ABSTRACT: This paper presents the results of stress measurements (HTPF method) made in 4 deep exploratory boreholes for the future base tunnel of the high speed railway project Lyon-Turin (Franco-Italian Alps). With respect to the expected values due to gravity forces alone, the results in the heart of the Arebin Massif globally show a marked deficit of vertical stress, and an excess of horizontal stress. The analysis of these results has been performed by using numerical 2D and 3D modeling, to study the respective influence of local topography and of global tectonic stresses
INTRODUCTION The 52 km running tunnel of the proposed Lyon- Turin railway link will be excavated through different geological formations with various tectonic contacts. The overburden over most of the route is greater than 1000 m and it reaches 2000 - 2500 m in the Arebin Massif, which is structurally a regular gueiss-micaschistic dome on the Franco-Italian border (Fig. 1).
The knowledge of the in situ stress field is fundamental to tunnel design, especially in cases (as this one) where excavations are to be made at great depth in complex tectonic massifs. The excavation deformation behavior and the occurrence of such phenomena as rockburst, bulking and squeezing highly depend on the in situ stress field. Efforts to reduce uncertainty in cost and time of the project has led Alp tunnel GEIE, in charge of tunnel design, to make stress measurements with the HTPF method (Hydraulic Testing on Preexisting Fractures, Comet 1986, Wileveau 1997), in 15 boreholes. The results of these tests show a scatter of information on values and directions of stress vectors. The interpretation of such results needs complex analysis of the topographical and geological-geotechnical context. For example, the stress measurements in the Arc river valley, where 8 HTPF measurements are available, generally show (Burlet et al. 1995) the major principal stresses horizontal and perpendicular to the valley axis.