THE NEW DESIGN AND THE UNDERPASSING OF LINE A
The station design thus underwent an overall revision, with the construction shortened from 250 metres to 140 metres, and the excavation bottom lowered by about 14 metres.
The deeper station body required not only lengthening the perimeter diaphragm walls that reached a length of about 56 metres, but also increasing their thickness from 80 cm to 120 cm. Lastly, the stability of the excavation bottom was guaranteed by heading the perimeter diaphragm walls in Pliocene clays, which show marked characteristics of impermeability, thus at the same time making it possible to eliminate the jet grouting treatment previously provided for in the tender-based design, but which could no longer be done as prescribed by the Special Superintendency for Archaeological Assets.
Another problem, which constituted a major constraint for the choice of the new route plan, was the presence of the foundation piles of the existing Line A station, 2000/2200 mm in diameter and 15 metres in length.
In this new configuration, Line C underpasses the existing Line A station for a length of 38 metres, by means of the excavation of two tunnels at a depth of approximately 30 metres below ground level.
These tunnels were made by traditional excavation, under the groundwater level, through the stabilisation and waterproofing of the soil by freezing technology. This is a method used for excavations under the groundwater level in difficult conditions with the presence of loose earth or fractured rocks.
The freezing technique is the most appropriate choice in cases particularly when working in urban settings, so as to guarantee the greatest possible level of safety, both for the labour that is employed, and for the pre-existing constructions on the surface. The difficult scenario in the area affected by the work site, with its high urbanisation and considerably complex traffic dynamics, led to the adoption of modern construction interventions, privileging the Top-Down system.
In order to adapt this excavation methodology to the station work site’s archaeological needs, it was necessary to make a change to the classic top-down excavation system, with the widespread use of prefabricated self-supporting structures in the casting phase.
In this setting, which requires the procedure of archaeological excavation for the first 18-20 metres below ground level, a construction method was put in place that, for each slab, involves an initial phase with the archaeological excavation down to the slab’s laying depth and the building of the perimeter curb, and a second phase proceeding with the completion of the slab, with the launch of self-supporting prefabricated structures in the casting phase only after having reached an excavation depth at least 3 m below the lower surface of the slab.
Given the impossibility of inserting intermediate supports, for the main structures of the station’s four intermediate slabs the choice was made to use a main weaving of precast mixed steel/concrete trusswork girders, with sections ranging between 80 and 100 cm, in such a way as to limit the slabs’ thickness, allow the systems to pass through, and guarantee the spaces’ usability.
The use of these main structures, completely self-supporting in the casting phase and with spans of approximately 20 metres, conveyed and facilitated the station construction operations, although careful logistics became necessary to transport, handle, and assemble these large elements. In fact, the almost 100 girders (25 for each of the 4 decks forming the station structure (Lobby level, First technical level, Connection level, and Second technical level) were handled inside the station using a special overhead crane located underneath the roof, bound from time to time to the bottom face of the slab placed above the one in the construction phase.