砂の弾塑性構成式の締固め砂質地盤への適用

Abstract

Sand compaction pile method is an effective soil improvement technique to prevent the liquefaction of loose sandy ground. Since the current design method of the sand compaction pile method is only incorporating the effect of the densification of sand due to compaction, the seismic design for this method has been reexamined after the 1995 Hyogoken Nambu Earthquake. However, the characteristics of compacted sand containing fine contents have not completely been clarified; it is difficult to incorporate all the complicated improvement effects to the current design code. In light of the recent trend of performance design, the dynamic numerical analyses by the finite element method have spread in practice. The soil water coupled liquefaction analysis, among many existing numerical analyses, can be particularly expected to evaluate the complicated improvement effects of compacted sand. Oka et al. have proposed the effective stress based liquefaction analysis using an elasto-plastic constitutive model for sand based on nonlinear kinematic hardening rule. In the present study, in order to explain the compaction effect of sand containing the fine contents, we have noted the relation between the parameters used in the elasto-plastic constitutive model and the state index of sand, Is. A series of undrained traixial compression tests of sand were conducted under the conditions that reproduce both the effective stress state and void ratio before and after compaction. We also simulated the triaxial test results by using the elasto-plastic constitutive model. It is found that the quasi overconsolidation ratio, OCR*, is correlated to the state index of sand and is one of the key parameters to evaluate the compaction effect of sand.