Establishing a constitutive model that reflects the local bonding breakage process has always been a core task in soil mechanics and is crucial for solving engineering stability issues. Based on thermodynamic principles and breakage mechanics, this paper proposes a macro-micro thermodynamic constitutive model. This model quantitatively describes the thermodynamic behavior of local bonding breakage and the non-uniform distribution of stress-strain at the microscale. It improves the prediction accuracy of the model for deformation characteristics, which is similar to the Cambridge model in mathematical form. Firstly, based on the law of conservation of thermodynamic energy, the mathematical expression of structural breakage work during compression deformation was determined. It was found that the dissipated energy of breakage can be mainly divided into two parts: the frictional effect between bonded elements and frictional elements, and the irreversible transformation from bonded elements to frictional elements. Furthermore, a macro-micro constitutive model framework considering the thermodynamic behavior of local bonding breakage was established. Secondly, based on the constitutive framework and the deformation mechanism of loess (frictional, bonded, and damaged), the expressions for free energy, dissipated energy, and damage dissipated energy were determined. The damage yield function and elastic-plastic constitutive model considering the evolution laws of volume breakage and shear breakage were derived. Finally, the established model was used to predict the experimental data of other scholars, and its rationality and simulation advantages were verified through comparison. This model aligns better with thermodynamic principles, and its parameters are easy to determine.

Moraine soils are widely distributed in southeast Tibet of China, which poses a serious threat to local railway construction. The mechanical behavior of moraine soil containing ice in cold regions is difficult to capture under the joint action of stress conditions and temperature. To study the strength characteristics of ice-rich moraine soil, a total of 112 groups of thermal-mechanical triaxial tests under different ice forms, ice contents, and temperature conditions are carried out. The test results show that the mechanical properties of moraine soil with crushed ice and block ice are different, showing the characteristics of strain hardening and strain softening, respectively. Overall, the peak strength of moraine soil with block ice is greater than that of crushed ice. The cohesion and internal friction angle of moraine soil decreases with the temperature rise. With the increase of ice content, the peak strength of moraine soil with block ice increases, while that of crushed ice shows the opposite trend. In addition, the increase of ice content increases the cohesion of moraine soil with block ice, but there is a threshold value of ice content (25%) for moraine soil with crushed ice, which leads to the maximum cohesion at this time. Based on the test results, a unified function is proposed to describe the quantitative relationship between the strength parameters of ice-rich moraine soil, temperature, and ice content. Finally, to explore the nonlinear strength behavior of moraine soil, a binary-medium model is introduced to describe its stress-strain relationship, and the evolution of the main parameters in the model is analyzed. Comparing theoretical and experimental results demonstrates that the established model is of satisfactory applicability to simulate the mechanical behavior of moraine soil with different ice forms.