Clay ground improved by the sand compaction pile (SCP) method is a composite of compacted sand piles and a soft clay layer. This type of composite ground shows complicated failure processes under backfill loading. This study was performed to determine the failure process of low replacement SCP ground under inclined loading through centrifuge model tests and numerical analyses based on the elasto-plastic constitutive model.
　First, two types of improved ground where sand piles did or did not reach a stiff layer with an underlying soft clay layer were subjected to many centrifuge model tests to investigate the deformation and failure patterns under various improvement
conditions. Second, undrained FEM analyses based on the elastic-perfectly plastic constitutive model were performed to examine the failure mechanism of improved ground under the limit state. Next, the method of ground stability analysis by the limit equilibrium method was proposed by modeling the balanced condition of improved ground based on the model tests and elasto-plastic analyses. The accuracy of this model was examined by comparison with the model tests and FEM analyses, and the model ground stability was assessed by a new method different from the present design procedure. Finally, the ground deformation under the limit state on the failure process is described. The mechanism of failure of improved ground subjected to the limit state was examined by soil/water coupling FEM analyses based on the elasto-viscoplastic constitutive model.