Experimental study on uniaxial compressive strength of ultra-low temperature frozen clay

Underground transportation, coal mining and other projects often encounter the construction problems of sandy soil crisscross and extremely rich groundwater in soft soil strata. Liquid nitrogen can be used to make the soil in the ultra-low temperature freezing state achieve the purpose of soil stability and reinforcement. Therefore, it is of great significance to explore the compressive strength of the ultra-low temperature frozen soil for the long-term stability and safety of engineering construction. In order to reveal the variation of uniaxial compressive properties of ultra-low temperature frozen clay, the uniaxial compression tests at -10°C to -180℃ are carried out on the soil samples with moisture contents of 17%, 20% and 23%. The results show that when the temperature of frozen soil is higher than -80℃, elastic-plastic failure occurs. When the temperature is lower than -80℃, shows brittle failure. As the temperature decreases, the compressive strength of frozen soil increases linearly first. When the temperature is lower than -80℃, the strength is basically stable, and the temperature is fitted by the compressive strength of frozen soil, and the fitting effect is good. When the moisture content is 17%~23%, the compressive strength of frozen soil increases with the increase of water content, and the elastic modulus of frozen soil increases with the decrease of temperature. Finally, the applicability of four kinds of stress-strain equations to the relationship of the ultra-low temperature frozen soil is compared and analyzed. It is found that the accuracy of the power function and hyperbolic formula to fit the stress-strain relationship of the ultra-low temperature frozen soil is low, and the fitting effect is not ideal. The composite power exponent model has a good fitting accuracy for the elastic-plastic failure process, and can accurately describe the yield and failure of the process, but it is not suitable for the stress-strain curve of brittle failure section, so the model has some limitations. The viscoelastic plastic equation has the best fitting accuracy for the stress-strain relationship of frozen soil. Then the temperature function is introduced to improve the viscoelastic plastic equation, and a composite equation related to the temperature of frozen soil is proposed. The fitting accuracy of the equation is higher, which improves the theory of stress-strain equation of the ultra-low temperature frozen soil, and can provide theoretical reference for practical projects.