Experimental Study of Freeze–Thaw Effects on the Dynamic Characteristics of Compacted Loess with Various Moisture Contents

Compacted loess exhibits significant degradation in its engineering properties after undergoing freeze–thaw (F-T) cycles, posing a substantial disaster risk under dynamic loading. However, the dynamic characteristics of post-F-T loess and the physical mechanisms governing these changes remain unclear. This study investigated compacted loess specimens with varying moisture contents. A series of laboratory tests, including F-T tests, dynamic triaxial tests, and scanning electron microscopy tests, was conducted to determine the influence of F-T cycles and moisture content on the dynamic stress versus dynamic strain (σd–εd) relationship, dynamic shear modulus, and damping ratio of compacted loess. The mesostructural evolution characteristics of compacted loess with different moisture contents under F-T cycles were analyzed. The dynamic deformation mechanism of compacted loess with high water content was explored. The results suggest that the σd–εd relationship of the post-F-T cycles compacted loess conforms to the Hardin–Drnevich model. The initial moisture content of the specimens significantly influenced the dynamic stiffness and profoundly affected the microstructure of the compacted loess. After the F-T cycles, the structural compactness of the compacted loess with a moisture content of 9.6% initially increased and subsequently decreased. The dynamic shear modulus increased and reached its peak value in seven F-T cycles and then declined with increasing F-T cycles, whereas its damping ratio initially decreased and then increased. The structures of the compacted loess specimens with moisture contents of 14.6% and 20.6% tended to loosen. The dynamic shear modulus decreased continuously with increasing F-T cycles, whereas the damping ratios increased continuously. Under long-term F-T cycles (greater than or equal to 15 F-T cycles), the dynamic shear modulus of the loess with a moisture content of 20.6% exhibited a rebound and increase, coupled with a decrease in the damping ratio.