The relationship between the thixotropic mechanism and the macroscopic thixotropic strength can be clarified by analyzing the changes in microstructure and pores in the loess thixotropic process. This approach is of significant importance for calculating the strength of compacted loess foundations. In the present study, a representative sample prepared from Xi’an loess was analyzed and eight resting ages were set. The micropore characteristics of the remolded loess and undisturbed loess at different resting ages were obtained using electron microscope observation and nuclear magnetic resonance testing. The results indicate that the thixotropy in the prepared loess samples is significant. It is also found that as the resting age grew, newly formed cements in the remolded loess continuously accumulated and filled in the microcracks between the aggregates. Consequently, the contact area of aggregates increased, thereby decreasing the width and length of the microcracks. The proportion of cementation pore and small microcracks gradually increased, while the proportion of large microcracks gradually decreased, indicating that thixotropy increased the cohesive force and friction force of soil structure at the mesoscale. This phenomenon also explains the increase of thixotropic strength at the macroscopic scale. The mesoscopic mechanism of loess thixotropic strength recovery is that the connection between soil particles is re-established after the break of the clay particle–water–charge system. Moreover, the elastic potential energy of soil particles generated by compression promoted the polymerization of clay particles dispersed in a pore water solution to produce flocculating aggregates during resting dissipation. The continuous consumption of clay particles expanded the processing time and flocs and continuously decreased the strength growth rate.

The cooling characteristics of liquid nitrogen can effectively weaken the mechanical properties of dry hot rock reservoirs, improve the porosity and permeability of dry hot rock, and thus improve the development and utilization of geothermal resources. In order to explore the weakening effect of liquid nitrogen cooling characteristics on high-temperature reservoir rocks, uniaxial compression tests, Brazil splitting tests, and three-point bending tests were conducted on high-temperature granite, and synchronous acoustic emission tests were conducted to monitor the damage evolution of rocks. It is found that under the action of liquid nitrogen, the elastic modulus, peak strength, fracture toughness, and other mechanical properties of granite decrease to varying degrees. At different initial temperatures, the damage degree of liquid nitrogen to granite is different. The higher the initial temperature of granite, the greater the damage effect of liquid nitrogen on granite. The low-temperature characteristics of liquid nitrogen not only have the effect of cracking damage on granite but also have the effect of thermal stress on granite. If the temperature difference between the two is large, the heat released in the combination process will be higher. The greater the temperature difference, the greater the damage to granite. Under the action of liquid nitrogen, the acoustic emission characteristics of granite also show different evolution laws, and the ringing count of rock increases significantly, which indicates that liquid nitrogen aggravates the internal damage of granite, making the rock more fractured during fracturing. The research results can provide some scientific guidance for the development and utilization of geothermal energy.

In this paper, a new definition of a structural parameter for soil is given to characterize the mechanical properties of soils and their changing patterns. The soil structural parameter is a quantitative descriptor of soil structural properties. Structural parameters are related not only to the grain size, density, and moisture content of the soil material composition and state, but also to the spatial arrangement of soil particles in the soil skeleton structure and the characteristics of intergranular associations. The new definition of the structural parameter, established from the comparison of loess structural stability and variability, is defined as the ratio of the shear strength of undisturbed loess to that of remodeled saturated loess. The patterns of moisture content, confining pressure, and dry density on structural properties were analyzed and the degrees of influence of each factor on structural properties were quantified. By analyzing the change rule of the structural parameter of loess with the difference of moisture content and plastic limit, its change rule with plastic limit and liquid limit, and the change rule of the structural parameter with the liquidity index, the essential relationship between the structural parameter and grain size, density, and moisture is revealed. The essential relationship between the structural parameter and grain size, density, and moisture were also revealed.

The structure of loess is an important physical property indicator. Just like grain size, moisture content and density, it also affects the physical and mechanical properties of loess. Based on the definition of the constitutive degree index, the definition of structural parameters was redefined. And conventional triaxial shear tests were conducted on undisturbed soil samples and remolded soil samples with different moisture contents and different dry densities. Firstly, the deformation characteristics and strength changes of the soil samples were analyzed. Then, based on the definition of structural parameters, the structural change laws of the soil were analyzed. Finally, the relationships between the structural properties and soil moisture, grain size and density were analyzed, thereby verifying that the newly proposed definitions of the structural parameters are reasonable.

This paper derives and modifies the structural quantitative indicator of loess—the structural index—based on the theory of limit equilibrium, thereby obtaining structural evolution parameters and structural modification parameters. However, since the structural index is defined for uniaxial stress states, this paper also introduces a structural parameter applicable to complex stress states—the complex stress structural parameter $m_{\tau }$. This parameter is not only applicable to complex stress states, but also enables the study of soils such as soft clay that are difficult to test under uniaxial compression. Additionally, it offers the advantages of requiring less testing effort, featuring simpler expression formulas, and possessing clearer physical meaning. Finally, the relationship between complex stress structure parameters and soil strength indicators was analyzed. We defined a quantitative indicator—the comprehensive structural state parameter $P_{cs}$—that reflects the combined effects of soil structural factors. By combining comprehensive structural state parameters with the physical properties of undisturbed loess (such as moisture content and dry density), it is possible to determine the strength parameters of loess under a given structural state over a wide range.

Based on the concept of comprehensive structural potential, many scholars have currently defined various structural parameters of loess. However, these parameters are mostly defined based on the deformation characteristics of soil samples. Due to the significant influence of the calculation process by experimental conditions and stress–strain states, these parameters are difficult to accurately represent the intrinsic relationship between the structural characteristics of loess and its mechanical strength. This problem has become a key bottleneck restricting the deepening of loess structural research. Therefore, this paper takes the shear strength theory as the core support and proposes a structural parameter of loess based on complex stress states. This parameter not only can directly reflect the coupling relationship between the structural characteristics and mechanical strength of loess, but also has the advantage of adapting to complex stress environments. More importantly, its calculation results are independent of specific stress states and experimental conditions, and have stronger universality. Based on the results of the three-axis shear test, this paper systematically analyzed the variation patterns of the shear strength and new structural parameters of loess samples under different moisture contents and dry densities; by deeply exploring the correlation mechanism between the shear strength of loess and its structure, the rationality and stability of this complex stress-type structural parameter were further verified. This study provides more reliable parameter indicators for the evaluation of loess structure, and also offers a new research perspective for revealing the structural control mechanism of loess mechanical behavior.