Abstract: In spring thawing period, slopes in seasonal frozen zone is susceptible to shallow slide due to thermal thawing. To investigate the stability of ecological slopes during this period, laboratory direct shear tests were conducted on the freeze-thaw interfaces of plain soil and root-soil composites. The root system was simplified as taproot type. The variation in shear strength indices of both soils was compared and analyzed. Through numerical simulation, the shear failure in the plastic zone of samples in the presence or in the absence of freeze-thaw interface was analyzed, and a model was developed to calculate the safety factor of slope stability when plant roots penetrate the freeze-thaw interface. Results indicate that as freeze-thaw cycles increase, the damage to the shear strength of plain soil at the freeze-thaw interface is greater than that of root-soil composite. Additionally, the cohesion of soil at the interface decreases, while friction angle increases for both plain soil and root-soil composite. The shear plastic zone of samples with freeze-thaw interface primarily develops within the soil body in the normal thawing zone and exhibits a significantly larger volume compared to samples with no freeze-thaw interface. By passing through the freeze-thaw interface, roots significantly raise the shear strength of the soil, thus enhancing the slope stability in spring thawing period. These findings provide a scientific foundation for the design, construction, and maintenance of ecological slope projects in seasonal frozen regions.

Key words: root-soil composite, freeze-thaw cycle, freeze-thaw interface, direct shear test, slope stability