Stress relaxation characteristics of warm frozen clay under triaxial conditions

Stress relaxation is one of the main concerns in geotechnical engineering, especially in permafrost regions with warm frozen soil. This paper investigated the mechanical behaviors of a warm frozen soil during relaxation process. A series of triaxial relaxation tests were carried out on the warm frozen soil samples under the temperature from − 1.0 °C to − 0.2 °C and confining pressure of 0.5 MPa to 3.0 MPa. Three stages were observed during the whole stress relaxation process, i.e. instantaneous relaxation, intense relaxation and slow relaxation. Relationship between deviatoric stress and time was described by a hyperbolic relationship. With relatively small pre-strain (3% and 4%), volumetric changes during stress relaxation contracted initially followed by expansion, but contraction dominates the relaxation process with the magnitude of pre-strain over 6%. Characteristic parameters such as the instantaneous stress relaxation, the critical relaxation duration and relaxation rate are defined and analyzed. It is found that the parameters are all dependent on soil temperature, confining pressure and magnitude of pre-strain.