A number of coastal dykes were damaged by the 2011 off the Pacific coast of Tohoku Earthquake. The main cause of the failures is assumed as the local scour at the landward toe. Furthermore, liquefaction damage caused by the earthquake has been reported. When tsunami strikes and overtops a coastal dyke, there is a possibility that liquefaction occurs by earthquakes and promotes scour at the landward toe. Liquefaction is caused by a decrease in effective stress. Therefore, it is important to evaluate the influence of the effective stress on the scour due to overtopping. In the present study, flume experiments were conducted. The effective stress was controlled by upward seepage to reproduce liquefied ground. As a result, it is found that scour profile is influenced by the effective stress, and backfilling of the scour hole due to slope failure is observed at low effective stress.
The 2011 off the Pacific coast of Tohoku Earthquake generated massive tsunami in the northwestern Pacific Ocean. Giant tsunami damaged coastal dykes and seawalls. From field surveys by Kato et al. (2012), scour holes due to overtopping tsunami at the landward toe of coastal dykes were found in many places along the Pacific Coast in Tohoku region. The scour at the landward toe has been considered the main cause of the failure of coastal dykes. Furthermore, liquefaction damage caused by the earthquake has been reported. Fukumoto et al. (2012) showed from the hydraulic data that tsunami attacks and the largest aftershock might occur simultaneously. On the basis of these reports, there is a possibility that liquefaction occurs when tsunami strikes. It was confirmed that the ground liquefied before tsunami struck as shown in Fig. 1.
In the previous studies, experiments and analyses on relationship between variation of water level and liquefaction have been conducted (Nago and Maeno, 1987; Zen and Yamazaki, 1990; Sassa and Sekiguchi, 1999). Variation of water level leads to change in pore water pressure in the ground. The rapidly decreasing water level brings a decrease in the effective stress of the ground. It is predicted that the ground around a coastal dyke liquefy by rapidly decreasing water level when tsunami backwash comes, and overtopping tsunami backwash causes scour around a coastal dyke. Actually, Sawada et al. (2014) pointed out from the wave data that liquefaction could occur due to tsunami under the condition that amplitude of changes in sea level was large and the drawdown rapidly occurred.
