Concrete blocks mining legacy

Stuart Marsh reports

If you’ve travelled on the Settle & Carlisle line at any time over the past 36 years, it would have been difficult not to notice the 30mph permanent speed restriction (PSR) which extends for almost a mile, just south of British Gypsum’s Kirkby Thore works. Subsidence problems in this area have stemmed from abandoned gypsum mine workings that were dug under the railway in the 1960s.

The New Stamphill Mine closed in 1972, leaving several mine tunnels passing beneath the railway on two levels. Those nearest the surface, some 10-15 metres below ground level, posed the main threat to the stability of the railway infrastructure. Natural processes too, involving surface water seepage into the mines, had caused solution cavities to form in the soft gypsum strata. These voids can migrate to the surface by progressive collapse of the cavity roof. Sinkholes had begun to appear on the adjoining land and differential subsidence had affected three railway bridge structures. Extensive track strapping had also been required. The decision was taken to eliminate these problems in conjunction with planned civil engineering and track renewal works scheduled for July.

Thinking ahead

Steven Townley was Network Rail’s Scheme Project Manger. He told me that “Planning for the Kirkby Thore remedial works began some 18 months prior to commencement of the works, with extensive ground investigations taking place. Core samples were taken and subterranean voids were located using acoustic techniques. There was also extensive sharing of information with British Gypsum. The result was an ambitious scheme that had to be shoe-horned into a scheduled 18-day blockade.”

Due to the volume and complexity of the works, the £5.2 million main contract was awarded to Birse Rail, the LNW Civils Framework Contractor, with all of the costs being met by Network Rail. The works were consistent with the route aspirations which include increased linespeed and passenger service frequency on the line. Civils and p-way works were awarded to Story Rail, with the embankment grading being undertaken by Stobart Rail.

Ecological studies carried out during the planning process highlighted a number of issues. As reported in last month’s issue of the rail engineer, specialist consultants Whitcher Wildlife Ltd, acting for Birse Rail, identified the northern worksite as an important habitat for great crested newts. And it didn’t stop there. Bat roosts were found under the affected bridges and a badger set had to be relocated. Once again, it was illustrated how the railway corridor can provide an important habitat for wildlife.

Divided attention

Activity was concentrated on two main worksites, each one dealing with mine working that passed at various angles beneath the railway. The most southerly site was close to milepost 280¾. Here, two accommodation underbridges – numbers 258 and 259, separated by just 100 yards – had both suffered badly from differential settlement, with fractures opening in the sandstone abutments. Bridge 258, carrying a concrete deck, had suffered significant full-height fracturing of the northern abutment and had become subject to additional examination at three-monthly intervals. The solution here was to infill the bridge.

The nearby underbridge 259 had steel trough girders supporting longitudinal timbers. The superstructure was propped in 2007 due to fracturing of the sandstone abutments whilst excessive deck movement had lead to track faults. As the landowner required a right-of-way to be maintained, this bridge was completely reconstructed using pre-formed reinforced concrete block units.

At the second worksite some 1,200 yards to the north, a similar situation had affected bridge 262, a three-arch sandstone overbridge with brick-lined arches carrying a single-track road. The Bridge Guard 3 Assessment failed the bridge qualitatively due to parapet and spandrel distortion as well as large diagonal cracks in the westerly arch barrel. The substructure needed substantial underpinning and remedial works undertaken to strengthen that arch.

All this had to be completed within an 18-day blockade from 9th-28th July. Beforehand, the embankment running northwards from bridge 258 was graded and covered with new topsoil. New cess drainage was provided, together with additional land drainage to prevent ponding at the toe of the embankment in the vicinity of bridge 262.

The remedy

So what was to be done about the subsidence and sinkhole problems? British Gypsum could not provide consent for the mine tunnels to be infilled so the novel solution adopted has been to cast reinforced concrete ground slabs under the track formation at the two problem sites. Stephen Townley explained that “These slabs were designed to span sinkholes up to 3 metres in diameter that might occur in the track formation area. A 270-metre ground slab, cast in five sections, was constructed under overbridge 262 and a 338-metre ground slab of six sections passes over underbridges 258 and 259.”

The construction of these slabs required both the Up and Down lines to be completely removed and the formation dug out to accommodate a concrete blinding upon which the 400mm or 450mm thick track slabs are cast in situ. A 300mm covering of ballast brought the formation up to sleeper level. The width of the slabs meant that embankment grading was required in order to provide sufficient width for cess drainage and walkways. Slightly increasing the angle of the embankment slopes meant that no additional land purchases were necessary. Rabbit netting was laid and staked over the finished topsoil profile.

