Investigating instability mechanism of loess: experimental insights from two constant shear modes

Loess slopes subjected to heavy rainfall or groundwater level fluctuations are highly susceptible to landslides, often resulting in severe consequences. To investigate the instability behavior of saturated loess under these particular conditions, a series of constant-shear drained (CSD) and reduced pressure drained (RPD) triaxial tests were conducted. The CSD test initiates shear instability in loess samples through manual elevation of the pore water pressure, under the constant deviatoric and the confining stresses. In the RPD test, the water of the sample can drain freely, while the confining pressure gradually reduced. An innovative method was developed for detecting shear instability initiation in loess samples, demonstrating high practical applicability and measurement accuracy. Several key factors influencing loess instability were examined. It was found that a higher loading rate significantly accelerates instability initiation in CSD sample. In RPD test, a slower reduction in confining pressure delays the shear failure, leading to larger axial strains at failure. For both shear modes, a straight line drawn from the origin of the stress-path diagram consistently fits all instability points with high coefficient of determination. The slope of this line, defined as the stress ratio at instability ( η s ), is independent of the initial stress state at the shear stage but varies with the shear mode. Comparing the two shear modes under similar initial conditions revealed that instability onset occurred earlier in RPD samples than in CSD samples. Additionally, effective principal stresses decreased more significantly during constant shear in CSD tests. These observed differences were attributed to dilative behavior occurring in CSD samples prior to instability. At micro-scale, it was inferred that localized high-pressure zones formed within CSD samples, with dilatancy developing to balance external loads. In contrast, water flowed freely through particle channels in RPD samples, where loess particles are prone to sliding upon overall effective stress reduction.