Engineers were able to keep the main London to Bristol railway line operational during major repair work thanks to a new rock-fall catch fence and drapery system from geotechnical specialists, Maccaferri.
As part of an on-going maintenance plan to assess the structural stability of track-side cuttings along the main Paddington to Bristol high speed rail link, a rock-combing and de-vegetation programme was undertaken on the upper slopes of the 23m deep, St Anne’s cutting at Brislington, near Bristol.
The work, by Greenman Environmental and Skyline Rope Access Ltd, identified a pre-existing, 8m long tension crack behind a large sandstone block 2/3rd of the way up the cutting face. The block was estimated at 8x7x3m and weighing in at a massive 300 tonnes. It sat on weak, shaley mudstone which had suffered progressive weathering, undermining the tenuous sandstone block above. Collapse of the block on to the 100mph rail line beneath could have had catastrophic results.
Consultants White Young Green [WYG] established a daily block movement monitoring regime, as part of a comprehensive geotechnical assessment and design. This work identified a number of options which were presented to their client, Earthworks Framework Contractors Dean & Dyball, on behalf of Network Rail.
As removing the block either by controlled blasting or by lifting out from above was ruled out – the line had to stay operational at all times and overhead power cables prevented crane access – the decision was taken to stabilise and secure the block using a combination of innovative techniques.
The initial stage of the stabilisation programme covered the installation of six vertical and two horizontal wire straps to hold the rock in place. Mass shuttered concrete was used to stabilise the undercut weathered shale directly beneath the rock.
The next phase involved the installation of a high tensile steel catch fence to prevent smaller debris falling on to the line. WYG consulted Maccaferri Ltd on their then, brand new rock-fall catch fence and debris flow barrier system.
Maccaferri proposed their OM250 barrier, which had independent certification to provide impact energy resistance of 250kJ. The barrier was to be two metres high and 35 metres long with support posts at five metre centres. Although not used before in the UK the system was part of a wider range of debris flow barriers which had been used extensively throughout Europe, particularly in the Alps.
The main catch fence consists of Maccaferri’s patented 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 four 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 the 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 barrier was then installed by Greenman & Skyline within two working days. Not an inconsiderable feat as they had not used the system before and they were working on a near vertical slope whilst the rail line remained open to 100mph trains.
Conventional Maccaferri rockfall netting was installed over the whole block as well as on other areas of the site to encapsulate smaller debris, where the risk of larger boulders did not require a catch fence.