Abstract: The foam concrete serving as a tunnel damping material plays an important role in resisting seismic compression-shear load. This study aims to investigate failure characteristics and shear stress evolution law of the contact surface between the lining and shock absorption layer under seismic loads. Thus, direct shear tests are carried out under different normal stress levels by using RMT-150C electro-hydraulic servo testing machine. We obtain the variations of shear stress-shear displacement curves, peak strength, residual strength, shear stiffness as well as the failure characteristics and physical state changes of the contact surface through tests. Meanwhile, according to the FEM simulation results, the distribution and evolution laws of shear stress in contact surface are studied. The results show that the failure characteristics of the contact surface between the lining and foam concrete are affected by the normal stress and the density of foam concrete. In addition, shear stress-shear displacement curves are divided into three types which could be converted to each other with the changes in foam concrete density and the normal stress. The influences of the foam concrete density and normal stress on the peak strength, residual strength and shear stiffness of the contact surface are mutual, and the influence of normal stress weights higher. Finally, according to the variation of shear stress-shear displacement curves, the composite index model is proposed to reflect the damage evolution and friction-slip process of the contact surface between the foam concrete and lining under the compressive-shear load. Therefore, this study can provide certain references for the effective aseismic design.

Key words: contact surface, direct shear test, numerical simulation, foam concrete, composite index model