A novel shear strength model for uncompacted loess from microstructure to macro-behavior

Loess, as a typical structural soil, exhibits shear strength characteristics significantly controlled by mesostructural evolution. This study investigates as-built loess in Xi 'an's Chang'an District through systematic triaxial shear tests combined with mercury compression and particle analysis. The research quantitatively reveals the macro-meso mechanical response mechanism of loess under coupled water content and confining pressure. Results demonstrate that as water content increases from 5% to 25%, the shear strength of loess decreases markedly: cohesion drops sharply from 92.52 kPa to 23.76 kPa (a 74.3% reduction), while the internal friction angle decreases from 26.57° to 25.22° with relatively gradual changes. Mesostructural analysis shows that the volume proportion of large and medium pores (diameter ≥ 0.38 µm) decreases significantly with increasing water content. Under 400 kPa confining pressure, this proportion drops from 77.6% at low water content to 50.5% at high water content, while small pore volume increases correspondingly, revealing the structural reconstruction process under combined loading and moisture effects. Based on fractal theory and pore distribution characteristics, a macro-meso correlation model integrating water content, shear stress, and large/meso-pore volume proportion was established. The model achieves an R² value exceeding 0.94 for experimental data fitting, accurately predicting shear strength evolution under various conditions. This study establishes a quantitative mapping relationship between macroscopic strength and mesostructure through physical mechanisms, providing reliable theoretical basis and computational tools for precise prediction and prevention of loess engineering disasters.