Performance-based analysis of Transit tunnels in the Chilean subduction zone

Abstract

In this article, we study the seismic response of a shallow metro tunnel in a subduction zone environment and the use of a performance-based approach to develop seismic demand hazard curves (SDHC) of key engineering demand parameters, such as drift ratio, surface settlement, bending moment, and axial loads. The tunnel consists of a 6-m diameter sprayed concrete lining in medium dense soil and is located in Santiago, Chile. To simulate its seismic response, a finite element model of the soil–tunnel system was implemented in OpenSees and validated against centrifuge test results, linear-equivalent solutions for a 1D soil column, and single elements in cyclic simple shear. The tunnel response was computed for 112 ground motions, selected and scaled using the conditional scenario approach. This approach assigns a rate of occurrence to each ground motion and therefore, allows for a direct computation of annual exceedance rates of the tunnel response parameters. For instance, at this specific location, the 2500-yr return period drift ratio in the lining is approximately 0.25% as a result of large deformations imposed by the surrounding soil. Likewise, axial loads between 0.3 and 1.0 MN/m and bending moments of ±0.2 MN∙m/m are apparent from the axial load-moment interaction diagrams; these results are of great value for the design and verification of the tunnel based on the collapse or life-safety limit states. Notably, the current formulation relaxes the assumption of scaling ground motions to a particular intensity measure and can be a computationally efficient alternative to standard incremental dynamic analyses.