A STUDY ON INFLUENCE OF PHYSICAL AND MECHANICAL PROPERTIES ON CHANGE OF SEDIMENTARY STRUCTURE IN SAND LAYER CAUSED BY LIQUEFACTION

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In Joetsu City and Muikamachi Town, Niigata Prefecture, which are one of the heaviest snowfall areas in Japan, the groundwater level is much lowered in winter due to the excessive pumping out of groundwater to melt snow. At Saijocho, Joetsu City, annual subsidence in heavy snowfall years is 10 times as much as in little snowfall years. The boring observation was put in operation two districts, which are located the urban Takada, Joetsu City and the urban Muikamachi, Muikamachi Town.Some standard consolidation tests and repeated consolidation tests on undisturbed soil, which was taken from these observation sites, were performed. In order to repeated consolidation tests, automatic oedometer apparatus was improved. The vertical displacement of specimens, which taken from sandy soil in both districts, was measured by digital gages with precision of 1 mum at 0.1 second intervals during the repeated tests. It is believed that the settlement due to repeated loading (unloading and reloading) is more severe than normal loading in the consolidation tests on saturated samples of sandy soil, as cohesive soil.It was developed a new observation system in order to monitoring of land subsidence. This observation system is used the wire made of poly amide for measuring subsidence and tried to miniaturized the observation system. These new observation system used to land subsidence at both sites. Subsidence has been advanced annual changes of groundwater level. Total amount of subsidence by this new system was any time an about equal to the measurement by double tube observation system.