Seismic stability analysis of piggyback dual circular tunnels in granular soils

The use of urban underground space is increasing due to the rapid urbanization of cities worldwide, and piggyback twin tunnels are inevitable in such cases. The present study uses the modified pseudo-dynamic approach to analyze the interaction effect between piggyback twin circular tunnels in a granular medium under vertically propagating shear and primary waves. Emphasis is given to determining support pressure and stress distribution in the finite element lower bound limit analysis framework and second-order cone programming. The support pressure has been evaluated using the Unequal Normal Stress Approach (UNSA). From the distribution of normal stresses obtained using UNSA, the support pressure for each tunnel must be greater than the maximum normal stress exerted by the soil on the tunnel periphery. The normal stress on the top tunnel’s invert and bottom tunnel’s crown was significant for closely spaced tunnels. Due to body wave propagation, the normal stress distribution is skewed towards the predominant direction of the horizontal acceleration. The variation of support pressures of both the tunnels with soil friction angle, cover depth, separation distance, peak base amplitude of seismic acceleration, dimensionless frequency, and time of seismic waves was presented. Tunnels placed at a close center-to-center spacing and a relatively shallow depth are highly vulnerable to seismic body wave propagation. The practical application of the current study has been demonstrated considering the 1995 Kobe earthquake in Japan. 

Gowtham, G., and Sahoo, J.P. (2025).  Effect of seismic body wave propagation on the stability of piggyback twin circular tunnels. Journal of Earthquake Engineering. https://doi.org/10.1080/13632469.2025.2515444