Significance of Primary Waves on the Seismic Stability of Underground Tunnels: A Numerical Perspective 

Generally, the influence of primary waves on the seismic stability of geotechnical structures is overlooked under the premise that structures are inherently stable to an increased vertical body force. No study is available to discuss the implications of including or not including primary waves in the analysis, especially for underground tunnels. The present study attempts to illuminate the importance of including primary waves in the seismic stability analysis of underground tunnels. Based on the support pressure computations for a circular tunnel placed in granular and cohesive-frictional soils, the present study aims to understand scenarios where including primary waves is inevitable and vice versa. The earthquake forces generated due to vertical propagation of shear and primary waves within the domain are simulated by horizontal and vertical harmonic base excitation. The dimensionless variation of dynamic increment to the static support pressure is analyzed in the frequency-time plane. It has been found that primary waves affect the support pressure the most for tunnels placed in soils with a higher shear strength (higher friction angle for granular soils and increased cohesion and/or friction angle for cohesive-frictional soils). Similarly, the influence of primary waves increases with the tunnel cover depth. For the input parameters used here, ignoring the vertical acceleration in the analysis underestimates the maximum dynamic support pressure by a maximum of 94%. This study also identified regions of negative dynamic support pressure, highlighting the importance of the primary wave’s direction, and reasoned the change in its extent in relevance to soil’s shear strength parameters and tunnel cover depth. 

Keywords: Tunnel, Dynamic support pressure, Primary waves, Limit analysis, Earthquake, Stability 

Gowtham, G., and Sahoo, J.P. (xxxx). Significance of Primary Waves on the Seismic Stability of Underground Tunnels: A Numerical Perspective. International Journal for Numerical and Analytical Methods in Geomechanics. Under Review