Abstract: In view of the effect of environmental factor on the strength and stability of subgrade, the clayey soil taken from Siping- Changchun section of Beijing-Harbin expressway was investigated，and its variations of mechanical properties affected by freeze-thaw cycles and water content were thoroughly studied and analyzed in laboratory tests. The influence mechanism was also discussed. The test soil samples with different water contents went through 0 to 16 freeze-thaw cycles, and then the triaxial compression tests were carried out indoor. Results show that with the increasing of freeze-thaw cycles and water contents, the stress-strain curve of the compacted clayey soil changes from strain-softening to strain-hardening when the water content is less than the optimum water content, and the failure mode of the test sample gradually changes from brittle failure form to plastic failure form. The ultimate strength, elastic modulus and cohesion all decrease with the increasing of freeze-thaw cycles, while there is no obvious relationship between the internal friction angle and the freeze-thaw cycle. However, the aforementioned mechanical parameters of the compacted clayey soil all tend to stabilize after 8 freeze-thaw cycles. In summary, the effect of water content on mechanical properties of compacted clayey soil is significant. Ultimate strength, elastic modulus, cohesion and internal friction angle all decrease drastically with the increasing of water content. Based on the consideration of the effect of freeze-thaw cycles and water contents on mechanical properties of compacted clayey soil, the drainage work of subgrade in the seasonal frozen area should be well conducted to keep the subgrade in a lower level of water content, and the engineering design values of mechanical parameters should be those of the soil that undergoes no less than 8 freeze-thaw cycles.

Key words: compacted clayey soil, freeze-thaw cycle, water content, stress-strain relationship, elastic modulus, shear strength parameter