Physical–Mechanical Properties and Microstructural Evolution Mechanisms of Typical Loess in the Yan’an Area

To elucidate the mechanical and seepage properties of typical Yan'an loess at different burial depths and its microstructural evolution patterns, this study systematically investigates four in situ loess layers (L₁–L₄) from a representative profile in Fuxian County, Yan'an. The objective is to clarify the synergistic mechanism of “structural densification–strength enhancement–permeability reduction,” providing scientific basis for regional loess stratum stability assessment and impermeability reinforcement design. Systematic investigations were conducted on the four loess layers using direct shear tests, permeability tests, and scanning electron microscopy (SEM) analysis. Pore parameters were quantitatively extracted using Avizo software. Data coupling analysis was performed via Pearson correlation analysis and principal component analysis (PCA). Results indicate: ① Shear strength exhibits a layered pattern with burial depth, showing significantly increased cohesion and a slight decrease in internal friction angle. ② The permeability coefficient decreases significantly with depth, exhibiting a “permeable upper layer, dense lower layer” characteristic, plummeting from 17.79×10^-5 cm/s in Layer L₁ to 7.55×10^-5 cm/s in Layer L₄. ③ At the microstructural level, particle contact patterns change from point contact and weak cementation in shallow layers to surface contact and strong cementation in deeper layers. Pores evolved from large pores (>35 μm) to medium-small pores (<30 μm), with shapes tending toward roundness. The orientation shifted from vertical dominance to uniform distribution. ④ Pearson analysis indicates a significant positive correlation between dry density and cohesion, while showing significant negative correlations with total pore area and permeability coefficient. PCA analysis reveals PC1 (contribution rate 70.8%) as the primary factor governing “structural densification-strength enhancement,” clearly distinguishing sample distributions between L₁ (loose, high permeability) and L₄ (dense, low permeability) layers. Conclusions indicate that the evolution of physical and mechanical properties in Yan'an loess is jointly governed by compaction and microstructural remodeling. With increasing burial depth, loess undergoes a “compaction-remodeling-densification” process: particles become more tightly packed, contact modes shift to surface contact, and large pores are compressed and destroyed, replaced by medium and small pores. Macro-scale manifestations include increased cohesion and significantly reduced permeability. Dry density, cohesion, and pore characteristics collectively govern the overall evolutionary trend of loess from loose and highly permeable to dense and low-permeability.