In order to explore the mechanical properties and microstructure changes of frozen saline silty clay in the Hexi region of Gansu Province, triaxial compression tests and scanning electron microscopy (SEM) analysis experiment were conducted to explore the effects of moisture content, confining pressure, and temperature on the stress-strain characteristics and failure modes of frozen soil, as well as the changes in the internal microstructure of the sample. The experimental results show that the strength of frozen sulfate saline soil first increases and then decreases with the increase of moisture content, and the maximum strength corresponds to a moisture content of 15%. The changes in confining pressure and strength have the same trend. The lower the temperature, the greater the strength of the sample. During the entire loading process, the specimens undergo a gradual transition from volume shrinkage to volume expansion. Due to the strain harden behavior of the stress-strain curve throughout the entire loading process, the failure mode of the specimens is plastic failure. The internal microstructure of the sample gradually transitions from point-point contact and edge-point contact before shearing to edge-surface contact and edge-edge contact after shearing, and the pore size inside the sample increases after shearing, with a loose arrangement of the particle skeleton. The above research conclusions can lay a certain theoretical foundation for the engineering design and construction of sulfate saline soil in cold and arid areas.

A novel unified hardening/softening model is presented, addressing challenges in the constitutive modelling of rocks' strain-hardening and strain-softening behaviours with brittle-ductile transition. The model highlights the impact of confining pressure on failure mode transition when capturing variations in initial yield, peak, and residual strength. The yield criterion and hardening/softening law are developed by a strength-mapping method, where the peak strength is considered the upper bound. The strength-mapping method relies on a mapping index formulated by plastic shear strains. The mapping index is then incorporated into the fractional plastic flow rule, leading to the proposed constitutive model with 9 easy-to-calibrate parameters. The model predictions have been validated by 4 series of rock samples on triaxial tests, where the brittle-ductile transitions have been well captured. The results indicate that it is reliable to capture the rocks' complicated mechanical responses, particularly the brittle-ductile transition, with our proposed strength-mapping method and fractional plastic flow rule.

In order to study the mechanical propertied and change rules of undrained shear behavior of saline soil under the freeze-thaw cycles, an improved constitutive model reflecting the effects of freeze-thaw cycles was proposed based on the traditional Duncan-Chang model. The saline soil in Qian'an County, western Jilin Province, was selected as the experimental object. Then, a set of freeze-thaw cycles (0, 1, 10, 30, 60, 90, 120) tests were conducted on the saline soil specimens, and conventional consolidated undrained triaxial shear tests were conducted on the saline soil specimens that underwent freeze-thaw cycles. The stress-strain relationship was obtained by the triaxial shear test. The model parameters have a corresponding regression relationship with the number of freeze-thaw cycles. Finally, based on the function expression of the model parameters, the modified Duncan-Chang model with the number of freeze-thaw cycles as the influence factor was established, whilst the calculation program of the modified model is compiled. Based on the test results, the stress-strain relationship of the saline soil specimen shows strain hardening. The shear strength gradually decreases with the increase of freeze-thaw cycle. The 10 freeze-thaw cycles are the turning point in the trend of changes of the mechanical properties of saline soils. The calculated and experimental stress-strain relationship are compared, and the comparison between the calculated value of the model and the experimental value showed that the two had a good consistency, which verified the validity of the modified Duncan-Chang model in reflecting the effects of the freeze-thaw cycle.

In this study, constitutive behavior of granular soils is modeled through a generalized plasticity-based theoretical framework. The soil hardening is addressed by a novel relationship proposed to calculate plastic strains and their evolution during loading history. The model is effective in predicting the response and incorporating it into a numerical scheme. Focus is given to stress ratios yielding liquefaction in a few stress cycles. The proposed hardening law is based upon a combined deviatoric-volumetric hardening rule updating the stress-strain relationship and plastic strain vector. Numerous undrained monotonic and cyclic triaxial tests are simulated for verification of the constitutive formulation. Results indicate that the developed model for sand-like cohesionless soils proves to match fairly well with the available experimental data. Plastic strains are calculated accurately and accumulated pore pressures are well captured. Triaxial test simulations exhibit a successfully improved way of capturing the essential static and cyclic behavior of granular soils.