ALPTRANSIT:GEOLOGICAL KEY PROBLEMS OF THE WORLD'S LONGEST TUNNEL SYSTEM UNDER CONSTRUCTION

The political and industrial centres of Europe are being brought closer together by means of a European high-speed railway network. The AlpTransit tunnel system, located in the central part of Switzerland, is an important element of this network. Currently the world's longest tunnel system under construction, the project consists of two lines - the Gotthard and Lötschberg Axes - which will pass through the heart of the Alps. Each of these axes consists of 2 or 3 base tunnels, the longest being the two-tube Gotthard (57 kms) and Lötschberg (35 kms) base tunnels. Construction work of these tunnels began in 1996 and is planned to be finished by the year 2012.

The new tunnels are of notable concern since the rugged topography in this young mountain belt reaches altitudes of up to 3000 m above the tunnel axes, resulting in overburdens of up to 2500 m. Here the base tunnels will intersect many of the tectonically deep units of the alpine mountain chain. Intersected rocks include fractured and karstic limestones and dolomites, evaporitic rocks, clastic sediments, and long section with older metamorphic and magmatic sequences. During Alpine orogeny these rocks were strongly deformed into complicated tectonic structures and metamorphosed at variable grade. Late or post alpine movements have reactivated some of the older fault structures and have generated new fracture sets which, in turn, have a pronounced effect on the geotechnical properties of the host rock masses.

Although the majority of the tunnel sections are to be constructed in rocks of fairly good tunnelling quality, the AlpTransit base line tunnels will encounter nearly every type of geological challenge an engineering geologist can think of: high water inflows along fractures and karst cavities, soil/debris inflows under high fluid pressures, caving and squeezing ground, rock bursting potential and surficial disturbances through drainage effects. Each of these hazards have been investigated during 10 years by surface based investigations and are now explored ahead of the moving tunnel faces. Examples and results from these extraordinary investigations will be presented, in addition to an overview of the engineering geology of this unique tunneling project