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.

In seasonal frost areas, an organic soil stratum is often encountered during engineering construction due to the widespread existence of organic soils. The soil stratum will experience frost heave in winter and thaw settlement in summer, resulting in a signiflcant variation in its engineering behaviour, especially for organic soil stratum. In this study, with the help of cement, fly ash, and fulvic acid, cement-fly ash stabilized organic soil (CFOS) specimens were prepared and the unconflned compressive (UC) and mercury intrusion porosimetry (MIP) tests were carried out on CFOS specimens. The effects of fly ash content, number of freeze-thaw cycles (FT-N), and curing period on the strength, resilient modulus, and porosity were investigated. Test results revealed that the fly ash content increased from 0% to the optimum content of 5%, the unconflned compressive strength (UCS) and resilient modulus of CFOS with FT-12 increased by 50.30% and 118.92%, respectively. The pore size distribution (PSD) curve and fractal dimension (Dn) of specimens were obtained from the MIP test. The proportion of macropores was the main factor affecting the UCS of CFOS. With the increasing FT-N, the macropore proportion of CFOS with 5% fly ash content increased by 333.33%, and the UCS decreased by 28.25%. Based on a freeze-thaw damage model, the damage parameters of the CFOS specimens were extracted with the Dn as an independent variable. The microscopic pore characteristics and the relationship between the strength, Dn and damage parameter were analyzed. The microscopic pore structure of specimens with different fly ash contents experienced a change subjected to freeze-thaw cycles. The lower the strength, the lower the Dn of CFOS. The damage parameters quantitatively reflected the damage degree of specimens after freeze-thaw cycles on the micro scale. The damage parameter of CFOS with 5% fly ash content increased by 341.57% with the increase of FT-N. The introduction of fly ash was able to reduce freeze-thaw damage to the specimens. The damage parameter and CFOS strength exhibited a signiflcant correlation. The relationships of the strength and microscopic pore structure of CFOS subjected to freeze-thaw cycles were conducive to a better understanding of the freeze-thaw damage mechanism of CFOS on the micro scale. These flndings are beneflcial for engineering construction design in seasonal frost areas.