Through Yorkshire’s rugged Pennine hills, the Penistone line links Huddersfield – the former capital of the woollen industry – with the steel city of Sheffield, some 25 miles further south. The northern section was opened by the Lancashire & Yorkshire Railway in 1850, wending its way through tunnels, cuttings and a collection of characterful villages. One of these is Denby Dale – famous for its enormous pies, the last of which weighed in at 12 tonnes and was baked to celebrate the turning of the Millennium.
Half-a-mile north of its station, the single-track line emerges from Cumberworth Tunnel into a 14-metre deep cutting, originally dug through lower coal measure formations comprising semi-competent sandstone interspersed with weak, friable siltstones and mudstones. The cutting slopes stand at angles of 45-55 degrees from the horizontal with individual rock outcrops of 1.0-2.0m in height. Weathering over the past 160 years has resulted in significant weakening of the cutting sides with the consequent risk of instability and debris fall.
Following detailed engineering assessment of the site in March 2009, Network Rail appointed Leeds-based Construction Marine Ltd (CML) as principal contractors for a ‘design & build’ programme of remedial works. In turn, it worked closely with designers URS Scott Wilson of Derby to develop a cost-effective engineered solution to provide a 60-year minimum design life for the works.
CML’s Managing Director Geoff Mortimer explains that “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 Down-side cutting slopes. As well as this, the curvature of the track and the tunnel resulted in reduced sighting distances. Consequently, trains entering the cutting were unlikely to have sufficient distance to stop if a block was present on the track.”
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. The restricted availability of line blockages also meant that much of the work would have had to be undertaken at night. This mix of challenges made the choice of solution a fine balancing act for both URS Scott Wilson as design consultants and CML as contractors.
The degree of potential hazard to the railway was considerable, as Adrian Koe, Principal Engineer for URS Scott Wilson explains. “Rockfall modelling indicated that lateral trajectories of falling rock were sufficient to enable them to reach the track below. With block sizes typically in the range of 200-600mm diameter, there was a significant risk these could cause a derailment should they become dislodged.”
Various options were considered at the design stage including spot rock-bolting and the physical removal of individual failures. But this was deemed impractical as the rock was likely to break up while drilling and, as weathering continued, further fractures were likely to develop between the rock bolts and scaled areas. In addition, the design life of a scaled slope – approximately five to ten years – was considered to be insufficient.
A further option was the installation of a rockfall drapery system to completely envelope the cutting slopes. Material and installation costs were felt to be uneconomic due to the time required to install the netting and the larger number of anchors that would need to be drilled. Ancient mine workings located at the crest of the slope also posed a risk to drilling operations.
Absorbing the energy
The solution ultimately chosen was the installation of a network of high strength, dynamic rockfall catch fences placed near the bottom of the slope, preventing debris from spilling onto the track. A Maccaferri CTR 05-07-B system was selected, 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.
“We devised a catch fence 2.0-3.0m in height and 160m long for the Up slope” recalls Adrian Koe. “This was positioned 2.0m up from the toe of the slope to clear the kinematic envelope of a passing train and allow for deformation of the fence during impact. It also allows clearance of debris to be undertaken at a safe distance from the line whilst trains were running. For the Down slope, a simple 1.0m wide x 1.0m deep rock trap at the toe was felt to be sufficient protection.”
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 the huge amounts of kinetic energy possessed by falling debris. Much of the development work is European-led and has resulted in the adoption of new European testing guidelines ETAG 027 (European Technical Approval Guideline 027). “This sets out the minimum standards for the manufacture, performance and ongoing product conformance testing of rockfall protection kits sold within the EU” David explains.
Safe and rapid assembly
Maccaferri’s CTR fence systems exceed the requirements of the most stringent Category A as defined by ETAG 027 and are supplied in kits which are designed to provide rockfall 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. The kits are supplied to site part-prefabricated for simple, safe and rapid assembly. They come with the majority of connections made in the factory so installation variables are minimised whilst reliable long-term performance is assured.
“The portability of the Maccaferri catch fence system was also felt to be a huge advantage” concludes Adrian Koe of URS Scott Wilson. “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 possessions were required which reduced the construction costs significantly.”
Construction of the catch fence and excavation of the rock trap was completed successfully during November 2009.