Study on effect of coarse-grained content on the mechanical properties of frozen mixed soils

In this study, the mechanical behavior of frozen mixed soils was studied by laboratory tests, and the effects of coarse-grained content on their mechanical properties were analyzed in detail. With the increase of coarse-grained content, the strength of frozen mixed soils decreases and then increases slightly. In regard to volume deformation, both the maximum volume expansion and maximum volume contraction of the frozen mixed soils decrease with the increasing confining pressure. All frozen mixed soil samples show strain softening during the shear process. The softening index, IS, defined here to analyze the degree of strain softening, increases as the mass ratio increases when the confining pressure is above 3.0 MPa, in which the mass ratio is defined as the ratio of coarse-grained mass to fine-grained one. The greater the IS, the more obvious of the strain softening. All stress-strain curves of the frozen mixed soils can be divided into three stages: (1) the first stage is the initial linear stage of εa ≤ εi, where the ice strength and deformation are dominant, and the linear limit stress point (σi) corresponding to εi increases with the mass ratio increasing. The axial strain range of the first stage in the experimental results of our test is about 0.874%; (2) the second stage is the hardening stage of εi < εa ≤ ε1f. In a macroscopic view, the samples show shear contraction. With the increases of mass ratio, the axial strain range of the second stage is reduced and the failure strain (ε1f) of the frozen mixed soils also decreases; and (3) the third stage is the softening stage of εa>ε1f. The samples exhibit dilatancy on a macro scale. Finally, the Mohr-Coulomb strength criterion is modified to discuss the strength variation of frozen mixed soils and an expression of stress-strain relationship for the three-stage characteristics of frozen mixed soils is proposed.