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Il metodo Nazzano tra passato e futuro. Storia e risultati della prima sperimentazione mondiale del sistema di ampliamento delle gallerie in presenza di traffico

Lunardi Pietro Cangiano M. Belfiore A.
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Gallerie e grandi opere sotterranee
Gallerie e grandi opere sotterranee N.100/2011

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Benché il mondo del sotterraneo si contraddistingua per la continua ricerca di innovazione da parte degli operatori del settore per far fronte alle sfide da affrontare quotidianamente, è tuttavia poco consueto che un’opera si caratterizzi, sin dall’iniziale concepimento, per la pervicace ricerca di soluzioni nuove, atte a rispondere alle peculiari esigenze del Committente. Ancor più rara è la realizzazione di un’opera che, oltre a definire una metodologia operativa rivoluzionaria, al contempo fornisca nuove possibilità a chi deve pianificare il potenziamento di infrastrutture in sotterraneo inserite in contesti territoriali molto complessi. Questo articolo vuole raccontare la storia dell’ampliamento in sede di una galleria autostradale a due fornici, la galleria Nazzano, realizzato, senza interrompere il traffico veicolare in transito nella galleria, tra il 2003 e il 2007, attraverso molte vicissitudini spesso non correlate agli aspetti tecnici ed operativi dell’opera stessa, grazie a una metodologia originale ideata dal Prof. Pietro Lunardi a metà degli anni 1990.

