One of the important factors that have to be considered in the design of piled-raft foundations is the selection of the spacing between the piles. In this study, a piled-raft foundation having different pile-spacing configurations and resting on clayey soil was modeled using PLAXIS numerical packages based on the 2D and 3D finite-element methods. The dimension effect is examined by comparing the 2D modeling analysis with the 3D modeling analysis. The results of the models with different pile spacings were compared and interpreted. As a general trend in accordance with the results of the two- and three-dimensional analyses, the total displacements, shear strains and volumetric strains decreased as the pile spacings (s/D) increased. In the case of s 6D as a threshold value there was no significant change in the aforementioned quantities (they remain constant) or decreased slightly. The maximum settlement was found to be greater in the 3D analyses than in the 2D analyses. It is thought that the dimension effect caused the differences in the results obtained with the 2D and 3D analyses.