Stability of Breakwater Foundation Under Earthquake and Tsunami: Centrifuge Tests

Abstract eng:
Breakwaters are designed to protect coastal areas and seaports from destructive effects of sea waves, currents and tsunamis by absorbing their wave energies and reducing overtopping effects. But, the extent of damage of breakwaters during the past earthquakes (e.g. the 2011 off the Pacific Coast of Tohoku Earthquake and 2004 Indian Ocean Earthquake) and tsunamis exposed the vulnerabilities of breakwaters. For example, large numbers of breakwaters were heavily damaged by the 2011 off the Pacific Coast of Tohoku Earthquake and subsequent tsunami. The world’s deepest and Japan’s 1st earthquake resistant breakwater at Kamaishi port was one of them. The breakwater at Kamaishi port failed to block the tsunami. The Kamaishi port suffered heavy casualties due to the tsunami. It was reported that the breakwater collapsed mainly due to failure of its foundation. The damage to the body of breakwater was not significant. However, exact mechanism of failure of the breakwater has not yet been completely known. Countermeasures are very important to reduce damage caused by such compound geo-disaster caused by earthquakes and tsunamis in the future. In order to develop countermeasures, it is important to understand the failure mechanism. Therefore, to determine the failure mechanism, and to reproduce the failure process of breakwater subjected to earthquake and tsunami, this paper describes the stability of breakwater foundation subjected to earthquake and tsunami using centrifuge tests. The main feature of the study is application of the centrifuge technique for compound geo-disaster caused by earthquake and tsunami. A series of centrifuge model tests were performed to determine stability of breakwater foundation subjected to earthquake and tsunami. The Breakwater at Miyazaki port (Miyazaki Prefecture, Japan) is chosen as the prototype. In addition to mound, two layers of foundation soils (seabed) was considered in the test. Centrifuge model tests were performed under 25 g gravitational fields. Foreshocks and main shock were used as earthquake, and effects of foreshocks and main shock were evaluated in terms of horizontal displacement and settlement of the breakwater. After the main shock, tsunami overflow test was performed on the same model. In the case of compound geo-disaster brought by earthquake and tsunami, a number of forces act on breakwater. Excessive pore pressure increases with the increase of acceleration of earthquake loading, and leads to high deformation of the foundation soil. It increases shear strain of the foundation soil. These factors decrease effective stress and shear strength, and finally reduce bearing capacity of the foundation. That resulted in flow of foundation soil below the breakwater, and led to settlement and horizontal displacement of the breakwater. Seismic inertia forces, generated by the earthquake, also affected the stability of the breakwater during earthquake. Acceleration magnitude of earthquake affects stability of breakwater significantly. Settlement and horizontal displacement of the breakwater increased with the increase in acceleration of the earthquake loading. Very high settlement and horizontal displacement of the breakwater was observed during main shock. It was observed that in addition to tsunami, earthquake was one of the main reasons of failure of the breakwater. High scouring of mound occurred on harbor side during tsunami overflow. Seabed was also scoured by the tsunami waves. Scoured sand was piled up on harbor side by the tsunami waves. Seepage of water occurred below the breakwater, and was concentrated mostly through the mound. However, significant amount of seepage was also noticed through the upper soil layer. The seepage force reduced confining pressure and shear strength. Thus, bearing capacity of the foundation soil reduced significantly during tsunami. Finally, these factors made the breakwater unstable.

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