The Neapolitan Subsoil

All the natural deposits within the depths of interest are geologically recent and were formed in a relatively short period of time. The Yellow Tuff and the Pozzolanas were deposited about 12,000 years ago, as a result of the volcanic activity of the nearby Phlegrean complex. The Yellow Tuff is a soft rock characterized by the occurrence of randomly distributed subvertical fractures, locally known as “scarpine”, generated by rapid cooling of the pyroclastic mass after eruption and deposition. Following deposition, erosion of the pyroclastic formations occurred over a period of about 2,000 years, causing material to be transported and redeposited.

The remoulded Pozzolanas are very well graded and not easily distinguished from the original intact pyroclastic deposits; they appear layered and sometimes interbedded with in situ Pozzolanas, or sometimes with marine sand deposits, such as encountered in the area of Municipio Station. After erosion and redeposition of the pyroclastic formations, a new explosive phase of the Phlegrean complex deposited the socalled Neapolitan Pyroclastic Pile, which consists of easily eroded alternating layers of pumices, ashes, pozzolana and lapilli and, where not eroded, has a thickness of about 15 m. The pyroclastic deposits are overlain by sands or silty sands of marine or fluvial-lacustrine origin. Most of the coastal areas of the city derive from relatively recent reclamations (1400 – 1800); this is reflected in the thickness of land-fill, which, in some areas, can reach more than 10 m. The water table is relatively close to ground surface. The
pyroclastic deposits are very common in the area of Naples and have very well known physical and mechanical properties because of their granular nature, they were characterized mainly by in situ penetration tests, such as CPTs and SPTs. In many ways, the most important aspect of the geotechnical investigation is the definition of the level of the roof of the Yellow Tuff formation.

Artificial Ground Freezing - AGF

Because of the water table level, very often underground works have to take into account possible water inflow. Municipio, Università, and Garibaldi Metro stations were carried out with the extensive use of the AGF, to ensure stability and waterproofing during excavation below the ground water table, through loose granular soils and the fractured tuff providing a recent example of an extensive and successful application of the AGF.

Here the technique was extensively used to ensure stability and control ground water flow during excavation of the station tunnel platforms and passageways and in some instances, the innovative techniques that were implemented had almost the character of full scale experiments; therefore, construction of the line was accompanied by an intense programme of monitoring designed to measure and/or control the construction processes and their effects on adjacent structures, which, for its extension and completeness, represented a unique opportunity to collect field data on the performance of AGF.

At present, the use of AGF at Municipio is being considered again, to construct two 40 metres long tunnels connecting the main access shaft and the TBM extraction shaft of Line 6 and preserve the archaeological remnants (fortification walls built in the sixteenth century) emerged on the surface above the tunnels during the works, with a modification to the original design, which was based on dewatering and excavations with conventional methods. At Duomo, where the station develops mostly in the Yellow Tuff, AGF was used only to excavate one inclined passageway, which runs partly in the pozzolanas. The experience gained in the works of Line 1 of Napoli underground is significant mainly because of the size of the intervention: for each station, a volume of approximately 33,000 m3of ground was frozen to permit excavation of the four station tunnels, with a cross section of 87 m2 and a length of 40 to 70 m, and of the four inclined passageways, with cross section of about 40 m2 and a length of 25 m.

The Vertical Shaft

During the works for the Metro Line 1 an advanced technology was implemented to build ten ventilation shafts. It is a mechanized method to dig and simultaneously case circular shafts, recently developed and called Vertical Shaft Sinking Machine (VSM). The method allows for the safe and controlled construction of shafts with diameters ranging between 4.5 and 9.0 m.

The VSM machine includes an excavation unit, a lifting/lowering unit, a separation plant and a remote control unit. The excavation unit is fixed to the bottom of the final lining while operating. It is composed by a cutting bottom and it works by reverse circulation within a floaded shaft; a submersible slurry pump, located just above the cutting drum, transports the slurry, containing in suspension the disgregated soil, to the separation plant located at the surface.

The lining is made by rings of precast r.c. segments, installed at the surface and sunk within the excavation as the excavation is deepened. The precast segment lining ensures watertightness due to the seal installed on each segment. The shaft in Naples were 4.5 m dia and 34 to 44 m deep and were all located in the historical centre, in a deeply urbanized area. The average construction performance rate achieved at Naples jobsite was 3 m/d (10 working hours per day) of dug and lined shaft. Nearby buildings monitoring and ground inclinometers adjacent to installed shafts showed negligible displacements with maximum settlement values of the order of 1 mm.

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