The works being carried out by the SELI/MONTI consortium for client Rete Ferroviaria Italiana between 121+038km and 123+885km, will upgrade the existing railway infrastructure, the realignment required in order to direct the railway line away from the Busa del Cristo area, where there is severe slope instability.
The tunnel is 2.71km long and maximum cover is 520m. The ‘S’ shaped alignment below Monte Zucco has a constant slope of 2.4%. Excavation, which started in September 2001, proceeds downhill from the northern portal, located in the Pieve di Cadore municipality, to the southern portal in the Valle di Cadore municipality to the left of the Boite stream. The realignment joins the existing railway route near the two portals.
Some 10 years ago, a 3.7m diameter pilot tunnel had been excavated for a length of 450m from the southern portal and abandoned.
The geology of the Monte Zucco hill is complex with sedimentary outcrops of the Triassic period overlying a crystalline metamorphic basement. The tunnel crosses a number of formations as follows:
The Durrenstein formation (Upper Carnian) formed by dolomites interbedded with micritic limestone. These only marginally affect the works;
The overlying dolomites are of two types: The first comprises light-coloured stromatolitic dolomites alternating with well-bedded massive micro-crystalline dolomites. The second is formed by brownish-grey micritic limestone in 10m layers;
The Raibl formation, through which the tunnel passes, is represented by an upper member formed by gypsums and yellowish-white anhydrides, by very pitted dolomites and grey and black-white brecciated limestone and by a lower member formed by conglomerates, sandstones, marls and marley clays.
The contact between the Raibl formation and the Main dolomites is being crossed under the maximum cover. Both formations are in Class III, according to the Bienjawski Rock Mass Classification. In fault zones, the presence of swelling ground with very high water inflows, the RMR index can fall to a value of 20, reducing the rock mass classification to Class V.
Tunnelling conditions
The geology along the tunnel was favourable for full-face mechanical excavation. The excavation system would have to provide the highest safety for the operatives as well as reducing to a minimum the risks associated with geological uncertainties.
The type of TBM was defined on the basis of the geology to be encountered and the critical situations which could be met during excavation of the tunnel i.e. karst phenomena, water under pressure and high ground cover. The differences between the limestone of the Raibl and dolomitic formations, which characterise the rock mass to be tunnelled, led to the assumption that there would be voids (or karst phenomena) which could be very large and filled with water, mud and other deposits. Since it is impossible to predict the presence of voids in advance, they represent a risk to safety and progress of excavation.
Identifying voids in advance is therefore a mandatory operation for the machine, which is equipped for drilling systematic and patterned exploratory holes ahead of the face. There is, alas, a requirement to deal with any karst conditions encountered. The TBM has to be equipped to treat the various types of ground by consolidation or filling prior to the passage of the machine. In addition, the TBM must deal with the effect of any remaining sludge deposits, as well as pressurised water tables.
Selection of TBM
The type of TBM was selected for two main reasons – its compatibility with the TBM specification requested for the project; and the availability of a suitable used machine, because the construction schedule did not allow enough time to manufacture a new machine. Although not specified for the project, the selected TBM is an EPB machine with an excavation diameter of 8.03m, suitable for supporting the face with the use foam or polymers.
For face confinement, EPB technology uses the excavated material, which is mixed with foam and accumulated in the chamber behind the cutting head to form a mass which is maintained at a controlled pressure to balance the soil or water table pressure, thus ensuring the stability of the face. The pressure in the excavation chamber is maintained by controlling the opening of the screw conveyor which regulates the flow of the spoil to the muck-out system. The TBM in this case is completely separate from the excavation environment. The shields and lining enable a differential pressure of 5 bar to be maintained between the inside and the outside of the shield.
The TBM selected was manufactured by NFM Technologies FRAMATOME, under license from Mitsubishi Heavy Industries, and was previously used on the ‘Passante di Milano’ in Italy. Modifications to bring the used machine up to specification were jointly developed by NFM and SELI technical staff. The machine and back-up system were assembled and tested at SELI facilities in Aprilia, near the company’s head office in Rome.
