Turin, Italy’s third largest city, is currently building its first metro line. The planned Line 1 alignment, which will run completely underground, will connect the western city of Rivoli to the southern neighbourhoods of Lingotto, where the main Turin exhibition centre is located, and further south to the Nichelino Municipality. The route will also connect the city with major transportation connections including the planned Lyon-Turin High Speed Railway.
The construction of Line 1, much of which needs to be operational by the start of the Winter Olympics being hosted by the city in 2006, has been divided into several consecutive phases to ease the tight schedule (Figure 1). So far, only the first two have been implemented.
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With a civil works investment of US$500M, this is the largest ever single work investment undertaken by Satti, the City Metropolitan Transport Operator. The Italian Transportation Ministry (60%), Piedmont province (8%), Turin Municipality (25%) and Satti (7%) share the project funding.
The total length of the line is about 25.2km and will include 31 stations, with an average inter-station distance of 750m. As mentioned above, the construction has been divided into various phases:
The underground section of the first phase is about 9.1km long, 8.5km of which will be in 6.9m i.d double-track, single bored tunnel, plus a short, 750m long, cut and cover section connecting the depot (Deposito) to Fermi station.
Design
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By GlobalDataThe design and works engineering supervision of phase one has been awarded to the JV of Systra and Geodata. The latter is an international geo-engineering firm based in Turin with wide experience in the design and supervision of large underground works and urban tunnels, including the Porto Metro (T&TI, December 2002, p16).
Design of Phase II is currently being undertaken by a JV of Systra, Geodata, Metropolitana Milanese (Italy) and Studio Quaranta (Italy).
The line’s route has been designed to run almost entirely under the city’s major avenues and streets, avoiding the city centre where over 2500 years of history is buried. Station design has been optimised to limit surface disturbance and interference of the 100 year old trees, which line the city Boulevards and are a major environmental concern for both the Client and the Municipality.
Design and construction methods
Stations
A standard design of 20m wide x 60m long has been adopted for all the underground stations, apart from the interchanges at Porta Susa and Porta Nuova railway stations. Due to limited space, the stations have been designed to maximise the use of what open space they have, thereby reducing any feeling of claustrophobia for users. The platforms will also been equipped with sliding doors for safety reasons and to avoid the unpleasant piston effect of an incoming train.
All the stations along the Boulevards have been, or are in the process of, being excavated by a top-down construction system, adopted to limit surface disruption and land requirement. Due to the geological conditions and the presence of the water table and a conglomerate strata, the use of hydromill technology has been widely adopted. Slurry walls 1m-1.2m wide and 26m-34m deep have been constructed and the bulk excavation of the station carried out below the roof slab to the bottom level, which, in some cases, has been pre-consolidated from the surface to improve waterproofing and avoid siphoning. Italian manufacturer, Mapei, is supplying contractors Metrocentro and Metrotre with admixtures for both the wet and dry shotcreting systems, used during station construction.
A grouted, consolidated block is provided at each end of the stations to provide a stabilised tunnel eye for safe TBM breakthrough and to ease TBM passage through the stations. Special care has been taken in the excavation of station slurry walls to avoid damaging the old trees that line the Turin avenues. A special ecological commission from the Municipality follows the works and takes care of the trees in cooperation with contractors. The construction of the stations along the major traffic arteries of the city could have generated major disruptions to the traffic. However, a careful information campaign and detailed diversion management has succeeded in keeping the disturbance at a low level, much appreciated by the people of Turin.
Shafts
The tunnel shafts have been designed to provide ventilation and emergency access points midway between the line’s stations. A standard 6m diameter circular shape has been adopted down to tunnel level. Hand-mined connection galleries varying in length from 5m-24m will link the shaft bottoms to the running tunnel. Dewatering facilities will be installed at shafts located at low points between stations. In some cases ventilation shafts are used to carry out the various ground treatment operations. In the case of Lot V, four shafts are also provided solely for treatment purposes.
The shaft construction design is based on the use of micro-piles (200mm-220mm diameter), with steel rib and shotcrete temporary support, followed by a permanent concrete lining. As per the stations, pre-excavation grouting will insure water tightness during excavation where the highly permeable ground, coupled with the presence of water, would make excavation difficult, if not impossible.
During construction of the connecting galleries (between the shaft bottom and the tunnel) above the water level, standard mining methods work well due to the strength of the conglomerate. In other cases, the water level is lowered to allow the excavation of a short treatment tunnel above the planned connection gallery. From here grouting with TAM pipe is carried out to consolidate the ground prior to the passage of the TBM constructing the running tunnel and to ensure the safe excavation of the connection gallery between the running tunnel and the shaft.
