Abstract;
The coast of Japan is an area subject to frequent earthquakes and tsunamis. Since the Great East Japan Earthquake and the resulting tsunami in 2011, countermeasures against earthquake and tsunami have been implemented rapidly and extensively in Japan for facilities such as breakwaters and quay walls of commercial and fishing ports. Countermeasures are also being considered for port facilities in remote islands. Japan's Ogasawara Islands, a world heritage site, is located about 1,000 km of Japan’s main island, south-southeast of Tokyo, and consists of 30 large and small islands. Among these, in inhabited Chichijima island (population approximately 2,000), Futami port functions to supply living goods to the island, move out from the island, and accept tourists. A regular service is provided by a cargo and passenger ship, Ogasawara Maru (11,035 tons, 150 meters long), from Tokyo port on a round trip a week basis. In addition, since Futami Port is the only port on Chichijima, it is also required to fulfill the function as an emergency transportation facility in the event of a disaster such as earthquake or tsunami, and the main mooring facility - 7.5 m quay must be able to withstand level 2 earthquakes and the resulting tsunamis.
In this paper, the result of seismic performance survey and tsunami wave performance survey on the quay of the pier structure will be presented, and countermeasures to satisfy the required performance are discussed. For the seismic performance, dynamic analysis was conducted on the level 2 earthquake by the effective stress analysis FLIP program. On the other hand, for anti-tsunami performance, a tsunami simulation based on nonlinear long wave theory for level 2 tsunami was conducted and stability checks for the tsunami level around the target facility were conducted. Based on these results, a structure that can withstand level 2 earthquake motions and tsunami, and is superior in economic efficiency and constructability was proposed. (Based on these results, studies and suggestions were made on structures that can withstand level 2 earthquake motions and tsunami, and that are superior in economic efficiency and constructability.) Note that the level 2 earthquake ground motion is the earthquake ground motion having the greatest strength from the past to the present and the future at the relevant design point. The level 2 tsunami indicates a tsunami occurring about once in 1000 years. Level 2 earthquake motions and tsunamis are applied to particularly important facilities among port facilities. The main conclusions are as follows.
Conclusions:
1) Level 2 tsunami to be designed for is the tsunami of the Nankai Trough massive earthquake. Based on the tsunami simulation for the level 2 tsunami by the tsunami fault model of the Nankai Trough massive earthquake, the maximum water level and minimum water level at Chichijima Futami Port were calculated. From the maximum water level and the minimum water level, the stability of the accidental condition against the level 2 tsunami was examined, and it was revealed that it possesses sufficient proof strength.
2) In addition, anti-tsunami resistance performance of the elements of the piers was examined by the two-dimensional numerical wave motion channel, and it became clear that the structure has sufficient resistance.
3) For the accidental state of the level 2 earthquake, dynamic analysis by FLIP was carried out, and the structure which ensures the stability (an allowable displacement in the horizontal direction of within 1 m) was examined. Based on the results, a ground improvement as follows was proposed as an earthquake-resistant reinforcement construction method; a ground improvement plan by the injection solidification method (medicinal fluid penetration solidification processing method) which suppresses liquefaction of the ground by consolidating the ground at the lower part of the facility with permanent chemical liquid, and reduces the stress of the steel material.
4) At present, detailed examination of the countermeasure construction method focusing on the construction plan is being carried out based on the ground improvement plan utilizing the injection solidification method (chemical infiltration solidification processing method), and hopefully the result will be presented in the main report.