Abstract: Using of the modified true triaxial apparatus in Xi’an University of Technology, the intact loess strength, deformation characteristics and failure modes of intact loess are investigated through lateral unloading plain stress tests of loess under different moisture contents and different confining pressures. The stress-strain relationship curve under different confining pressures shows hardening behavior at the different moisture contents, similar as ideal plasticity curves. The lateral deformation of loess has a non-linear relationship with its vertical deformation. Lateral strain increment is larger than the vertical strain increment, indicating dilatant volume deformation. The failure strain is smaller in the lateral unloading plain strain test than vertical loading plain strain and conventional triaxial test. In brief, the failure strength of loess is closely related to moisture content, confining stress and spheric stress state. With the moisture content increasing, cohesion reduces and internal friction angle decreases slightly. The intact loess is develops dilatant slip failure modes under lateral unloading conditions.

Key words: intact loess, plain strain test, lateral unloading, stress-strain relation, failure modes