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The scour observed at the landside ground behind coastal protection facilities, such as seawalls and coastal dikes, due to tsunami overflows is the main reason for failures in such constructions. In this study, we focused on sheet-like (two-dimensional) scour countermeasures, which do not require large-scale excavation of the ground, and experimentally verified the effects of geonet and mesh sheets. By applying a vertical jet directly on the ground and increasing the external force, a situation close to the real scale conditions was reproduced. The experimental results show that the geonet material is able to significantly reduce the scour depth on both non-liquefied and liquefied grounds. We show that in the most effective case, the maximum pore-water pressure and the scour depth are reduced by about 91% and 86%, respectively, by the mesh sheet on the sand-bed surface. [ABSTRACT FROM AUTHOR]

Following the Great East Japan Earthquake, a massive tsunami caused serious damage to coastal dikes. Local scour at the ground behind these dikes reportedly causes dike failures. It is important to reveal the scour process to establish methods for effective scour countermeasures. Herein, we focus on geogrids. To verify this proposal and examine its design, flume-model experiments were performed for non-liquefied and liquefied grounds behind coastal dikes. The results showed that the scour amount and area were reduced drastically when geogrids were present. Geogrids on the sand-bed surface changed the direction of overflow water. Thus, a scour hole developed away from a coastal dike model. Furthermore, geogrids exerted a scour-reduction effect even during liquefaction. [ABSTRACT FROM AUTHOR]

In the present study, hydraulic model experiments were conducted to clarify the influence of liquefaction on the scour behind coastal dikes due to tsunami overflow. In particular, the variation of the scour profile (scour depth, scour length and scour area) due to liquefaction was discussed. Degree of liquefaction was controlled by upward seepage, which was induced by the head difference between a water tank and a sand bed. Six hydraulic gradients were applied to the sand bed. The experimental results showed that liquefaction had a significant impact on the scour profile. At high hydraulic gradient, slope failures occurred due to reduction of effective stress of the sand bed. As a result, scour length was increased by the slope failures. Furthermore, the maximum scour depth and scour area for high hydraulic gradient became smaller than that for low hydraulic gradient. [ABSTRACT FROM PUBLISHER]