Methodology

On the embankment sections, each slab measures 8550mm wide by 450mm deep, with a 1 in 60 fall each side of the centre line. In cuttings, the slabs are narrowed to a width of 7300mm and have a reduced depth of 400mm. The slabs vary in length between 50 metres and 56 metres and have a 50mm Aerofill expansion gap between them. Each slab section is linked to the next by a line of ‘Ancon DSD’ stainless steel shear keys spaced at 600mm centres. At 5 metre intervals along the slab centre line, 50mm galvanized steel monitoring pipes pass through the slab and blinding to allow future ground assessment works to be undertaken.

The reinforcement cages for the slabs were constructed within the two site compounds and then placed in position by 100 tonne crawler cranes. Concrete pouring was undertaken as a 24-hour operation using two suppliers, Tarmac and Hanson, with a third supplier being kept on standby as a contingency. All in all, about 300 lorry loads of pre-mixed concrete were required. Fortunately for some, a forecasted spell of very hot weather did not arrive. Had it done so, the stressing and concrete pouring would have become a night-only operation.

Bridge on a bridge

Overbridge 262 presented an interesting challenge. This three-arch masonry structure had suffered from differential settlement, causing serious cracking to the westerly arch, less significant cracking of the central arch, dropped voissoir stones and spandrel distortion. Old mine workings pass directly under this bridge and it was clear that some serious stabilisation works would be required. Firstly, the cracks in the central and westerly arch barrels were repaired with stainless steel stitching bars and then grouted. L-Bars were grouted into the underside of the westerly arch and stainless steel mesh reinforcement attached. This arch was then strengthened with sprayed concrete to a thickness of 200mm.

Meanwhile, at each end of the bridge, a line of seven mini piles was driven vertically from the roadway through the abutments and underlying shale to meet the bedrock. Further stabilisation work involved major underpinning of the two piers by means of transverse steel beams passing under the formation. Five 356x406x287mm UC steel beams were pocketed completely through the piers, effectively tying them together and providing vertical support. Grout injection insured that the beams fully supported the masonry piers. An additional UC steel beam passes on each side of the bridge piers. All seven beams were incorporated into a cast concrete ground slab of depth 725mm. This was linked to the conventional ground slabs each side of the bridge by shear pins.
The leaning spandrel walls also required stabilisation. A 775x400mm concrete backing was cast below road level and then pinned at 500mm centres to the spandrel walls and arch barrels by means of resin-anchored rebars.

Track renewals

The permanent way sections over the new ground slabs have had the Up line reinstated as continuous welded rail whilst the Down has been upgraded from jointed track on timber sleepers to concrete sleepers with CWR. The section between the ground slabs and within the limits of the PSR required the Down line to be renewed with concrete sleepers and CWR between 280m 1100 yards to 281m 946 yards. This equated to about 3 miles of track to be renewed and tamped. Tamping alone required the use of an on-track machine for four days. The blockade was to end on 28th July, with further tamping taking place the following weekend. All works associated with the project are to be completed by 11th September.

Performance benefits

“These intervention works will provide valuable journey time reductions for all stakeholders” explains Stephen Townley. “Restoring the linespeed of 60mph will reduce passenger train journey time by 1½ minutes on each line and 4½ minutes for freight.” Northern Rail’s average PPM (Passenger Performance Measure) currently stands at 86.62%. A 1½-minute performance improvement is likely to take it above the 90% target.
In the longer term, this project should help to realise capacity benefits. Network Rail’s Lancashire and Cumbria Route Utilisation Strategy aspires to a one-hourly passenger service and the goal is to raise the linespeed along the route to 75mph. Following the ‘Network Change’ process, involving consultation with TOCs, FOCs and signal sighting checks – the full benefits of these works – the lifting of the PSR – is expected during the autumn.

One interesting footnote from these works reveals just how far ahead Network Rail is planning. Electrification of the S&C is not even remotely on the horizon but the track slab under bridge 262 is lowered by 250mm to allow overhead line equipment to be accommodated. Standard procedure perhaps but imagine Ribblehead under the wires. Sparks might fly, and not just from the pantographs. What a project that would be!

Article courtesy of the rail engineer magazine.

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