Tunnels
The bored tunnel length has been divided into three lots with an average length of 2.7km, requiring the use of three TBMs. This was due to the tight project schedule, that requires the the stretch between Collegno and Porta Nuova to be in operation by November 2005. An average advance of 10m/day was therefore planned for each lot.
Although the contracts left the freedom to choose between slurry and EPB shields, all contracting JVs preferred the use of EPB, considering it better suited to the existing geological and environmental conditions.
The design calls for the use of 6+1, 300mm thick reinforced segments for the tunnels’ single pass lining, in this case using bolted segments with standard EPDM gaskets. On average the reinforcement quantity is about 80kg/m3 and a universal tapered ring (72mm taper) has been adopted. Turin ready mix company, Calcestruzzi SpA is supplying the pre-cast segment concrete, with Mapei providing the concrete’s modified acrylic based admixture. The 1.5m wide ring, combined with its conical shape, allows the lining of the maximum 180m radius curve needed by the alignment.
Despite the high tunnel gradient, up to 6%, spoil removal from the back-up is being carried out with four muck wagon trains (22m³ each). This arrangement allows muck extraction from intermediate stations, shorter train runs and simultaneous finishing in already passed stations.
According to the contractor, the planned average advance cycle allows 40 minutes for excavation and 30 minutes for ring building, for a total of 70 minutes; resulting in an average excavation speed of about 38mm/min.
Further into tunnelling, the average TBM working time (excavation + ring erection) has been estimated as 32% of the total time, giving an average advance rate of 10m/day. The maximum expected advance rate is about 18m/day, corresponding to a TBM working time of 58% when considering the indicated cycle.
As mentioned before, three TBMs were required in the tender documents, one for each Lot: III, IV and V. After careful selection, two new 7.8m diameter Lovat EPBMs were purchased for Lots III and IV, whilst a refurbished 8m diameter NFM machine is foreseen for Lot V. The tender documents drew attention to several factors for the TBMs design. The alignment’s grain distribution shows the presence of cobbles with a maximum diameter of 400mm-500mm and a very high compressive strength of up to 300MPa with high abrasivity due to their crystalline origin. This lead to the adoption of a disk cutter equipped cutting wheel, with full wear protection.
A state of the art conditioning system was considered necessary to compensate for the lack of fines, in the range of 5%, and the fact that the presence of silts and clays will be limited to 20-25%.
On the other hand, the high variability of cemented and loose strata, combined with its coarse nature, requires a proper dimensioning of machine trusts as well as the need for constant face support to avoid instability. Following the tender recommendations, the contractors and the TBM manufacturer took these important aspects into consideration for the design of the new machines.
Considering the coarse and cemented characteristics of the ground, the new TBMs are equipped with 56 twin tip disc cutters. This particular disc type is deemed more suitable for crushing cobbles and boulders, since the double tip can crush the cobble while avoiding its displacement or rotation.
The foam injection system on the new machines has been designed and manufactured to allow for a wide range of conditioning agents including soap, polymer, water, bentonite slurry and air, which can be independently mixed and injected at the face, in the mixing chamber or along the screw conveyor.
Works progress
The first Lovat TBM started on Lot III from Fermi station in October 2002 towards the Pozzo Strada reception shaft. As expected, work involved a steep learning curve to adequately cope with the encountered ground conditions. During the first metres of excavation the ground behaved like a stable material at the face, but became unstable in the crown. This resulted in a very localised instability that, with low overburden, could have reached the surface.
The coarse nature of the ground, means that establishing an effective “EPB” pressure can be somewhat difficult, due to the lack of a proper supporting “paste”. Tests are still underway with minimum EPB pressure in the crown to limit ground instability above the cutting wheel.
The second Lovat TBM started boring Lot IV in February 2003 (see cover) from Principi D’Acaja to Pozzo Strada.
The third TBM, constructing Lot V, is scheduled to start in July 2003 from a launch shaft 60m from Principi D’Acaja station. The connection between the shaft and the station was constructed after a minor consolidation campaign, using TAM grouting. This TBM will bore all the way to Porta Nuova where an extraction shaft is currently under construction.
Presently the works are on schedule for opening in time for the Winter Olympics Games in 2006.
Related Files
Longitudinal section of the Lot V alignment (see boxed text at end of article)
Fig 2 – The tunnel’s finished cross section showing the pre-cast concrete segment’s configuration
Fig 1 – Map of the Turin metro’s Line 1 showing the three bored tunnelling Lots
