Finite element evaluation of the effects of faulting on a shallow tunnel in alluvial soil

A numerical estimation is presented on the effects induced in an existing tunnel by the development of a fault from the deep bedrock during a seismic event. The spreading of the fault within the alluvial deposit hosting the tunnel, and the consequent effects on its permanent liner, are studied in static conditions through a series of elastic-plastic, plane strain finite element analyses. They account for the reduction of the shear strength and stiffness characteristics of the faulting zone with increasing irreversible strains. Even though the calculations require only “standard”, e.g. peak and residual, material parameters it is shown that these properties can hardly be obtained for the alluvial deposit at hand. To overcome this drawback a relatively large scale in-situ tests could be performed and its results could be interpreted through a suitable back analysis. This would permit characterizing the numerical model to be subsequently adopted for the analysis of the faulting process.