Influence of freeze thaw on stress-strain characteristics and microstructure of cement and fly ash stabilized organic soil

Organic soil is often encountered in seasonally frozen areas in China. Before construction, the organic soil is required to be treated to improve its engineering performance due to the high moisture content and low bearing capacity. Cement and fly ash were adopted in this study to treat organic soil subjected to natural freeze-thaw cycles. The influences of freeze-thaw cycles on the stress-strain behavior and microstructure of cement and fly ash-stabilized organic soil (C-F-S-O-S) were evaluated using unconsolidated undrained triaxial (U-U), mercury intrusion porosimetry (MIP) and CT experiments. With and without freeze- thaw cycles, results indicate that the specimen with 20% cement and 5.0% fly ash content performed the best in strength and was selected to evaluate the influence of freeze-thaw cycles on C-F-S-O-S mechanical and microstructure characteristics. The strength, elastic modulus (E-M), cohesion, and internal friction angle of the specimen show the largest decrease of 9.27%, 13.97%, 3.45%, 5.19% after the first freeze-thaw cycle and then slow decreased with further increase of the number of freeze- thaw cycles. The strain corresponding to the peak stress increased with increasing freeze-thaw cycles, and the increase was the largest with a value of 10.19% after the first freeze-thaw cycle. Relationships between the number of freeze-thaw cycles and above parameters were established. A generalized model was also established to predict the stress-strain curve of the C-F-S-O-S. The applicability of the proposed model was validated with published experiment data. The specimen porosity increased first (by 11.03%) and then gradually stabilized after a series of freeze-thaw cycles as revealed by the MIP. Consequently, MIP and CT analysis reveals the soil structural variation since the freeze-thaw cycle is the main reason of the reduction of the specimen strength after the freeze-thaw cycle.