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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 The hybrid application of deep mixing Columns with walls featuring a new three-dimensional improvement form, based on the combined use of wall-type and pile-type improvements, was developed for application in foundations of high embankments on soft, viscous ground. Compared to conventional methods, this method is advantageous in that it is highly effective in controlling changes in the ground surrounding embankments. To reflect the effects of three-dimensional improvement forms on design, a detailed study using three-dimensional FEM analysis, not generally used for ground improvement design, is conducted. This study reveals that the maximum benefits in control of change can be achieved by locating a wall-type improvement at the top of an embankment slope via three-dimensional FEM analysis using the MuDIAN analysis code. This method is therefore more economical than conventional pile-type improvement. Proof is also given of the ability to use the method for detailed study through simulation of trial works on the Kumamoto Udo Road designed using FEM analysis.

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