Research on Physical and Mechanical Properties of Granitic Residual Soils at University Town in Guangzhou

Based on the consolidation fast shear tests, the shear strength curves of undisturbed granite residual soil and remodeled soils obtained from Guangzhou University Town were described, and the granite residual soil particle grading curves, chemical composition and their micro-structure changes along depth direction were investigated by XRD, particle size analyses and SEM tests. The results showed that: (1) Compared to soil at a depth of 3 m, the proportion of coarse particles in soil at a depth of 18 m is significantly smaller, while the proportions of silt and clay particles are slightly larger. Iron oxides and quartz caused by weathering and leaching are enriched in the shallow soil layer. The evolution of particle size gradation and the redistribution of secondary minerals are the main factors contributing to the differences in particle size gradation curves and physical and mechanical properties of soil at different depths. (2) The preconsolidation pressure (140 kPa) of undisturbed soil at a depth of 3 m is higher than its current self-weight stress, primarily due to the periodic increase and decrease in matrix suction induced by fluctuations in groundwater level and climatic wetting and drying cycles, leading to gradual compaction of the shallow soil during repeated drainage-suction processes. The hysteresis effect revealed by the measured soil-water characteristic curve in this paper confirms this mechanism. (3) the granite residual soils obtained from Guangzhou University Town were mainly composed and mixed by both sand-silt bonding aggregate and kaolin soil piling aggregate. For the undisturbed residual soil, the pore opening diameter inside the aggregate was less than 1µm which belongs to the category of microporosity, and the pore opening diameter between adjacent aggregates was usually bigger than 1µm. (4) through consolidated-quick shearing test of the granite residual soils, the difference in shear strength between the undisturbed and the remodeled soils was caused mainly by the particle cementation and the dilitancy degree induced by shearing, and the dilitancy degree of the undisturbed soil was usually greater than that of the remodeled soil under over-consolidated state.