Quantification of the spatial-temporal evolution of loess microstructure from the Dongzhi tableland during shearing

The physical and mechanical properties of loess are significantly influenced by its microstructural characteristics. The microstructure evolution is of great importance to better understand the loess deformation mechanism. To quantitatively and qualitatively characterize the microstructure evolution of intact loess during shearing, SEM and EDS were carried out at 5 typical strain states during triaxial tests. In addition, the microstructure at different positions inside the samples was also observed to study the spatial evolution. The results indicate that the intact Malan loess studied has a metastable structure with a high porosity of 0.45, widely distributed spaced pores, point contacts and weakly cemented chains. During deformation, the microstructure in the middle part of the sample is most sensitive to deformation and has the significant variations and local difference. The microstructure evolution can be described in four stages: weak compaction stage, initial stage of structural collapse, massive structural collapse stage and structural stability loss stage. These four stages correspond to the four special stages of deformation, and the deformation process is expounded from the perspective of microstructure. The peak stress point is the critical point at which the external stress exceeds the initial structural yield strength. The failure of weak cementation and unstable contact, which leads to the massive collapse of the spaced pores, is the intrinsic factor of structural system instability. The appearance of local microcracks indicates that the shear strength of loess sample begins to decrease, and the deformation velocity will increase. These results from the study of the spatial–temporal evolution of loess microstructure contribute to a better understanding of the catastrophic disaster–causing behavior and the physical mechanism of deformation in the study area. •The microstructure in the middle part of the sample is most sensitive to deformation.•The microstructure evolution corresponding to shear deformation is characterized by four stages.•The instability of metastable structure systems is the direct factor for strength reduction.•The collapse of spaced pores (>200 μm2) provides space for shear deformation.