Volume changes and mechanical degradation of a compacted expansive soil under freeze-thaw cycles

Expansive soils located in permafrost and seasonal frozen regions can easily suffer from the action of frost heaving and repeated freezing-thawing. When exposed to freeze-thaw (F-T) cycles, it may pose risk to civil engineering structures and thus causes heavy economic losses. In this study, a series of cylindrical expansive soil specimens were compacted at three different molding water contents (15%, 20% and 23%) and then subjected to a maximum of 12 closed-system F-T cycles. Besides, selected specimens compacted at the optimum water content were also tested under cyclic freezing-thawing with varying freezing temperatures (-5 degrees C, -10 degrees C and -20 degrees C). After each cycle of F-T, volume changes were measured and unconfined compression testing was also performed to estimate stress-strain behavior, resilient modulus and failure strength. Moreover, meso-structural analysis was conducted by using a simple optical test system to quantitatively extract the surface porosity and the pore orientation degree of expansive soil specimens after different F-T cycles. It is found that: 1) Volume changes for expansive soil specimens with higher and lower saturations present opposite directions and different magnitudes upon freezing, but show a similar trend of volume expansion after thawing. A moderate freezing temperature (i.e. -10 degrees C, in this study) has the greatest effect on volume changes. 2) Expansive soils tend to exhibit strain-softening behavior under unconfined compression conditions. The resilient modulus and failure strength decrease significantly at the first cycle of F-T and then reduce gradually to a stable value with increasing F-T cycles. The higher molding water content and a moderate freezing temperature will lead to a more pronounced degradation of mechanical behaviors with the F-T cycles. 3) It is suggested from the meso-structural analysis that the internal pores of expansive soils after a sequence of F-T cycles tend to become larger and more uniform, especially for the soil with higher water contents or experienced at a moderate freezing temperature.