Abstract
The paper is focused on the assessment of seismic fragility curves for circular tunnels under moderate to severe earthquakes with the aim of improving the reliability of the risk assessment of underground infrastructural networks. Non-linear two-dimensional dynamic analyses were performed on different tunnel and soil configurations by using the finite-difference method implemented in Flac2D software. The applied input motions were selected considering their amplitude and frequency content variability. The response accelerations and predominant frequencies computed at ground level, above the tunnel, were compared with the corresponding free-field results to distinguish the effects attributable to the tunnel presence from those due to the site amplification. Tunnel safety was assessed through fragility curves, taking into account the dependence of tunnel lining bending resistance on the axial force variation during the earthquake. The more effective intensity measure was investigated correlating the tunnel performance to peak ground accelerations and peak ground velocities computed at the ground level and at the bedrock depth, respectively. The resulting fragility curves showed a satisfying matching with the empirical ones, generated on the basis of the observed seismic damage on tunnels.
Highlights
Nowadays, many countries in the world are facing a continuous demand for urbanisation, structuring the underground space to develop their physical interconnectivity (Broere, 2016)
A comparison between the results revealed that the lining axial forces and acceleration would not be significantly affected by the modelling approach
The computation of fragility curves in terms of PGAim was repeated assuming zero axial force acting in the lining and the lowest bending resistance of the section, which is superimposed on Fig. 8 through the horizontal line
Summary
Many countries in the world are facing a continuous demand for urbanisation, structuring the underground space to develop their physical interconnectivity (Broere, 2016). When the tunnel is deep and the sand is dense, the presence of a void zone (i.e. V1 – dashed line) reduces the ground motion with respect to the free-field results (V3 – continuous black line) This trend most probably confirms the ‘shadowing effect’ observed under the low-amplitude excitation (see Fig. 5 in the earlier section entitled ‘Numerical models’), due to the loss of highfrequency content when the seismic wave crosses the tunnel. Despite the influence of soil plasticity on the accumulation of residual bending moments at the end of shaking (experimentally confirmed by Cilingir & Madabhushi (2011a) and Lanzano et al (2012)), the comparison shown in Fig. 8 of numerical predictions (that include different plasticity models) with the classical Wang’s solutions (that assume an equivalent linear behaviour) is legitimate since the calculated steady-state cyclic changes of bending moments occurring during shaking are considered. The numerical results of this study (empty markers), that take into account stiffer linings (F ranges between 37 (dense sand) and 11 (loose sand) for G0, and between 11 (dense) and 3 (loose sand) for 0·3G0), are closer to the analytical solutions, and in many cases fall within their observed range
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