Stability of square and rectangular tunnels in sand under seismic loading

The stability of square and rectangular tunnels with rounded corners placed in sandy soil subjected to seismic forces was examined. The pseudo-static method was used in the framework of lower bound finite element limit analysis based on the distribution of stresses in a statically admissible stress field for the stresses generated in the soil due to seismicity. The support pressure, the minimum pressure required for tunnel stability, was calculated as the maximum of the normal stresses exerted by the soil surrounding the tunnel perimeter. The variation of support pressure with tunnel cover depth and the aspect ratio of the tunnel was extensively reported. The support pressure was found to increase for an increase in the seismic coefficients and a decrease in the friction angle of the soil. Also, the support pressure for narrow rectangular tunnels was found to be less than the square or wide tunnels. The stress is noted to be higher along the wall as compared to the roof and base in the case of tall rectangular tunnels and square tunnels, implying the collapse of these tunnels commences from the tunnel walls. In wide rectangular tunnels, the maximum stress occurs along the roof compared to the side wall and base, where the failure begins from the roof.