Internal erosion refers to the seepage-induced movement of fine particles within a soil matrix and is a common failure mechanism in earthen hydraulic structures. Understanding soil microstructure changes and the associated fabric evolves during fine particle transport is crucial for explaining the underlying mechanisms of post-erosion shearing behaviour. However, data concerning fabric evolution during erosion and subsequent anisotropic shearing behaviour under different stress paths is unknown. Moreover, there is a lack of knowledge on how internal erosion affects the maximum shear modulus in different directions and influences the stiffness degradation curve across various small strain ranges (0.0001% to 1%). This study aims to investigate any erosion-induced changes in the anisotropy of maximum shear modulus and post-erosion shearing and volumetric behaviour. A new apparatus, back-pressure-controlled triaxial permeameter that is equipped with Hall-effect transducers and bender elements was developed for testing. Changes in fabric anisotropy during erosion under two isotropic confinements (i.e. 50 and 100 kPa) were measured by sending shear waves in the horizontal direction but polarising in both vertical and horizontal directions. Subsequently, constant-mean-effective stress compression and extension were applied at each confinement to determine any changes in the stress-strain behaviour within a small strain range (i.e. up to 1%). Any erosion-induced evolutions in shear modulus and its anisotropy under low and high confinements will be presented and discussed. Effects of stress path on the post-erosion shearing and volumetric behaviour including dilatancy also will be investigated. Based on the interpretation of the new test data and existing state-of-the-art understanding of internal erosion, a conceptual model will be proposed to illustrate and provide insights into the mechanisms of fine particle transport and soil microstructure changes under isotropic confinements and how fines migration affects the anisotropic post-eroded shearing and volumetric behaviour under different stress paths.

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