A unified formulation of stress-strain relations considering micro-damage for expansive soils exposed to freeze-thaw cycles

Expansive soils, which contain the clay mineral montmorillonite, are a worldwide problem and occur in many parts of the world. Some cutting slopes at Yanji along the Jilin-Tumen-Hunchun high-speed railway in Northeast China as typical seasonally frozen region consist of expansive soils. The mechanical and microstructural properties of these soils subjected to periodic freeze-thaw (F-T) cycles may change and further produce potential risk to the earth retaining structures in the slope. In this study, a multi-scale approach was implemented and the microstructural and mechanical changes of expansive soils exposed to F-T cycles were investigated. To carry out these tasks, mercury intrusion porosimetry (MIP), X-ray computerized tomography (X-ray CT), and consolidated-drained (CD) triaxial tests were conducted. The microstructural analysis results reveal that a bimodal distribution of pore sizes exists for the expansive soils and the effect of freeze-thaw cycles is concentrated in pores of more than 5 μm. Voids observed by X-ray CT scanner are generated and extended as increasing the number of F-T cycles. The results of mechanical properties demonstrate that when the number of freeze-thaw cycles exceeds a threshold value (i.e., 7), the effect of F-T cycles on the shear strength and volumetric strain loses significance. Based on the experimental results, a unified expression taking into account the micro-damage for stress-strain behaviors of expansive soils exposed to F-T cycles was developed. An average damage variable defined through the CT values was adopted to quantify this micro-damage. The agreement between experimental and predicted results confirms the possibilities of reproducing some most important features of expansive soils suffered to F-T cycles.