Abstract

The collapse mechanism of breakwater and countermeasures under the continuous tsunami overflow were investigated using a drum centrifuge. A 1/70-scaled model, representing a 10 m high prototype breakwater, was set in the waterway in the drum centrifuge to 1) reproduce the caisson collapse mechanism and 2) investigate the effect of countermeasure by placing the embankment over the breakwater foundation mound using rubble materials and filter units. The study found that the caisson failure was due to the scouring of rubble mound and the seepage flow. Moreover, placing the embankment over the mound by using rubble materials or filter units was confirmed effective against breakwater instability.

Introduction

In the 2011 off the Pacific Coast of Tohoku Earthquake, many breakwaters were severely damaged by the tsunami. As the results of disaster investigations, four major disaster patterns that cause damages to breakwaters were suggested, namely: ‘wave force’, ‘overflow and scouring’, ‘breakwater head scouring’, and ‘wave backrush force’ (Tohoku port and harbor, tsunami and earthquake prevention disaster technical committee, 2011). As for the "overflow and scouring" pattern, tsunami overflows the breakwater and scours the backfill materials behind the caisson. Finally, the caisson slips, overturns, and collapses. This type of mechanism and effect of embankment behind the caisson were investigated by 1G large-scale waterway model tests (Arikawa et al. 2013). They found that the breakwater failure was caused from backfill scouring by tsunami overflow which reduce the bearing capacity of foundation.

Imase et al. (2011) and Kasama et al. (2013) quantitatively investigated, from numerical simulations and model tests, the possibility of breakwater failure caused by seepage flow originated from the differential water pressure of the inner and outer breakwater. Sassa et al (2014) and Takahashi et al. (2014) investigated the failure mechanism of breakwater and quantitate the effect of the embankment from the points of both overflow scouring and seepage flow by using centrifuge model tests. However, the influence of scouring ground around breakwater and seepage by continuous tsunami overflow including checking method were not clarified.

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