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Internal erosion is a major cause of dam or dike failures and incidents. In this study, laboratory tests were conducted to investigate the deformation of two gap-graded soils during internal erosion and the stress-strain behavior of the soils that experienced loss of fine particles due to internal erosion. An erosion-controlled experimental method was adopted to achieve a designated loss of fine particles during internal erosion by adding a predefined amount of salt into the soil sample during sample preparation and dissolving the salt in water under a controlled stress condition. Both the radial and axial deformations during erosion were measured using a photographic method. Subsequently, drained triaxial compression tests were performed to study the stress-strain behavior of the soils that had lost different amounts of fine particles. The peak friction angle and critical friction angle of the soil decreased with the loss of fine particles. After a significant loss of fine particles, the stress-strain behavior changed from the initially dilative behavior to a more contractive behavior. [ABSTRACT FROM AUTHOR]

Internal erosion involves selective loss of fine particles within the matrix of coarse soil particles under seepage flow, which affects the hydraulic and mechanical behavior of the soil. In this research, extensive laboratory internal erosion tests were conducted under complex stress states following three stress paths: isotropic, drained triaxial compression, and triaxial extension stress paths. These tests were designed to investigate the initiation and development of internal erosion and the effect of stress state on critical hydraulic gradients. The entire erosion process can be divided into four phases: stable, initiation, development, and failure. Accordingly, three critical gradients termed as initiation, skeleton-deformation, and failure hydraulic gradients, can be defined. These critical gradients correspond to the onset of erosion of the fine particles filling the large pores of the skeleton, the buckling of the strong force chains formed by the coarse particles, and the soil failure, respectively. The initiation gradient under compression stress conditions generally increases with the shear stress ratio first and then decreases when the stress conditions approach failure. The tests under isotropic stress conditions show the largest initiation and skeleton-deformation gradients at the same porosity. [ABSTRACT FROM AUTHOR]