How to predict rock temperatures for deep Alpine tunnels

Abstract

Reliable assessments of the underground temperature are needed for construction projects like deep tunnels, shafts and storage facilities. In areas of pronounced topographic relief special attention must be given to the influence of three-dimensional topography on the subsurface temperature field. Futher parameters needed for the prediction include ground surface temperature, local heat flow density, geological factors (structure/schistosity, thermal conductivity, erosion rate, water circulation). A first prediction was attempted for the planned Gotthard railroad tunnel (NEAT). This tunnel will have a total length of 56 km with a maximum cover of 2500 m. The temperature predictions were calculated, based on the Rimini topography array, for points every 1 km along the planned axis. Maximum temperature (conservative upper limit) is about 45°C; the error of the predicted temperatures could be, in view of the uncertainties involved, in general ca. ±5−10°C. Since little is known at present about the deep water circulation system in the realm of the planned Gotthard tunnel the prediction calculations assume heat transport by pure conduction. Significant subsurface water flow would mainly lead to the reduction of rock temperatures by cold infiltrations from the surface as demonstrated by observations in the Simplon and Mont Blanc tunnels. In fact, the deviation of actual measurements (performed right behind the advancing face) from “conductive” predictions should be used as a diagnostic tool of large water flows which could be encountered during tunnel construction.