the mining companies rich,
but creating huge cavities under the ground into which the modern town could suddenly sink. Salt was mined for more than 2,000 years at Northwich, right
up until 2006.
A three-year project to fill the disused mines and put an end to subsidence is now nearing an end. It is hoped that this will lift planning blight, and allow Northwich to grow. “There was an 80% risk that the mines could start to collapse 20 to 25 years from now,” says Pete Skinner, geotechnical engineer for
project manager Wrekin
C onstruction, appointed by
Vale Royal Borough Council
to stabilise the mines.
Skinner is referring not to
a gentle roof, but a catastrophic collapse that could destroy
the town – even swallowing buildings.
Rampant and unregulated salt extraction has left these disused mines unstable. Today, a responsible miner would leave at least 30% of the salt as pillars to prop the subsidence. But some of the older mines have only 5% left as salt pillars. The risk of collapse was so great that in 1994, the local council imposed a moratorium on new developments.
The mines have been filled with a saturated brine solution since the 1920s in an attempt to support the roof of the excavation. This was only a temporary measure, however, as water leaking into the mine from the surrounding water table diluted the saturated mixture, dissolving even more salt from the pillars.
No existing recipes for stabilisation mixes included salt, so before pumping material into the mines, the team had to examine what was possible.
Wrekin brought in Tom Brooks, an associate at Arup’s Birmingham office who had experience of working with subsidence from limestone mines in the Midlands.
“We used a PFI, 3% cement and brine mix. The brine was extracted from the flooded mines,” explains Brooks. This averts the possibility of water bleeding from the mix, dissolving more salt and increasing instability. Brooks employed researchers at City University in London to help model the behaviour of the salty grout suspension entered the mines.
Their research was promising. It concluded that the grout would sink through the brine, initially forming grout cones on the mine floor. As more of the mix was introduced, the cones would grow until they eventually merged. The aim was to fill the mines with grout by continually pumping in the mixture and allowing it to settle.
Brooks modelled the mines using finite element analysis
to try and estimate the quantities required to fill the empty spaces – 780,000m3. But,
the team could not adequately model obstructions like
disused mining equipment.
He also modelled the progress of the grouting work using the same technique.
Unexpected blockages in the mine prevented some areas from filling with the salty grout, so the aim was to fill these areas with as much grout as possible, seal them and move on.
“We built a new pumping station, 2.5km from the town to receive the ingredients for the grout,” explains Wrekin regional manager Chris Osborne.
To introduce the grout into the mines, brine was pumped from them to the pumping station, then mixed with cement, and PFA brought in by train from Drax power station in Yorkshire. The mix was then pumped back to the mines via boreholes inserted in the ground.
Some of the proposed borehole locations were occupied by buildings, so Wrekin employed a directional drilling specialist to reach areas underneath them and under a local river. Special valves to release trapped air, which could have lain under the mine roof for decades under intense pressure, were installed at the borehole locations. “One leaked for two days when we found it,” says Skinner.
There had to be special care with the drilling, to ensure fresh water did not mix with the salt and cause further instability. The drilling contractors used a brine flush, which caused a great deal of wear to the drilling rigs. The brine corrodes everything is comes into contact with. At the pumping station, equipment only three years old appears ancient, having been ruined
by salt corrosion.
A feature of the job is that
it caused little disruption
to the town. At the start of
the process, 250 boreholes
were drilled and pipes laid,
but once that had been
done, most of the work
Osborne is very proud of
a letter sent to the local newspaper. “One resident complained that the works were delayed because he had not heard
noise from the pipes at the
bottom of his garden, [but] 500,000 tonnes of mix had actually passed through,” he said
Started: February 2004
Completes: this month
Client: Vale Royal Borough Council
Design: Wrekin, Arup
Drilling and ground investigations:
M&J Drilling and Grouting