When a subway built to improve rail safety became a risk to rail traffic, the Coal Authority called on Soil Engineering to help develop a grouted solution to the problem
Most grouting work carried out for the Coal Authority involves backfilling abandoned mine shafts and shallow mines, but recent work at a site in South Yorkshire presented a different challenge for Soil Engineering.
Denaby Main Bath House Subway near Doncaster was built in the 1950s under live rail conditions to provide miners with safe access between the colliery and the bath house. Sixty years on and the colliery and bath house have been abandoned but the presence of the subway called for regular inspections and ongoing monitoring.
The subway’s route under the Doncaster to Sheffield railway line means that it is classed as an underbridge by Network Rail and the inspection regime started to raise questions over the stability of the backfilled underpass.
CCTV surveys confirmed that the tunnel was filled with debris and water so the Coal Authority opened a small section of the roof of the tunnel to take samples of the fill material for testing. The initial idea was to form an access into the tunnel, remove the fill and then stabilise the tunnel using foamed concrete. However, analysis of the samples removed showed some of the debris contained white asbestos.
The Coal Authority agreed with Network Rail that the asbestos-contaminated debris should be left in place and encapsulated using drilling and grouting techniques. The aim was to fill the voids to mitigate the risk of large-scale progressive collapse and associated ground movement causing damage to the railway line above the subway.
With a method of work agreed, the Coal Authority brought in its framework agreement contractor Soil Engineering to develop and deliver the solution.
The physical constraints of the site made access very difficult – there was insufficient room between the road and the railway line to position drilling rigs to form angle holes for the placement of the grout, so it was agreed to expose the tunnel in a pit and place the grout horizontally.
Before Soil Engineering moved onto site R&K Construction formed a 4m deep coffer dam to create the pit with a “clean” working platform and sump for dewatering to the base of the pit. Two sides of the tunnel were broken out from the pit by TRD Total Reclaims Demolition and concrete stoppers were installed at either side of the excavation, while seven 225mm diameter ducts were cemented in place on the side nearest to the railway line to avoid the need for concrete drilling to allow grout placement.
Finally, Soil Engineering could gain access to get the subway grouting underway but there was still the challenge of rig selection. The rig needed to be capable of placing the grout horizontally via the concrete ducts set at various heights within the concrete stopper, so Soil Engineering fitted a TEI drill mast to a 30t excavator to undertake the task.
The excavator was set up at ground level and used to drill six horizontal holes through the ducting to allow the injection of a grout – formed using a mix of PFA (pulverised fuel ash), cement and water – during a series of track possessions and overnight closures.
Water flush was used for the drilling work and a total of seven treatment boreholes with a total length of more than 162m were created. The debris backfill meant that constructing the boreholes was not straightforward and the site team encountered brick and rubble, as well as lots of voids – and after the initial borehole was grouted, the subsequent boreholes also encountered grout.
The grouting was monitored by CCTV equipment installed in one of two 35 degree breather boreholes which were constructed using a rotary percussive drilling rig.
More than 78t of grout was injected during the work.
Work has now been successfully completed and the site reinstated, so the only record of the subway that once linked the bath house with the pit head is the Coal Authority’s photograph archive. Nonetheless, Soil Engineering’s involvement is not finished yet – the company is arranging further track possessions with Network Rail to carry out probing to validate the grouting work.
Path to the bath
Located on a strip of land between the River Don and the Doncaster to Sheffield railway line, Denaby Main colliery was first mined in 1868 and by 1893 the colliery was in full production with 1,000 colliery houses build on land located to the south side of the railway line.
These cottages were modest to say the least and none of them had bathrooms or toilets, so the miners would leave the pit head, cross the railway line and return home to have a wash in a tin bath at the fireside of their cottage.
In the 1950s a law was passed that all collieries had to have a pit bath house but when it came to building Denaby’s the constraints of the site caused a problem. The site was bounded to the north by the River Don and the Doncaster to Sheffield railway line traversed the site from east to west, while to the south and immediately next to the railway was the first row of miners’ terraced cottages.
The decision was taken to construct the bath house on land further south of the cottages and provide a tunnel from the bath house direct to the pit head to safely take the miners under the road, row of houses and the railway line. The cottages were underpinned as the construction of the box tunnel progressed below by means of hand-poured concrete before the route went under the railway (pictured) and road by similar means. The bath house subway was in use right up until the closure of the mine in 1968.
The pit head site has since been redeveloped with the Dearn Valley Leisure Centre and the bath house is currently derelict.