A simplified analytical method for the tunnels’ mechanical behaviour under the action of active fault zones

Abstract

A fault zone consists of a fault core and a fault damage zone, and its width varies from tens to hundreds of metres. The permanent displacement of a stratum during rupture of a fault is obviously nonlinear, which would result in significant damage to the tunnels crossing active fault zones. At present, a series of mechanical models have been proposed for the preliminary design of tunnels under the action of active fault zones. However, in the current analytical models, the fault zone is simplified to a thin line, the nonlinear permanent stratum displacement due to rupture of a fault is simplified as a uniform displacement, and the mechanical characteristics of different fault types are not fully considered. As a result, the application of these analytical models is limited. In this study, a simplified procedure for analysing the mechanical behaviour of tunnels under the action of active fault zones is established. Specifically, the effect of the fault zone width on the mechanical behaviour of a tunnel is considered by reducing the stratum stiffness. In addition, the nonlinear displacement of a stratum under the action of five different fault types is considered by using Okada’s model (Okada, 1992), and the differential equations for tunnel displacement are solved by employing the finite difference method. Then, the proposed model is verified by comparisons with both 3D solid and beam-spring numerical models, and the results are in excellent quantitative agreement. Finally, the parameter analysis shows that the mechanical response of the tunnel is greatly affected by the width and mechanical properties of the fault zone, and ignoring the effect of the fault zone (as assumed in the current analytical models) would cause an overestimation of the internal forces on the tunnel. The proposed model can be used as an efficient tool for the design of tunnels that cross active fault zones.