The sand compaction pile(SCP) method can provide not only bearing capacity increasing effects because SCP form composite grounds together with the original ground but also long-term drainage effects. It can also effectively provide ground improving effects through reduction in consolidation time and drainage distances resulting from stress concentration. Recently, the SCP method has been mainly studied in relation to changes in the behavior of composite grounds when construction work has been performed with low replacement ratios. Along with the foregoing, the partly penetrated SCP method used to form pile bodies underground with partly penetrated SCP instead of penetrating the SCP to the support layer existing below the soft clay layer of the ground to be improved is also attracting gradually more attention. However, studies on the behavior of partly penetrated composite grounds are insufficient and in particular, experimental studies in relation to penetration ratios are quite insufficient. Therefore, in the present study, centrifuge model tests were conducted in order to grasp the deformation characteristics and behavior of partly penetrated SCP reinforced grounds in relation to replacement ratios and penetration ratios. Preliminary consolidation was performed to form grounds that would maintain a certain level of strength in order to simulate stress conditions of the ground, SCPs were installed using a compaction method, the relationship among loads and settlements and stresses were assessed under a rigid load condition and the patterns of deformation of piles were compared and analyzed. In addition, finite element analyses were conducted in order to verify the results of centrifuge model tests and assess stress change or settlement reduction effects in relation to penetration ratios and depths. The results of centrifuge model tests and numerical analyses conducted in order to grasp the deformation and stress behavior of SCP reinforced grounds in relation to replacement ratios (10, 30%) and penetration ratios(50, 65, 80%) are as follows.

1. When the replacement ratio was 10%, the results of the centrifuge model tests under rigid load conditions showed a tendency that the deformation of partly penetrated compound grounds was similar to that of non-reinforced grounds and when the replacement ratio was 30%, unlike the case when the replacement ratio was 10%, the results showed a tendency that as the penetration ratio increased, settlement reduction effects became clearer so that the amount of settlement decreased.
2. Reinforced areas showed typical bulging failure patterns and the movements of grounds in these plastic areas induced shear failure. Whereas the fore-ends of the piles failed to resist against deformation when the penetration ratios were 50 and 65%, they resisted against deformation while delivering loads from the top of the piles when the penetration ratios were 80 and 100%. In addition, a phenomenon can be identified that as penetration ratios increase, the conical failure area decreases while rising from the area of the fore-ends of the piles toward the area of the heads of the piles.
3. Stress share ratios were assessed to show a tendency to gradually decrease as the depth increased with the rate of decreases increasing as penetration ratios decreased while showing almost similar ranges regardless of penetration ratios at points close to the surface layer where the sand piles were installed.
4. In the results of the centrifuge model tests and numerical analyses, stress concentration ratios were shown to be proportional to penetration ratios although they showed similar values regardless of penetration ratios at points close to the surface. In particular, the values showed very close ranges when penetration ratios were 100% and 80%.