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日本語

The impact of earthquakes, tsunami, and waves on the stability of the seabed is a very important research topic in evaluating the soundness of harbors and coastal structures. Until now the soundness of harbors and coastal structure foundations has usually been evaluated by means of multiple practical tests. However, there are still many uncertainties in clarifying the phenomena caused by the interactions between waves and the ground and structures, such as the damage which was inflicted on breakwaters and seawalls due to the tsunami caused by the Great East Japan Earthquake.

We are studying the soundness assessment of harbors and coastal structures, focusing on the stability of the supporting ground in respect to earthquakes, tsunamis, and high waves. In this research, we evaluate the soundness of harbors and coastal structures on multiple scales, ranging from the movement of very small soil particles to the evaluation of the stability of huge civil engineering structures, while simultaneously aiming for research fusion by collaborating with water engineering. We believe that it has a high value both academically and socially.

In order to understand how to reliably defend structures against earthquake tremors and tsunami, we have clarified the damage mechanism of the structures that accompanies the destabilization of the supporting ground, by using numerical simulations using a method called centrifuge model test or particle method, and we have studied drastic measures and design methods based on fracture control design.

Recently, we have been focusing on the complicated sediment transport phenomenon, where the driving force to the soil particles in the ground caused by the flow rate on the surface of the seabed at the wave field and the permeability generated by the water pressure fluctuations in the ground act in combination. We have been observing this process using wave-making channel experiments. The experiments are carried out on a reduced scale model of the actual conditions to be targeted. Such a scaled experiment creates the problem that the full scale phenomenon cannot be exactly reproduced in this way.

Accordingly we must consider not only the geometric scale on the length scale, but also on the mechanical/dynamical scale Normally, because influence by gravity is dominant in wave reproduction experiments, they are conducted in consideration of the similarity law according to the Froude number (the ratio of the inertial force of the fluid to the gravity). Furthermore, we are conducting our studies under experimental conditions that take into account the scale law with the actual size, focusing on the speed of soil particles falling in the water in order to examine the reproducibility of the fluctuation of the pore water pressure (water pressure in the ground due to groundwater), in addition to the movement phenomenon of the seabed ground.

Specifically, we have installed sensors to measure the water pressure in the ground and to record changes in the ground pressure where waves act. Then, to interpret the probability of the numerical values measured in the experiments, we compared the data with the numerical values obtained by the numerical solution to the scale in the real phenomenon. As a result, the experimental values of the ground pressure change that we obtained were very close to the value obtained by the mathematical solution, and we were able to determine that the stress change of the ground at the wave field could be reproduced appropriately in the experiment. It is our hope that by such methods we may come to a deeper understanding of what kind of structural damage is caused to foundations by the movements of soil, sand and waves.