Hard going

In an effort to alleviate the high traffic volumes entering Lugano's city centre the 2,63 km long Vedeggio-Casserate road tunnel under the Trevano Hill, Switzerland, has started, with a completion date of mid-2010.

The tunnel's geology – 2370 m in rock, 260 m in soft ground – meant constructing a 70 m long, 28 m wide excavation pit at the tunnel's eastern entrance in soft ground. From here the tunnel was advanced using drill and blast.

For the pit Bauer Spezialtiefbau Schweiz, part of the Consorzio Jetting Lugano Joint Venture, constructed secant bored pile retaining walls – 362, 1m diameter piles were drilled to depths of up to 31 m. The walls were anchored back with up to four levels of anchors. The work, which took four months, used two Bauer heavy-duty rotary drilling rigs – a BG 40 and a BG 28.

During construction of the pit, considerable difficulties were experienced when Bauer encountered two artesian groundwater horizons, which resulted in significant artesian pressures. In addition, boulders up to 4 m in diameter, had to be drilled through at depths up to 28 m.