Effect of the seasonal freeze–thaw cycle of the active layer on the seismic response of the free field in the permafrost region

Seasonal freezing and thawing of the active layer in permafrost can significantly alter the seismic response of the free field. In this study, a specific laminar shear model box with a soil-freezing system was designed, and shaking table tests of free fields under different soil-freezing conditions were conducted for ground motions with different intensities. The lateral displacement and acceleration responses of the free field were collected. Based on these results, the acceleration spectrum, shear stress–strain relationship, and frequency component of the ground motions of the free field were analysed. During the tests, the lateral displacement of the free field during both warm (thawed active layer) and cold (frozen active layer) seasons transitioned from linear to nonlinear relationships. The permafrost layer suppressed the amplification effect under ground motion with low intensity, and the frozen active layer enhanced the global stiffness of the free field during the cold season. As the seismic intensity increased, the restraining effect of the permafrost layer on ground motion decreased to a certain extent, and the frozen active layer had a restraining effect on the propagation of seismic waves. In addition, the transition layer (the unfrozen interlayer between the frozen active layer and the permafrost layer) during the cold season undergoes compaction during the nonlinear deformation stage. In the event of strong earthquakes, different soil layers exhibited selective amplification effects, and the frozen active layer exhibited an obvious influence on the filtering effect of the lower soil layers. These results can provide a reference basis for the seismic risk analysis of free field in permafrost regions.