Reconnaissances pour le projet de tunnel ferroviaire Maurienne–Ambin (52 km), reliant France et Italie: adaptation des méthodes et modifications de tracé

Resumé

Cet article a pour but de présenter l'évolution des reconnaissances géologique, géotechnique et géophysique menées sur un projet de tunnel, à travers l'exemple concret du tunnel d'Ambin, maillon clé de la partie internationale du projet de nouvelle liaison ferroviaire transalpine Lyon-Turin. A travers la description et les commentaires des résultats obtenus par plus de 80 forages (de 50 à 1520 m de longueur), plus de 100 km de sismique de surface et de la sismique de puits réalisée dans une trentaine de forages, nous montrons la progression et l'adaptation des reconnaissances dans une suite logique d'investigations. Nous présentons également les adaptations apportées en cours d'investigation, entreprises en temps réel au vu des premiers résultats, ainsi que les conséquences sur l'évolution du tracé prévisionnel du tunnel et de certaines galeries de reconnaissances.

Abstract

 This article presents the evolution of a geological, geotechnical and geophysical survey system for tunnel projects, using the Ambin Tunnel as an example. After a short introduction to the Lyons–Turin project, the paper describes in chronological order the systems and methods used to obtain geological and geotechnical information. Finally, the paper examines the modifications made to this particular survey programme as a result of the data obtained. The consequences for the evolution of the provisional layout of the tunnel are examined. The project to construct a new Lyons–Turin rail connection is part of the European transport scheme and is a key element in the Trans-European Network. It is one of the 14 priority projects of the European Union. This new link is of great importance for passenger transportation between France and Italy in terms of the potential reduction in journey time; the travel time from Lyons to Turin, instead of the current 3 h 55 min, being reduced to 1 h 20 min. It will also accelerate the transportation of goods. Thus, it will provide the best solution to the problems of transalpine traffic congestion and enhance environmental protection. The key element in the international section of this new link is the Ambin Tunnel, which is over 52 km long, running between Saint Jean de Maurienne (France) and Susa-Bussoleno (Italy). The tunnel will go through the main geological units of the Western Alps from the ultra Dauphinese area (external zone) to the Piemontese area after cutting through the Brianconese reliefs (inner zone). Since 1990, an extensive site investigation survey has been conducted by the SNCF (the French national railway company) and the FS (the Italian 'Ferrovie dello Stato'). Since 1994, this survey has been undertaken by the European Group of Economic Interest, Alpetunnel GEIE. The investigation aimed to accurately define the layout of the tunnel and to reduce as many uncertainties as possible with regard to conditions, time and cost of building. The survey initially utilised information on ground conditions from detailed geological maps and air photographs, allowing the construction of a provisional geological cross section of the tunnel. This cross section highlighted a few areas that were poorly understood. These areas were then studied using seismic refraction and reflection, often in tandem with the drilling of boreholes for increased accuracy. Many boreholes have been drilled to determine the geotechnical characteristics in addition to understanding the geology. Many tests and in-situ measurements have also been undertaken to study the hydrogeological and geotechnical conditions in the mountains that will be bored through. In 1999 and 2000, further drilling will be undertaken in areas requiring more detailed study. Also two directional drilling projects will be started vertically and then run obliquely until reaching the horizontal. This will produce more than 1000 m of horizontal core, allowing a better knowledge of the nature of the ground along the axis of the tunnel to be gained, including geotechnical measurements, hydrogeological data and in-situ geotechnical conditions. Later on, these studies will be extended to allow the construction of three or four survey galleries with the same dimensions as the tunnel in order to check and test the in-situ rocks. Some surveys have been changed midstream as a result of initial results. For example, deeper drilling was undertaken following the realisation that a seismic reflector was present a few metres below . It has also proved necessary to change the layout of the survey gallery at Modane and to change its cross section after the results of the first set of boreholes were analysed. Layers of karst, water-filled cavities, soft ground and flowing water were all found. The layout of the tunnel itself has been changed in several places following the results of the boreholes, seismic data or geotechnical tests. Examples include the first mile of the French side, several miles in the Brianconese coal measures, and the underground service installations. These results have led to an increased construction of survey galleries.