Weathering of steep slopes on the Longfield cutting in north Kent called for a mix of soil nailing and rock bolting to prevent future failures affecting rail services.
Parsnips, Chalk mines and domestic gardens are just a few of the issues Bam Ritchies is dealing with on a slope stabilisation scheme it is currently undertaking on a rail line in north Kent. While the parsnips and domestic gardens were a known part of the access challenges, the discovery that the cutting was sliced through the mine – a denehole – was a surprise to the site team.
The Chalk cutting that is the focus of Ritchies’ work is at Longfield Hill on the Chatham Main Line linking Victoria to the Medway towns, Dover and Ramsgate. Weathering of the Chalk on this key rail route resulted in several minor landslips in January last year and the current work is designed to prevent a major incident at the site.
The 750m-long cutting has steeply sloping sides that are up to 14m deep with exposures of the Thanet Sands over the Chalk. In some parts of the cutting the outcrop of the sand is up to 10m deep.
Before starting the work on site, Ritchies worked with Donaldson Associates to value engineer the initial designs undertaken by Network Rail and Costain through optimisation of the soil nail layout and facing mesh.
Remediation is taking place on both sides of the cutting over a total length of about 750m and the completed work will see 4,000 soil nails, 1,500 rock bolts – with 20km of drilling to install them – and 22,500m2 of facing mesh installed at the site.
Installation of the soil nails, that vary in length from 3m to 7m, is being undertaken by a mix of track work and top-down access.
Work started with the installation of soil nails close to the track using excavator mounted drilling masts mounted on Megarailer and Gigarailer road to road vehicles (RRVs) during track possessions. A small amount of re-grading was also carried out at this stage to remove some of the overhanging material.
Work higher up the slope is being undertaken by two rope supported Ripamonti hydraulic rigs, suspended from temporary Platypus anchors.
The rigs were first used on the GE Award-winning Hooley Cutting Stabilisation project without track possessions but that wasn’t possible at Longfield, although the soil nailing is being carried out with line restrictions rather than full possession. A small amount of hand drilling will also be required in hard to access places, mainly adjacent to the bridges on the site.
The soil nails are self-drilling hollow core bars with galvanised sections near the surface. They are fully grouted with cementitious grout placed under pressure during installation. The stabilisation work also includes installation of 1,500 rock bolts and top crest bolts formed from 3m-long dowels.
Following the soil nailing the face of the slope is being treated with 22,500m2 of Geobrugg Deltax mesh with some areas using Greenax, which is a Deltax mesh combined with erosion control matting Techmat 400.
However, before work on the stabilisation of the slopes could get underway there were a number of challenges to be dealt with in the form of access to the full length of the cutting on both sides. Bam Ritchies senior geotechnical engineer Andy Morris said: “Access on sites like these is always a primary consideration and we have needed to create temporary working spaces and access across a number of properties including domestic gardens and fields – one of which is full of parsnips.”
Work was also needed to secure the carriageway of the busy B260 road ahead of the work on the cutting slopes. The road crosses the cutting on a bridge and runs alongside the crest of the cutting on either side of the bridge for a short distance. Ritchies has constructed a short length of retaining wall and capping beam to ensure the road’s long-term safety.
Work then turned to clearing the vegetation, which was when the denehole was exposed. Historic use of the local area for chalk mining and production of lime in kilns was known but the exposure of the domed brick shaft cap at the eastern end of the site was not anticipated. Discovery of the shaft that had been cut through during the cutting construction called for some modification of the soil nailing solution in that area to secure the feature.
On the up side of the cutting the vegetation clearance also uncovered some badger setts that needed an approved programme of isolation this year to allow the badgers to relocate before final remediation of this section. Ritchies has said that the setts are likely to be grouted before soil nailing is undertaken.
According to Ritchies, its use of BIM on the scheme has helped with dealing with the subsequent redesign work following discovery of the denehole and badger setts.
“BIM is being used to help deliver the project with 360 Field being used to capture site data including safety tours and plant inspections,” says Bam Ritchies geotechnical engineer Emily Wood, who is leading the application of BIM on the project. “It ensures that site records are comprehensive with a progress chart populated automatically from the data entered on the iPads on site. A point cloud is being developed of the full topography and soil nail positions with photos and as-built information recorded into the system.”
Wood moved to the Longfield project from working on chalk mine remediation in Hertfordshire, but hadn’t expected that experience to prove useful on her current task. “Finding a Chalk denehole on a cutting stabilisation project is unexpected but my Chalk mine experience has already given me a good understanding of the risks associated with these features,” she says. “Using BIM should help us and Donaldson Associates come up with useful suggestions as to how to deal with the denehole and the badger setts.”
Ritchies started work on the scheme for Network Rail’s contractor Costain in September last year and, despite the challenges, work on the Longfield cutting is progressing well.