Location map

Key findings

A major fractured structure carries a very large and persistent groundwater flow

The Mont Blanc Tunnel intercepted a major, steeply oriented tectonic/fracture zone in the crystalline massif. During construction, it produced exceptionally large water inflows of about 1,000 L/s, and decades later the same structure was still draining roughly 200 L/s. This demonstrated that crystalline Alpine massifs can contain major, persistent groundwater-flow structures at great depth.

The deep groundwater is supplied by high-altitude infiltration

Your hydrothermal modelling showed that the cold-water anomaly observed around the structure could be explained by infiltration of surface water from the Alpine environment. The simulations indicated that cooling of the massif has been occurring since the end of the last glacial period, over roughly 12,000 years, and that the thermal system responds extremely slowly

Temperature can be used to identify and quantify deep groundwater circulation

An analytical relationship between rock-temperature anomalies and groundwater discharge through a fracture. applied to the Mont Blanc case showed that the thermal anomaly could be used to estimate the magnitude of groundwater flow and potentially anticipate major water inflows during tunnel excavation. Rock temperature provides a proxy for deep groundwater circulation in fractured crystalline massifs.

The most recent finding: groundwater–seismicity coupling

The enhanced seismic catalogue showed that a strong seasonal seismic signal appeared after 2015. Hydrological observations indicate that young meltwater reaches the major fault system, and modelling indicates that meltwater-induced pore-pressure changes can account for most of the seasonal seismicity. A second, migratory component appears to involve a different mechanism, with tectonic processes and aseismic slip becoming important after the initial hydrological triggering.

Hydrogeological model

Picture Gallery

Related publications