That sinking feeling
‘Northwich is a dream town, a fanciful creation of a disordered brain. The streets swagger at eccentric angles, and the houses lurch forward drunkenly. I entered the library and experienced sensations usually associated with the deck of a ship in a heavy sea. Walking made me dizzy, and I expected every moment to collide with the wall.’
These aren’t the wild ramblings of a confused drunkard, but observations written 100 years ago by a reporter sent by the now-defunct Sunday Chronicle to investigate strange goings-on in the Cheshire market town of Northwich. The malady afflicting this ‘topsy-turvy town’ was subsidence, itself the result of a long history of rock-salt mining.
Today, Northwich (population 20,000) appears relatively normal and positively perpendicular, but its long-term future has only recently been secured following a two-year £32million stabilisation project that involved the injection of more than a million tonnes of grout into four holes under the town centre.
Early industry
During the 1800s, salt mining brought prosperity and employment to Northwich, swelling its population from less than 1,500 to more than 15,000. However, the industry also created huge, unstable underground caverns. Back then, shops, houses and even whole streets would regularly collapse. Indeed, the town’s rickety, cockeyed buildings became something of a tourist attraction during the Victorian era as visitors flocked to see what was little more than a picturesque ruin.
One early 1900s resident, describing the effect of subsidence on the buildings, explained: ‘They don’t go down suddenly; it’s a slow process, and when we think they have gone far enough we lift ‘em up again with hydraulic jacks. I have heard of folks going to bed with saws so that if the door would not open in the morning, they could saw themselves out.’
A young girl whose family shop had sunk nearly two metres below the road level found the town’s situation equally disorientating. She complained: ‘Nothing will hang straight on the walls, and you are always running into corners that ought not to be there. Besides, we never know when we shall slip through the earth altogether.’
An ingenious form of architecture was developed to deal with the subsidence problem. Buildings were constructed entirely within a wooden frame, so that when the ground collapsed underneath, they could simply be jacked up or even transported to another location altogether. This is what has given Northwich its distinctive, attractive black-and-white Tudor-style architecture, which can still be seen in the town today.
Brine in the mine
Salt has been extracted from beneath Cheshire since Roman times, but Northwich’s troubles started back in 1779, when a deeper bed of pure rock salt was discovered. Two years later, mine shafts were sunk to extract it.
‘The mines are only about 90 metres deep,’ says Dave Billington of Vale Royal Borough Council, who manages the stabilisation project. ‘The engineers left large pillars of salt unexcavated to support the ceiling. They felt that eight per cent of the floor area was sufficient to support the mine roof, but later that was considered to be totally inadequate.’
Part of the problem was that water began to seep in to fill the void left when the salt was removed. It’s common knowledge that water dissolves salt; however, at first, the water became saturated with salt ions, and the space remained fairly stable. Problems arose later, during the 19th century, when fierce competition encouraged industrialists to re-flood the old mines with fresh water and pump out what was known as ‘bastard’ brine. Vital pillars supporting the roof crumbled and the ground above became unstable.
In 1880, the brine pumping was to have devastating consequences. Many of the mines were interconnected, and when one shaft suffered slippage, it often affected a wide area. On 6 December, there was a sudden collapse so cataclysmic that the event became known as the Great Subsidence.
The movement of earth and water was so violent that the River Weaver reversed its flow, buildings collapsed, roads broke up and huge pockets of air and water burst through the surface, creating explosive mud volcanoes. Part of a salt works and a large chimney were simply swallowed up.
Subsequent subsidence events were less dramatic but equally distressing for the people who lost their homes and livelihoods. On several occasions between 1880 and 1920, the local authority was forced to raise road levels in order to maintain drainage and sewerage systems. However, this was an expensive exercise and caused disruption to homeowners and shopkeepers, who found their ground floors disappearing as much as two and a half metres below street level.
Pillars of the community
Safe methods of brine pumping were eventually introduced some years ago, but the dangerous legacy of the original exploitation of the four mines under Northwich remained. The full extent of the problem was only uncovered in 1988, when a property developer wanted to expand the town centre northwards. Engineers drilled a few test boreholes to satisfy financial backers, only to discover that the existing salt pillars were unstable. All major development in Northwich was immediately halted.
During the late 1990s, Vale Royal Borough Council instigated a detailed investigation into the mines. ‘This involved a number of bore holes to determine the strength of the salt pillars,’ says Billington. ‘Sensitive extensometers were installed in a grid across the site and levelling points were established at the surface to measure any ground movement. The report concluded that there was a strong possibility of mine collapse – so doing nothing wasn’t an option.’
This is where Weaver Vale MP Mike Hall took up the cause and provided a vital key to unlock a safe future for Northwich. Although former coal-mining areas are eligible for government assistance, there
was no existing legislation in place to protect areas devastated by other types of mining.
