Relation between temperature and intensity of groundwater circulation in alpine massifs as a tool for predicting water inflows in tunnels
Numerous long tunnels have been and will be drilled at great depths in mountainous alpine massifs. Water inflow temperatures in five existing long alpine tunnels (Vereina, Gothard-N2, Mont-Blanc, Simplon and Gothard-AT) have been studied and compared with the volume of water inflows. The Vereina railway tunnel drilled in Austroalpine nappes encountered little water inflow. The linear discharge rates vary between 0.003 and 0.006 L/s/m. Water temperatures series have been observed in both the northern and southern parts of the tunnel trace: the northern thermal gradient is equal to 0.018 °C/m, whereas the southern thermal gradient is not very different with a value equal to 0.016 °C/m. No special thermal anomaly has been observed at this site. The Gothard-N2 road tunnel (National route number two) intersects the Aar and Gothard External Crystalline Massifs. A general thermal gradient equal to 0.015°C/m is observed in the southern part of the tunnel trace in the Monte Prosa massif. Positive thermal anomalies have been measured in both the northern and central parts of the tunnel trace. They are due to topographical effects: in this region, the tunnel is situated beneath the Reuss river valley. Water inflows are weak in this tunnel: about 0.020 L/s/m in the Monte Prosa zone, for example. The Mont-Blanc road tunnel intersects the Mont-Blanc External Crystalline Massif. A water thermal gradient equal to 0.016 °C/m has been observed on the northern part of the massif, at depths less than 1000 meters. This region corresponds to a low-permeability crystalline schist zone. The linear discharge rate is equal to 0.008 L/s/m in this zone. A large negative thermal anomaly was measured during the drilling of this tunnel. The water temperatures decreased from 32°C to 11.5°C beneath the Pointe Helbronner. This decrease corresponds to large water inflows (about 1000 L/s) in a strongly fractured zone. A second water thermal gradient (very weak: 0.007 °C/m) corresponds to the granitic unit which is globally more permeable than the schist with a linear discharge rate equal to 0.193 L/s/m. The Simplon railway tunnel, drilled through the Penninic nappes, is also characterized by a negative thermal anomaly situated in the very permeable marbles of the Teggiolo zone. In this tunnel, the water temperatures decrease from 55°C in the Berisal gneissic zone to less than 15°C in the Teggiolo zone. The water thermal gradient in the northern part is high, in conformity with the weak water inflows (linear discharge rate less than 0.001 L/s/m). A third zone is observed in the Veglia marbles: it is characterized by a water thermal gradient equal to 0.010 °C/m for a linear discharge rate equal to 0.203 L/s/m. The Gothard-AT gallery has been drilled in Penninic gneiss. A water thermal gradient equal to 0.013 °C/m has been measured over the first 3000 m. A negative thermal anomaly was encountered at the end of the gallery, due to the presence of very permeable metasedimentary rocks with important water circulation. These results show that the water temperature in underground works is strongly dependent on the massif permeability and the existence of groundwater flows. Cold waters coming from high infiltration zones have a refrigerating effect on the massif. Thus, measuring water temperature during drilling constitutes a prediction tool for eventual water inflows. Two cases are possible: the observation of a local thermal anomaly due to a very localized aquifer zone, or the decrease of the water thermal gradient due to diffuse water inflows in the massif. Local thermal anomalies, correlated with large water inflows along discrete zones, have been shown in the Simplon, Mont-Blanc and Gothard-AT tunnels. Such thermal anomalies can be measured hundreds of meters before the intersection of the tunnel with the aquifer zone: temperature monitoring thus constitutes a prediction tool for large water inflows localized in a particular aquifer zone. The use of 3D numerical simulations allows one to improve the prediction quantitatively, by taking into account the problem geometry, the heterogeneity and anisotropy of the thermal and hydrogeological properties of rocks, and the boundary conditions. The comparison of water thermal gradients at the massif scale with linear discharge rates in the tunnels through the massif allows us to determine a mathematical relationship between these characteristics of the massif. This relation permits one to predict the water quantity expected during drilling, knowing the water thermal gradient. These results show that water temperature measurements during drilling of an underground work constitute an efficient and cheap predicting tool for water inflows. Anomalies due to relief must be taken into account; these can be very important in such mountainous massifs. A 3D modeling of heat transfer in the massif is, in all cases, necessary to improve the precision of predictions.