Response of urban single and twin circular tunnels subjected to transversal ground seismic shaking

The response of tunnels subjected to ground seismic shaking is commonly evaluated, disregarding the existence of other structures in the near area. However, significant interaction phenomena may take place between the tunnel and existing structures in dense urban environments, altering the seismic response of the tunnel compared to the one predicted for 'green field' conditions. This study aims at investigating the effect of heavy buildings on the response of urban single or twin circular tunnels, when subjected to severe ground seismic shaking in the transversal direction. For this purpose, a numerical parametric study was conducted on representative tunnel soil-buildings systems, employing the finite element code ABAQUS. Several configurations were examined, including single or twin circular tunnels, embedded in clayey soil deposits. The high-rise buildings were simulated as equivalent single degree of freedom oscillators (SDOFs), founded on rigid raft foundations. Salient parameters that affect the seismic response of tunnels, namely (i) the burial depth of the tunnels, (ii) the number of buildings-SDOFs and their position relative to the axis of the tunnel and (iii) the soil properties and response during ground shaking were investigated within this parametric study. The computed responses of the investigated configurations, in terms of (i) lining deformations, (ii) dynamic earth pressures and seismic shear stresses developed along the perimeter of the tunnels and (iii) dynamic lining forces, were compared with the cases where the buildings-SDOFs at the ground surface were neglected. The effect of buildings-SDOFs was quantified by means of response ratios mu, defined as the ratio of the maximum envelope value of the response parameter computed at critical lining section at the presence of the buildings-SDOFs, to the relevant value estimated for green-field conditions. The response ratios were plotted against the flexibility ratios, F, of the investigated soil-tunnel systems, highlighting the effects of the salient parameters mentioned above on the interaction phenomena. Generally, the consideration of the buildings-SDOFs at the ground surface resulted in an increase of the response of the tunnels, compared to the one predicted for 'green field' conditions, both in terms of tunnel deformations and dynamic earth pressures and soil dynamic shear stresses around the tunnel, as well as in terms of dynamic lining forces, with the effect being generally higher for shallow tunnels.