So Hall set about lobbying a series of government ministers, leading up to then Deputy Prime Minister John Prescott, who eventually agreed to visit the town on a fact-finding mission. ‘He was left in no doubt that we would have an environmental disaster on our hands if we did nothing,’ Hall says. ‘The study had reported that there could be a collapse of the town centre within five to ten years. ‘I told him that it wasn’t a situation where we had a choice. If we didn’t do anything, we would end up with a huge hole in Northwich.’
Prescott was convinced: in April 1999, the government modified the Land Stabilisation Fund to include areas affected by mining other than coal. This clause was to be the saviour of Northwich. In July 2002, English Partnerships granted £28.9million to fund the Northwich Mines Stabilisation Project.
The first step was to come up with a recipe for the material that would be used to fill in the four mines at Baron’s Quay, Witton Bank, Neumann’s Mine and Penny’s Lane. The mixture had to be fluid enough to travel 2.5 kilometres down a pipeline from the mixing plant in nearby Winnington, yet able to set fairly quickly once pumped underground.
And they were going to need plenty of it. With the mines up to 90 metres deep and the worked areas around five metres from floor to ceiling, the total volume to be filled was about 780,000 cubic metres, equivalent to 290 Olympic swimming pools. This would require a staggering 1.3 million tonnes of grout.
The final recipe consisted of 96 per cent pulverised fuel ash (PFA) – which arrived twice daily by rail from the Drax coal-fired power station in Selby, North Yorkshire – and four per cent cement, with a splash of salt brine to keep it fluid. The infilling process started in January 2005 and was completed in November last year, paving the way for new development of the town centre.
Crisis averted
Today, Cheshire still earns a living from salt mining, and the county contains Britain’s only dedicated working rock-salt mine at Meadowbank in nearby Winsford, which is operated by US-owned company Salt Union. However, local residents will be pleased to know that water can no longer penetrate into the mine and there is no threat of subsidence.
In fact, the company is so confident of the integrity of the mine structure that it has diversified into waste disposal and secure document storage, exploiting the huge voids left behind by the mining of rock salt. The mine is home to around 800,000 document boxes from a variety of sources, including councils and health authorities, Network Rail and the precious National Archive; there are even valuable documents and rare books from Oxford’s Bodleian Library.
Salt, that delicate white substance so vital to daily life, is indelibly woven into the history of Cheshire. It has been the source of both the county’s wealth and its troubles and will, no doubt, continue to play a role well into the 21st century.
January 2008
These aren’t the wild ramblings of a confused drunkard, but observations written 100 years ago by a reporter sent by the now-defunct Sunday Chronicle to investigate strange goings-on in the Cheshire market town of Northwich. The malady afflicting this ‘topsy-turvy town’ was subsidence, itself the result of a long history of rock-salt mining.
Today, Northwich (population 20,000) appears relatively normal and positively perpendicular, but its long-term future has only recently been secured following a two-year £32million stabilisation project that involved the injection of more than a million tonnes of grout into four holes under the town centre.
Early industry
During the 1800s, salt mining brought prosperity and employment to Northwich, swelling its population from less than 1,500 to more than 15,000. However, the industry also created huge, unstable underground caverns. Back then, shops, houses and even whole streets would regularly collapse. Indeed, the town’s rickety, cockeyed buildings became something of a tourist attraction during the Victorian era as visitors flocked to see what was little more than a picturesque ruin.
One early 1900s resident, describing the effect of subsidence on the buildings, explained: ‘They don’t go down suddenly; it’s a slow process, and when we think they have gone far enough we lift ‘em up again with hydraulic jacks. I have heard of folks going to bed with saws so that if the door would not open in the morning, they could saw themselves out.’
A young girl whose family shop had sunk nearly two metres below the road level found the town’s situation equally disorientating. She complained: ‘Nothing will hang straight on the walls, and you are always running into corners that ought not to be there. Besides, we never know when we shall slip through the earth altogether.’
An ingenious form of architecture was developed to deal with the subsidence problem. Buildings were constructed entirely within a wooden frame, so that when the ground collapsed underneath, they could simply be jacked up or even transported to another location altogether. This is what has given Northwich its distinctive, attractive black-and-white Tudor-style architecture, which can still be seen in the town today.
Brine in the mine
Salt has been extracted from beneath Cheshire since Roman times, but Northwich’s troubles started back in 1779, when a deeper bed of pure rock salt was discovered. Two years later, mine shafts were sunk to extract it.
‘The mines are only about 90 metres deep,’ says Dave Billington of Vale Royal Borough Council, who manages the stabilisation project. ‘The engineers left large pillars of salt unexcavated to support the ceiling. They felt that eight per cent of the floor area was sufficient to support the mine roof, but later that was considered to be totally inadequate.’
