This paper analyzes the macroscopic and mesoscopic damage evolution of frozen compacted loess under loading and unloading by combining real-time computed tomography (CT) observation technology with mesostructure change and mechanical behavior. The failure mechanism of the internal structure of the soil undergoing loading and unloading pressure is revealed. With increasing loading and unloading, the internal cementation of the sample gradually weakens, while the dissipation energy and elastic strain energy increase and then decrease. During the strain softening stage, the sample undergoes mainly strain at the plastic limit, the dissipation energy tends to be stable, and the elastic strain energy decreases. Mesoscopic CT image analysis reveals irreversible structural damage after multi-stage loading and unloading, with mesoscopic damage variables increasing as the load increases. In the elastic stage, the analysis of macroscopic and mesoscopic damage variables is crucial for studying the damage evolution of frozen loess. In the viscoplastic stage, different macroscopic and mesoscopic damage variables effectively characterize geometric deformation, strength, and stiffness of the material, providing valuable insights to guide the further study of the mechanical properties of frozen compacted loess.
To solve the problem that the mechanical behavior of undisturbed loess in seasonally frozen soil area is affected by freeze-thaw action, triaxial shear tests of undisturbed loess under freeze-thaw condition were carried out. The results show that the mechanical properties of undisturbed loess are greatly affected by factors including freeze-thaw process, water content, natural density and confining pressure. Freeze-thaw action has a certain impact on the failure surface shape and stress-strain curve. Before and after freeze-thaw, the shape of the shear failure surface is complex, including single oblique failure surface, double oblique failure surface, vertical failure surface, X-shaped failure surface, bulging failure, etc. And under the conditions of low water content, low confining pressure and high dry density, the stress-strain curve tends to be softened. Conversely, the curve tends to harden. Freeze-thaw action can make the stress-strain curve transition from softening to hardening. In addition, the freeze-thaw action significantly weakens the failure strength, shear strength, cohesion, initial tangent modulus and failure ratio of undisturbed soil, but does not change the internal friction angle obviously. Also, the heterogeneity of natural soil is also an important factor affecting the mechanical parameters, failure surface shape and stress-strain curve of undisturbed loess.
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