April 6th, 2021
Soletanche Bachy Tunnels: Breakout of the Cliets tunnel in Savoie
Since 2019 mid-October, Soletanche Bachy Tunnels teams have been hard at work alongside other VINCI group contractors (VINCI Construction France TP Lyon (main contractor) and Dodin Campenon Bernard) to build a tunnel which will reopen in few weeks the crossing near Cliets, an area currently closed for safety reasons.
The RD1212 departmental road runs through the Arly gorges over 15km and carries average traffic of 5000 vehicles/day in both directions, with peaks of 10,000 vehicles/day during busy tourist periods in the winter. This mountain road, at an altitude of 665m, passes through the old Cliets tunnel for 60m and crosses a river. Like many roads in this type of environment, this section of the RD1212 is dominated by an imposing cliff with a series of steep rocky outcrops stepping down nearly 120m from the top to the bottom of the tunnel.
For several decades, major landslides have regularly occurred in the area, which is known to be unstable, leading to closure of the RD1212 for several months at a time.
The site is subject to regular monitoring using extensometers, enhanced in 2003 by a geodetic device. Monitoring was stepped up from 2014 and an acceleration in movement and automatic monitoring allowed measurements to be taken every three hours, enabling closure of the road before any major event.
Following the massive landslide of 9 February 2019, however, road traffic was suspended, closing the Cliets crossing. This prolonged closure has allowed the local authorities to arrange a major programme of safety work without any traffic.
The contract involved in the construction of a curved tunnel 240m long, to be excavated using explosives, through mica-schists with varying degrees of instability and fractures, potentially including sections through karst. Following reinforcement of the entrance and exit, tunnel was taking place beneath pipe roofing. According to the breakdown of the contract by rock type, the retaining sections used comprise HA anchor bolts, fibre-reinforced shotcrete and HEB 180 arches.
Worksite teams have faced and will continue to face a number of challenges at the end of the project. Due to a sensitive environment (cliff and bridge), vibration thresholds needed to be painstakingly monitored for neighbouring areas. Finally, at a technical level, shotcrete waterproofing needs to be implemented within the structure.
