۱- Introduction

۲- Finite element modelling

۳- The effect of lining model on tunnel response

۴- The effect of lining model on ground response

۵- Conclusions

References

Abstract

Major seismic events have shown that tunnels in cohesionless soils may suffer extensive seismic damage. Proper modelling can be of great importance for predicting and assessing their seismic performance. This paper investigates the effect of lining structural modelling on the seismic behaviour of horseshoe-shaped tunnels in sand, inspired from an actual Metro tunnel in Santiago, Chile. Three different approaches are comparatively assessed: elastic models consider sections that account for: (a) linear elastic lining assuming the geometric stiffness; (b) linear elastic lining matching the uncracked stiffness of reinforced concrete (RC); and (c) nonlinear RC section, accounting for stiffness degradation and ultimate capacity, based on moment-curvature relations. It is shown that lining structural modelling can have major implications on the predicted tunnel response, ranging from different values and distributions of the lining sectional forces, to differences in the predicted post-earthquake settlements, which can have implications on the seismic resilience of aboveground structures.

Introduction

Tunnels constitute critical underground infrastructure, vital for urban transportation and logistics, and thus for the economy of major urban conurbations. In many cases they are built in high seismicity areas, and therefore their seismic design can be of paramount importance. Determination of their seismic response is challenging due to the large number of parameters affecting behaviour, including those associated with nonlinear soil response, soil–structure interface behaviour, and nonlinear structural response. In general, their seismic performance is better than above-ground structures since inertia effects are not significant, with the main source of loading being of kinematic nature, stemming from the dynamic response of the surrounding soil, which can be carried efficiently by the tunnel acting as a pressure vessel ([1–۶]). Despite their advantages over above-ground infrastructure, tunnels have experienced severe earthquake–induced damage, such as the collapse of the Daikai metro station during the 1995 Kobe earthquake, of various tunnels in Taiwan during the 1999 Chi-Chi earthquake, and of the Bolu tunnels in Turkey during the 1999 Kocaeli earthquake ([7–۱۳]). Therefore, the assessment of tunnel seismic response has become the objective of many previous studies, which focussed on tunnels of circular or rectangular cross-section, in idealized nonlinear soils representing clays or sands (e.g., [5,6,13–۲۰]). Centrifuge modelling has been employed to validate numerical models, focusing on nonlinear soil response ([5,6,17–۱۹]). The nonlinearity of the tunnel lining response, however, has not been studied in detail so far. Purely elastic structural behaviour is typically considered for the structural elements that represent the tunnel lining (e.g., bending stiffness EI and axial stiffness EA, based on the diameter, wall thickness and Young’s Modulus of the lining material). Such an idealized elasticity approach cannot be considered adequate for reinforced concrete (RC) tunnel linings, where EI and EA must be defined considering the interaction between the concrete loaded in compression and the steel loaded in tension (e.g., [21]). Nonetheless, Argyroudis and Pitilakis [22] introduced strength and capacity of an elastic tunnel through different damage indices (DI), which were then used by Argyroudis et al. [23] to estimate fragility curves accounting for lining corrosion. Furthermore, Lee et al. [24] accounted for the nonlinear behaviour of rectangular concrete tunnels by conducting pseudo-static analyses, replacing the soil with equivalent springs along the normal and the shear direction. Aiming to bridge the apparent gap in the literature, this paper examines how the structural modelling approach used for the tunnel.