Relationship between macro-mechanical properties and microstructures of Youxi granite residual soil

This study firstly investigates the particle size distribution curve, mineral composition, basic physical properties, and soil-water characteristic curve of the Youxi granite residual soil through laboratory tests. Subsequently, by utilizing environmental scanning electron microscopy tests on compacted soil sample and compacted saturated soil sample, as well as consolidated undrained (CU) triaxial test results on compacted saturated soil samples, the influence of the soil's microstructure on the macroscopic stress-strain curves and strength parameters is analyzed. Investigation results that shows: (1) Under the same dry density, the deviatoric stress-axial strain curves of both dry-side and wet-side compacted saturated soil samples generally exhibit strain hardening behavior. The shear strength of the saturated granite residual soil samples decreases with an increase in compaction water content, while the cohesion decreases with increasing compaction water content. The internal friction angle slightly decreases with increasing compaction water content but shows little variation. (2) Under the same dry density, the pore pressure of both dry-side and wet-side compacted saturated soil samples increases nonlinearly with axial strain. After reaching the peak, the pore pressure of the samples shows a slightly and slowly decline or remains stable. The pore pressure of dry-side compacted saturated soil samples is lower than that of wet-side compacted saturated soil samples. (3) Scanning electron microscopy tests reveal that the dry-side compacted granite residual soil exhibits a flocculated structure, while the wet-side compacted soil shows a mixed structure of dispersed and flocculated arrangements, with the dispersed structure being dominant. These structural differences are primarily controlled by the magnitude of matric suction within the soil. (4) After saturation, the larger matric suction in dry-side compacted soil samples decreases to zero, leading to relatively greater movement of aggregates. In contrast, the smaller change in matric suction in wet-side compacted soil samples results in relatively smaller movement of aggregates. Consequently, the void ratio of dry-side compacted soil samples after saturation is smaller than that of wet-side compacted soil samples. This explains why the peak strength of dry-side compacted saturated soil samples is greater than that of wet-side compacted saturated soil samples, and also why the pore water pressure of dry-side compacted saturated soil samples is lower than that of wet-side compacted saturated soil samples.