The Project

2 October 2023

The Construction of Shaft 3.2 for Line C

The Construction of Shaft 3.2 for Line C

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The construction of the Pozzo 3.2 in Rome’s Line C is one of the most significant undertakings in the currently under-construction T3 section. It involves a combination of various construction methodologies and adopted technologies. Pozzo 3.2 is located midway between the Fori Imperiali station and the Amba Aradam/Ipponio station, inside the garden of Piazza Celimontana, near the Celio Military Hospital. It has a circular footprint with an outer diameter of approximately 35 meters, and its floor level is at an elevation of -15.50 meters above sea level. This results in a significant excavation depth, reaching 60 meters below ground level.

Apart from serving as a ventilation shaft (housing ventilation systems), the Pozzo also functions as the access point to the railway for the Fire Brigade in case of emergencies. Most importantly, it enables trains to switch from the even track to the odd track to maintain a train frequency of 4 minutes on the Fori Imperiali – Alessandrino section. The construction of the Pozzo was particularly complex due to its large size, considerable depth, the need for archaeological excavations, and the presence of a high groundwater table: the water table in the area was measured at approximately 12-14 meters above ground level, or 45 meters above the excavation floor.

The Structure

The underground structure consists of 8 horizontal levels, including the roof and foundation slabs. The significant excavation depth necessitated the construction of two levels of excavation support structures:

  • The first level comprises diaphragm walls with a thickness of 80 cm and a length of 20.0 meters, arranged along a circumference with an internal diameter of 32.40 meters.
  • The second level consists of diaphragm walls with a thickness of 120 cm and a length of 56.0 meters, arranged along a circumference with an internal diameter of 28.0 meters, reaching a depth of approximately 72.50 meters below ground level.

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Given the hydrogeological context in which the Pozzo is situated, the stability of the excavation floor is ensured by the presence of pliocene clay layers. The second-order perimeter walls extend for over 35 meters within these clay layers. The permeability of the pliocene clay was extensively investigated during the design phase through laboratory tests and in-situ flow tests.

Construction Phases and Site Logistics

The chosen excavation technique for constructing the Pozzo is of the “bottom-up” type. After the first-order diaphragm walls are constructed, archaeological excavation is carried out to a depth of approximately 18 meters below ground level, in the absence of intermediate elements between the diaphragms. Once the archaeological excavation is completed, the second-order diaphragm walls are constructed, and the hydrofraise machine is lowered to the bottom of the Pozzo.

Archaeological excavations, as seen in many other Line C construction sites, led to the discovery considered one of the most interesting by the Archaeological Superintendent in recent years. Various artifacts were found, including a section of a Roman aqueduct, possibly the Aqua Appia or the Anio Vetus, ancient roadways leading from the Colosseum to the Navicella, and an Iron Age tomb dating back to the 10th century BC.

The excavation continues with the downward construction of annular ring beams, each with a thickness of 1.50 meters, corresponding to various horizontal levels, and the counterforts for each interlevel.

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The load-bearing structures that make up the horizontal levels are constructed in an upward direction once the slab with an extrados at an elevation of -5.00 meters above sea level is completed. Subsequently, the shaft is prepared for the passage of both TBM machines by creating the slab at an elevation of -5.00 meters above sea level, positioned above the train running paths, with a covering of approximately 5 meters above the excavation machine’s shell. During the passage, the TBMs pass through the diaphragm walls reinforced with fiberglass bars and brackets.

This construction method allows for the decoupling of the internal structures of the shaft from the passage of the two TBMs. In fact, once the slab at an elevation of -5.00 meters above sea level is completed, and while waiting for the completion of the mechanical excavation of the underground tunnel, which is linked to the completion of the mechanized excavation of the subway tunnels, intermediate horizontal levels can be constructed, leaving an adequate central operational hole. After reaching the excavation floor and completing the foundation slab, the construction of two branches of traditional excavation tunnels (each approximately 40 meters in length) is carried out. These tunnels are necessary for installing the track switch system inside the shaft, which will allow the connection of the two train running paths. To mitigate the stresses induced on the diaphragm walls, it was necessary to construct the linings of the last interlevel before beginning the excavation of the tunnels.

The construction site logistics, along with the need to move the position of the overhead crane in various phases of the shaft’s construction, required the use of a 32-meter-long steel truss girder to support the crane’s running tracks. The crane has a capacity of 30 tons.

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Construction Technologies

Both containment structures are excavated using a hydrofraise. This technological choice is related to several factors:

  • The need to create milled joints between the diaphragm walls, ensuring adequate hydraulic sealing, especially in relation to pressures that reach 4 bars at the excavation bottom.
  • The need to create a circular crown with interpenetrating panels to ensure suitable transfer of compression forces in the circumferential direction, resulting from radial pressures exerted by the soil and groundwater on the exterior walls of the structure.
  • The need to limit deviations from the vertical of the panels, given the significant depth of the diaphragm walls. Excessive deviation of the panels from the vertical would reduce the contact area between the diaphragms, leading to an increase in transfer stresses associated with the joints between the diaphragms.

The choice to construct the shaft in a “bottom-up” manner, along with the need to cover significant spans with horizontal levels and the impossibility of installing intermediate supports for the floors (due to interference at platform level with the switchgear park), has led to the extensive use of prefabricated structures, which are as self-supporting as possible during assembly and casting of the slabs.

This choice provides clear advantages, especially in terms of execution speed.

Supporting Structures for the First 5 Intermediate Floors:

  • 10 inclined precast concrete struts arranged radially;
  • 1 in-situ concrete ring on which the inclined struts are connected, designed to absorb radial horizontal forces transmitted by the struts;
  • 10 prefabricated concrete beams arranged radially, supported by the inclined struts and the perimeter diaphragm;
  • Sections of slabs made using ribbed precast elements supported by the main prefabricated beams and connected to them through in-situ castings;
  • 1 circular in-situ concrete element for closing the central hole, constructed using slabs resting on the inner rings.

All prefabricated structures are designed to be self-supporting during casting, and the only temporary elements are 10 circular columns used to temporarily support the inclined struts before the casting of the ring beam.

These metal frame elements are designed to be modular and easily adaptable to all levels. The intermediate floor at a level of -5.00 meters above sea level, located immediately above the train tracks, the service platform, and the foundation slab, are constructed with cast-in-situ concrete. The internal counterforts vary in thickness, increasing with depth. Counterforts with a thickness of 75 cm are used for the first two levels, increasing to 1.0 meter and 1.3 meters for the last two levels.

Considering the significant depths to be reached, approximately 60 meters, which need to be traversed with 30 flights of stairs, each composed of 12 steps, prefabricated elements were chosen for the construction of the structural elements of the stairwell and elevator shaft. Billets were used for creating the perimeter walls, and ramps were equipped with intermediate landings and steps.

Progress in Construction Activities

To date, the excavation of the shaft has reached a depth of -7.00 meters above sea level, and the slab at -5.00 meters above sea level, located above the train tracks, has been completed. While awaiting the completion of the mechanized tunneling for the Line C, scheduled for November 2019, the construction of the internal structures of the shaft has begun.