Abstract: Rainfall infiltration and groundwater level fluctuations are important factors to induce loess slope instability. In this paper, triaxial drained shear tests were conducted under two constant stress modes with increasing pore pressure (CQD) or decreasing confining pressure (RPD), the strength attenuation and shear deformation of saturated remolded loess under two pore pressure models were studied. The incremental strain rate method was proposed to identify the instability point of loess samples. The test results show that: In the CQD test, when the pore pressure growth rate of the sample increases, it is more prone to instability; In the RPD test, when the confining pressure of the sample decreases faster, the instability occurs earlier, and the initial consolidation pressure and initial void ratio have influence on the occurrence of instability in the sample under the two loading modes. For saturated loess samples with similar initial state (void ratio and confining pressure), the mean effective stress is higher at the instability point in the RPD sample and the occurrence of instability is also earlier. During the process of instability to the end of test, the reduction of effective principal stress in the RPD sample was found about twice that of the CQD sample. It can be inferred that in the CQD test, the reduction of effective stress of the loess sample is in a local range due to the accumulation of local excess pore pressure, while in the RPD test, the overall effective stress of the saturated loess sample decreases due to the influence of static pore pressure, and it is more prone to instability than in the CQD case.

Key words: constant stress drained shear, loess, pore pressure development law, instability, effective stressconstant stress drained shear, loess, pore pressure development law, instability, effective stress