PSI - Issue 84

Enrico Mittiga et al. / Procedia Structural Integrity 84 (2026) 867–873

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In this sense, Anas is implementing the use of precast lining for road tunnels renovation: precast elements are put in place by using specific equipment and trained personnel, to reduce as much as practicable closures and traffic disruptions. After a pilot application in Central Italy (‘Cesaronica’ tunnel on National Road no. 685), Anas adopted this technique for renovation works of ‘Pallotta’ tunnel on the very busy arterial road between Perugia and Rome-Florence motorway (central Italy). The ‘Pallotta’ is a 315-metre-long twin-tube tunnel located on the outskirts of Perugia within its urbanised area. Any disruption to the tunnel may have serious consequences for the flow of city traffic. 3. Project 3.1. Conceptual Framework The concept behind the proposed solution stems from the need to combine speed of execution and structural reliability. Traditionally, when renovating existing tunnels, the lining is often improved using shotcrete, localized reinforcement measures or new in-situ concrete casting with formwork. Although well-established, these techniques have obvious limitations in terms of execution times, casting quality and interference with road operations. The approach described involves the construction of a new precast lining consisting of shell elements (Fig. 4) and designed to be structurally effective from the initial stages of assembly. The precast lining is then connected to the existing lining by filling the annular space with self-compacting concrete (SCC), thus forming a composite section capable of ensuring adequate structural performance and long-term durability.

Fig. 4. Precast element.

The most significant aspect of the design is that the most critical verification phase is not the final service condition, but rather the transitional phase corresponding to the pouring of the filling concrete. During this phase, the new prefabricated lining is subjected to high stresses, comparable to hydrostatic pressures, which must be considered without the use of shoring or complex temporary support systems. 3.2. Structural Design The design was based on experience gained in the field of newly constructed artificial tunnels, where the static three-hinged arch scheme is widely used. This scheme provides an isostatic structure that is therefore able to adapt to small geometric variations; it is therefore particularly suitable for existing tunnels, which are characterized by limited space and reduced construction tolerances. The precast arch is assembled on site using elements connected to each other to form a self-supporting structure capable of supporting its own weight and the loads induced during the transitional construction phases. The adoption