Blind hole

The worsening problem of urban traffic and the consequent need to minimize the impact of new works on road surfaces led MM, in the course of operations on Line 2 in Via Pacini, Corso Garibaldi and Via A. Doria, to try out some techniques of blind hole excavation. The most problematic aspect was linked to the lack of cohesion of the ground, which therefore required preventive consolidation action to allow the ground to support itself during excavation: over the years numerous mixtures have been tried out, taking the utmost care to verify their acceptability in terms of the need to protect the water table: those most widely used today are the cement and microfine cement mixtures, and stable silicate mixtures, which are exempt from the phenomenon of syneresis.

These mixtures injected with ‘tubes-a-manchette’ into loose soil, have been used, particularly in Naples, even in lithoid formations with MPSP technology, to cement the cracks in the tuffaceous mass, rendering it waterproof. Today, especially when it is not possible to operate from street level, consolidation in advance of the tunnel is increasingly used, with the formation of sub-horizontal jet-grouting columns.

The particular difficulty and slowness of the excavation and lining phases of a traditional blind hole tunnel has led to the increasingly widespread use of the TBM (Tunnel Boring Machine): the speed of progress of a traditional tunnel, which provides for successive stages of ground consolidation, excavation and spoil removal, the laying of the ribs, lining with shotcrete, installation of waterproofing, installation of reinforcement cages and then the final spray lining, can be estimated at no more than 2 ribs per day; however, using a TBM that speed has risen to values that can easily reach 15 metres of finished gallery per day.

In 1983, Metropolitana Milanese started the first trials of these machines, which were very advanced for the time, to create the advance shafts on some lotes of Line 3 and the Underground Urban Rail Link, namely the shafts of 3…3.3 m (10…11 ft) diameter, from the inside of which the radiating injection bores were drilled: it was a rather rudimentary machine, open-fronted, lined with wooden ribs and rafters. After a further experiment with a tunnel borer (Westfalia type), also open-fronted, in a single-track tunnel in Naples, all the later Metropolitana Milanese experiments were performed with EPB (earth pressure balance) type machines where the front is kept under pressure from the excavation soil itself, appropriately fluidised by the introduction of surfactants. This technology was used to create the single-track tunnels of the Underground Urban Rail Link from Garibaldi to Bovisa, the tunnels of the Line 1 extension from Molino Dorino to Rho, the tunnels of the lower section of Line 1 in Naples, and now the twin-track tunnel of Line 6 in Naples.