Nell’ambito dei lavori di realizzazione della “Variante di Valico” dell’Autostrada A1 Milano-Napoli, nel tratto tra Sasso Marconi (Bologna) e Barberino del Mugello (Firenze), la costruzione della galleria “Sparvo” rappresenta una sfida unica nel mondo del sotterraneo, sia per il difficile contesto geologico-geomeccanico da affrontare, sia per l’ampiezza dello scavo che si sta realizzando utilizzando per la prima volta al mondo una TBM EPB da oltre 15 m e mezzo di diametro. L’articolo che segue, nel quale gli autori illustrano con ricchezza di dati e preziose informazioni l’esperienza sin qui maturata nello scavo della galleria, è la naturale prosecuzione di quanto già pubblicato nel n. 98.
Evidences collected during the excavation of the first tube of the “Sparvo” tunnel by using “Martina”, the world’s largest EPB-TBM The paper presents the first evidences collected during the excavation of the first tube of the “Sparvo” tunnel, bored by means of an EPB-TBM having the record diameter of 15.62 m and completed in July 2012. Particular attention is paid to the most critical point, from both a geotechnical viewpoint and due to the presence of gas deposits, consisting of the APA formation. The main operative parameters are commented and confronted with those that had been projected during the design stage. Rocksoil was involved in the final and detailed design of the tunnel and in technical assistance during TBM excavation. Analyses of the geological conditions of the tunnel were carried out during work progress, through constant control of the material being extracted and transported on the conveyor belt; through periodic inspections of the excavation chamber during stops for verification of the excavation cutters’ wear and necessary substitution; and through the use of the “BEAM system”, which proved to be an excellent instrument for inspection during work progress and to detect in advance the contact position of the main formations encountered by the TBM. In general, the geological data collected during the excavation corresponded well to that which had been projected during the design stage; just in the final sector, in correspondence to SCB-APA tectonic contact, a slight northward translation of all stratigraphic and tectonic limits was observed (about 30-40 m). About production, after a first period of tuning of the operative modality, the construction process was industrialized up to an average production of 13.2 m per day (with a record of 22 m/d) in the period of March-July 2012, above expectations. An important aspect is that production values were not significantly conditioned by the lithologies bored, with only a reduction of 30% in the most difficult formation of APA, where a conventional tunnelling process would have required extensive ground improvement (9.7 m per day in APA with respect to 12.9 m per day in SCB, 12.3 m per day in MOV and 14.7 per day in BAP). The pressure parameters at the face were in line with design projections, needing high pressure levels of 3.0-3.5 bars when boring through the heavily covered APA; in this sector the expected squeezing behaviour of the mass took place (the ground converges upon the shield and the back-filling volumes remained constantly at minimum levels, equal to 22-23 m³ on respect the maximum value 29.5 m³), although the necessary thrust for the progress of the TBM (maximum value equal to 160 MN) did not go over the dimensioning limit of the machine (270 MN up to a spike force of 390 MN), also thanks to the confinement operated at the face. This evidence confirms - also in the field of mechanized excavation - the importance of face confinement in keeping the stability and the deformation control of the cavity. The tuning of the conditioning parameters (FIR equal to 80-120%, FER 6-8 and foam percentage in the range 2.2-3.0%) of the material was of particular importance, as it avoided the clogging of the excavation chamber and consequent negative effects on the TBM’s performance and the tool’s wear; in particular, 6 liquid input lines were added to the centre of the main drive. Geotechnical monitoring during the work made it possible to evaluate the interaction between the excavation and the existing buildings on the surface. In APA formation, despite the high level of coverage present and the use of mechanized excavation, a significant deformation response at the ground level took place, in an area which had already been compromised by past movements of the slope: settlements equal to 40-70 mm and movements of 50-110 mm; the ground level started to be disturbed when the excavation face was 50-70 m from the topographic measure point, while a tendency towards asymptoticity was observed once the excavation face went 150-200 m beyond each measure point (around 1 month after the TBM had passed). Monitoring of the gas concentrations in the excavated material (which reached values up to 0.15-0.20%, on average equal to 0.03%-0.05%), by means of specific procedures, made it possible to manage this delicate aspect of construction.
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