In-situ freezing characteristics of subgrade silty clay under traffic-induced cyclic stresses

Subgrades in cold regions undergo the coupling effect of freezing and traffic loads. In-situ frost heave plays an essential role in total frost heave deformation for subgrades with low permeability or low groundwater levels. A series of freezing tests were conducted under a closed system, considering the effects of traffic-induced vertical cyclic stress and horizontal cyclic shear stress. The experimental results show that the freezing process consists of supercooling, free water freezing, and film water freezing processes. Increasing vertical cyclic stress amplitude promotes consolidation and reduces water content, enhancing localized supercooling and critical nuclei formation probability. Loading frequency and amplitude of vertical cyclic stress are both influencing factors for soil freezing process. Under horizontal cyclic shear stress, the spontaneous nucleation temperature rises first and then falls with increasing stress amplitude and frequency. When vertical cyclic stress and horizontal cyclic shear stress are applied together, the critical crystal nuclei get much easier to form due to the larger consolidation and lower initial water content. In the closed system, in-situ frost heave of saturated Harbin silty clay includes the volume expansion of water-ice transformation and phase change induced unfilled micropores in the soil skeleton. Cyclic stress increases the in-situ frost heave ratio of the soil by 25 %–63 % per unit volume of pore water phase transformation, with effectiveness from low to high are vertical cyclic stress, horizontal cyclic shear stress, and complex cyclic stress combining vertical cyclic stress and horizontal cyclic shear stress. Finally, an in-situ frost heave ratio prediction model of saturated Harbin silty clay was established, considering the effects of vertical cyclic stress and horizontal cyclic shear stress.