The influence of freeze-thaw cycles on the shear strength of Illite clay

Abstract

Thermo-active structures, such as energy piles, are used to store and extract heat energy from soil. In areas with seasonal freezing and thawing, such infrastructure is subjected to changes in the soil structure and, consequently, the soil strength. This research investigated the influence of repeated freeze-thaw (FT) cycles and different freezing rates on the shear strength of a frost susceptible Illite clay. Samples of saturated clay were subjected to between 1 and 20 FT cycles, and the shear strength was determined using undrained unconsolidated triaxial tests on the thawed samples. Soil subjected to freezing exhibited a reduction in shear strength compared to never-frozen soil. Slower freezing rates (warmer surface temperatures) resulted in lower shear strength. Results indicated an inverse relationship between the number of FT cycles and the shear strength with a constant applied freezing temperature. Strength recovery occurred between 1 and 3 freezing cycles. Between 3 and 7 FT cycles, the shear strength decreased, after which it approached an equilibrium shear strength between 7 and 10 FT cycles. The increase in shear strength between 1 and 3 FT cycles at a high freezing rate was not identified in the literature reviewed, but coincides with a decrease in stiffness and ice lens thickness. The reduction in shear strength with increasing FT cycles was attributed to movement of pore water through the sample and formation of ice lenses, which damaged the soil microstructure. The ice lenses formed via cryogenic suction pulling unfrozen pore water towards the freezing front and layer where a new ice lens was growing. Macro-CT scans showed decreasing size of ice lenses with increasing FT cycles, and denser ice lens formation near the freezing surface. The largest change in shear strength and ice lens formation occurred between a surface temperature of -5 and -10°C. Ice lenses increased in size moving away from the freezing surface, and a saturated ‘slurry’ layer formed when the samples thawed. Samples failed along the plane of the largest ice lens. Slower freezing rates resulted in thicker ice lenses and slurry layers, which resulted in lower shear strength and stiffness. At high freezing rates, the soil stiffness was almost double that of the never-frozen clay due to local consolidation of clay fragments. With decreasing freezing rates, local consolidation was offset by formation of large horizontal ice lenses. CT scans indicated that after being exposed to multiple FT cycles, the microstructure of the clay was destroyed. The evolution of shear strength should be taken into account in geotechnical design, as a period of thermal consolidation may be required prior to full loading and the rate and number of times a soil is frozen significantly impacts its shear strength.