforward and we would use the system again in similar circumstances”