Constructing a double-lined water supply tunnel in central Italy

The 8km long Chiascio water supply tunnel is designed to draw off up to 20m3/sec from the 200M m3 multi-purpose Valfabbrica reservoir on the Chiascio river in central Italy. It is part of a large water scheme which is to permit irrigation of 65,000ha of agricultural land in the Tiber valley between the Umbrian historical cities of Perugia, Todi and Spoleto.

The use of full face shielded TBMs for mechanised excavation and installation of precast reinforced concrete segment lining rings immediately behind the machine is, nowadays, a standard choice for the construction of long tunnels.

At Chiascio, high pressure water tunnel excavation was followed by the first phase lining: a 20cm thick segment lining designed to withstand TBM and outer rock mass pressures. This had to be followed by an inner, second phase, 30cm thick strongly-reinforced concrete shell designed to resist the operational pressures from both inside and outside the tunnel.

Excavation of the circular 4.55m diameter cross-section and first phase precast 20cm thick segment lining was completed on 9 September 1999, when the Lovat RM184SE broke through the slope face at the reservoir end of the tunnel. Production was an average of 20m/day, reaching a maximum of nearly 50m/day in a marly sandstone formation of varying strength.

Lining of the tunnel in the second phase is now in progress. A great amount of steel is required for reinforcement because of the high internal water pressure during operation: up to 6bars of static head plus 1bar for water hammer. Steel bar placement was accelerated by using pre-welded steel mesh panels. The outer layer consists of six panels of 2-2.8m. The inner layer is similar but with one smaller panel. The 8mm diameter steel bars were placed at intervals of 5-10cm. Zinc coating is used for protecting the steel reinforcement from possible cracking of the concrete lining. Although there is a low probability of this happening, it is possible due to the internal pressure and outer anisotropy. Close spacing and small diameter bars were adopted to limit the size of concrete cracks. Apart from greater quality assurance and precision in placing the bars, the system adopted has more than doubled the forecasted production rate: from 18m/day with traditional reinforcement to 35-40m/day with pre-welded steel mesh, of which 47tons were put in place in a single day.

Geodata, an international geo-engineering company specialising in foundation and underground structures, was responsible for the detailed design and construction follow-up on behalf of the contractor De Lieto of Naples, Italy.