This paper will provide an overview of mechanized tunnel boring methods implemented at major alpine railway tunnel construction projects in the past years. A focus will be on the Amsteg/Erstfeld Lot (Gotthard Base Tunnel in Switzerland) and the Koralmtunnel Lot 2 in Austria. The lots were both driven through crystalline geological formations using open-gripper hard rock TBM's at Amsteg/Erstfeld and hard rock double-shield TBM's at KAT2. The difficult ground conditions for both drives will be described as well as solutions for mitigating these difficulties. The performance of the different machine concepts in the encountered ground conditions will be compared and illustrated. Another challenging aspect of handling difficult and frequently varying ground conditions is the contract execution during these trouble areas.
Since the 1990's the European Union, Switzerland, and member countries have made large investments in the railway infrastructure for areas which lie in the heavily mountainous alpine region. The goal is to reduce the transport costs and, in addition, the increasing pollutant emissions from transport trucks by transferring the traffic flow from the roads to the rails.
The above-mentioned projects were successful with regard to the type of tunneling, be it conventional or mechanized, as well as overcoming the challenges of ground conditions, associated support processes during construction and the construction sequence.
Mechanized tunneling (TBM) is applied generally to longer tunnels with homogeneous geologies, whereas drill-and-blast (conventional) is better suited for varying geological conditions or otherwise in TBM start areas, caverns, cross-passages, accesses, and connecting structures.