The paper presents results of stress measurements made in exploratory boreholes for the future base tunnel of the high speed railway line Lyon-Turin (HTPF method). With respect to expected values due to gravity forces alone, the results globally show a shortage of vertical stress value especially marked in the heart of the Ambin massif, and an excess of horizontal stress values. The analysis of these results has been performed by using numerical 2D and 3D modelling. It shows the respective influence of local relief consequences of more global tectonic stresses applied to the massif.
L'article presente les resultats des mesures de contraintes effectuees dans les sondages de reconnaissance pour le futur tunnel de base de la ligne TGV Lyon-Turin (methode HTPF). En reference aux valeurs attendues des seulesforces de gravite, ces resultats font apparaître globalement un deficit de contrainte verticale, particulièrement accentue au ceeur du massif d'Ambin, accompagne d'un excès de contraintes horizontales. L'interpretation de ces resultats à l'aide de modelisations numeriques 2D et 3D montre le rôle respectif des effets locaux de relief et d'un effort tectonique plus generalise applique au massif.
Dieser Artikel liegt die Ergebnisse der im Rahmen der Erkundungsbohrungen des kuenftigen Eisenbahntunnels der Hochgeschwindigkeitslinie Lyon-Turin ausgefuehrten Spannungsmessungen vor (HTPF Methode). Im Vergleich zu den erwarteten Werte der einzigen Schwerkrafte stellen diese Ergebnisse ein Defizit der vertikalen Spannung, insbesondere in dem Ambin Gebirgskernvund ein Übermaß der horizontalen Spannung aus. Die Auswertung dieser Ergebnisse mit numerischen 2D- und 3D-Modells stellt die örtlichen Wirkungen des Reliefs und der am Gebirge verallgemeinerte aufgewandte tektonische Kraft aus.
The 52 km base tunnel of the future Lyon-Turin railway link will be excavated through different geological formations with various tectonic contacts. The overburden of most of the layout is higher than 1000 m and it reaches 2000–2500 m in the Ambin massif, which consists of a regular gneiss-micaschistic structural dome across the French-Italian border. The knowledge of the in situ stress field is fundamental for the tunnel design, especially in cases (such as this one) where excavations are foreseen under high overburden in complex tectonic structures. In fact, the ground response to excavation and the occurrence of such phenomena as rockburst, bulking and squeezing depends highly on the in situ stress field.
The concern to reduce the time-cost uncertainties in the project led Alpetunnel GEIE, in charge of the tunnel design, to carry out stress-measurements with the HTPF (Hydraulic Testing on Preexisting Fractures) method, in IS boreholes. The results of these tests show rather scattered information on the magnitude and orientation of stress vectors. The interpretation of such results needs complex analysis of topographic and geological context. For instance, stress measurements completed on another site near the Arc river valley, where 8 HTPF measurements were available, exhibited, generally major principal stresses horizontal and perpendicular to the valley (Fabreet al. 1996).
In this paper we concentrate on the Ambin massif, where four boreholes have been tested with HTPF:
F16 in the Ambin valley (1525 m long), started vertically, but deviated with a final 40° inclination.
