Study on the effects of cooling phase and construction technology on the fire performance of R/C tunnels

Fire performance of tunnels can represent a critical issue in the design phase, even more than for other structures and infrastructures, due to some inherent features such as (a) the fire compartment geometry often leading to very high temperature (also making difficult the intervention of fire brigades), (b) the structural redundancy caused by soil restraint fostering the development of relevant indirect actions, and (c) the high compression state in the lining (all the more during the fire exposure) that increases the spalling propensity and severity. Within this context, the role played by key parameters such as lining thickness and stiffness are investigated by comparing the fire performance of two different technological solutions for the lining: (1) traditional cast-in-situ lining and (2) pre-cast segmental tunnel lining. The fire scenario also considers the cooling phase, in order to discuss the main critical points to be solved when facing the final stage of the fire. 3D finite element analyses have been performed, proving that (I) higher thickness and stiffness does not necessarily correspond to a higher safety factor due to the indirect actions, and (II) fire cooling phase (if any) can be even more critical than the heating phase.