Experimental investigation on strength behavior and microstructure of unsaturated net-like red soil

The relationships between matric suction, microstructure, mineral composition, and shear strength of unsaturated net-like red soils are important for engineering practice. Here the strength behavior and microstructure characteristics of an unsaturated net-like red soil from Xuancheng under various matric suctions, net vertical stresses, and the additions of clay minerals were systematically investigated by means of unsaturated directional shear experiments, scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) test, and digital image processing (DIP) quantitative analysis. The results showed that more quartz amassed in the white vein, and more aggregation of Fe oxide occurred in the red matrix. Different clay minerals had different effects on the unsaturated strength. The addition of montmorillonite can increase the water content, and the microstructure of soil is mainly in the surface-to-surface configuration, so the unsaturated strength decreased by 5.38%, 14.52%, and 18.82% with the increment of montmorillonite content. The microstructure of the soil mixed with illite minerals showed a strong structure, which enhanced the strength of the soil by up to 6.45%.