not-yet-known not-yet-known not-yet-known unknown 1 3 RESULTS AND DISCUSSION 1.0.1 3.1 Interfacial shear stress-shear displacement curve Figure 7 illustrates the influence of different test temperatures on the interfacial shear stress-shear displacement curves (w=13%, k=1100kPa/mm). Overall, during the initial stage of shearing, the deformation of the interface is primarily elastic, and the shear stress increases linearly with the increase in shear displacement 25. A higher normal stress results in a greater initial shear stiffness of the interface. Under the same normal stiffness, as the temperature decreases, the initial shear stiffness gradually increases. This is due to the increasing content of ice crystals at the interface and the corresponding increase in bonding strength as the temperature drops. Subsequently, as the shear displacement increases, the shear stress exhibits a nonlinear growth pattern, and the shear stiffness of each curve gradually decreases, indicating a slowdown in the growth rate of shear stress. The shear stress behavior of the interface can generally be classified into strong hardening, weak hardening, and softening types 26,31. As seen in Figure 7(a), the curve at 20℃ exhibits strong hardening characteristics. As the shear displacement increases, the shear stress continues to rise, and the growth rate of shear stress in the nonlinear stage decays relatively slowly. From Figures 7(b), (c), and (d), it can be observed that under negative temperature conditions, the interface primarily exhibits weak hardening characteristics. In the nonlinear stage, as the shear displacement increases, the growth rate of shear stress gradually decays. Under the same normal pressure conditions, as the temperature decreases, the interface curve transitions from strong hardening to weak hardening, and the initial shear stiffness also continuously increases. This is because as the temperature decreases, the content of ice crystals at the interface gradually increases. On one hand, this enhances the interfacial shear strength and initial shear stiffness. On the other hand, the cementation effect of ice crystal particles increases the strength of the soil in the shear zone, making it less likely for particles at the interface to roll or exhibit other similar behaviors during shearing. Therefore, the shear curve transitions from strong hardening to weak hardening. FIGURE 7 Shear stress and shear displacement curves (k =1100kPa/mm,w =13%) Figure 8 displays the shear stress-shear displacement curves of the interface under different water contents (k=1100kPa/mm, T=-4℃). It can be observed that the morphology of the interface curves does not vary significantly under different water content conditions, all exhibiting a weak hardening behavior. Since the temperature remains constant, both the content of ice crystals and the unfrozen water increase simultaneously, resulting in subtle changes in the curve morphology. Under the same normal pressure, as the water content increases, the shear stress value in the stable stage of the curve gradually rises. This is because as the water content increases, the number of ice crystals formed within the soil and at the interface gradually increases. At lower normal pressures, the final shear stress increases slightly with the increase in water content. However, at higher normal stresses, the final shear stress increases relatively more significantly with the increase in water content. This indicates that under negative temperature conditions, the increase in the content of ice crystals can alter the apparent friction angle of the interface to a certain extent. FIGURE 8 Effect of water content on the shear stress and shear displacement curves (k =1100kPa/mm,T =-4℃) Figure 9 illustrates the relationship between interfacial shear stress and shear displacement under different normal stiffnesses and test temperatures (σ N=100 kPa, w =13%). As shown in Figure 9(a), under thawed conditions, the curve morphology does not exhibit significant changes as the normal stiffness increases, exhibiting strong strain hardening behavior with a relatively small elastic deformation stage. The shear stress at the end of the test gradually increases with the increase in normal stiffness because a higher normal stiffness constrains the rolling of interfacial particles, leading to more plowing and frictional motion among particles. Both of these motion forms require greater loads, resulting in an increase in the final shear stress 12,21,22. Comparing Figures 9(a) and (b), it can be observed that when the interface temperature decreases from room temperature to sub-zero, the curve morphology transitions from strong hardening to weak hardening. The initial elastic stage of shearing becomes more pronounced, corresponding to a larger displacement. In the residual stage, the increment of shear stress with increasing shear displacement is smaller. The final shear stress is greater than that at room temperature due to the formation of ice crystals at the interface under sub-zero conditions. Figure 9(c) presents the interfacial curves at -4℃. It can be seen that the curves exhibit softening behavior when the normal stiffness is 200 and 500, while hardening behavior is observed at 800 and 1100. Research indicates that the softening phenomenon at frozen interfaces under constant normal stress conditions is caused by brittle fracture of interfacial ice crystals. However, Figure 9(c) reveals that the type of interfacial shear curve is not only related to the brittle fracture of interfacial ice crystals but also depends on the normal confinement conditions of the interface. Different normal confinement conditions alter the motion state of interfacial soil particles, which, combined with the deformation of ice crystals, leads to changes in the type of interfacial shear curve.Comparing Figure 9(b), as the normal stiffness increases, the final shear strength also increases significantly, indicating that the deformation of interfacial ice crystals or ice-soil aggregates during shearing is significantly influenced by the normal stiffness confinement conditions.Figure 9(d) shows that the interfacial curve exhibits strain hardening behavior. With the increase in normal stiffness, the increment of interfacial shear stress becomes more pronounced. At the same normal stiffness, the final interfacial shear stress increases as the temperature decreases.。 FIGURE 9 Effect of temperature and normal stiffness on the shear stress and shear displacement curve(σ N=100 kPa,w =13%) 1.0.2 3.2 Interfacial Shear Sterngth Figure 10 illustrates the impact of different test temperatures on the peak shear strength at the interface. Typically, the shear strength at the soil-structure interface can be fitted using the Mohr-Coulomb strength criterion, allowing for the calculation of strength parameters. It can be observed that the shear strength of the interface satisfies the M-C shear strength criterion with respect to normal pressure. At a constant temperature, the interfacial shear strength increases linearly with the increase in normal pressure. At a constant normal pressure, the shear strength gradually increases as the test temperature rises, and the increase is more significant under high normal stress. As the temperature decreases, both the interfacial cohesion and the interfacial friction angle gradually increase, indicating that the interfacial ice crystals have an impact on both of these strength parameters. 1 3 RESULTS AND DISCUSSION