Response of a tunnel in double-layer rocks subjected to harmonic P- and S-waves

• Seismic responses of tunnels at various depths in double-layer rocks are studied numerically.
• Seismic induced stresses are maximal when normalized tunnel depth is 0.25 multiplying an odd.
• Wave reflection from free surface and layer boundary and tunnel scattering amplify stress.
• Impedance ratio links seismic responses of double rock layers subjected to an incident wave.
• Impedance ratio affects the seismically induced stress increments in a tunnel lining.

The combination of tunnel depth and incident wavelength has been numerically revealed as a key factor dominating seismic responses of a tunnel in a homogenous rock layer. To investigate further the seismic response of a tunnel in double-layer rocks and to clarify the effect caused by the layered rocks, this study applies the dynamic finite element method and investigates seismically induced stress increments in the lining of a circular tunnel subjected to an incident harmonic P- and S-wave. Analysis results reveal that the normalized seismically induced stress increments are maximal when normalized tunnel depth is 0.25 multiplying an odd number. Additionally, normalized seismically induced stress increments are minimal when normalized tunnel depth is 0.25 multiplying an even number. Further, the impedance ratio links the seismic responses of upper and lower rock layers subjected to incident harmonic waves, which may significantly amplify incident stress in the upper rock layer, thereby affecting the seismically induced stress increments in a tunnel lining.