Seismic damage evolution and dynamic characteristics of the surrounding rock in tunnel portal anti-dip slopes reinforced with frame beams

A large-scale shaking table test of an anti-dip rock slope at the tunnel entrance with retaining structures was designed and conducted, and the dynamic responses of the slope and the surrounding rock were analyzed. To investigate the damage evolution law in the surrounding rock, dynamic parameters of the rock mass and marginal spectra were used to study the dissipation law of seismic energy and the distribution characteristics of seismic energy across different frequency bands, respectively. The research findings indicate that the retaining structure effectively inhibits toppling failure at the tunnel entrance; however, considerable toppling deformation persists in the slope, and a through-going crack has developed at the invert section of the tunnel lining. The acceleration amplification effect in the shallow buried section of the tunnel is stronger than that in the deep buried section. Under vertical seismic action, the acceleration amplification effect at the tunnel entrance is significant. The damage analysis based on dynamic parameter changes shows that, with increasing input acceleration, the damaged area is mainly concentrated in the surrounding rock at the lower part of the lining and gradually extends deeper into the slope. Marginal spectrum analysis shows that, under vertical seismic action, the damage to the surrounding rock at the lower part of the lining can be identified according to the energy distribution characteristics in the 9-12 Hz frequency band. The failure state of the lining can be effectively identified through changes in the dynamic parameters and attenuation of the marginal spectrum amplitude of the surrounding rock. For the seismic design of anti-dip rock slopes at tunnel entrances, the seismic performance of the inverted arch in the lining should be improved, and reinforcement measures for the surrounding rock beneath the lining should be intensified. Greater attention should be given to the low-frequency components of vertical seismic waves.