Microscopic Damage Evolution of Moraine Soils under Freeze-Thaw Cycles Based on PFC^2D Simulation

In order to investigate the microscopic damage mechanism of the degradation of the properties of freezing-thaw (F-T) damaged moraine soils, a method of simulating F-T damage of soils through water particle expansion is proposed based on the discrete element theory. Using the particle flow software PFC^2D to simulate the triaxial compression test, combined with the comparative analysis of the test results, this method is accurate and reliable in modeling the changes in mechanical properties of moraine soils and reveals the evolution of microcrack; displacement field; force chain field and rupture characteristics of F-T moraine soils during the loading process. The results show follows (1) The microcracks in the F-T process show a trend of "cumulative evolution" that arises from the surrounding area and gradually expands to the middle, and the tensile microcracks are dominant; at 2-5 times of F-T processes, horizontal compression between particles dominated and a large number of tension microcracks inclined at 90° were developed. (2) The deterioration of moraine properties caused by F-T is particularly obvious in the early freeze-thaw period (2-5 times), the cohesion c decreases as a negative exponential function with the number of F-T cycles, while the internal friction angle φ shows a small fluctuation. (3) The specimen loaded process is dominated by shear cleavage, with the trend of "first slow, then steep, and finally slow" evolution, and the stress-strain curve is divided into four deformation stages according to the evolution characteristics. (4) When the sample is loaded after freeze-thaw, the transition point B of deceleration slope moves before the peak stress point C, indicating that point B can be used as a "precursor feature" in the process of microcrack expansion-through-formation failure; The specimen with 20 F-T cycles were more severely damaged when loaded and formed distinct shear zones.