CIVILS, TUNNELS & EMBANKMENTS
Swansea District Line railway cuttings restrained
David Gibson, BAM Ritchies’ business development manager, describes a successful stabilisation project. T
wo railway rock cuttings in the Swansea area are being stabilised to ensure safe railway operations for decades to come.
Over many years, weathering of the rock faces has taken place. To ensure future resilience, a comprehensive solution involving rock bolts and a tensioned mesh facing system has been installed above the Swansea District Line by a joint BAM Nuttall and BAM Ritchies team on behalf of Network Rail, to designs prepared by Tony Gee and Partners.
Pentwyn Farm Cutting
The rock cutting adjacent to Pentwyn Farm, near Skewen, had been designated as having a risk of failure on both the Up and Down sides of the cutting by the Network Rail earthworks examiner. As a result, Network Rail secured funding for a stabilisation project.
The design of the stabilisation works features a carefully-placed grid of rock bolts and a high- tensile mesh tensioned onto the face. This system ensures that not only are potential loose blocks of rock prevented from falling to the track, but the active support produced prevents their movement in the fi rst place.
This means that Network Rail achieves a long term and durable solution that will be maintenance-free and avoids the need to bleed bulges in the mesh that could encroach into the kinetic envelope on the passing trains, since any material is prevented from moving in the fi rst place.
To ensure the suffi ciency of this solution, the bolt positions in the design are initially set out using annotated photographs and fi ne-tuned (within maximum designated spacings) in response to actual circumstances found on site, so as to eliminate tenting effects on the mesh and to maximise the physical support of the face blocks from the mesh.
The bolts themselves also secure any rock blocks that require immediate stabilisation.
Whilst one rock bolt specifi cation has been produced and site-specifi c items (e.g. number and positioning of bolts) are shown on the design drawings for the section of cutting being remediated,
54 | rail technology magazine Jun/Jul 14
as the work progresses the designer can reduce or increase either or both the free length or overall length, depending on the drillers’ logs and suitability testing.
The mesh/rock bolt design is based upon relevant Network
Rail standards and
proprietary mesh system standard details. The fi nal design location and spacing of bolts has been decided following de-vegetation and inspection.
By agreement between the client, designer and contractor,
Geobrugg’s high tensile
TECCO mesh has been adopted with rock bolts comprising Dywidag’s GEWI high tensile threaded bar.
During drilling, care is being taken to ensure that none of the potentially detached rock is shattered. Drilling is being done by excavator and road rail plant-mounted drill rigs along with rope- supported ‘Terrapin’ rigs, depending upon access and reach. The face height is up to 25 metres high and is steep. Rock conditions are hard abrasive consistent coal measures that feature in the locality.
At present, it is anticipated that some 823 rock
bolts will be needed at Pentwyn Farm cutting, using 25mm diameter GEWI bars grouted into 100mm holes three metres long on the face and 40mm diameter GEWI bars grouted into 125mm holes fi ve metres long on the crest, which are also used to anchor both the operatives and rope supported drill rigs.
The TECCO mesh is standard G65/3, which is
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