ENGINEERS IN Cheshire this week began a two year, £33M project to stabilise four huge salt mines under Northwich town centre using around 1Mt of grout.
In recent decades, the mines have caused subsidence which has damaged housing and blighted town centre development.
Work will be funded with a £33M grant from the English Partnerships land stabilisation programme.
The four mines date from 1670 and are currently full of brine from water which was pumped in to stabilise the workings when they shut in the 1920s.
The brine has to be pumped out of the mines in balance with the grout injection in what is believed to be a unique operation.
Contractor Wrekin Construction will pump a total of 1M. t of a 97% PFA/3% cement grout mix into the mines over the next two years, fi lling 800,000m 3 of underground void 90m below the market town. Salt will be added to the grout to prevent further dissolution of supporting rock salt pillars.
The void has placed an area of around 30ha at risk of subsidence.
Approximately 500 vertical and inclined boreholes have been sunk in preparation for the operation.
'The brine will be extracted by pipeline to a handling lagoon nearby, ' said Vale Royal Borough council project manager civil engineer David Billington.
The brine is to be tankered to British Salt's workings 15km away at Middlewich, where the salt will be extracted.