German contractor Bilfinger Berger has successfully completed excavation of the 1.8km long, twin-tube Steinhaus road tunnel in Semmering, Austria. Breakthrough of the second tube was achieved in October after two years of drill and blast tunnelling utilising a top heading, bench and invert advance through chalk, phylitte, calcite and quartzite.

The tunnels, each with a full cross sectional area of 75-90m2, were driven using a fleet of three Atlas Copco Rocket Boomers 352s and one Rocket Boomer L2 C for blasthole and rock reinforcement drilling. In the weaker ground near the portals 10-20 blastholes were required in the top headings, increasing to 80-90 through the alignment’s more competent rock. Due to the relatively soft ground, each advance was restricted to 1.5m-1.7m followed closely by a strict support regime. This consisted initially of up to nine x 5.3m long Atlas Copco self drilling anchors with mortar injection, followed by 1.3m long grouted rebars inserted into 33mm diameter dry drilled holes and self drilling rock bolts. Finally three to four arches were placed at 1.5m intervals during the 24hr cycle of three shifts.

A unique feature at Steinhaus was the central pilot tunnel at the west portal within which the buttress separating the two main tubes was cast using self-compacting concrete. The buttress was designed to replace the local ground that was too weak to support construction of the inner flanks of each tube.

The two lane unidirectional road tunnels, with three 57m2 cross sectional area cross-passages, are being constructed for the Austrian Highways Authority Ösag as part of the new S6 highway through the ski resort of Semmering.