A network of rock-fall prevention measures has recently been installed at a site of special geological significance next to the busy A58, near Halifax, West Yorkshire. The measures include a rock-fall drapery system, catch fencing and the placement of over 80 tonnes of sprayed concrete and stabilising grout.
The A58 is a major arterial route linking the trans-Pennine M62 with the busy industrial and financial centre of Halifax. On its southern boundary, the road passes through Godley Cutting, a 10.0 – 15.0m deep, steep sided cutting, which climbs from the bottom of The Shibden Valley to a high point before dropping into the town.
The sides of the cutting comprise alternating layers of hard sandstone and shaley mudstone, with a thin coal seam underlain by seat earth, which show evidence of deep, longitudinal mining [now redundant]. Where the road passes through the cutting, the strata are revealed so clearly that the site is recognised as a Regionally Important Geological Site (RIGS) and is monitored by the West Yorkshire Geological Trust.
Over the years, the cutting slopes have suffered selective weathering with resultant instability and frequent spalling and rockfall, most of which has been retained by a catch-trench at the foot of the slope. In places, steel girders have been used to support undercut strata.
Although no serious instances of rockfall onto the road had occurred in the recent past, Calderdale MBC recognised the potential risk and, through their Professional Engineering Services Provider, Mouchel, devised a rockfall mitigation scheme with the dual purpose of protecting road users from potential hazards and, at the same time, preserving and improving the quality of the geological exposure.
A detailed ecological survey by Mouchel’s Leeds Office was conducted to investigate the presence of bats within the mine openings. There was no evidence of bat roosting but consultation with Natural England and Calderdale’s Conservation Officer led to an agreement to retain openings in the mine adits (tunnels) to encourage bat roosting in the future. The construction works revealed the passages to be extensive, estimated to be in excess of 100m deep into the hillside.
Mouchel specified a programme of scaling to clear loose material followed by the installation of a rockfall drapery system and clearing of the rock catch trench. Mine openings would be supported or, where collapsed, the ground strengthened and unstable facing strata supported to prevent future collapse. It was also necessary to complete the bat access work by the end of October 2009, in time for the start of the bat roosting season in late autumn.
Phil Bolton, project manager for Mouchel explained: "One of the main issues we faced was access to the site, as the cutting is a pinch point in the road with no suitable diversion route. Shuttle working with traffic lights to allow access from the carriageway during peak hours would have caused major traffic disruption so working downwards from the crest of the slope was the only viable option."
CAN Geotechnical of Chesterfield won the contract and work began in August 2009. Their experience in rope access methods allowed work to continue without disruption to the road below. CAN staff used abseiling techniques to reach their work areas and all materials were either brought in from above or delivered at night-time or weekends during temporary lane closure.
The existing catch trench behind a 1.8m high dry-stone wall adjacent to the road was cleared and after removal of loose surface material, approximately 25.0 cubic metres of sprayed concrete was pneumatically applied onto exposed shale faces and beneath overhangs to prevent further weathering and collapse.
The majority of the 350m long cutting comprises near vertical open strata up to 10m high, topped with rough grass to a maximum of 15.0m in height. Complete rockfall prevention was felt to be impractical over such a large area so a drapery system from erosion prevention specialists Maccaferri was installed to envelope the whole face.
The Maccaferri system is a double twist woven steel wire netting which, on steep or nearly vertical slopes such as Godley Cutting, is anchored at the top and bottom of the cliff but left unanchored along the slope. This allows small rocks and debris to fall safely to the foot of the slope whilst remaining contained between the rock face and the mesh.
To limit the containment area between the mesh and rock face and prevent "bagging", some nailing or pegging of the mesh at intervals between the top and bottom anchorage is occasionally necessary. In the case of the Godley Cutting installation, this was not felt to be required as the drapery span was relatively short.
According to Maccaferri, the most important factor is to have a safe and continuous anchorage at the top and provide sufficient space to allow rocks and debris to move downwards. On moderately steep slopes, or slopes where some vegetation may grow, the mesh is kept as close as possible to the slope face.
"It is a relatively straightforward process to install the drapery system when there are no restrictions at the foot of the face," commented Steve Gates, CAN Geotechnical project manager. "However, working with active traffic lanes immediately beneath, required careful management and lowering of the drapery netting in pre-cut lengths."
Driven earth anchors were used at intervals along the top of the slope to fix a lateral cable onto which the 2.0m wide drapery panels were folded over and securely laced. At the foot of the slope, an array of 25mm solid bar rock bolts were drilled and grouted in position to take a similar "soft detail" fixing method. Approximately 4600sqm of Maccaferri PVC coated and galvanised rockfall netting with a hexagonal aperture size of 100 x 80mm, was used in the project.
Because of the importance of the site, The West Yorkshire Geology Trust (WYGT) requested that an area of the cutting face was left uncovered. Jeremy Jones, senior geotechnical engineer for Mouchel Ground Engineering explains. "The site is a classic example of a coal measures sequence of sandstone and mudstone and we were able to accommodate leaving exposed an area at the head of the cutting, which is clearly visible from the road opposite, by modifying the protection system adopted."
Instead of enclosing the face with drapery netting, CAN operatives installed lightweight rockfall catch fence at the foot of the slope to prevent any debris reaching the road.
According to the WYGT, there is a long history of coal mining in the area and the south side of Godley Cutting has at least three adits which exploited the thin coal seam and underlying seat-earth (fireclay) running back from the face. These adits opened into chambers in which pillar and stall mining had taken place, probably over 150 years ago. Mudstone has also been extracted from the cutting area to supply raw materials for local brick-makers.
The mine adits were deemed hazardous and CAN installed block-work walls to close the mine openings, one at the immediate entrance and a second approximately 4m back from the face. Lengths of pipe-work, up to 500mm in diameter were placed between the walls to allow bats to access the subterranean chambers and the voids between the walls were filled with pumped-in grout. On the advice of specialists from Natural England, un-treated, rough textured timber was fixed to the pipe invert to help the bats negotiate the entrance.