A Wirth Tunnel Bore Extender (TBE) machine, broke through the first tube on Zurich’s 4.46km long Uetliberg tunnel in early February. The TBE had been used to widen the heading to 160m2 from a 5m diameter pilot bore.
Wirth’s machine is claimed to be the largest diameter hard rock machine of its kind in the world. The machine cut a section 14.4m x 14.2m, a flattening of the circular shape of the tunnel that gave more favourable characteristics for the lining design. Wirth said this resulted in roughly 200mm less of shotcrete being required to be applied in the roof than if the circular section had been kept.
Excavated through hard molasses that exhibited sudden changes in the strata sequence of sandstone, siltstone and marl, primary support consisted of steelfibre or mesh reinforced shotcrete with rockbolts. A waterproofing layer with membrane with then be applied, followed by a final cast concrete lining.
The machine will now be moved to drive the second tube of the twin tunnels. Amberg Engineering, who also produced the design for the underground works for client, Tiefbauamt des Kantons Zürich, is supervising the US$935M highway tunnel project.
