References

The Chavanne tunnel, part of the Rhin Rhône new High Speed Railway Line, crosses the relief made by the ridge of the Lassus woods. Two “false” tunnels, 190m and 50m long respectively, extend the tunnel to the West in the Chavanne cuesta and to the East to the Rupt valley crossed by the Pertuis viaduct.
The geological model predicts a combination of Bajocian calcareous soils and Trias marl, with a layout due to different fault systems created by the Alps and Rhine tectonics. Beyond the brutal facies variation and occasional water inflows caused by the fault zones, an additional issue resulted from the swelling of the Domerian and Toarcian marls, more particularly the lower Toarcian called “paper shale”.
TERRASOL was the geotechnical consultant of the Project Manager (SETEC), after the study phase led upstream by SETEC/TERRASOL. During the works, the follow-up of the excavation by the geotechnical engineers on site resolved different issues related to:

• check of the proper setting of the geological model, and more particularly the location/crossing of the different faults ;
• check of the in-situ soil properties with respect to excavation short-term stability issues and solicitations in the retaining structure, by monitoring the in-situ instrumentation.

The assignement of the geotechnical engineers on site also included the participation of TERRASOL experts for more delicate issues, such as the tunnel face instability in the paper shale (observed at PM110-West attack), or the definition of retaining structures and convergence criteria in swelling marls.

Several aspects can be pointed out after works completion: proper setting  of the geological model (fault zone properly located), resolution of immediate stability issues of the tunnel face in the lower Toarcian (45° slip plane), and of arch stability issues in disturbed marls.