Undrained cyclic behaviour of loess with initial shear stress: A focus on failure mode

Soils in the field conditions, such as in cut slopes or under embankments, often sustains an initial static shear stress prior to the cyclic loadings. Current knowledge on the undrained cyclic behaviour with the presence of initial shear stress has been mainly built on sands. Experimental studies for loess that contains adequate information in this respect is lacking. To fill this gap, this paper presents results from a series of undrained cyclic triaxial tests on the reconsititued and intact loess samples, subjected to different initial static shear stresses. It was found that the existence of initial static shear stress has a significant impact on the failure modes of loess. Three failure modes were observed in the loess samples, including the flow failure, the cyclic mobility and the plastic strain accumulation, among which the flow failure in the reconsitituted loess sample was more destructive because of its sudden nature. On the basis of the experiments, we propose a chart to facilitate the identification of the failure modes of loess by taking into account the influence of initial stress state and the excess pore water pressure (Δu). Specifically, the changes of Δu in the first two cycles are used to distinguish the flow failure and other failure modes. The proposed metod is further validated using the literature data, showing a satifactory predictive peformance. The outcome of this study provides a guideline that is useful not only for evaluating undrained cyclic behaviour of loess, but also for predicting the failure modes without the need to load samples to failure.