Spalling Mechanism: Key for Structural Fire Resistance of Tunnels

The need to consider tunnel safety, especially with respect to fire, has been highlighted in the past by various severe tunnel fires that have occurred throughout Europe. This article provides a general overview of the current rules and regulations governing the fire resistance design of tunnel structures. Subsequently, the engineering perspective is presented, focusing on the assessment of fire severity and the resulting thermal loading on the tunnel structure, which may result in spalling of concrete. Thereafter, a scientific perspective is provided, reflecting recent advances made in numerical calculations with respect to the spalling mechanism. From the numerical results and comparison with observations from full-scale tests, it is concluded that thermal restraint is dominant for spalling. Spalling is triggered by the buckling of concrete layers loaded with strains imposed by temperature, which is a new perspective for future scientific research and innovative engineering solutions.