Tehran Subway Tunnel Effect on the Seismic Response of the Ground Surface with Linear Soil Behavior: An Experimental and Numerical Study

In the current study, a series of 1 g shaking table tests was performed to study the Tehran subway tunnel effect on the ground surface acceleration response. Two reduced-scale 1 g shaking table models, designated as FF and SF, were constructed in 1/32 scale. The FF was constructed to study the seismic response of the soil layer in free field condition, while the SF model includes a subway tunnel to study its effect on the acceleration response of nearby ground. In prototype scale, the subway tunnel with 8 m diameter and 0.35 m thickness was embedded in a soil layer with 32 m thickness. The soil was dense sand with 70% of relative density. The models were constructed in a rigid box made from Plexi-glass with dimensions of 178*80*120 cm (L.H.W.). Lateral boundaries of the models were covered with conventional foam in order to reduce the lateral boundary effect on the seismic response of the soil layer. The constructed SF model is depicted in Figure (1a). The accelerometers and LVDT transducers installed in the models to record the acceleration in the soil and settlement at model surface are illustrated in Figure (1b). The experimental study revealed that the tunnel does not affect the incident waves with dimensionless period (λ/D) larger than 10. Previous numerical studies [1-2] also demonstrated that an underground tunnel does not affect the free field response at λ/D greater than 10. Up to now, this finding has not been demonstrated by any experimental research. However, the physical modeling performed here is suffered from some limitations regarding the applied frequencies. Therefore, a numerical model was developed based on the results of the shaking table tests, and the effect of the tunnel on the excitations with higher frequency ranges was investigated. Besides, the effect of different parameters such as shear wave velocity of the soil, flexibility ratio and depth of the tunnel on the acceleration at the ground surface was numerically determined. Figure (2) shows the numerical model of the SF model in prototype scale. Six real earthquakemotions that were matched to the response spectrum of ground type I in Standard 2800 were used in the parametric analyses. PGA of the motions was scaled to 0.35 g. The push of the amplification during the analyses were considered as the maximum response and depicted as amplification pattern at the ground surface. Amplification pattern at the ground surface for a tunnel at h/a=1.5 in soils with different shear wave velocities (VS) is depicted in Figure (3). As presented, the maximum amplification occurred at X/a = 1.5 for all Vs. Moreover, as the shear wave velocity increases, the amplification ratio decreases. The study revealed that the amount of the amplification on the ground surface depends on the tunnel depth and shear wave velocity of the soil. The maximum amplifications at the ground surface was equal to 5, 8 and 10 percent for the tunnel depth rations of 1.5, 2 and 3, respectively, in a soil medium with 175 m/s of the shear wave velocity. The effect of tunnel depth on the amplification pattern was investigated in the parametric study. It was concluded that as the tunnel depth increases, the amplification ratio decreases. The tunnel depth affects the location of the maximumamplifications. As the tunnel depth increases, the location of the maximum amplification gets away from the tunnel center and occurs at longer distance from the tunnel center. The effect of the Tehran subway tunnel on the response spectrum at the ground surface in different soil for different ratios of the tunnel depth was investigated. It was concluded that the subway tunnel in soils with different shear wave velocity affects the different ranges of the periods. A subway tunnel with 8 m diameter influences the seismic response of the buildings with the period lower than 0.4 sec or the buildings smaller than 10 m. It means that the tunnel has an adverse effect on the short buildings.