SS No. 27 “Gran San Bernardo” – Road upgrading works between Etroubles and the motorway junction for the Gran San Bernardo tunnel – Variant bypassing the built-up areas of Etroubles and Saint-Oyen from km 15+180 to km 18+700 (re-tender).

Project objective

The design scope consisted of completing the bypass variant of the built-up areas of Etroubles and Saint-Oyen, from km 15+180 to km 18+700 of SS 27 “Gran San Bernardo”, connecting Etroubles to Cret through the St. Oyen natural tunnel, replacing the existing alignment that crosses several villages towards the Tunnel.
At the time, tunnel excavation and lining works were only partially executed, from the North portal up to chainage 1,883.00 and from the South portal up to chainage 1,012.20, due to instability phenomena and significant water inflows.
The works also include the Window Tunnel, intended as an emergency exit, fully excavated but with the final lining not completed along its entire length.
Other completion activities concerned, in Etroubles, the viaduct over the Artanavaz Stream connecting the South portal of the tunnel to the roundabout junction with SS 27 and, in Cret, a second viaduct with on- and off-ramps connecting to the existing State Road.

Main works of competence

St. Oyen Tunnel
To reconstruct the geological framework ahead of the remaining section to be excavated, forward investigations were carried out consisting of two 200 m continuous-core boreholes drilled from the two excavation faces at the work stoppage points, collecting samples for laboratory testing, performing in-hole dilatometer tests, investigating the presence of water in the rock mass and carrying out borehole video inspections.
The outcomes of these investigations enabled the definition of stabilisation interventions for the homogeneous sections identified.

Window Tunnel
Connected to the main alignment approximately at mid-length (chainage 1+928.57 km), orthogonal to it, with an overall length of about 492 m: the completion of the final lining (invert arch and/or crown) was redesigned over almost the entire length where not executed in segments, together with the waterproofing, consistent with the excavation geometry in the crossover connection area with the main tunnel.