STUDY ON THE RESTORATION METHOD USING A TWO-LAYER FILTER FOR CAVITY COLLAPSE CAUSED BY COASTAL INTERNAL EROSION

Abstract

 As a countermeasure against coastal internal erosion of ground behind seawalls and quay walls, a method using a two-layer filter, i.e. upper filter layer to prevent internal erosion of backfilling sands and lower filter layer with relatively large grain size to keep stability on backfilling stones, has been developed and put into practical use. The purpose of this study is to investigate the application of the above method to the restoration of the cavity collapse caused by coastal internal erosion. Two restoration methods were devised, depending on the depth of the location of washout, as follows. In the case of coastal internal erosion occurring deeper than the low-tide groundwater level, the two-layer filter is filled into the sinkhole due to the cavity collapse in the form of the lower filter layer being surrounded by an upper filter layer with appropriate layer thickness. On the other hand, in the case of coastal internal erosion occurring at a relatively shallow depth and when the ground around the location of washout is unsaturated during low tide, the two-layer filter is placed to cover the origin of the coastal internal erosion by excavating the residual backfilling sand in the sinkhole during the exposure period. Prototype-scale visualization tests showed that the two-layer filter with appropriate grain sizes can prevent recurrence of coastal internal erosion even under strong hydrodynamic forcing in both restoration methods.