Motorists on the A58 in Halifax can now travel safely without fear of rocks falling on live traffic. GE reports
Rockfall has long been a problem on a stretch of the A58 that passes through the 10m to 15m high, steep-sided Godley Cutting, near Halifax.
Local highways authority Calderdale Metropolitan Borough Council (MBC) has decided to take action and invested in several prevention measures to eliminate this risk.
These include the installation of a rockfall drapery system, catch fencing, and spraying more than 80t of concrete and stabilising grout onto the cutting walls.
The A58 is a major arterial route linking the trans-Pennine M62 with Halifax.
It winds through Godley Cutting, climbing from the bottom of Shibden Valley to a high point just before it reaches the town centre.
The cutting runs through alternating layers of hard sandstone and shaley mudstone, with a thin coal seam underlain by seat earth, showing evidence of the deep, longitudinal mining once carried out in the valley.
Where the road passes through the cutting, the strata are revealed so clearly that the site is recognised as a Regionally Important Geological Site and is closely monitored by the West Yorkshire Geological Trust.
Over the years the cutting’s slopes have suffered selective weathering, resulting in instability, frequent spalling and rockfall. Most of this 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 rockfall incidents have occurred in recent times Calderdale MBC recognised the risk to motorists.
“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,”
The council’s engineer Mouchel devised a rockfall mitigation scheme to protect road users and preserve and improve the quality of the geological exposure.
The dark, cool environment in disused mines makes them a perfect roost for bats.
A detailed ecological survey was conducted to investigate their presence within the mine openings.
There was no evidence of roosting, but consultation with Natural England and Calderdale’s conservation officer led to an agreement to retain mine entrances to encourage bat roosting in the future.
The construction works revealed the mine passages to be extensive, possibly in excess of 100m deep into the hillside.
It was also necessary to complete the bat access work by the end of October 2009, in time for the bat roosting season in late autumn.
Mouchel specified a programme of scaling to clear loose material before installing a rockfall drapery system and clearing the rock catch trench.
Mine openings were then to be supported or, where collapsed, the ground was to be strengthened, with unstable facing strata supported to prevent future collapse.
“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,” says Mouchel project manager
“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 won the contract to carry out the work and began on site in August 2009.
The firm’s experience in rope access methods allowed work to continue without disruption to the road below.
Can staff abseiled to their work areas, and materials were either brought in from above or delivered during temporary road closures at night or weekends.
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 25m3 of sprayed concrete was pneumatically applied onto exposed shale faces and beneath overhangs to prevent further weathering and collapse.
Most of the 350m long cutting comprises near vertical open strata up to 10m high, topped with a rough grassy mound to a maximum height of 15m.
Complete rockfall prevention was felt to be impractical over such a large area so a drapery system from erosion prevention specialist Maccaferri was installed to envelope the whole face and ensure falling rocks do not reach the road.
The system comprises double twist woven steel wire netting which is anchored at the top and bottom of the cutting but left unanchored along the slope.
This allows small rocks and debris to fall safely to the foot of the slope while remaining contained between the rock face and the mesh.
To limit the containment area between the mesh and rock face, and to prevent “bagging”, some nailing or pegging of the mesh at intervals between the top and bottom anchorage is occasionally necessary.
But in the case of the Godley Cutting installation, this was not required as the drapery span between the top and bottom of the slope was relatively short.
According to Maccaferri, the most important factor is to have a safe and continuous anchorage at the top and to provide sufficient space to allow rocks and debris to fall 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,” says Can Geotechnical project manager Steve Gates.
“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 2m 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.
Approximately 4,600m2 of the PVC coated and galvanised rockfall netting was used.
The netting has a hexagonal aperture size of 100mm x 80mm.
Because of the importance of the site, The West Yorkshire Geology Trust requested that an area of the cutting face be left uncovered.
“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,” says Mouchel senior geotechnical engineer Jeremy Jones.
Instead of enclosing this section of the face with drapery netting, Can Geotechnical operatives installed lightweight rockfall catch fence at the foot to prevent any debris reaching the road.
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 or fireclay.
These adits opened into chambers in which pillar and stall mining took place.
Mudstone has also been extracted from the cutting area to supply raw materials for local brickmakers.
The mine adits were deemed hazardous, and Can Geotechnical installed blockwork walls to close the mine openings, one at the immediate entrance and a second approximately 4m back from the face.
Lengths of pipework 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, untreated, rough textured timber was fixed to the pipe invert to help the bats negotiate the entrance.
Work began on the Godley Cutting rockfall prevention project in August and finished in December.