Suffusion at soil–structure interfaces under downward seepage: an experimental study

Internal erosion at soil–structure interfaces is an important cause of failure in hydraulic structures such as dams and levees, and suffusion is one form of internal erosion. However, the influence of soil density on interface-induced suffusion remains inadequately understood. This study experimentally investigates the effects of soil–structure interfaces and relative density ( Dr ) on suffusion in gap-graded soils. Using a newly developed apparatus for downward seepage testing, specimens compacted to Dr = 30%, 60%, and 90% were tested with and without a cylindrical concrete block. Local Darcy velocity and post-test particle loss (%) were measured spatially. The concrete block produced a localized increase in Darcy velocity near the soil–structure interface. Compared with soil-only specimens, Darcy velocity at S1 increased by factors of 1.76, 1.63, and 1.40 at Dr = 30%, 60%, and 90%, respectively. At Dr = 30% and 60%, the increased local seepage velocity was accompanied by pronounced particle loss at the interface, whereas particle loss remained low at Dr = 90% despite the increase in velocity. These results indicate that densification restricts fine-particle migration even where local seepage velocity increases. Soil–structure interfaces can therefore act as localized weak zones for suffusion, while high relative density substantially reduces particle migration. However, the gap-graded grain size distribution leaves the soil geometrically susceptible to suffusion even at Dr = 90%. Thus, densification reduces particle migration but does not eliminate the susceptibility associated with the grain size distribution. These findings highlight the importance of adequate compaction for reducing interface-induced internal erosion in hydraulic earth structures.