Laboratory study on soil behavior in loess slope subjected to infiltration

• The failure mechanisms of loess slopes are examined by performing stress path tests.
• The behavior of anisotropically consolidated saturated loess is presented.
• The failure of saturated soil is mimicked by rising pore pressure at constant load.
• The failure of unsaturated soil is mimicked by reducing suction at constant load.
• The soil–water characteristic curves of loess are presented.

In recent years, Heifangtai Plateau in Gansu Province, China, has received significant attention as a loess region in which landslides frequently occur. This study aims to examine the soil behavior and failure mechanism(s) of loess slopes subjected to water infiltration. High-quality samples were retrieved behind the failure plane of a landslide in Heifangtai. Three sets of stress path tests were conducted to mimic the process of loess slope failures; these tests include undrained compression on anisotropically consolidated saturated samples, drained shear by decreasing mean effective stress at constant axial load on saturated samples, and wetting by decreasing matric suction at constant axial load on unsaturated samples. Loess behavior was evaluated in terms of shear strength, deformation upon shearing, critical state, state parameter, and soil water characteristic curve. Three major observations were drawn from the test results. First, the anisotropically consolidated saturated loess exhibits a noticeable strain-softening behavior upon undrained compression. The peak shear strength is mobilized at less than 1% axial strain. The ultimate shear strength is only 20–30% of the peak value. Second, when subjected to shear by decreasing mean effective stress at constant axial load, the saturated loess exhibits sudden failure when the confining stress is low. At higher stresses, the soil exhibits progressive failure. Third, unsaturated loess deforms progressively when subjected to a reduction in matric suction (i.e., wetting) at constant axial load. The failure mechanisms of the loess slopes are discussed based on the experimental findings.