Alptransit: Engineering Geology Of The World?S Longest Tunnel System

The Swiss AlpTransit System (also called NEAT) is an important element of the new European high speed railway network. This system, which is currently under construction, consists of two railways axes- the Gotthard and Lötschberg Axes – which will pass through the western and eastern parts of Switzerland. Each of these axes consists of 2 to 3 base tunnels, the longest being the two-tube Gotthard base tunnel (57 kms in length), which is currently the world's longest tunnel under construction. Within this system, an existing base tunnel will be used (the Simpson base tunnel), a second is to be located in the pre-alpine foreland (the Zimmerberg tunnel), and the remaining three are to built within the Alpine region (the Gotthard, Lötshberg and Ceneri base tunnels). These final three tunnels are of notable concern since the rugged topography in this young mountain belt reaches altitudes of up to 3000 m near the tunnel axes, resulting in an overburden of up to 2500 m. Here the new base tunnels will intersect many of many of the tectonically deep units of the alpine mountain chain: Mesozoic to tertiary sediments and the crystalline basement of the helvetic and penninic domain. During alpine orogeny these rocks were strongly deformed into complicated tectonic structures and metamorphosed at variable grades. While the longest sections of the base tunnels will be drilled in fairly competent ground, the AlpTransit project will also be confronted with a large variety of geologically controlled hazards. The most important of these include: high water inflows along faults and karst cavities, inflows of rock debris and surficial disturbances through drainage effects. The results from these investigations are presented in this paper together with an overview of the engineering geology of this unique tunneling project.