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Deterrent Effect of Two-layer Filter using Rock Debris against Coastal Internal Erosion of Sea Sands under Various Hydrodynamic Forcing

Collapse of cavities caused by coastal internal erosion has occurred at the back sides of
seawalls and quay walls all over the world. As a countermeasure against the coastal internal
erosion, a method using a two-layer filter has been developed by the authors. In this study, the
applicability of rock debris as the two-layer filter was comprehensively examined through a
series of sand filtration tests and prototype-scale experiments under various dynamic external
forces acting at the waterfront. As a result, in cases where sea sands with low grain sphericity
are used as backfilling sands, the two-layer filter composed of granulated rock debris showed
a sufficient deterrent effect under various external forces involving tides, waves, seepage flows,
rainfalls, overtopping waves, and earthquakes, when the uniformity coefficient of the filter
(UcF) was higher than or equal to 2.5, and the median particle diameter ratio (DF50/DS50) was
less than or equal to 25.