Abstract: To accurately describe the diffuse instability behavior of sand-fine mixtures under different loading paths, the influence of fines content on the evolution mechanism of stress increments was systematically investigated based on the H-model theory and the discrete element (DEM) strain probe analysis. The results indicate that the effect of fines content on the direction of stress increments is mainly concentrated in the initial shear stage, whereas its influence on the magnitude of the stress increments depends on the stress path and is independent of the current strain state. On this basis, the particle size ratio is further introduced, and an improved H-model comprehensively considering the effects of fines content and size ratio is established. By incorporating five parameters, this model describes the instability response of sand-fine mixtures under multi-factor coupling and different loading paths using only one set of parameters. Proportional strain loading tests and constant shear drained tests on sand-fine mixtures were simulated, and the results show that the predictions of the improved H-model are in good agreement with the laboratory test results, verifying that the model can effectively capture the coupled effects of fines content and size ratio on the instability characteristics of sand-fine mixtures under different loading conditions.

Key words: sand-fine mixtures, fines content, size ratio, constitutive model, stress paths