Work resumes this month on the last stage of an underground flood protection scheme in the Moselle region of eastern France.
Problems for the town of Moyeuvre-Grande began in 1998, when the iron ore mine below it was sealed and its drainage pumps shut down.
Since then many cellars have been regularly flooded because of water level changes in the vast underground reservoir that was created, fed by 120M. m 3of water a year.
High capacity pumping wells installed as an emergency measure proved incapable of permanently solving the problem, forcing some inhabitants to leave their homes.
A new drainage tunnel through the mine will help to lower water levels, connecting to a channel taking excess water to the nearby River Orne.
The US$12M project will lower the water table by between 2.8m and 3.8m using gravity outlets capable of handling a flood flow of 10m 3tunnel has a 0.2% gradient and will cut across an inclined tunnel and old working tunnels.
Client DRIRE Lorraine (Direction Regionale de l'Industrie, de la Recherche et de l'Environnement) began work in summer 2001, when river levels were low. The first phase comprised extensive surveys to ascertain the complete geological picture and pinpoint the mine workings.
Subhorizontal sampling boreholes were drilled from a reopened inclined mining tunnel, which will be enlarged and lined as part of the scheme. Work also included a survey of a 'Winze' (a fully immersed shaft) by a team of divers.
Underground works are being carried out by contractors Bec Freres and Soletanche-Bachy under manager DDE (Direction Departementale de l'Equipement) de la Moselle. Project manager is Groupement Coyne & Bellier and Ingerop Grand Est.
Tunnelling, using mechanical excavation, steel arches and sprayed concrete lining, began in summer 2002. Blasting could not be used because of limits on vibrations beneath the town. Tunnelling is due to finish towards the end of October.
The 5m wide, 450m long drainage channel was built by GTM last year, mostly by cut and cover, except for a 45m long section beneath a SNCF railway line. Subcontractor CSM Bessac worked round the clock for four days with a small TBM to create the 2.9m diameter tunnel with just 3m of overburden.