The seepage and inertia effects on the rate-dependent subgrade reaction of a single pile in liquefied soil are clarified through numerical studies. The quasi-static (for seepage effect) and dynamic (for inertia effect) numerical analyses are performed with a soil-water coupled formulation and a simplified cyclic elasto-plastic constitutive model for liquefied sand. The constitutive model can explicitly deal with the liquefaction intensity by changing the lower bounds of the mean effective stress. The liquefied soil at a certain depth around a pile is modeled with finite elements under a plane stress condition. The results of the quasi-static analyses under monotonic loading conditions show that the apparent rate-dependency of the subgrade reaction is caused by the seepage of pore water around the pile due to soil positive dilatancy under large strain range. The results of the dynamic analyses under cyclic loading conditions show that the positive correlation in a p-v (subgrade reaction-pile relative velocity) relation can be explained with the phase difference in movement between a pile and the neighboring soil around the pile due to inertia under small strain range.
2008 The Japanese Geotechnical Society