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

Geo-energy infrastructure, such as ground source heat systems (thermo-active structures), induce thermal cycles that can result in changes of the bearing capacity of soil by changing, for example, the void ratio, soil structure, unit weight and hydraulic conductivity. The influence of repeated freeze-thaw (FT) cycles and different freezing rates on the shear strength of a frost-susceptible illite clay was investigated. Samples were subjected to between one and 20 FT cycles, and the shear strength of the thawed material was determined using undrained unconsolidated triaxial tests. After the shear strength decrease due to the first FT cycle, a transitory shear strength recovery occurred between one and three freezing cycles, followed by a shear strength decrease between three and seven FT cycles, which then approached an equilibrium value. Computed tomography scans showed ice lenses increased in size moving away from the freezing surface, and more uniform ice distribution with increasing FT cycles. Changing the freezing rate yielded differences in the formation and structure of ice lenses perpendicular to the freezing direction. The observed failure plane typically coincides with the plane of the largest ice lens due to formation of a slurry layer after thawing.