Effects of drying-wetting cycles and free iron oxides on the mechanical behaviors of a partially decomposed granite residual soils

It is widely acknowledged that granite residual soil (GRS) is highly structured and water-sensitive due to the interparticle bonding of free iron oxides (FIOs). To further understand the effects of drying and wetting (D-W) cycles and the content of FIOs on the mechanical responses of GRS, a series of tests were presented to reveal the microstructure characteristics and mechanical behaviors of GRS under different D-W cycles (0, 1, 2, 4). The particle size distribution (PSD) curve exhibited evident bimodal peaks by the dynamic light scattering (DLS) analysis, then changed to single peak curve after the D-W cycles or the removal of FIOs. Quantitatively investigated by the dithionite-citrate-bicarbonate (DCB) method, the content of FIOs in GRS showed a decreasing trend but finally tended to be a stable value after the D-W cycles. Due to the intrinsically cemented structure, GRS presented complicated shear behaviors in response to D-W cycles. A phase transition was clearly observed in the stress-strain relationship of the soil. The GRS at 60 kPa which is undergoing the 0th and 1st D-W cycles exhibited weak strain-hardening type, but it would transform into the strain-softening state after the 2nd D-W cycle. The yield stress reached a maximum value at the 2nd cycle and then decreased with the subsequent D-W cycles. Similarly, cyclic D-Ws resulted in obvious increase in the small-strain stiffness, especially in the 1st cycle. The complicated mechanical properties of GRS are characterized by coupling effects among the irreversible volume deformation and the content of collodion and the development of microcracks.