Swelling of clays due to water saturation causes structural damage in civil engineering structures. At the microstructural level, swelling is caused by interlayer and interparticle (double-layer) forces. The present article is focused on the interlayer force with the specific objective of developing a force-displacement relation. On the basis of the variation of the pressure tensor with distance, the interlayer repulsive force is evaluated. The interlayer force-displacement relation is thus established. It is found that (a) the interlayer force for small particle separations can be hundreds of times the double-layer force at the corresponding separations, and (b) the interlayer force decays very quickly with increasing particle separation. The results indicate that the fundamental mechanism controlling swelling changes as the specimen's density changes, implying that mitigation strategies should depend on the density of the soil. The interlayer force-displacement relation is needed in assembly analyses such as the discrete element analysis. An ongoing study indicates that the interlayer force does not contribute to the overall swelling pressure when the void ratio is larger than a certain value.