ELASTIC AND INELASTIC RESPONSE OF TUNNELS UNDER LONGITUDINAL EARTHQUAKE EXCITATION

This paper addresses the problem of the behaviour of underground tunnels subjected to longitudinal earthquake excitation. State-of-the-art solutions, which are relative to infinite length tunnels in the stationary case, with linear models for both the ground and the structure, are first recalled. These models are either static or dynamic; results generally indicate large strains, often incompatible with reinforced concrete structures, and small crack widths.

For finite length tunnels, in this study the dynamic solution for the linear case is developed and it is shown that maximum strain is an increasing function of the tunnel length so that, in many situations, the behavior of the lining is satisfactory also under the aspect of crack formation. When the nonlinear behaviors of the tunnel and soil are taken into account, segments of reinforced concrete tunnels, after cracking, may undergo considerable relative displacement, undesired both for serviceability and ultimate limit states. After review of the constitutive relationship for the ground, a parametric study on the crack openings for an example tunnel is presented. It is shown that crack widths computed with linear static analyses underestimate the real value for tunnels embedded in stiff grounds and that the lining longitudinal reinforcement can reduce crack width so as to cope with serviceability and ultimate limit states.