Investigation of unsaturated frozen soil behavior: Phase transformation state,post-peak strength,and dilatancy

In this study, a database of triaxial compressive tests on unsaturated frozen soils is compiled to investigate the mechanical behavior that has not been considered in previous studies. The results for the stress-strain volume changes are presented first. Then, the physical mechanisms that might control the deformation of unsaturated frozen soils, namely, volumetric compression and frictional sliding, are used to interpret the changes in volume and deviator stress during the tests. The relationship between the compression rate due to the mean stress and the dilation rate due to shearing determines the changes in sample volume and shear stress with an increasing axial strain. The test results indicate that confining pressure and temperature significantly affect the phase transformation state, the post-peak strength, and the maximum dilation ratio. A higher post-peak strength ratio is observed in tests performed at higher confining pressure or under a higher temperature. As the confining pressure is increased, the shear stress at the phase transformation state initially increases and then decreases or stabilizes. However, the maximum dilation ratio decreases considerably and tends to reach zero after a certain confining pressure is reached. Both the deviator stress at the phase transformation state and the maximum dilation ratio are higher under a lower temperature given constant confining pressure. Ice cementation and pressure melting are attributed to the specific features of frozen soils compared to those of unfrozen soils. This paper provides new insights into the mechanical behavior of frozen soils.