## 1 Introduction
Recent advances in F-T research have shifted from single-scale analyses of particle arrangements or pore morphology to multiscale frameworks integrating macro-scale physical, hydraulic, and mechanical properties with microstructural characterization techniques [45]. Contemporary investigations emphasize the coupled effects of multiple factors--such as cycle frequency, moisture content, and particle gradation--on pore structure regulation, elucidating the dynamic evolution of multiscale pore networks [46]. Emerging approaches employ constitutive models [47] complemented by numerical simulations [48] to predict microstructural alterations under cyclic F-T conditions, advancing mechanistic understanding of cryogenic weathering in geomaterials. Experimental studies demonstrate that F-T cycling induces coarse particle fragmentation and fine particle aggregation in soils [49; 50; 51]. Paralleling mechanical property degradation, the initial F-T cycle exerts the most pronounced impact on pore structure [52], with subsequent cycles progressively reducing particle size and increasing porosity [53; 54; 55; 56; 57]. This manifests as a decrease in gel micropores (<10 nm), accompanied by proportional increases in capillary pores (10-5000 nm) and macropores (>5000 nm) [58]. Microporosity diminishes while meso- and macroporosity expand [59], transforming initially dense, low-porosity aggregates into porous, interconnected frameworks [38]--microstructural alterations that directly mirror macroscopic strength deterioration [60]. During freezing, pore water crystallization weakens interparticle cementation, destabilizes aggregates, and enlarges intergranular voids [61]. Thawing generates frost-heave cracks on aggregate surfaces, temporarily reducing pore spacing; however, the irreversible nature of F-T damage prevents structural recovery to the initial state, perpetuating mechanical degradation [62; 63]. Prolonged cycling ultimately loosens soil fabrics, inducing significant structural weakening exacerbated by elevated moisture content [64; 65; 66; 67]. These findings underscore the critical role of cryogenic microstructural evolution in governing the hydro-mechanical deterioration of seasonally frozen soils. Although numerous domestic and international studies have investigated F-T-induced degradation mechanisms in rock masses, loess, and conventional red clays, research specifically targeting the F-T damage evolution of northern Hipparion red soil remains scarce. Critical knowledge gaps persist regarding the quantitative relationship between F-T cycle frequency and the characteristic mechanical strength deterioration patterns (e.g., cohesion decrease rate, internal friction angle reduction), as well as the coupled evolution of micropore structure reorganization. Furthermore, the intrinsic multiscale response mechanisms linking macroscopic strength degradation to microscale structural alterations in this unique Neogene deposit require systematic exploration. Therefore, this study focuses on the Hipparion laterite from the slip zone of the Nao panliang landslide (Figure 1) in the northern Jin-shann Canyon of the Yellow River Basin, aiming to reveal the evolution of mechanical properties and pore characteristics under F-T conditions. Through innovative integration of ring-shear tests (simulating the in situ shear behavior of slip zones) and quantitative SEM image analysis (based on pore parameter extraction and fractal theory), we systematically analyze the macro-micro response mechanisms of the soil under hydro-thermal-mechanical coupling processes. This work addresses the research gap in understanding the engineering geological properties and failure mechanisms of Neogene Hipparion laterite in seasonally frozen regions. The conclusions provide theoretical foundations and practical insights for the early warning and mitigation of loess landslides on the Loess Plateau. Figure 1: Distribution of frozen soil and general situation of Nao panliang landslide in China.