Work begins to stop town centre from collapsing into 260ft hole

A £28m project was launched yesterday to stop an entire town centre collapsing into a deep hole.

A £28m project was launched yesterday to stop an entire town centre collapsing into a deep hole.

After 10 years of anxiety, the Cheshire town of Northwich began work to stave off a potential calamity caused by the long tradition of salt mining beneath the town. The industry shut down 82 years ago, but it left behind four cavernous mines which were destined eventually to collapse and bring the town centre in on them.

Those mines, now flooded, are 260 feet below street level but that will not stop a crevasse of more than 12 feet forming, bringing buildings down, when the rock pillars which support them finally give out, possibly within five years. It might not be a geological time bomb of San Franciscan proportions but the threat of a collapse - which would occur over several months - is enough to have kept all developers out of Northwich since 1988, when the problem was detected.

Now, what is believed to be the first attempt to stabilise flooded rock salt mines anywhere in the world is under way. In the town's Marks & Spencer car park, a borehole rig has been erected to dig a 90-metre shaft - the first of 11 to be sunk in preparation for the lowering of sonar equipment that will map out a computerised model of the mines.

Engineers are planning to suck out the reservoirs of underground brine before pumping in a million tons of grout to fill the void.

Northwich, which has lobbied hard for English Partnerships grants to resolve its problem, has all too graphic an understanding of the threat it faces. In the late 1800s, entire buildings toppled and roads and bridges cracked as salt was aggressively mined by companies attracted by its quality and proximity to the surface, but who ignored the dangers in pursuit of profit.

This explains why hundreds of the town's older properties have the curious quality of "movability" - they were erected on timber frames which could be jacked up hydraulically or even moved wholesale in the event of ground slippage.

The timbered Bridge House, a former inn in the town centre, gives a whole new definition to "moving house". It was moved 100 yards on rollers without a single pane of glass breaking in 1913 and has since been shifted again.

The town thought all these problems were in the past until geotechnical consultants acting for a potential retail developer bored into one of the Barons Quay mines in 1988.

The prognosis was shocking. The old mining methods of leaving pillars of undisturbed salt to support the mine roof had been flawed.

The rock salt was deteriorating and the pillars would collapse in domino fashion. Although many mines had long since caved in, at least four, covering 60 acres of land, were still intact.

"We knew at that point that subsidence had returned to Northwich," said George Westlake, mines project manager at the local Vale Royal Borough Council. "We are not sure how ground above the mine and below will behave once it's subject to collapse. It is unlikely that any building [above the mines] would remain standing."

The problem is so serious that Vale Royal council immediately banned all new construction until it was solved.

There is some compensation for those affected by the remnants of what was once a honeycomb of 250 brine shafts and mines here: the Brine Compensation Board, set up in 1891, still pays out sums ranging from £100 to £200 for subsidence damage - an average of 100 claims a year since 1990.

But the commercial effects are severe. Estate agents are desperate for action to help boost the local property market. A spokesman for the Beresford Adams agency said: "Some homes are sinking faster than others. Purchasers often pay for a brine survey as well as their local survey."

The lack of new retail development also means that Northwich is being eclipsed by the Trafford Park retail park in Manchester and the vibrant Cheshire Oaks outlet village near Chester.

Among some householders, traces of fear are also creeping in. "We've known about itfor a while now, but it's crept up on us and it worries people," said Steve Beauchamp, 40, whose home borders the threatened area.

The council's policy is clear. "We're being deliberately up-front about what we're doing here," said Mr Westlake.

"The rigs draw attention to the problem but they're 90ft high and bright yellow. You can't hide them."