Experimental Investigation of Loading and Unloading of Frozen Calcareous Clay under True Triaxial Stress State

In the process of freezing method construction of underground engineering in Huainan and Huaibei mining areas, the deep frozen soil is subjected to loading and unloading in different directions at the same time, showing a dynamic change from the initial original stress state to the increasing stress on one side and the decreasing stress on the other side. The uneven distribution of stress leads to different stress paths of the soil in front of the working face, so that the soil is in a three-way unequal stress state, which in turn affects its mechanical properties. Based on this, in order to further study the mechanical behavior of frozen soil under true triaxial stress state, this study analyzed the effects of different temperatures, initial stress states, and loading and unloading rates on the strength and deformation characteristics of frozen calcareous clay using a self-developed true triaxial apparatus for frozen soil. The test results show that under the same test conditions, when the loading rate increases from 0.4 mm/min to 0.8 mm/min, the slope of the generalized shear stress q and the generalized shear strain curve γ of the frozen calcareous clay gradually increases, that is, the slope of the stress-strain curve of the frozen calcareous clay increases with the increase of the loading rate, and the peak stress corresponding to the strain softening group also gradually increases. In addition, with the increase of initial stress and loading rate, the stress-strain curve of frozen calcareous clay changes from strain softening to strain hardening as a whole. Under different stress paths, the strengthening effect of loading rate on frozen calcareous clay gradually decreases with the increase of loading rate, and satisfies the power function relationship. Under the same freezing temperature T and loading rate v, the triaxial strength of frozen calcareous clay conforms to the Drucker-Prager criterion in the principal stress space, and the relationship expressions of α and K with freezing temperature T and loading rate v are given respectively. The research results can provide a theoretical basis for deep freezing shaft sinking.