The TBM Tunnels from Fori Imperiali to Piazzale Appio
The section currently under construction called T3 extends for a total length of about 2.8 kilometers starting from Via dei Fori Imperiali, near Caesar’s Forum, until Piazzale Appio, where the line tunnels cross underneath the existing station of San Giovanni on Line A. Starting about 100 metres before Piazza Venezia, the tunnel lines follow the axis of the via dei Fori Imperiali, passing adjacent to Caesar’s Forum, crossing under the Cloaca Maxima and continuing below the via dei Fori Imperiali, where, in the stretch between the Basilica of Maxentium and the Colosseum piazza at the boundaries of the Velian Hill, the station of Fori Imperiali is located.
The route subsequently passes the Colosseum and heads towards the Celio Military Hospital in Piazza Celimontana, within which a ‘pari/dispari’ railway communication is placed. In this section the line tunnels reach the maximum depth ever reached by the Line C, being in the layer of Pliocene clay for about 15 metres, at about 55 metres from street-level with a hydraulic head at the crown of the tunnel, which runs for about 40 metres.
The line then continues in a south-easterly direction, skirting the church of San Stefano Rotondo passing between the latter and the church of Santa Maria in Domenica. Passing the slopes of the Celio, the line progresses below the Aurelian Walls at Porta Metronia and continues parallel to the Walls until it reaches Piazzale Appio. In this last stretch is placed the Amba Aradam-Ipponio station and the multifunctional 3.3 shaft, respectively located in the garden of Viale Ipponio and inside the gardens of Via Sannio. The tunnels are made by mechanized excavation with a TBM (Tunnel Boring Machine) of the same EPB (Earth Pressure Balance) type and manufactured by HERRENKNECHT. The machines are equipped with a cutter of a diameter of 6.70 metres while the final coating of the tunnel consists of precast segments of 30cm thickness to form an inner ring with an internal diameter of 5.80 metres.
Each precast ring is 1.40 metres long and consists of 6 main segments plus 1 key segment. The rings are of a “universal” type and adapt to 19 different possible positions depending on the radius of the curvature to be obtained. The segments are made in a prefabrication plant by means of a fixed installation with steam curing.
The choice to carry out the mechanized excavation with a TBM of a diameter of 6.70 metres instead of 10.10 metres as foreseen in the preliminary project based on the tender, was made following the stringent design constraints dictated by the Archaeological Superintendence at the end of the first phase archaeological investigations. The Preliminary Project envisaged the location of the stations in surroundings in which it was impossible to perform works excavated entirely in the open air, such as with the Colosseum station, located under the existing Line B structure, and the Amba Aradam station located partly under the Aurelian Walls at Porta Metronia. The net internal section of the tunnel excavated with the TBM with a diameter of 6.70 metres did not allow the contemporary housing of the railway train and its station platforms whereas the use of the large diameter TBM avoided the need to perform, in correspondence to each station, long tunnels with traditional blind-hole excavation in complex hydro-geological contexts, which would also interact with the monumental heritage.
Following the restrictions placed by the Superintendency at the end of the archaeological investigations of the first phase, which required archaeological excavations up to about 15-20 metres from street-level to the landfill bench and prohibiting the use of any consolidation technique and waterproofing of the land in the absence of prior archaeological excavation, the stations have been necessarily relocated in larger settings and transformed into the type defined as “box”. This type allowed the open air excavation incorporating the platforms directly into the building without the need to excavate the tunnels traditionally. After this constraint was overcome, it was preferable to use small diameter tunnels with obvious benefits also in terms of the subsidence that the excavation of the tunnels could induce on the monumental heritage. In this particularly urban area where the T3 section sites are installed it has been necessary to reduce to a minimum, with consequent repercussions on the design and operational choices, the encumbrance of the excavation machines.
The Via Sannio shaft located in the gardens of Via Sannio was chosen for the insertion of the two TBMs and the subsequent power supply of the excavation of the first section between the shaft and the Amba Aradam-Ipponio station, for a length of about 380 metres for each tunnel. The shaft has a length of about 62 metres and a width of 22 metres and, in the final phase it is designed to house some technological rooms functional to the operation of the Line. The dimensions of the shaft are rather reduced compared to what is generally necessary to allow the operation of two machines such as those used by Line C, which have a length of about 100 metres. Moreover, at the same time as being the 2 TBM power supply, from the inside of the shaft the soil consolidation activities also began to go ahead for the construction of the tunnels with traditional excavation as far as the station of San Giovanni. These logistic restraints imposed a series of operative choices aimed at reducing the dimensions of the equipment to be used together with the need to excavate the two tunnels separately.
The two TBMs were assembled on the surface and then placed inside-the weight here being about 450 tons each-through an opening in the center of the shaft, using a Liebherr LG1750 lattice crane.
Once the two machines were placed on the metal saddles arranged at the bottom of the shaft, they were moved up to be housed on reinforced concrete cradles in correspondence with the starting point. After this, the assembly of the contrasting structures necessary to balance the thrust of the machine in the starting phase was completed.
The excavation of the section between the shaft 3.3 and the Amba Aradam-Ipponio station began with the departure of the TBM for the excavation of the ‘dispari’ track tunnel in the middle of March 2018 and was completed with the entry of the two machines inside the structure of the station respectively in the middle of June 2018 the first, and in the first days of August 2018 the second.
The average daily production was about 10 rings a day with 14 ring tips, working on 3 daily shifts for 5 days a week. The geotechnical and structural monitoring installed at street-level and on the buildings which are situated in the potential subsidence basin of the excavation showed a behaviour of the pre-existing and current ground in line with what the project had forecast. The excavation of the section between the Amba Aradam-Ipponio station and the current end of the route, located along the via dei Fori Imperiali in correspondence to the Caeser’s Forum of a length of about 2.6 kilometres, will start this month of November with the power supply site of the two machines located in the Amba Aradam-Ipponio station. The design decision to move the supply site of the mechanised excavation derives from the need to complete the construction of the shaft 3.3 and of the tunnels between the shaft and San Giovanni station, performed with traditional excavation, approximately one year before the completion of the entire T3 section, in order to improve the cadence of the stretch in operation bringing it to about 240 seconds between the San Giovanni station and Alessandrino station.
Eliano Romani – Engineering Manager
Valerio Foti – Construction Manager TBM – San Giovanni Station T3 Civil Plant