Permanent increments of internal forces on a tunnel lining due to dynamic loadings

The seismic design issues for shallow tunnel design are often poor or absent in the current national or international recommendations. Moreover, the available simplified formulations for the evaluation of tunnel internal forces, induced by seismic actions, account only for transient effects due to wave passage. Some centrifuge tests on circular tunnels in dry sand, recently performed, showed that permanent increments of internal forces clearly arise in the tunnel lining after seismic events. This evidence is also confirmed in real scale monitoring of tunnels,which suffered residual stresses after strong earthquakes. In order to capture the permanent increments of internal forces on a tunnel lining due to dynamic loadings, a series of numerical analyses have been performed by using a material model which includes hardening plasticity and the strain dependency of stiffness and damping.A experimental set of centrifuge tests in sand was used to validate the numerical analyses. The influence of the soil-lining relative stiffness and of the shear stress transmission at the interface between the soil and the lining are discussed in the paper.