Abstract
This paper examines the seismic response of a “horseshoe–shaped” tunnel, inspired by a recently constructed Metro tunnel in Santiago, Chile. A FE analysis has been conducted, investigating the effect of soil density, apparent cohesion, the interface between the tunnel and the surrounding soil, the intensity of the seismic excitation and the effect of volume loss due to tunnel construction on the seismic behaviour of tunnels. The presence of apparent cohesion leads to a reduction of tunnel distress and to smaller post-earthquake ground settlements over a reduced distance from the tunnel. The consideration of volume loss does not significantly affect the acceleration field around the tunnel, but does beneficially decrease the lining forces. Furthermore, although it leads to an increase of the pre-earthquake settlements, it is found to decrease the co-seismic settlements. Finally, it was found that the most conservative model regarding the design detailing of the tunnel lining would be considering a rough interface, zero cohesion, and negligible volume loss (i.e., an ideally-excavated tunnel).
Highlights
Underground structures, such as tunnels, typically constitute critical infrastructure whose serviceability needs to be maintained after major earthquakes
The investigated parameters include: (a) soil properties, covering a wide range of stiffness and strength parameters for dense coarse-grained deposits; (b) apparent cohesion due to a cohesive matrix material; (c) the properties of the soil–tunnel interface; (d) the intensity of the seismic excitation; and – most importantly – (e) the effect of volume loss due to tunnel construction
The constitutive model used for the analyses had been previously validated against centrifuge model test data for wished-in-place tunnels, and was further validated here in terms of capturing pre-earthquake settlement troughs due to construction-induced volume loss against other previously published centrifuge data
Summary
Underground structures, such as tunnels, typically constitute critical infrastructure whose serviceability needs to be maintained after major earthquakes. Previous research on the subject has focussed on wished-inplace tunnel linings, ignoring the effects of the construction sequence, except some isolated studies (see Kontoe et al, 2008) Another important effect of tunnelling is the ground surface settlements during tunnel excavation, which are a function of stress relief and volume loss. The investigated parameters include: (a) soil properties, covering a wide range of stiffness and strength parameters for dense (non-liquefiable) coarse-grained deposits; (b) apparent cohesion due to a cohesive matrix material (e.g., fine plastic silts occurring alongside larger particles in historic river deposits, which are characteristic of local site conditions in Santiago); (c) the properties of the soil–tunnel interface; (d) the intensity of the seismic excitation; and – most importantly – (e) the effect of volume loss due to tunnel construction. Eoed (kPa) unloading-reloading stiffness, Eur (kPa) small-strain stiffness, G0ref (kPa) shear strain, s,0.7 peak friction angle, φ′(°) dilatancy angle, ψ(°) apparent cohesion, c (kPa) m
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