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 behavior and mesoscopic mechanism of internal erosion under fluctuating hydraulic condition were paid little attention. In this study

a setup of transparent soil seepage test was developed. Seepage tests of two kinds of soils with different internal stabilities 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 hydraulic gradient

the increase of the hydraulic conductivity under fluctuating hydraulic condition was faster than that under steady hydraulic condition for internally unstable soil

manifesting that hydraulic fluctuation aggravated the migration of fines. In order to further reveal the mesoscopic mechanism

a three-dimensional reconstruction method was employed to rebuild the mesoscopic fabric of soil based on the two-dimensional cross-sectional images obtained by the planar laser scanning; and a three-dimensional 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.