The “Nazzano Method” for widening road, motorway, mainline and metro rail tunnels without interrupting traffic was used for the first time in the world to widen the Nazzano twin bore tunnel from two to four lanes (including the emergency hard shoulder) in each direction. The tunnel is 337 m. in length, located in the municipality of Nazzano on the Milan-Rome A1 motorway. From a geological viewpoint, the tunnel runs through sandy and silty-clayey soils of LUNARDI P. (1999) – Construction des stations de grandes portées pour métro, ETH-Tunnelbau- Symposium 99, Zurigo, 18 marzo 1999. LUNARDI P. (1999) – Une methode de construction innovante pour elargir les tunnels routiers, autoroutiers et ferroviaires sans interrompre la circulation; son application au tunnel de Nazzano sur l’autoroute A1 Milan-Naples, Atti della Conferenza su “Instandsetzung von Tunneln” - Olten, 21 Ottobre 1999. LUNARDI P. (2000) – The construction of largespan stations for underground railways, Tunnel, n. 8 (dicembre), anno 2000. LUNARDI P., CALCERANO G. (2001) – A new construction method for widening highway and railway tunnels, Atti del Congresso Internazionale su “Progress in Tunnelling after 2000”, Milano, 10 ÷ 13 giugno 2001. LUNARDI P. (2003) – Un metodo costruttivo innovativo per allargare gallerie stradali, autostradali o ferroviarie senza interrompere il traffico: l’applicazione alla galleria Nazzano sull’autostrada A1 Milano-Napoli, Strade & Autostrade, n. 2. LUNARDI P. (2003) – Widening the load at Nazzano, Tunnels & Tunnelling International, Luglio. LUNARDI P., LUNARDI G., CASSANI G. (2007) – Widening the Nazzano motorway tunnel from two to three lanes + an emergency lane without interrupting traffic, Atti del convegno Internazionale su “Tunnels, drivers of change”, Madrid, 5-7 Novembre 2007. LUNARDI P. (2007) – Progetto e costruzione di gallerie-Analisi delle deformazioni controllate nelle rocce e nei suoli (ADECO-RS), Ed. HOEPLI, 575 pagine. TOLENTINO F., FREDIANI A. (2007) – Ampliamento della galleria di Nazzano senza interruzione del traffico, Strade & Autostrade, n. 4. TOLENTINO F. (2008) – Le gallerie nel progetto di ampliamento a 3 corsie dell’A14 da Rimini a Pedaso, Gallerie e Grandi Opere Sotterranee, n. 2, aprile-giugno. the plio-pleistocene series. The water table is located at the level of the side walls of the tunnel and the overburdens range from a few metres to a maximum depth of 45 m. The inspiration for the method came to the author during the work performed for the construction of the “Baldo degli Ubaldi” single vault underground station on line A of the Rome metro. The construction system adopted to excavate the crown (approximately 20.5 m. wide and 9 m. high) employed advance reinforcement of the face-core with fibre glass structural elements combined with mechanical precutting technology and an “active arch” lining, while the work cycles were extremely industrialised by employing high degrees of mechanisation. Finally, in order to mould all these technologies into a single and highly efficient construction system, a dedicated machine was constructed and put into service consisting of a large steel portal with a geometry designed to fit the profile of the crown of the station tunnel. It rested on stabilisers set on longitudinal beams installed in the side-drifts excavated for the side walls of the tunnel in order to allow it to traverse longitudinally. In addition to the equipment needed to perform the mechanical precutting, the equipment for handling and erecting the prefabricated concrete segments of the final “active arch” lining was also installed in the portal. As a result of the use of that machine and the accessory equipment, the volume of ground below the springline of the tunnel, with a cross section similar to that of a normal motorway or mainline rail tunnel was not affected to the slightest extent by the construction operations. It therefore clearly emerged from that observation, that the technology existed to widen a tunnel while it remained in service, at least in terms of advance ground improvement and reinforcement and the placing of the preliminary and final linings. All that needed to be found was an appropriate system to protect traffic travelling on the motorway below, under the construction site. To achieve this, a steel tunnel shield was constructed approximately 60 m. in length, termed a “Traffic Protection Shield” placed inside the existing tunnel with an appropriate structural design. This system would guarantee the performance needed for the safety of traffic and construction workers, by separating the working areas from the road area. Initial design and first contract The initial design for the enlargement of the Nazzano tunnel included two tunnel section types: a principal type with advance ground reinforcement using mechanical precutting technology and an alternative section type consisting of a double ring of jet-grouting in advance around the cavity. The latter could be employed if problems arose with the integrity of the precut, given the predominantly sandy nature of the ground. On that basis the project was put out to tender in 1998 and the contract was awarded to a Spanish firm with a bid at a 25% discount. In order to justify that discount the contractor had presented a major modification to the design, which in practice discarded the option of using mechanical precutting in favour of a single section type using jet-grouting. The client accepted that proposal, but subject to field tests of the proposed technology, to be performed on site after handover of the site for the project. Unfortunately the result of the field tests was negative, which caused the client to request the use of the tunnel section type with mechanical precutting technology. After lengthy negotiations the parties agreed to withdraw from the contract without a single metre of the tunnel having been widened. Second contract The works to enlarge the Nazzano Tunnel were put out to tender a second time in July 2002, after a new design was drawn up to take account of external work already completed and above all of what had emerged during the first contract. The winner of the new contract was a group of companies led by Cossi Costruzioni S.p.A. Once the construction site had been prepared, the first excavation work to widen the tunnel commenced in January 2003, but significant problems immediately arose, attributable mainly to the underpowering of the multifunctional equipment and the operating space which was too restricted. After in-depth reconsideration of the operating methods and a substantial overhaul of the multifunctional equipment, work resumed in March 2004 with improvements which were seen immediately: production now stood constantly at greater than an average of 0.5 metres of finished tunnel per day. A further modification which increased the length of the precutting blade of the multifunctional equipment from 4.5 m. to 5.5 m. then made it possible to raise average daily production to 0.8 m/d of finished tunnel. Enlargement of the north bore of the tunnel was completed in November 2005: of the seven and a half years that had lapsed since the works first began, only one was actually spent on the completion of 75% of the work to widen the north bore. Construction of the south bore Now that it had been seen that the methodology tried out on the enlargement of north bore was reliable, it was decided to apply those same methods to the south bore, in order to widen it while maintaining it in service. Experience acquired during the widening of the first bore was used to immediately achieve even faster average production rates of close to 0,9 m/d with peaks of around one metre per day, which enabled widening of the entire bore to be completed by May 2007, or in other words in just 15 months, while constantly maintaining the motorway in service at a high level. Completion of the two carriageways The work to enlarge the Nazzano Tunnel still had to be completed by placing the tunnel invert in both bores. In order to maintain at least two lanes open to traffic on the carriageway affected by the works, concrete was poured in longitudinal sections with the consequent adoption of connecting sleeves between reinforcement rods, while the safety of traffic and site workers was constantly guaranteed by appropriate methods to separate the in-service road area from the adjacent construction site. The work to cast the tunnel inverts, which first began in the south bore and then continued in the north bore, lasted for around six months from June until December of 2007. Final considerations On completion of the project it can be seen that the period of eight years and nine months (between September 1998 and May 2007), which lapsed between the initial commencement of the works and the completion of the enlargement of the second bore of the Nazzano Tunnel was spent as follows: – 44% on the difficulties of the first tender and the formulation of the procedures necessary for the second tender; – 25% on preparation of the construction site for the north bore after the second handover of the site and problems with the fine tuning of the construction systems to overcome difficulties related to the characteristics of the ground and the first time ever use of the operating methodology; – only 31% of the remaining time was spent on actual excavation to widen the two bores of the motorway tunnel. There is probably no evidence more valid than this of the one hundred percent reliability of the “Nazzano Method”.