121 / 2025-07-14 13:27:24

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

internal erosion, geotechnical filter, median particle diameter ratio, rock debris, sea sands

 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 coastal internal erosion, a method using a two-layer filter, i.e. upper filter layer to prevent coastal internal erosion of backfilling sands and lower filter layer with relatively large grain size to keep stability on backfilling stones, has been developed by the authors. Since the filter is crushed stone, it is able to follow deformation under various dynamic external forces acting on the waterfront area such as earthquakes, waves, and currents, and can stably and continuously deter coastal internal erosion. Therefore, the geotechnical filter is considered to be an effective prevention method of coastal internal erosion.

Rock debris is a byproduct of rock mining and civil engineering work. It is a natural material with unadjusted grain size and it contains large quantities of rocks and sediment. If rock debris can be applied to two-layer filters, it will be possible to deter coastal internal erosion more economically. The aim of this study is to systematically investigate the applicability of rock debris as a filter material and the performance of a two-layer filter using rock debris as a countermeasure against coastal internal erosion. First, we investigated the physical properties of rock debris and examined how to divide the grain size into each layer. Then the deterrent effect against coastal internal erosion of two-layer filter using rock debris 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 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 (D_F60/D_F10 ; D_F60 : Particle size corresponding to 60% finer for filter, D_F10 : Particle size corresponding to 10% finer for filter) was higher than 2.5, and the median particle diameter ratio (D_F50/D_S50 ; D_F50: Particle size corresponding to 50% finer for filter, D_S50: Particle size corresponding to 50% finer for sand) was less than or equal to 25.