The Masjed-e-Soleyman dam is a 177 meters high rockfill dam with central clay core, located in southwestern Iran. This dam has exhibited a deformation behavior that could be considered unique in comparison with other known high rockfill dams in the world. During construction and first impounding, a considerably high excess pore water pressure has been developed inside the core and it has been being dissipated with a very slow rate, so the consolidation deformations have been insignificant. However, there have been reports of noticeable internal deformations in the different parts of the dam -especially inside the core- during the construction, and the crest has also exhibited significant quick settlements during the first impounding. Therefore, the main objective of this paper was to identify the deformation mechanism of this dam. To achieve this objective, first the data recorded by the instruments installed inside the dam body were carefully studied and then a three-dimensional numerical model of the dam was developed to simulate its deformation behavior. The calculations were carried out via simultaneous use of flow and mechanical formulations. The mechanical behavior of embankment materials was idealized by a hardening / softening constitutive model. This model was based on Mohr-Coulomb model and its strength parameters are functions of plastic shear or tensile strain. In this paper, authors proposed a numerical method, based on this constitutive model and Rowe’s stress–dilatancy theory, to simulate the deformation behavior of coarse-grained materials, like rockfills, due to particle size distribution (PSD), particle breakage, rotation, and rearrangement under shearing. The results show that significant development of pore water pressure in the core and its insignificant dissipation, plastic shear deformations inside the core, significant volumetric strains in coarse-grained zones, and extensive collapse settlements of the upstream rockfill shell are the main causes influencing the deformation mechanism of this dam during the construction and first filling

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