In February 1.4 million metric tons of spoil material shifted from the Hatfield Colliery in the United Kingdom onto a nearby rail line, at right. All four tracks on the line, which operates east of the city of Doncaster, were lifted and twisted 5 m vertically and 15 m laterally. Network Rail
Four months ago, a landslide at the Hatfield Colliery, located near Doncaster, United Kingdom, wreaked havoc with a 250 m stretch of rail line. A solution has now been selected.
July 25, 2013—Undertaking damage from a large landslide can be a herculean task at the best of times, but it is made all the more difficult when the slide has destroyed crucial transportation infrastructure. That is the problem that faced the United Kingdom’s Network Rail—the owner and operator of Britain’s railway infrastructure—four months ago, when slippage from a mining spoil heap located alongside a rail line east of the city of Doncaster caused the ground to contort and four tracks to buckle along a 250 m stretch. A plan to repair the line that involves the relocation of 1 million m3 of material and the reconstruction of the rail bed has been selected.
On Saturday, February 9, 2013, a train driver reported a “rough ride” along the line, according to material contained on Network Rail’s website. By the time ground movement had stopped a few weeks later, 1.4 million metric tons of spoil material had moved onto the nearby rail bed. All four of the line’s tracks were lifted and twisted 5 m vertically and 15 m laterally, according to Tony Heap, Network Rail’s project engineer for the reconstruction work, who wrote in response to written questions posed by Civil Engineering online.
The colliery used the spoil heap to dispose of waste generated by its deep coal mining operations, according to Heap. “This by-product is mainly mudstones, in both granular [form] and a fine slurrylike material formed during the washing and coal fines reclamation,” he noted. The granular matter was used to create an embankment to contain the slurry mixture, also known as MRF, which has a moisture content of approximately 40 percent.
“When the slip occurred, the railway ground was both uplifted and in effect shunted sideways by the weight of material above,” Heap explained. “Though the slip material that finished up blocking the railway line was of either the coarse material or made from the adjacent local ground, it could not be immediately removed as it was supporting the cell of MRF material.” This slurry cell was enormous: It measured approximately 20 m deep and is estimated to have contained 200,000 m3 of material.
According to Heap, the engineers from Network Rail worked with the mine owner, Hatfield Colliery Limited, to create seven potential scenarios for reinstating the affected rail line. Options included building a diversion, adding extensive piling to create a protective embankment, and building an elevated overpass, among others. “All options were considered, and a carefully considered decision was taken to reinstate the railway on the present alignment as the option that was achievable in the safest possible manner, that would ensure no repeat occurrence, and [that] could be achieved within the shortest possible period,” Heap said.
The solution involves moving the MRF to a valley that is located within the colliery’s compound and that will be encompassed by embankments. A water main that serves a nearby village and a bridleway previously located in the valley are being relocated as part of the preparatory work for the MRF’s relocation, according to Heap. The remainder of the spoil heap will be relocated away from the rail line, to a recently purchased site on the opposite side of the colliery’s grounds.
As part of the project, an enormous slurry cell measuring
approximately 20 m deep that is estimated to have contained
200,000 m3 of material is being relocated to a nearby valley.
Future spoil material will be stored away from the rail line and slip
zone, on the opposite side of the colliery’s grounds. Network Rail
Once the material located beneath the rail bed is removed and the ground leveled, a reinforced geotextile mat will be placed along the slip zone, extending 10 m beyond the zone’s edges in each direction, with a low spot, or sump, built in to drain water. Atop this will be a roughly 500 mm thick layer of clean granite, each piece of gravel having a diameter ranging between 20 and 40 mm. A second layer of geotextile will be laid atop that, and a second layer of granite; together, these layers form what Heap calls a “floating raft.”
Track work will include the replacement of 500 m of each of the four tracks impacted by the ground heave; switches, crossings, signaling, and power supplies that serve the four tracks will also be replaced, according to Heap. Track repair work will take five weeks, according to Heap. Of this, three full weeks will be spent reconstructing the subformation level in the damaged section.
Initial Network Rail estimates in February anticipated that it would take eight weeks from the event to repair the damage caused by the slide. As the complexity of the problem was realized, however, those estimates were revised. Network Rail announced in mid-June that a single passenger line will reopen in less than a month, by early July, with full service reintroduced by the end of the month.
All passengers have been routed via buses for the duration of the rail line closure, while the 140 daily freight trains that used the line—20 percent of Britain’s daily total freight volume—have been rerouted to alternate tracks, according to Network Rail’s website.
For legal reasons, Network Rail was unable to discuss the cause, cost, or liability of the spoil heap slip. The spokesman for Durham, U.K.-based Hargreaves Services, which manage the Hatfield Colliery, did not respond to Civil Engineering online.