THE 18th century stone mines used to build the City of Bath are to be pumped full of concrete to stop them collapsing.
Bath and North East Somerset Council is looking to appoint between one and four firms for a £50 million framework deal to stabilize the Combe Down mines.
Costain, Wrekin, Nuttall, Norwest Holst, Skanska, Bristol firm Hydrock and West Bromwich contractor Forkers have until October 13 to submit bids.
The council has been carrying out emergency work since 2001 but has decided a more permanent solution is needed.
One project insider said: 'It has been quite an extensive pre-qualification process. It's far from straightforward because it is essentially a mining job. The lads will have to go underground, whereas normally the work is done from above.'
Winners will be appointed later this year, with work starting next Easter and finishing in 2010.
Problems were first found in 1986 when a workman digging a trench exposed an underground tunnel. A survey carried out in 1994 showed that in some places, tunnels are just 2 m below the surface.
Approximately 1,660 people live above the planned work site, while a primary school, a nursery and three churches are also in danger. The mines are located in a Site of Special Scientific Interest and a conservation area that protects a number of listed buildings as well as a Grade I aquifer.
The mines are also home to a variety of bat species, which are protected by UK and European law. A number of the underground roadways that will be built to provide worker access will be left open to provide flight ways.