JACK IT UP

This article reports a new tunnelling method, using pipejacks, which has been devised for an underground station in Milan. The ground below Milan's surface has never been easy to tunnel, as it is an uneven mixture of fills, sands and alluvial deposits, usually unconsolidated and fairly unstable. The water table begins a few metres down. In the past, cut and cover could have been used for tunnelling through such ground, but, today, such methods would disrupt street traffic too much. Thus proper tunnelling methods had to be found for the underground railway link across Milan, northern Italy's busiest city. The 23 km of tunnels will connect the northern and southern regional railway network in a U91.5 million unification scheme. Shields and the New Austrian Tunnelling Method (NATM) will be used for the 8.8m diameter railway tunnels, but other methods are also needed for some of the station works. One of the techniques used is to construct a station arch from multiple tubes formed in the ground, before excavating the main chamber. Concrete arches, which later support this tubular roof, are built before main excavation by working outwards from the tubes. A starter pilot tunnel and initial side tunnels are also built then.