Collieries and railways, in their various forms, have a long history of happy coexistence. Wooden wagonways were commonly carrying minerals to river wharves back in the 17th century. As new coal mines appeared in the landscape, tracks were soon laid to serve them. And their produce did of course power the railway’s locomotives throughout the steam age, ever since Salamanca’s first outing at Middleton pit in 1812.
As a child of West Yorkshire, I grew up accustomed to a skyline broken by pit headgear and t’slag ’eaps. How times have changed. Given the prevailing health and safety culture, it seems inconceivable today that colliery tips were entirely unregulated until 100,000 tons of slurry slid down a valley side to consume fog-bound Aberfan on 21 October 1966. Lost that morning in South Wales were 144 lives, most of them school children.
The formal inquiry pulled no punches. Underlying the tragedy was “a terrifying tale of bungling ineptitude” on the National Coal Board’s part: it had no tipping policy in place and offered no guidance to its workforce on how spoil should be managed. Despite previous slips in 1944 and 1963, evidence of which were clear on the hillside, the tips at Aberfan were not routinely inspected. Legislation bolted the stable door in 1969, courtesy of the Mines and Quarries (Tips) Act.
Railways have never claimed immunity from mining’s impact. The route through Bolsover’s notorious tunnel had to be singled, with the track slewed onto its centreline, after subsidence and distortion caused by local coal workings. Finally abandoned in December 1951, it then suffered the ignominy of being backfilled with 150,000 cubic yards of colliery waste. Annesley and Clifton Hall tunnels were similarly treated. More recently, however, mining had not imposed any significant operational constraints on the railway until the events at Stainforth, South Yorkshire, in February this year.
The earth moves
During the First World War, Hatfield Colliery established itself alongside the line linking Doncaster with Goole and Scunthorpe via Thorne Junction – a four-track route now used daily by First TransPennine Express, Northern Rail and about 140 freight trains serving power stations, steel works, waste plants and assorted petrochemical facilities. By volume, about 20% of the UK’s rail freight passes along this corridor. Under British Coal’s ownership, the pit closed in 1993 but it has since witnessed several reincarnations, the latest coming in 2007. Hargreaves Services runs the current operation.
No problems had been recorded with the section of track running past the colliery for about 20 years. But on Saturday 9 February, routine inspections identified a track defect on the Up Slow, nearest the pit. This coincided with a driver reporting a rough ride. Remedial lifting and packing failed to resolve this and more reports came in the following day. Overnight Monday into Tuesday, the track was moving at a rate of 1mm per hour; tension in the rails was increasing audibly. By 7am on Tuesday 12 February all lines had been closed.
Driving this movement was a failure of the adjacent spoil heap. By the time it had stabilised, more than a week later, four running lines and a siding were displaced by as much as five metres vertically and 20 metres horizontally over a distance of 300 metres. Forming the slip was excavated mudstone waste, known as coarse discard, as well as slurry-like MRF (multi-roll filter), with 40% moisture content, produced during the washing and reclamation of coal fines. The role of the coarse discard was to support a 20-metre deep cell of MRF – some 200,000 cubic metres of it – but the slip had allowed this to shift towards the railway. Potentially dynamic conditions created by fractures in the heap prompted a period of apparent inactivity. With surveys impossible on foot, remotely controlled drones flew over to confirm the heap had stopped moving.
But much was going on behind the scenes as Arup developed the design solution. This featured three distinct elements: stabilisation of the heap, reinstatement of the railway and the subsequent protection of the live lines.
Stabilisation has involved a vast muck-shifting exercise which will ultimately encompass around one million cubic metres of material. Taking the tip back to a gentle slope involves complex and careful sequencing, all within the weather’s influence. Charlie Chell, Network Rail’s senior construction manager and a seasoned railway firefighter, had many a sleepless night listening to the rain thumping on his roof. “Not good.” Favours have been called in to secure sufficient plant – high-capacity excavators and dumpers, with competent operators – at a time of year when such resources are hard to come by.
Relocating the MRF cell was an early priority as this was still contained by the landslip, thus preventing clearance of the railway alignment. An opportunity was identified in a valley between the colliery’s two main heaps – the intention being to erect bunds at either end and then infill it with the slurry.
As well as inevitable ecological concerns, two constraints emerged here in the form of a public bridleway and water main – the only supply into the adjacent village of Stainforth. Apart from comprising a concrete asbestos spun pipe, little else was known about it: condition, depth, location. Once found, it was initially sleeved before a permanent diversion was laid around the site’s eastern end. The bridleway will follow a similar path when it reopens.
Early on, bunded areas close to the village received around 200,000 cubic metres of material, a process that advanced more rapidly than expected thanks to decent weather making the MRF less fluid. Space within these tips soon became restricted so they started backfilling the haul roads.
