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A Catch-All Solution

High-strength rock-fall catch fences have been installed along the main Huddersfield to Sheffield rail line as part of a four-year maintenance review programme for Network Rail. GE takes a look

A four-year programme of maintenance and upgrades is tackling unstable and potentially dangerous cuttings in Network Rail’s London and North-East region.

One recent scheme at Cumberworth, West Yorkshire, has been carried out on the single-track Penistone Rail line.

This connects the once busy wool town of Huddersfield, to the city of Sheffield 40km to the south through the picturesque Pennine hills, often in tunnels and steep cuttings.

At a point near the village of Denby Dale, famous for its giant pies, the line emerges from the Cumberworth Tunnel into a 14m deep cutting, originally dug through lower coal measure formations comprising semi-competent sandstone interspersed with weak, friable siltstones and mudstones.

The 160-year-old cutting slopes stand at angles between 45-55o from horizontal with individual rock outcrops between 1m and 2m in height.

The weathering of this rock over the years since the line was built has created instabilities and loose patches and a heightened risk of debris fall.

Following detailed engineering assessment of the site, Network Rail appointed Leeds-based partnership contractor CML as principal contractor for a design and build programme of remedial works.

CML, in turn, worked closely with its designer, URS Scott Wilson, 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.”

“This was positioned 2m up from the toe of the slope to clear the kinematic envefor 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”

“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 were present on the track,” he added.

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.

This mix of challenges made the choice of solution a fine balancing act for both URS Scott Wilson as design consultant and CML as contractor.

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 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 design stage, including spot rock-bolting and the physical removal of individual failures.

This was considered to be 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 the scaled areas.

Maccaferri_Cumberwoth_WIP_1

In addition, the design life of a scaled slope - approximately five to 10 years - was considered to be insufficient.

Another option was to install a rock-fall drapery system to completely envelope the cutting slopes.

Material and installation costs were felt to be uneconomical 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.

The solution ultimately settled on was a network of high-strength, dynamic rock-fall catch fences installed near the bottom of the slope to prevent debris spilling onto the line.

The catch fencing used was Maccaferri CTR 05-07-B, 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 2-3m in height and 160m long for the up slope,” says Koe. “This was positioned 2m up from the toe of the slope to clear the kinematic envefor 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 1m wide by 1m 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 generated by falling debris.

Much of the development work is European-led and has resulted in the adoption of new European testing guidelines.

“Trains entering the cutting were unlikely to have sufficient distance to stop if a block were present on the track.”

David Cheer explains: “European guidelines set out the minimum standards for the manufacture, performance and on-going product conformance testing of rock-fall protection kits.

Maccaferri’s CTR fence systems exceed the requirements of “Category A” in the European Technical Approval Guideline 027.

They are supplied in part-prefabricated kits which are designed to provide rock-fall protection from 250kJ up to a maximum impact energy of 5MJ within 5.6m displacement.

Most of the connections for the kits are made in the factory so installation variables are minimised.

“The portability of the catch fence system was also felt to be a huge advantage,” says URS Scott Wilson’s Koe. “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.”

The construction of this initial rock fall catch fence and excavation of the rock trap was completed successfully as one of 18 cuttings upgrades in the current four-year programme.

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