A network of high strength rock-fall catch fences from Maccaferri has been installed along the main Huddersfield to Sheffield rail line in Yorkshire. Maccaferri takes a look at the project and assesses the design and construction challenges faced by the installation team.
In the rugged, Pennine hills of South Yorkshire, the Penistone Rail Line links Huddersfield with the steel city of Sheffield, some 25 miles further south.
Originally built by the Lancashire and Yorkshire Company in the 1850’s, the Penistone Line winds its way through tunnels and cuttings and a string of pretty hillside villages. Just north of the pie village of Denby Dale, the single track rail line emerges from the Cumberworth Tunnel into a 14m deep cutting.
The cutting slopes stand at angles between 45° – 55° from horizontal with rock outcrops between 1m and 2m in height. Weathering over the last 160 years has resulted in significant weakening of the cutting sides with consequent risk of instability and debris fall.
Following detailed engineering assessment of the site in March 2009, Network Rail appointed Leeds-based partnership contractors CML as principal contractors for a design and build programme of remedial works. CML in turn, worked closely with designers, URS Scott Wilson of Derby, to develop a cost-effective engineered solution to provide Network Rail with a 60 year minimum design life for the works.
CML managing director, Geoff Mortimer explains: "The main failure mechanism observed in the cutting was block fall from the intermediate layers of sandstone, most likely caused by ravelling failures from the underlying mudstone. Potentially loose blocks were observed on both the up and the down side cutting slopes."
Access to the site was difficult as the cutting is some distance from the nearest road. Wooded areas at the head of the cutting made it extremely difficult to bring plant or lifting equipment close enough to be effective. Restricted line-possession also meant that much of the work would have had to be undertaken at night.
The degree of potential hazard to the rail line was considerable, as Adrian Koe, principal engineer for URS Scott Wilson explains: "Rock-fall modelling indicated that lateral trajectories of falling rock were sufficient to enable them to reach the track below. With block sizes in the range of 200mm – 600mm diameter, there was a significant risk these could cause a derailment should they become dislodged."
The solution chosen was a network of high strength, dynamic rock-fall catch fences, 2m -3m in height, installed near the bottom of the slope to prevent debris spilling onto the 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, one of a wide range from the company is capable of withstanding 500kJ impacts for maximum energy level (MEL) designs.
Adrian Koe continued: "The catch fence was positioned 2m up from the toe of the slope to clear the kinematic envelope of a passing train and to allow for deformation of the fence during impact."
According to Dr David Cheer, rockfall mitigation specialist for 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 ongoing 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 5,000kJ – the equivalent of stopping a 16.5t lorry travelling at 57mph, within 5.6m displacement.
Maccaferri fence kits are supplied to site part-prefabricated for simple, safe and rapid onsite 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.
Adrian Koe of URS Scott Wilson concluded: "The portability of the Maccaferri catch fence system was also felt to be a huge advantage. Top down rope access techniques were used and this allowed installation of the fence whilst trains were still running. In fact, only a small number of night time possession were required, which reduced the construction costs significantly."