Northwich's salt miners surfaced for the last time in the 1920s but their potentially catastrophic legacy is one that today's citizens are having to face.
There are four mines beneath the Cheshire town: Witton Bank, Baron's Quay, Penny's Lane and Neumann's. All were excavated using the room and pillar method: as they worked, miners left pillars of salt to support the roof. Modern miners typically leave 25% of material unmined for this purpose, but those at Northwich only left about 7%.
So far so bad, but the problem at the site is worsened by the effect pressure has on salt. Tom Brooks, project manager for consultant Arup, says: 'The way salt responds to load is to creep. After a period of time it goes into tertiary creep and this can go towards failure. Salt is unusual as theoretically every pillar can go into tertiary creep without an increased load.' Most of the movement at the surface is due to dissolution of the upper salt layer, which is at least 10m above the lower salt band where the mines sit, although there is probably a small effect of creep from the mines themselves. A glacial channel that cuts through the top salt may also be exacerbating the dissolution there.
In the town the effects of settlement are clear. Some of the brick and timber buildings have jacks, and in the car parks vehicles wallow at bizarre angles on the undulating surfaces, as if frozen in the middle of an earthquake.
But the danger of collapse in the mines themselves meant Vale Royal Borough Council had to take action.
Wrekin Construction secured the £28M design and build contract, funded by English Partnerships.
Site manager Richard Stirling says: 'In 15 to 20 years they [the mines] may collapse and it's blighting the development of the town.' Brooks adds: 'It's inevitable, given enough time.' After salt extraction from the lower layers was wound up 80-odd years ago, brine was pumped down to stabilise the chambers 90m below the surface. The decision to install a longer term answer will see the use of a process that Wrekin claims is a world fi rst.
'No one has been presented with this problem before and the solution is to remove the brine and replace it with 1.3Mt of grout, ' Stirling says.
Brooks says the project is one of the biggest infill operations in the UK. The grout is unusual as the mix is 97% pulverised fuel ash (PFA), 3% cement, and, crucially, brine is being used instead of fresh water. This averts the possiblity of water bleeding from the mix into the mines and causing salt dissolution, thereby increasing instability.
Wrekin operations director Geoff Martin says a brine-grout mix has been used before, on a German salt mine project, but adds: 'They wouldn't tell us the mix.' Initially a mix of lime waste from a nearby lagoon was considered as a stabilisation material. While the trials were reasonably successful, there would have been handling diffi lties meaning prohibitive expense with such a large quantity of the material, which would not pour from an upturned bucket.
Lime waste also fell out of favour because its use would have required a waste management licence, although by the time the job started a licence was needed for PFA.
An Atkins non-technical summary produced in May 2001 for a planning application states: 'In strict technical terms, there is no need to fill the Penny's Lane and Neumann's mines. They are much smaller than the other two and the investigations have shown that they are long-term stable.' But Brooks says: 'We think the two little mines do need to be done because collapses in them would cause significant settlements.' M&J Drilling Services won the contract to drill the 200 boreholes required. The ground is comprised of gravels and sands over Mercia Mudstone, top salt, middle marl and bottom salt. The mines, measuring 5m from floor to ceiling, sit in this bottom layer.
The subcontractor is drilling 190 boreholes for grout to be pumped in, and 10 to carry brine out. Some need to reach positions under buildings so these are being drilled at up to 22¦ from vertical.
Grout lines have 219mm diameter casings that extend to about 20m depth into the glacial till to prevent the holes collapsing. A narrower 168mm casing continues from here to 45-55m depth depending on the level of the top salt, and this is reamed into the wet rockhead to seal off the water table. A fi nal 112mm casing extends from here to the mines.
For the brine lines, a 244mm diameter casing goes to 20m with another at 219mm going to 60m depth where a pump is located.
From here a 120mm uncased hole goes to mine depth.
The ground does not make for easy drilling. Brooks says: 'The particular concern is that if drilling is done in an uncontrolled way it could cause settlement in the top salt. High porosity in the top bed can cause deviation.'
The rig crews must be mindful of air pockets at the top of the bottom salt layer as these add an extra element of risk. Drillers are using a blowout preventer to deal with the 10bar air in the mines. During the site investigation three years ago, an air pocket in the mines was hit, blowing a casing about 30m out of a borehole. At the time the drillers had thought the mines were completely full of brine.
Martin says: 'In some respects it was a good thing to happen because it highlighted what we need to be aware of.' Now the lesson has been learned the crews can use the blowout preventer to shut off any pressure and then vent it under control.
But air pockets pose another threat. Blowouts could mean a lowering of the pressure in the mine, which could increase the load on the salt pillars by 30%. Such an incident could also introduce fresh water into the top salt layer, causing dissolution and then movement and settlement beyond the levels the people of Northwich are accustomed to.
Regardless of the blowout risk, the interface between the brine and fresh water is a critical consideration.
Apart from the hazard of water getting into the top salt, the level must be monitored to ensure brine does not rise into the water table.
So an operator, watching the level from a control centre in the grout batching plant, controls the amount of brine being removed as the grout is pumped in.
A pump at the batching plant propels grout up to 2.6km to the furthest borehole, but the route crosses roads and the River Weaver.
Stirling says site workers used thrust boring to form routes beneath the roads to prevent traffic disruption, but the river was another matter.
Here, LMR Drilling UK used directional drilling to install a 470m, 560mm plastic pipeline. This carries fi ve lines, two for grout and two for brine, with the fi fth kept spare in case a grout line becomes blocked.
Although the Northwich setup allows 500m 3/day of brine to be discharged into the nearby river, there is still a surplus to be disposed of. Wrekin has hired a railway engine to haul the surplus to a British Salt site at Middlewich.
Wrekin started on the project in January 2004 and mine work began last October. Completion is scheduled for September 2007.