Microstructural Mechanisms Influencing Soil-Interface Shear Strength: A Case Study on Loess and Concrete Plate Contact

Understanding the shear behavior of loess–concrete interfaces is essential for foundation design in collapsible loess regions, yet the pore-scale mechanisms remain unclear. This study investigates the relationship between interface shear strength and loess microstructure at different burial depths. Direct shear tests were conducted on undisturbed loess samples under stress conditions simulating in situ confinement. High-resolution SEM images were analyzed via Avizo to quantify pore area ratios at multiple scales, fractal dimensions, and directional probability entropy. Pearson correlation, principal component analysis (PCA), and hierarchical cluster analysis (HCA) were employed to statistically interpret the microstructure–mechanics relationship. Results show that interface shear strength increases significantly with depth (35.2–258.4 kPa), primarily due to reduced total porosity and macropore content, increased small and micropore fractions, and enhanced isotropy of pore orientation. Fractal dimension negatively correlates with strength, indicating that compaction-induced boundary regularization enhances particle contact and shear resistance, while entropy positively correlates with strength, reflecting structural homogenization and isotropic pore orientation. PCA and HCA further confirm that small and micropores are the dominant contributors to interface resistance. This study provides a quantitative framework linking microstructural evolution to mechanical performance, offering new insights for optimizing pile–soil interface design in loess areas.