## 1 Introduction  
F-T cycling, an intense physical weathering process in nature [1], alters soil's granular skeleton and spatial architecture through water phase transitions, thereby modifying its physico-mechanical properties [2, 3, 4, 5]. Over half of China's landmass lies within seasonally frozen regions [6, 7], with the Loess Plateau being nearly entirely subjected to these cyclic effects [8; 9; 10]. Beneath the Quaternary loess deposits, a well-preserved Neogene Hipparion laterite stratum (termed Northern Red Clay) predominates across the Loess Plateau and adjacent areas [11; 12]. Composed predominantly of montmorillonite and illite clay minerals, this hydro-sensitive material [13; 14; 15] undergoes swelling-induced strength reduction upon hydration, critically governing loess landslide and collapse mechanisms [16]. Investigating F-T impacts on its mechanical behavior therefore carries critical implications for geohazard mitigation in these vulnerable terrains. Recent climate change and infrastructure expansion in cold regions have heightened research interest in F-T mechanisms affecting soil mechanics and geotechnical stability [17; 18; 19]. F-T processes fundamentally reconfigure soil systems through multi-dimensional modifications: (1) granular rearrangement and pore structure evolution, (2) physicochemical property transformations, (3) mechanical parameter degradation. These alterations collectively drive significant variations in soil strength, compressibility, and long-term stability. Soil mechanical responses under cryogenic conditions are governed by a complex interplay of intrinsic and extrinsic factors, including but not limited to moisture content and phase transition dynamics, thermal gradients and F-T cycle frequency, porosity and pore size distribution heterogeneity, particle size gradation and mineralogical composition, confinement stress regimes (e.g., overburden pressure), soil fabric architecture, and diagenetic history [20; 21; 22; 23]. Contemporary consensus identifies F-T cycling as the primary causative agent of progressive mechanical deterioration in cold-region soils [24]. Experimental studies have demonstrated that the initial F-T cycle induces the most pronounced reduction in soil strength [25; 26; 27], with significant divergence observed in stress-strain curves after three cycles, transitioning from pronounced strain-softening to moderate softening behavior [28]. Progressive mechanical degradation occurs with increasing F-T cycles [29], exemplified by reductions of 85% in unconfined compressive strength and 92% in elastic modulus after seven cycles [30]. Shear strength parameters exhibit distinct deterioration patterns: subgrade clay experiences a rapid cohesion loss within the first few cycles [31], sulfate-rich saline soils show abrupt declines in internal friction angle between cycles 9-20, with cohesion reaching minima post-20 cycles [32], while chloride-dominated saline soils exhibit cohesion and internal friction angle reductions of 54.4-71.4% and 28.3-29.6%, respectively, after 30 cycles [33]. These trends confirm that sustained F-T cycling systematically weakens soil strength and stiffness [10; 24; 34], markedly reducing peak and residual strengths through microstructural alterations, including pore proliferation, particle rearrangement, and microcrack propagation. Consequently, elucidating the microstructural evolution under F-T cycling is imperative for understanding the fundamental mechanisms driving this macroscopic mechanical degradation [35; 36; 37; 38; 39; 40]. F-T cycling profoundly reshapes soil microstructural porosity through mechanisms including phase transitions of pore water, ice crystal growth, and thermal stress generation, consequently altering the soil's physical and mechanical properties [41]. The degradation of soil mechanical behavior arises primarily from microstructural damage induced by cryogenic processes [42; 43; 44].