UNTERSAMMELSDORF TUNNEL - CHALLENGES, SPECIAL MEASURES AND USE OF SPECIAL SHOTCRETE DURING TUNNELING IN LACUSTRINE CLAY

摘要

The 665 m long, twin-track Untersammelsdorf Tunnel is being built in the course of the construction of the Koralmbahn line between Graz and Klagenfurt. The tunnel is located in extremely challenging subsoil conditions consisting of silty to fine sandy lacustrine deposits. The developed tunneling concept provided wide-ranging special underground engineering measures as piling, jet grouting and temporary struts and represents a unique construction method to date. Accordingly, numerous challenges arose in the design phase, which could be verified by carrying out extensive trials and investigations in the preparatory period or for which fallback levels had to be provided for the support system. In order to guarantee the stability of the face, two to four jet grout columns were provided at a longitudinal spacing of 2.03 m (see figure 1). A challenge was the transfer of vertical forces from the jet grout arch into the bored piles due to the lack of reliable parameters for friction and cohesion forces in the joints between jet grout body and bored piles. Therefore load transfer from the shotcrete lining through notches in the bored piles was provided as an additional measure (see figure 2). In order to transfer concentrated loads through a relatively small area, technically correct construction according to the design of the shotcrete lining in the notch was essential. But precisely here in the bearing area there was a danger of honeycombing due to rebound and spraying through several layers of reinforcement. Therefore a special shotcrete has been developed and testiest in extensive trial spraying. As the bench and invert were advanced, the 10 to 12 m long unsecured excavation area represented the critical loading case for the bored piles at the sides. In order to avoid heavy bending reinforcement in the bored piles, dewatering boreholes were drilled to reduce water pressure acting from outside and temporary bracing between the bored piles was planned in some sections (see figure 3). Now tunneling work is complete, the experience can be reported.