But still to be dealt with was an even larger quantity of spoil, upwards of 300,000 cubic metres. Its new home would be across the road in fields for which the colliery had already been granted outline planning permission. These were acquired, the topsoil removed to form a sound barrier and an aquifer capped to prevent groundwater pollution. A three-quarter mile haul road has been established alongside the railway, passing beneath two bridges, to access this area without disrupting the highway. 60-tonne laden dumpers process perpetually to and fro, and will continue so doing for at least another 12 months.
Convention dictates inclusion of a few words here about the collaborative approach taken by all those involved. But this has been a project with tensions, perhaps understandably given the circumstances that have thrown the parties together. Forging progress where it’s really been needed has involved a relentless push push push. “It’s two separate worlds meeting,” asserts the aptly named Tony Heap, Network Rail’s project engineer.
Ground investigators were brought in to support the team in challenging the colliery’s restrictions on rail- side activity. Incremental benefits ensued, resulting in the railway being largely cleared of debris, piecemeal fashion, ahead of the time programmed for actually starting that work. “At every opportunity, we’ve been looking to take seconds off,” insists Charlie.
Road to recovery
So July will see the railway fully operational again, the substantive work to reinstate it having begun in mid- June. Amco is acting as civils contractor (see page 48) whilst the track, signalling and telecoms work is being fulfilled by Babcock. Although only 300 metres of track were damaged, around 500 metres are having to be replaced as a result of the design solution and adjacent S&C. Despite everything going back like-for-like, service reintroduction will be phased due to issues surrounding route learning.
Discounted at an early stage was a diversion of the tracks to the south through Doncaster Council’s household waste recycling centre, separated from the railway by a power line to the colliery. Also ruled out due to time constraints and geological/groundwater implications was a piling solution.
Instead a 500mm sump has been formed using granular Type B material on a reinforcement geotextile, extending 10 metres beyond the slip zone in both directions. Associated with this is a 300mm toe drain, installed prior to the main dig to assist with drainage. This lies below the Down cess and empties into the colliery ditches. A floating raft is then being constructed, keyed back into the existing formation by means of 20-metre transition ramps. Comprising it are a geogrid base and a layer of Type 1 stone, falling towards the Down side at a gradient of 1:50. 300mm of ballast then sits on top of it.
The muck shift will continue for months beyond the railway’s reopening, demanding a set of robust protection measures to prevent any incursions from the haul road. There’s also a highways interface at the bridges. Rail and colliery operations rarely come into such proximity, imposing particular challenges for the team which has had to draw down on mining & quarrying, CDM (construction design and management) and highways regulations to develop an approach that’s fit for purpose. So going in on a foundation strip is 1km of concrete L-units with engineered fill at the back, providing a minimum 1.5 metre barrier between trains and dumpers. The bridges have concrete slabs beneath them to maintain levels whilst four A-frames will provide solid impact protection.
As you’d expect, ground monitoring continued throughout the recovery works and a LiDAR was undertaken across the site to produce a plot of all levels. Inclinometers, taken out for the railway reinstatement phase, will go back in until the muck shift is completed. The flank protection will also be monitored.
All of which leaves an elephant in the room: what triggered the slip? With legal action pending to recover what it describes as “multi-million pound” costs, Network Rail is unable to comment. But notwithstanding any unique local factors, only three basic actions will affect a tip’s stability according to Paul Nathanail, Professor of Engineering Geology at the University of Nottingham: taking support from the toe, adding load at the top, or introducing water.
Whilst there is no evidence that material had been removed, this was certainly an active tip and the winter had been wet. In which case, what’s happened here was effectively a rebalancing as the load on the tip increased and overcame the resistive forces of looser natural ground above a sandstone layer 7 metres below track level. This shifted along a plane of weakness, perhaps exacerbated by the high groundwater. The slip’s toe started to daylight after passing beneath the tracks and adjacent power line, causing one of the pylons to lean back towards the railway. “The ground moves because it’s unstable and tries to find equilibrium,” Paul contends. “Then it stops of its own accord. It can find that equilibrium either because, in moving, it releases water pressure or, as the tip’s geometry changes, the toe eventually becomes able to support the weight of whatever is driving it.”
Courage and conviction
Industry firsts are rare occurrences. Stainforth was one such, certainly within living memory. Just days after the slip, Network Rail’s mining team had performed its own surveys of similarly-built structures to reassure the industry that no repetition was likely elsewhere. Once is obviously enough.
Whilst the impact on customers of five months’ service disruption shouldn’t be underestimated, recovery from this event had the undoubted potential to drag on into the far distance, just as the muck shifting will. With trains about to start running again rather against the odds, we must doff our hardhats in the direction of the project leaders whose willingness to bring their steel toe caps into the negotiating room paid real dividends.
There is a broader lesson here: if you know you’re right, don’t take no for an answer.