Internal erosion is a prime cause of dam failure. Despite studies over the years, there are still many ambiguities in the post erosion response of an internally unstable soil. In this study, a new combined erosion-triaxial apparatus was developed to investigate the post erosion behavior of a gap-graded soil (vulnerable to internal erosion). This new apparatus eliminated the need to remove the soil sample which thus prevented any loss in saturation and furthermore minimized testing disturbance. Photogrammetry and X-ray tomography techniques were effectively employed to assess deformations during the erosion phase. It was found that the primary soil structure became temporarily unstable after the initial erosion of some fine particles. However, this temporary instability was found to be reversed after rearrangement of the remaining coarse particles. This was detected by the measurement of sudden spikes in vertical deformations on the sample surface. This phenomenon was also visible in the 3-D Computed Tomography scans. Although dilation tendency during shearing was found to decrease after erosion, the measured increase in undrained shear strength at small strains was likely to be caused by enhanced mechanical interlock between the coarse particles.