Model experiments to study the hydrothermal variation and resilient modulus of soil subgrade subjected to freeze-thaw conditions

Freeze-thaw (FT) cycles are highly destructive actions on roads in seasonal frozen regions. To examine the mechanical behavior of the subgrade subjected to seasonal FT conditions in close to the actual hydrothermal environment. A hydro-thermal-mechanical (HTM) coupling model test apparatus was developed, FT tests under closed and open boundary conditions were conducted on it. In the tests, hydrothermal variations were investigated and variation behaviors of resilient modulus (MR) were analyzed and compared from the perspective of HTM coupling. The results showed that (i) the models of closed system test (CST) and open system test (OST) experienced similar freezing processes, but the difference in boundary conditions caused a significant freezing rate gap between them; (ii) hydrothermal variations in the two tests were greatly different during the freezing, moisture field variation in CST was relatively typical, while it was more complicated in OST, especially in the unfrozen zone of OST model. In addition, continuous replenishment caused the water content in OST model to be much higher than that in CST model after thawing; (iii) MR was principally related to the state of upper soil layers in both FT tests due to the distribution characteristic of additional stress, and was also affected by the water content of other frozen and unfrozen soil layers. The influence mechanism of water content on OST MR was sophisticated because of the complicated moisture field variation process; (iv) the CST MR tended towards stability after 3 FT cycles. The open system was more adverse to the bearing capacity of the subgrade, with the OST MR after 1 FT cycle even lower than that in CST after 6 cycles.