Liquefaction evaluation of large pier foundations by undisturbed sampling method called GS sampling for coral pebble mixed soil

抄録

At the Okinawa Island, a construction project of large-scale jacket pier for large cruise ships is proceeding with the in-crease of cruise passengers from overseas. However, very loose coral pebble mixed soil was deposited thickly in the project site, and there was concern that liquefaction would occur in the coral pebble mixed soil. In order to evaluate the liquefaction characteristics of coral pebble mixed soil, it is necessary to collect samples with undisturbed condition, but it was predicted that it would be difficult to collect by conventional sampling method used in Japan. Therefore, we decided to apply a special sampling method called GS sampler. GS sampler is high-quality sampler for collecting loose sandy soil, gravel, fractured zone and waste matter which have been very difficult to obtain good samples with undisturbed condition. A total of eleven samples were recovered from coral pebble mixed soil layer with the thickness of about 15m to 20 m. When the sample collected by GS sampler was photographed with X-ray, it was confirmed that the sample was collected in a good condition without disturbance. Based on the undisturbed samples, the cyclic triaxial test was carried out, and it was found that the liquefaction strength, R_L20, of the coral pebble mixed soil was range of 0.22 to 0.27. Based on this result, as a result of evaluated the liquefaction, the input ground motion of level 1 earthquake, the liquefaction resistivity, F_L, was 1.6 to 3.4, and the result not liquefied was obtained. In this way, because of conducting a detailed liquefaction investigation by GS sampling and the cyclic triaxial test, it was unnecessary to initially improve the ground so that it was possible to greatly reduce the cost of the ground improvement.