Mesure et modélisation des contraintes naturelles. Application au projet de tunnel ferroviaire Maurienne–Ambin

Résumé

Dans le cadre de la conception d'un futur grand tunnel à travers les Alpes, des mesures de contraintes naturelles ont été faites dans 13 forages profonds, dans des contextes géologiques et topographiques variés. L'article en propose une analyse critique en insistant sur l'influence des méthodes d'interprétation employées, et s'appuie pour les interpréter sur des modélisations simplifiées prenant en compte la topographie et la tectonique. Malgré la dispersion des résultats, trois grands types d'états de contraintes se dégagent. A proximité de la vallée de l'Arc, les contraintes horizontales sont fortement anisotropes, orientées suivant la vallée et les contraintes verticales sont fortes, ce qui traduit un effet topographique classique. Sous deux vallons du massif d'Ambin, les contraintes horizontales sont anisotropes et supérieures aux verticales. Enfin, sous la retombée orientale de ce massif, on trouve plutôt un état de contraintes de "type K₀". Tous ces résultats sont en bonne concordance avec les modélisations présentées, à l'exception des faibles contraintes verticales du massif d'Ambin.

Abstract

 The 52 km running tunnel of the proposed Lyon–Turin railway link will be excavated through different geological formations across the Franco-Italian Alps. The overburden of most of the route is greater than 1000 m, and it reaches 2500 m in the Ambin Massif, a regular micaschistic dome. As part of the reconnaissance program for this project, stress measurements using the HTPF (Hydraulic Testing on Pre-existing Fractures) method have been carried out in 13 deep boreholes, which provides a unique opportunity to better understand the regional stress field. After a presentation of the HTPF method and of its possible interpretation methods, the paper presents and discusses the results obtained. The influences of topography and tectonics are highlighted with the support of numerical modelling with the Finite Difference Method (FLAC code). Despite a great scatter of information on stress values and directions, three main stress regimes can be distinguished in three different geological-topographical contexts. The first one is found near the Arc valley, which is the main topographic feature in the area: the vertical stresses are greater than the weight of overburden and the horizontal stresses are highly anisotropic, σ_h being approximately perpendicular to the valley. This suggests a strong topographic influence of the Arc valley, especially for those measurements closest to its axis. Two other regimes have been encountered in the Ambin Massif. On the one hand, in the heart of the massif, under two small valleys, horizontal stresses are anisotropic with σ_h parallel and σ_h perpendicular to the valleys; but curiously vertical stresses are nearly 20% lower than the weight of overburden and are in both cases the minor principal stresses. On the other hand, under its Italian border and in a slope situation, the measured states of stress are those classically obtained in soils: vertical stresses are equal to the weight of overburden and horizontal stresses are nearly isotropic, equal to the half of the vertical ones. In order to better understand these measurements and their relation to the regional state of stress, a series of numerical calculations of increasing complexity were undertaken. They all take into consideration the effects of both the simplified topography and, in a parametrical manner, the tectonic stresses. The results are consistent with most of the global trends presented above, and particularly for the first and third stress regimes. For the second one, however, in the heart of the Ambin Massif, modelling gives only a first qualitative explanation for the low measured vertical stresses.