Celimontana
shaft

The ventilation shaft is located midway between the Fori Imperiali station and the Amba Aradam/Ipponio station, within the Piazza Celimontana garden, near the Military Hospital of Celio. It has a circular plan with an outer diameter of approximately 35 meters, while the track level is at an elevation of about –15.50 meters above sea level. This results in a significant excavation depth, reaching 60 meters below ground level.

The underground structure is of great importance for the entire line: in addition to its ventilation shaft function, it will also serve as an access point to the line for the Fire Brigade in case of emergencies. Furthermore, it will enable the passage of railway trains from the even track to the odd track, ensuring a train frequency of 4 minutes on the Fori Imperiali-Alessandrino section.

The stratigraphy of the area is characterized by the presence of a thick layer of anthropogenic deposits with a thickness of about 20 meters, which overlies the Pleistocene clays (AR). Below the clays, geotechnical surveys have revealed the presence of a layer of sandy gravel (SG) with a thickness of up to 10 meters, resting on the Pliocene clays (APL), which constitute the typical substrate of the city of Rome.

In addition to its large dimensions and considerable depth, the project is particularly complex due to the presence of a high hydraulic head: historically, the water table in the area has been measured at about 12-14 meters above ground level, which is 45 meters above the excavation bottom.

Given the hydrogeological context in which the shaft is situated, the stability of the excavation bottom is ensured by the presence of the Pliocene clay layer, within which the perimeter diaphragm walls of the second order are embedded for over 35 meters. The permeability of the Pliocene clays has been extensively investigated during the design phase through laboratory tests and in-situ flow tests.

The excavation technique chosen for the construction of the shaft is of the bottom-up type. After the execution of the first-order diaphragm walls, the archeological excavation is carried out, reaching a depth of approximately 18 meters above ground level, in the absence of intermediate diaphragm elements. Once the archaeological excavation is completed, the second-order diaphragm walls are constructed, and the hydrofraise machine is lowered to the bottom of the shaft. The excavation continues with the downward construction of annular ring beams with a thickness of 1.50 meters at various levels and retaining walls for each inter-floor level.

The load-bearing structures that form the horizontal levels are instead constructed upwards. Subsequently, the shaft is prepared for the full passage of both TBMs by creating the slab at an elevation of -5 meters above sea level, positioned above the train running tracks, with a cover of approximately 5 meters compared to the excavation machines’ shield. The diaphragm walls affected by the passage of the TBMs are reinforced with fiberglass bars and brackets to facilitate milling by these machines. This construction method allows for the decoupling of the internal structures’ construction within the shaft from the passage of the two TBMs. In fact, once the slab at an elevation of -5 meters above sea level is completed, while waiting for the shaft’s mechanical excavation to be completed, which is tied to the completion of the mechanized excavation of the line tunnels, intermediate horizontal structures can be built, leaving an adequate central operating ring.

Archaeology

The archaeological excavation of Well 3.2 in Piazza Celimontana began in October 2014 and was completed in December 2016. The archaeological activities were carried out by archaeologists and specialized workers on behalf of the General Contractor Metro C, with on-site coordination by the Special Superintendent of Rome.

The excavation reached a depth of approximately 18 meters below the current ground level, a depth that had never been explored in previous excavation tests conducted in Piazza Celimontana. The opportunity to investigate the subsurface at this depth allowed for the exposure and documentation of an exceptional stratigraphic sequence, ranging from the Iron Age – represented by a burial with grave goods dating back to the late 10th and early 9th centuries BC – to the Modern Age.

The internal flowing surface consists of a thick layer of opus signinum (cocciopesto) found in perfect condition, with a slight slope from East to West. On both sides, the aqueduct shows traces of a consolidation intervention carried out by constructing spurs made of yellow tuff. At the northeastern end of the investigated area, the aqueduct rests on a pre-existing basin, also constructed with capellaccio blocks.

Based on stratigraphic data, the construction of the aqueduct can be dated to a period shortly before the mid-3rd century BC. The most widely accepted hypothesis for its identification is with the Aqua Appia, the oldest aqueduct in Rome, which undoubtedly crossed through this neighborhood at a considerable depth. Both the section of the aqueduct intercepted by Well 3.2 and the block basin have been dismantled in a controlled manner and preserved for future reassembly.