Sand compaction pile method (SCP) has been utilized for the improvement of the bearing capacity, stability, horizontal resistance of a soft soil, in which many compacted sand piles are driven in the ground.Bearing capacity of the SCP improved ground has been thoroughly investigated by many centrifuge model tests, field test and numerical analysis. Some model tests and field measurements have also been performed on the horizontal resistance of the SCP improved ground, but uncertainties still remains on this subject.
　In this study, a series of centrifuge model tests was carried out in the 50g centrifugal acceleration in order to evaluate the horizontal resistance of a single pile embedded in the SCP improved ground. In the model tests, the single pile was statically loaded to investigate the effect of the width, depth and improvement area ratio of the improved ground on the horizontal resistance. The test results were compared with the numerical analysis based on Kubo's formula in which sub grade reaction coefficient of the improved ground was incorporated. It was found that the sub grade reaction coefficient of the SCP improved ground was much dependent upon its width, depth and improvement area ratio, and that the horizontal resistance of the improved ground can be evaluated by incorporating the sub grade reaction coefficient of the improved ground.