Part of the problem was that water began to seep in to fill the void left when the salt was removed. It’s common knowledge that water dissolves salt; however, at first, the water became saturated with salt ions, and the space remained fairly stable. Problems arose later, during the 19th century, when fierce competition encouraged industrialists to re-flood the old mines with fresh water and pump out what was known as ‘bastard’ brine. Vital pillars supporting the roof crumbled and the ground above became unstable.
In 1880, the brine pumping was to have devastating consequences. Many of the mines were interconnected, and when one shaft suffered slippage, it often affected a wide area. On 6 December, there was a sudden collapse so cataclysmic that the event became known as the Great Subsidence.
The movement of earth and water was so violent that the River Weaver reversed its flow, buildings collapsed, roads broke up and huge pockets of air and water burst through the surface, creating explosive mud volcanoes. Part of a salt works and a large chimney were simply swallowed up.
Subsequent subsidence events were less dramatic but equally distressing for the people who lost their homes and livelihoods. On several occasions between 1880 and 1920, the local authority was forced to raise road levels in order to maintain drainage and sewerage systems. However, this was an expensive exercise and caused disruption to homeowners and shopkeepers, who found their ground floors disappearing as much as two and a half metres below street level.
Start the slideshow (6 pictures)
Pillars of the community
Safe methods of brine pumping were eventually introduced some years ago, but the dangerous legacy of the original exploitation of the four mines under Northwich remained. The full extent of the problem was only uncovered in 1988, when a property developer wanted to expand the town centre northwards. Engineers drilled a few test boreholes to satisfy financial backers, only to discover that the existing salt pillars were unstable. All major development in Northwich was immediately halted.
During the late 1990s, Vale Royal Borough Council instigated a detailed investigation into the mines. ‘This involved a number of bore holes to determine the strength of the salt pillars,’ says Billington. ‘Sensitive extensometers were installed in a grid across the site and levelling points were established at the surface to measure any ground movement. The report concluded that there was a strong possibility of mine collapse – so doing nothing wasn’t an option.’
This is where Weaver Vale MP Mike Hall took up the cause and provided a vital key to unlock a safe future for Northwich. Although former coal-mining areas are eligible for government assistance, there
was no existing legislation in place to protect areas devastated by other types of mining.
So Hall set about lobbying a series of government ministers, leading up to then Deputy Prime Minister John Prescott, who eventually agreed to visit the town on a fact-finding mission. ‘He was left in no doubt that we would have an environmental disaster on our hands if we did nothing,’ Hall says. ‘The study had reported that there could be a collapse of the town centre within five to ten years. ‘I told him that it wasn’t a situation where we had a choice. If we didn’t do anything, we would end up with a huge hole in Northwich.’
Prescott was convinced: in April 1999, the government modified the Land Stabilisation Fund to include areas affected by mining other than coal. This clause was to be the saviour of Northwich. In July 2002, English Partnerships granted £28.9million to fund the Northwich Mines Stabilisation Project.
The first step was to come up with a recipe for the material that would be used to fill in the four mines at Baron’s Quay, Witton Bank, Neumann’s Mine and Penny’s Lane. The mixture had to be fluid enough to travel 2.5 kilometres down a pipeline from the mixing plant in nearby Winnington, yet able to set fairly quickly once pumped underground.
And they were going to need plenty of it. With the mines up to 90 metres deep and the worked areas around five metres from floor to ceiling, the total volume to be filled was about 780,000 cubic metres, equivalent to 290 Olympic swimming pools. This would require a staggering 1.3 million tonnes of grout.
The final recipe consisted of 96 per cent pulverised fuel ash (PFA) – which arrived twice daily by rail from the Drax coal-fired power station in Selby, North Yorkshire – and four per cent cement, with a splash of salt brine to keep it fluid. The infilling process started in January 2005 and was completed in November last year, paving the way for new development of the town centre.
Crisis averted
Today, Cheshire still earns a living from salt mining, and the county contains Britain’s only dedicated working rock-salt mine at Meadowbank in nearby Winsford, which is operated by US-owned company Salt Union. However, local residents will be pleased to know that water can no longer penetrate into the mine and there is no threat of subsidence.
In fact, the company is so confident of the integrity of the mine structure that it has diversified into waste disposal and secure document storage, exploiting the huge voids left behind by the mining of rock salt. The mine is home to around 800,000 document boxes from a variety of sources, including councils and health authorities, Network Rail and the precious National Archive; there are even valuable documents and rare books from Oxford’s Bodleian Library.
Salt, that delicate white substance so vital to daily life, is indelibly woven into the history of Cheshire. It has been the source of both the county’s wealth and its troubles and will, no doubt, continue to play a role well into the 21st century.
January 2008
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