An effective stress analysis can evaluate the seismic behavior of liquefiable ground and it is ideal to consider this behavior in a design of the liquefaction countermeasure such as densification methods. Especially for planning the appropriate densification range, it is important to take groundwater movements into account because the development of the excess pore water pressure in the densified area can be predicted properly due to the reproduction of the seepage into a densified area from an adjacent undensified area. To consider groundwater movements in seismic response analysis, governing equations of a two-phase system (u-w formulation) were introduced into a general-purpose 3D-FEM program "TDAPⅢ". Additionally, an elasto-plastic constitutive law (Stress-Density model) was incorporated as a three-dimensional constitutive model to express the stress-strain relationship of liquefied sandy soil.

To investigate whether the effects of the densification on liquefiable ground and pile foundations are properly evaluated by implementing the new features, simulation analyses by "TDAPⅢ" were carried out on a series of centrifuge tests. The subjects of these tests were pile foundations surrounded by the ground where the densification is conducted.

As a result, the liquefaction suppression effects due to the densification were expressed in the simulations, and the influence of seepage on the behaviors of the pore water were explained by visualizing the flow direction of the groundwater. Furthermore, the trends of the bending moments and the horizontal subgrade reactions of the piles under the different densification range conditions were able to be evaluated in the simulations.