Abstract: Internal erosion is a phenomenon that fine particles migrate through the channels within coarse matrix under seepage flow. Previous studies mainly focused on the internal erosion under steady hydraulic gradient, while the studies on the behavior and mesoscopic mechanism of internal erosion under fluctuating hydraulic condition were rarely reported. In this study, a setup of transparent soil seepage test was developed. Seepage tests on two types of soils with different internal stability were conducted under steady and fluctuating hydraulic conditions respectively to investigate the internal erosion behaviors under fluctuating hydraulic condition. The macroscopic experimental phenomenon showed that when the hydraulic gradient exceeded the critical value, the hydraulic conductivity under fluctuating hydraulic condition increased faster than that under steady hydraulic condition for internally unstable soil, manifesting that hydraulic fluctuation aggravated the migration of fine particles. In order to further reveal the mesoscopic mechanism, a three-dimensional (3D) reconstruction method was employed to rebuild the mesoscopic fabric of soil based on two-dimensional cross-sectional images obtained by the planar laser scanning; and a 3D visualization digital model including coarse matrix, inter-granular pore channels and fine particles was established. By observing the distribution of fine particles in pore channels, it was found that fine particles might clog and accumulate at narrow pore throats under steady seepage. While the perturbation caused by hydraulic fluctuation could break these weak stable structures of clog and accumulation, and then restart the migration process of fine particles.

Key words: internal erosion, hydraulic condition, transparent soil, three-dimensional (3D) reconstruction, mesoscopic mechanism