DSI Hollow Bars and Lattice Girders secure Driving of the Sten Tunnel

Skofja Loka, a town some 25km northwest of the Slovenian capital of Ljubljana, looks back on a long tradition. First mentioned in historical records in 973, the town was rebuilt after a major earthquake in 1511. Even today, the town is largely conserved in this style and ranks amongst the most significant medieval cities of the area. The construction of a 3.9km long bypass was chosen to shield the many medieval buildings and the inhabitants from the steadily increasing traffic volume.

Key projects on this bypass are several bridges as well as the 613m long Sten Tunnel. Attached to the actual tunnel, there are galleries on both sides, extending the total tunnel structure to 712m. Tunnel driving was performed by drill-and-blast and using excavators in accordance with the New Austrian Tunneling Method (NATM). DSI supplied R32 DSI Hollow Bars together with the required injection pumps for the starting gallery and the initial support. In addition, DSI provided three 70/30/20 Lattice Girders. The girders were installed in combination with shotcrete for the immediate support in the excavation area along the entire length of the portal. In contrast to standard steel sections (solid-web girders), Lattice Girders are entirely integrated in the shotcrete lining, avoiding porous zones and shotcrete spray shadows. The owner decided to also use DSI Hollow Bars for the support of the tunnel because their characteristic application as injection anchors without the need for casing makes them very suitable especially for non-stable boreholes. The self-drilling installation is done using the anchor bar as a drill rod with a single-use drill bit. According to the requirements during installation, the anchor lengths can easily be extended using couplers. Work on the new Sten Tunnel was successfully completed mid-2012.