Fine particles may migrate in the preexisting pores of an internally unstable soil matrix caused by water flow. This migration changes the fine particle distribution and content at different zones and can affect the mechanical properties of these soils. Due to the different roles that fine particles can play in the force chains of an internally unstable soil, the available geometrical assessment methods do not predict post-erosion behavior of the soil. The fine particles may sit loose in the voids, provide lateral support for the primary soil matrix, or participate directly in stress transfer. This will depend on the fine content, particle size distribution, constriction size, relative density, stress path, and particle shape. However, to evaluate the post-erosion behavior accurately, computational modelling or experimental investigation needs to be conducted. A modified triaxial apparatus connected to a water supply system and collection tank was developed to investigate the post-erosion behavior of an internally unstable cohesionless soil under different loading patterns in undrained conditions. This system allowed all test phases to be completed, including erosion inside the triaxial chamber to remove any possible impact of specimen disturbance. The results suggest that the undrained shear strength of the eroded specimen increased at small vertical strains (0–4 %) under monotonic and cyclic loadings, whereas the initial modulus of elasticity remained unchanged. Also, the eroded specimen showed much higher resistance against cyclic loadings, whereas the non-eroded specimen was liquefied during less than five cycles of loading. This improvement was due to a better interlock between coarse particles due to erosion of fine particles. The hardening strain behavior of the non-eroded specimen changed to limited flow deformation due to a decrease in the fine content. The flow deformation of the eroded specimen at medium strain may be due to the local increase in lubrication effect of fine particles in the eroded specimen.