Research on seismic damage of anti-dip rock slope at tunnel entrance and seismic performance of retaining structures

Tunnel earthquake disaster investigations show that damage at tunnel portals is the most severe. During the Wenchuan Earthquake, retaining structures effectively prevented landslides on portal slopes. Therefore, research on the seismic performance of retaining structures at tunnel portals is of great significance. In this paper, a large-scale shaking table test was designed and conducted on an anti-dip rock slope at a tunnel portal reinforced with a prestressed anchor cable(bolt) frame beam. The seismic damage evolution of the slope and the seismic performance of the retaining structure were investigated. The test results indicate that although the support system effectively suppressed macroscopic landslides, significant toppling deformation of the slope still occurred. Vertical seismic waves exhibit a pronounced amplification effect at the tunnel portal. As the input acceleration increases, damage initiates in the shallow rock mass near the frame beam; damage to the surrounding rock beneath the lower portion of the lining intensifies and gradually propagates into the deeper part of the slope. Before the input intensity reaches 1.0 g, the axial force at the anchor head located at the slope crest is maximal; after it exceeds 1.0 g, the axial force at the anchor head of the frame beams above the tunnel portal becomes maximal. Throughout the duration of earthquake excitation, the frame beams on the tunnel portal slope are subjected to alternating tensile and compressive forces. When the input seismic intensity exceeds 0.6 g, all frame beams maintain a stress state of outer - side compression and inner - side tension after the earthquake. The seismic design of slope retaining structures should consider both the force and deformation characteristics during and after the earthquake.