Abstract: This study investigates the mechanical properties and microstructural evolution of peaty soils under freeze-thaw (F-T) cycles through unconsolidated undrained (UU) triaxial shear tests and scanning electron microscopy (SEM). The effects of F-T cycles (0−30), confining pressure (100−400 kPa) and fiber content (0%−12%) were systematically evaluated. Results indicate that the ultimate strength and shear strength decrease as the number of freeze-thaw cycles increases, with the most pronounced reduction occurring during the first 5 cycles. Beyond 15 cycles, the rate of decrease diminishes, and the curves tend to flatten. The fiber reinforcement significantly mitigates the strength degradation caused by freeze-thaw cycles. The most significant improvement is observed at a fiber content of 12%. The fibers act as bridging elements that improve soil particle connectivity, thereby strengthening cohesion and mitigating particle displacement and deformation during freeze-thaw processes. SEM analysis reveals that fiber-soil interaction mechanism undergoes a progressive transformation with increasing fiber content, evolving from localized fiber embedding to comprehensive network formation through fiber entanglement. This structural evolution establishes robust inter-aggregate connections that enhance the soil matrix integrity. Following 30 freeze-thaw cycles, no apparently penetrating fissure was formed although the freeze-thaw process damaged the connection between fibers and soil aggregates, demonstrating the effectiveness of fiber reinforcement in mitigating freeze-thaw damage. These findings provide critical insights into the microstructure-mechanical properties relationships of peaty soils, offering practical guidance for foundation treatment in seasonal frozen peat regions and controlling the engineering diseases problems.

Key words: peaty soils, freeze-thaw cycles, mechanical properties, microstructural evolution, fiber reinforcement