Mechanical behavior of clay ground improved by SCP with low replacement area ratio could be more significantly affected by mechanical interaction between sand piles and clays than that of clay ground improved by SD or by SCP with high replacement area ratio. It is essential to elucidate the mechanical interaction in the improved clay ground, in order to accurately estimate behavior on reducing settlement of the improved ground and increasing strength of clays. In this paper, through a series of model tests and numerical simulations of clay ground improved by SCP with low replacement area ratio, each mechanical behavior of sand piles and clays in the improved ground during consolidation is elucidated, together with stress sharing mechanism between sand piles and clays.