METRO C: DISMANTLING OF THE TBM IS NEARING COMPLETION
Excavation of the tunnels of Line C between the San Giovanni station and Piazza Venezia using TBMs (Tunnel Boring Machine) ended on 29 August 2020, and in September started the two machines and their equipment dismantling.
The two TBMs had previously, and until 2011, built the tunnels of section T4 (from via dell’Acqua Bullicante to Largo Brindisi – San Giovanni). Once excavation of section T4 was completed, they were dismantled, overhauled and then reassembled at the via Sannio shaft, through which they were brought back to the excavation level.
Between March 2018 and August 2020, they built the tunnels of section T3 (initially up to via dei Fori Imperiali and then, following the design modifications that occurred at the end of 2019, up to Piazza Venezia).
TBM DETAILS AND COMPOSITION:
The two TBMs (Tunnel Boring Machine) with which the tunnels of section T3 were built use EPB (Earth Pressure Balance) technology. They are German manufactured (Herrenknecht) and can be roughly described as cylinders with a diameter of 6.70 meters and a length of approximately 11 metres, with an attached convoy consisting of 7 wagons (back up) containing all the equipment and the technological systems and plants necessary for their operation, for a total length of approximately 95 metres.
They are mainly made up of:
- A cutter head containing the soil excavation tools and openings suitable for the transference of the soil into the excavation chamber behind;
- A cylindrical metal “mantle”, consisting in turn of:
- A “front shield” housing the “main drive”, the main bearing of the machine, responsible for rotating the cutting head, with its relative geared motors and rotary joint for injecting the materials necessary for conditioning the soil in the excavation chamber.
- A “rear shield” containing the functional systems for advancing the machine into the ground and mounting the tunnel lining.
- A “tail shield” inside which the final lining of the tunnel, made of circumferential segments in reinforced concrete, is mounted.
- A convoy of 7 back-up wagons, containing mainly the following facilities/equipment:
- Wagon 1 contains the control cabin, the pumps for the injection of the two-component mortar for clogging the back of the segments with relevant tanks, control and command panels).
- Wagon 2 contains the sliding beams of the “crane” (system for transferring the segments inside the shield); it also contains the hydraulic pumps for the circuits that allow the TBM to operate and the pumps for the injection of the sealing grease for the tail brushes and of the oil and grease of the various circuits; the electric motors for the forward thrust and the heat exchanger for cooling the circuits.
- Wagon 3 contains the “crane” just above described and the electrical panels for operating the machine.
- Wagons 4, 5, 6 & 7 contain further components necessary for the operation of the machines (in particular the cable reel for the Medium Voltage transformer; the elements of the cooling and compressed air circuit of the machine, the hose reels of water and cement mortar).
The shields and the tail are connected by cylinder systems that allow them to move in relation to each other, which is necessary to accommodate the curves of the tunnel construction route.
The end of the tail is equipped with systems (“brushes”) that prevent, during periods of excavation or permanence of the TBM in the ground, the flow into the macchine of both ground water contained in the soils and of the cement mortars injected by the TBM on the back of the tunnel lining, to prevent subsidence of the surrounding soils.
STAGES OF THE TBM DISMANTLING:
It is planned to leave the shields of the two macchines in the ground after they have been secured by filling the excavation chamber with cement mortar and creating a reinforced concrete “lining” inside the cylindrical mantle, pending their retrieval once the perimeter box of the “Venezia” station has been built.
This is to avoid, even in the long term, any structural failure of the TBMs or the entry of materials into the machines themselves.
At the end of the excavation, in order to allow for the completion of work in both station and shafts affected by the passage of the machines, as well as the construction of all the plant installations (railway equipment and relative technological systems) in the tunnels created by the machines, all the equipment and systems necessary for the operation of the TBMs are completely dismantled.
In particular, at the Amba Aradam station, which due to its location and the logistical conformation of its site, was used as a logistical base for mecchanized tunnel excavation, the systems managing the excavation soil conveyor belts and all the supply systems necessary for tunnel construction are dismantled.
At the same time as the dismantling in the tunnels, the seven back-up wagons that form the convoys containing all the equipment and technology required to operate each TBM are also extracted.
Finally, the main components of the machines (screw conveyor, erector with its relative bridge and H-beam, thrust and articulation cylinders, hperbaric chamber and glove box, rotary joint and main drive geared motors) are removed in order to proceed with the reinforced concrete lining of the inside of the machine shield.
The dismantling of the equipment serving the TBM machines in the tunnels and on the logistics site of the Amba Aradam station under construction has been completed.
The back-up wagons of the two TBMs have been disconnected and removed. All equipment and installations inside the TBM shield on the even-numbered track were removed and the reinforced concrete lining inside the shield was finished (similar work is also being completed on the TBM on the odd-numbered track).