Retaining structures are used to hold earth and maintain a difference in the elevation between two levels. Soils behind a retaining system could be expansive materials, which may result in significant lateral swelling pressure in addition to the active earth pressure due to soil own weight. Swelling pressure is mainly correlated with soil plasticity index (PI) and change in moisture content. Retaining structures such as diaphragm walls that are constructedin front of such expansive soilsare subject to damage and cracks, as the swelling pressure is not usually considered in the design due to the complexity of its assessment. This study aimed to assess the effect of changing the PI on lateral swelling pressure for different retaining systems using the finite element program GeoStudio. After validating the model outcomes, a parametric study was carried out to determine the relation between swelling pressure and PI, and it was found that they are directly related to each other. In addition, the rate of change in the swelling pressure declines along the bottom third of retaining walls due to relative stabilization in the moisture levels.