Strength Characteristics of Unsaturated Compacted Loess Under Complex Stress Paths

A series of (45 sets) true triaxial shear tests with controlled suction and under conditions of equal-b and equal-p are performed on unsaturated compacted loess. By incorporating the effects of matric suction in unsaturated loess and the influence of middle principal stress conditions, the characteristics of the stress–strain curve evolution, strength failure line and related strength parameters of the unsaturated compacted loess under complex stress paths are analyzed, and the applicability of different strength criteria is analyzed. The results indicated that both the matric suction and the middle principal stress condition have significant effects on the magnitude of shear stress. While matric suction exerts minimal effect on the configuration of the stress–strain curve, the middle principal stress condition markedly affects its shape. A function is established to approximately describe the correlation between the strength-related parameters and the middle principal stress coefficient. A comparison of the failure lines predicted with several strength criteria and the experimental data indicates that the experimental results of unsaturated compacted loess under a controlled suction state are consistent with the results of the predicted failure lines of the Lade–Duncan strength criterion. The findings can provide crucial theoretical support for engineering design and disaster prevention in loess regions.