Study on the allowable deformation of structures in liquefied grounds and appropriate measures against liquefaction

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Liquefaction causes deformation of structures such as settlement of buildings and floatation of manholes. In current design procedure for liquefaction, however, the occurrence of liquefaction is judged only. Deformation of the structures is not evaluated. In the design for Level 2 earthquake motion, which has been introduced recently in new design codes after the 1995 Hyogoken-nambu earthquake, it is necessary to evaluate the deformation because many current countermeasures can not prevent the occurrence of liquefaction. Then, in this research, appropriate evaluation methods for liquefaction-induced deformation of structures and allowable deformation in liquefied grounds were studied by conducting several site investigations, shaking table tests and residual deformation analyses. Simple measure methods against the settlement of houses and the floatation of conduits were developed also. Effectiveness of the developed methods were confirmed by carrying out shaking table testes and seismic response analyses. Main conclusions derived from the research were as follows :1.A site investigation on the liquefaction-induced settlement of timer houses during the 2000 Tottoriken-seibu earthquake showed that the allowable angle of tilting was almost 1/100.2.Settlement of tanks and floatation of buried conduits can be evaluated by a residual deformation analysis "ALID." Effectiveness of ground improvement by compaction was also evaluated by this analysis.3.Effectiveness of gravel drain method can be evaluated by focusing on the settlement of structures even though the improved ground is liquefied under Level 2 earthquake motion.4.A simple countermeasure method by attaching brads to buried conduits was effective to prevent liquefaction-induced floatation.5.A lot of sewage manholes floated up during the 2003 Tokachi-oki earthquake. Detailed site investigation revealed that the floatation induced due to the liquefaction of backfilled soils.