Behavior of shallow tunnel in soft soil under seismic conditions

In tunneling practices, it is vital to have an accurate estimate of the depth of embedment of the tunnel; lining thickness and shape of the tunnel, which enables resisting of stresses; and deformation generated by the surrounding soil under seismic loading conditions. The present study highlights the behavior of shallow tunnel in soft soil under seismic conditions by using the finite element (FE) analysis. The developed numerical model is compared with available analytical solutions. Thereafter, a series of parametric studies are carried out by varying the tunnel embedment ratio, soil-tunnel interface conditions, lining thickness, shape of the tunnel, and input ground motion. It has been observed that distortion in the tunnel lining is dependent on the depth of embedment and the flexibility ratio of the tunnel. Ovaling (in a circular tunnel) and racking (in a rectangular tunnel) are found to decrease significantly when embedment ratio is greater than 2. Nearly 6-18% of greater distortion and 20% of greater bending moment are obtained in the full-slip interface condition when compared to the no-slip interface condition. The maximum induced bending moment in the tunnel lining is directly proportional to its flexural rigidity. An unconventional square tunnel with rounded corners, yields 55% lesser bending moment than the square tunnel under the same seismic loading condition. This study also highlights the importance of the input ground motion characteristics that govern the development of the maximum dynamic earth pressure around the lining of the tunnel, and the heaving of the ground surface just above the crown. The outcomes of the present study will be useful in design through understanding the effects of various influencing parameters that control the stability of the tunnel in soft soil under seismic loading conditions.