Mechanical and microscopic properties of soil according to the rate of increase in pore water pressure

Increases in pore water pressure affect the effective stress state of loess soil, resulting in deformation and even soil erosion. However, it is still unclear how the mechanical behaviours and microstructures of soil are influenced by the rate of increase in pore water pressure (RIPWP). To better understand the mechanical failure process and properties of soil under different RIPWPs, a series of constant-shear drained (CSD) triaxial tests were carried out on Q 2 loess (silt loam, a loose aeolian deposit) in the South Jingyang Platform of China. Further, scanning electron microscopy (SEM) tests were conducted on the samples before and after the triaxial tests to observe the microscopic characteristics of the soil, and reveal the microscopic mechanism of liquefaction failure in soil under CSD triaxial tests. The results showed that the failure process was divided into two stages of 1) stable creep and 2) sharply unstable deformation. In the second stage, rapid-slow strain development was observed at relatively low RIPWPs. With increases in RIPWP, the critical pore pressure ratio u / σ c increased within the range of 0.48–0.66. Microscopic analysis of the samples after shearing found clay particles filling the pores and obvious aggregation structures. In addition, macropores and mesopores were transformed into micropores. With increases in RIPWP, the orientations of particles and pores increased and the abundance and fractal dimension of pores decreased. In conclusion, the microscopic mechanism of soil instability under CSD triaxial testing was as follows. Increases in pore water pressure led to the dispersion of fine particles, weakening of cementation, and the rotation and recombination of coarse particles within the soil. In addition, the clay particles rolled, slipped and filled into pore throats, resulting in the blockage of seepage channels and, correspondingly, rapid increases in local pore water pressure. Finally, the loess liquefied due to a sudden collapse of its structure under the effect of long-term high pore water pressure. These results can provide a theoretical guidance for soil management and slope instability prevention. • The rate of increase in pore water pressure (RIPWP) was as a variable. • Constant-shear drained (CSD) triaxial test samples were sheared by increasing the back pressure. • The critical pore pressure ratio was lower than those of traditional triaxial tests. • The formation of rapid-slow strain states was explained. • The microscopic mechanism of loess slope instability was revealed by combining mechanical tests and SEM tests.