This paper presents and evaluates a new reinforcement technique, based on the use of rubber sheets and Permex blocks to negate the effects of tsunami-induced instabilities (seepage flow and scouring) on a caisson type composite breakwater. In this study, a series of hydraulic model tests (on a 1/100 scale prototype) were performed and the effectiveness of the countermeasures was evaluated based on the displacement of the caisson, mapping the seepage flow pattern within the mound and calculating the stable weight of the mound. Results obtained from this study verified that the proposed countermeasures significantly improved the overall stability of the breakwater.
The 2011 earthquake off the Pacific coast of Tohoku (Mw=9.0 earthquake) resulted in the biggest and deadliest tsunami in the history of Japan. The tsunami reached a run-up height of 22 meters at the bay mouth of Kamaishi (Mori et al. 2011), while a GPS wave meter (18 km from the coast of Kamaishi) measured a 6.7 m high tsunami (Sugano et al. 2014). The Kamaishi breakwater, the world's deepest breakwater of that time, failed to stop the tsunami and was left heavily damaged.
Previously the breakwaters designed were more focused on being resilient towards wind waves and therefore the researches related to the behaviour of breakwaters during tsunami waves (especially those caused due to massive earthquakes) were fairly limited. All that changed after the 2011 Great East Japan Earthquake. And Kamaishi breakwater became a hot topic for researchers, especially in Japan. Arikawa et al. (2012) and Maruyama et al. (2014) separately conducted researches to determine the cause of the failure of caisson type composite breakwater and reached the common conclusion that the inadequate resistance against tsunami-induced instability problems (tsunami-induced seepage flow and overflow) played a major role in the failure of the breakwater. Takahashi et al. (2014) evaluated the stability of caisson type breakwater under tsunami-induced seepage flow by carrying out tests based on the centrifuge technique and performed FEM analysis (Finite Element) to conclude that the seepage force decreases the bearing capacity of the mound. This conclusion (of reduction of bearing capacity due to seepage flow) was also reached by Kasama et al. (2016) who also conducted a stability evaluation study of a composite breakwater under tsunami flow. He further noted that using a countermeasure model based on gabions is useful in reducing the likelihood of seepage failure, however the rubble filled gabion was not effective in improving the bearing capacity of the mound since its effective weight was not sufficient to contribute to the overall strength of the harbor side rubble mound. Hazarika et. al (2015) experimentally verified that the whirlpools generated by the overflow of the water during the tsunami caused a scouring of the harbor side rubble mound while the seepage flow in the region below the caisson resulted in settlements and displacements of the caisson.