Description of tunnelling machine
The main characteristics of the machine are:
- Cutterhead – This is a four-arm type with an opening ratio of about 30%. It has 52 disc cutters
interchangeable with ripper teeth and is fitted with 220 cutting tools.
- The main drive system – This has a rated power of 2,000kW (ten 200kW variable displacement pumps and 14 hydraulic motors); the maximum torque is 15,000kNm at 0-1 revs/min, with 5,125kNm torque at the maximum speed of 2.4revs/min.
The two-part shield is articulated by means of eight 1,400kN jacks. This enables the machine to carry out corrections to the alignment, down to a minimum radius of 150m. The maximum operating thrust is 91,350kN at a maximum pressure of 500 bar in the hydraulic circuit (exceptional operation), with a normal thrust of 63,800kN at the standard pressure of 350 bar. Three rows of brushes are fitted at the rear of the shield. These are filled with grease to prevent flow of grout between the shield and the segments. A row of steel paired scales is also installed toward the excavation profile.
The 165mm annulus between the segment and the excavation is continuously filled with grout injected through the rear shield by means of six pairs of 50mm pipes (six used and six spare). The pressure is automatically controlled at the injection points. The ring type segment erector uses a mechanical clamp-on screw fixed to the reinforced concrete segment.
The back-up train is around 100m long and is formed of 10 decks holding the ancillary equipment. The decks are rigidly connected to the EPBM and follow its advance. Personnel, materials and muck are transported to the surface by two trains for each 1.5m advance stroke. The trains are powered by a locomotive with a 600hp diesel engine weighing 45t; an upper loading bed for transporting lining segments; a flat-bed pick-up and a loading area for the remaining lining segments; an 8m3 silo for transporting the grout; and six 23m3 muck cars for removing spoil. The train is positioned inside the back-up where segments are unloaded by lifting gear and placed in the segment conveyor.
Lining the tunnel
A 350mm thick ring, consisting of seven reinforced pre-cast segments, is being used. The universal type ring is able to follow the curves along the tunnel (R=600m) as well as the straight sections, and permits correction of any deviation by the machine.
The main characteristics of the tunnel lining are: internal radius 3.5m; external radius 3.85m; average length of the ring 1.5m. The ring is completed by an invert element, with a flat upper side, which is placed on a mortar bed on the tunnel invert.
Water tightness of the lining is obtained by EPDM gaskets in each segment. The seals are stuck to the surface of the concrete and provide water tightness at hydraulic pressures of up to 6 bar, which is the maximum design pressure. The connection between segments of the same ring and between adjoining rings is by means of bolts sloping at 60° to the surfaces to be connected. The bolt, once inserted in a segment, is tightened to a plastic insert embedded in the adjoining segment
Organisation of works
The activities carried out on site are mainly the tunnel excavation and related works (spoil disposal, production of grout, and so on). Activities were carried out initially in three 8h shifts/day for five days a week, and subsequently in four shifts, enabling the works to progress without interruption, in order to meet the very tight construction schedule. The operations are carried out by 12 workers per shift, eight working underground and four on the surface.
Conclusions
Apart from the initial period, when the setting up of plant and training of personnel influenced progress, the average daily advance is 10m, with peaks of 12m. A total of 700m has been excavated to date, more than satisfactory considering the challenge presented by the geological conditions along the alignment, as well as local phenomena (faults, swelling ground, high water inflows and karst phenomena). The solution of overhauling and upgrading an existing TBM was a sound precaution for increasing the level and reliability of tunnelling operation.
The selection of the ‘universal ring’ instead of the proposed left and right tapered rings is an example of keeping up to date with new technology to ensure design and operational activities meet the highest standards.
Related Files
Figure 2: Geology along the 2.71km alignment
Figure 3: Cross section of the Monte Zucco Tunnel showing segmental lining design
Figure 1: The alignment of the Monte Zucco Tunnel
