STUDIES ON SOIL LIQUEFACTION AND SETTLEMENT IN THE URAYASU DISTRICT USING EFFECTIVE STRESS ANALYSES FOR THE 2011 OFF THE PACIFIC COAST OF TOHOKU EARTHQUAKE

The 2011 off the Pacific coast of Tohoku Earthquake caused soil liquefaction over a wide area. In particular, severe soil liquefaction was reported in the northern parts of the reclaimed lands around Tokyo Bay, even though the seismic intensity in this area was only about 5 on the JMA scale with low acceleration. The authors surveyed the residual settlement in the Urayasu district and then conducted effective stress analyses of sites affected and not affected by liquefaction. The analyses results were compared with the acceleration waves monitored with K-NET Urayasu or ground settlements surveyed. They were based on the acceleration observed on the seismic bedrocks in earthquake engineering in some other districts adjacent to Urayasu. Much of the settlement was due to the long duration of the earthquake, with further settlement resulting from the aftershocks. The study shows that the effects of aftershocks need to be monitored. The simplified liquefaction prediction methods using the factor of safety, FL, also need improvement.