Mechanical properties of ice-frozen soil interface under cementation of ice crystals

Water migration often occurs in the permafrost slopes during seasonal variations. The water can be accumulated in the vicinity of the freezing front, and then congealed into ice, due to the influence of gravitational and temperature fields. The underlying ice layer was gradually formed with the increase of ice thickness and area. The interface between the ice layer and the overlying frozen soil can play a crucial role in the stability of the permafrost slope. The mechanical behaviors of ice-frozen soil interface can provide the theoretical support for the stability assessment of permafrost slopes over the long term. This research aims to investigate the mechanical properties of the ice-frozen soil interface. A series of direct shear tests were conducted under the temperature of −3 ℃. The shear stresses of the ice-frozen soil interface and the ice-soil particles interface were investigated under different normal stresses, initial water contents, and initial void ratios of the frozen soil. The experimental results showed that the shear stress of the ice-soil particles interface exhibited the hardening behavior, and then increased with the decreasing void ratio of the frozen soil. Softening behavior was found in the shear stress of the ice-frozen soil interface, particularly under lower normal stresses. The shear stress also increased with the increasing initial water content of the frozen soil. The structural coefficient, damage variable, and structural strength of the ice-frozen soil interface were defined, according to the mechanical properties and the failure evolution of the ice-soil particles interface and ice-frozen soil interface. There was an increase in the shear displacement corresponding to the structural coefficient peak point of the ice-frozen soil interface with the increase of initial water content. Furthermore, the initial water content of frozen soil increased from 14% to 18%, while the shear displacement increased by 32.7%, 41.3%, and 52.1%, respectively, when the initial void ratios of frozen soil was 1.0, 0.8, and 0.6. A nonlinear elastic damage model was established to consider the bonding characteristics of ice crystals at the ice-frozen soil interface using the Duncan-Chang model, where the tangent stiffness was obtained. The calculation results demonstrate that the shear stress curve suddenly decreased after reaching the peak strength under low normal stress, when the ice content at the interface was high, indicating the noticeable brittleness interface. The mechanical properties of the ice–frozen soil interface shifted gradually from brittleness to plasticity, as the normal stress increased. There was the significant decrease in the magnitude of the shear stress after the peak strength. Therefore, the inhibitory effect of the increasing normal stress on the brittleness failure of the ice-frozen soil interface. The calculation was matched better with the experimental before the peak shear stress. By contrast, the decrease rate of the calculated shear stress curve was significantly lower than the experimental after the peak strength. The tangent stiffness of the interface increased with the increase of normal stress under the same initial void ratio and water content of the frozen soil. The normal stress increased from 50 to 200 kPa, while the tangential stiffness peak point of the interface increased by 63.7%, when the initial void ratio and water content of frozen soil were 1.0, and 18%, respectively. The increasing normal stress was induced a shear movement of soil particles at the ice-frozen soil interface, thus promoting the pore healing and strengthening the interface. Conversely, once the normal stresses were the same, the tangent stiffness of the interface increased with the increase of initial water content of the frozen soil, as the shear displacement was smaller than u₀. While the shear displacement was larger than u₀, the tangent stiffness gradually decreased with the increase of shear displacement and approaches zero.