For liquefaction countermeasure based on grid-form ground improvement, the grid interval has conventionally been designed using the excess pore water pressure ratio or the liquefaction resistance factor, FL value, and conditions under which liquefaction does not occur in the ground within the grid. However, in the case of seismic motions with a long duration, such as that which occurred during the 2011 Off the Pacific Coast of Tohoku Earthquake, used as a design seismic motion, the increase in the excess pore water pressure in the ground within the grid becomes greater than that for a seismic motion with a shorter duration and an equivalent acceleration level. Costs therefore increase due to a narrower grid interval.
Centrifuge model vibration testing was conducted to obtain the knowledge required for a performance design focused on settlement occurring in the ground within the grid. Test results indicate that settlement is smaller with a smaller grid area, that the influence of liquefaction occurring at depth in the ground within the grid is small, and that the influence of the rise of excess pore water pressure at shallow locations is significant.
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