<p>Coarse alluvial sediments consisting of silt–sand–gravel mixtures are usually internally unstable and are likely to internal erosion and suffusion. During the first impounding of the Bar Dam, sinkholes, surface deformation, and concentrated leakage were observed. This study investigates the causes of these accidents by integrating in-situ investigation, theoretical criteria, and large-scale suffusion experiments. Field observations revealed that sinkholes developed not only in coarse sediments but also in zones covered by a clay blanket, indicating the simultaneous occurrence of suffusion and contact erosion. Laboratory tests using a large permeameter confirmed that hydraulic gradient, particle size distribution, and relative compaction strongly influence fine particle migration and internal erosion. Negligible amounts of particles are lost at the low hydraulic gradient of 0.1, while considerable fine particles are eroded at a gradient of 1. Fine particle loosening during the suffusion increases with the finer fraction. Further, the <i>relative density</i> is found to affect markedly the <i>fines migration</i> and potential of suffusion<i>, where</i> increasing the relative compaction from 84 to 96% results in a descending in washed-out particles of about 2%. The outcome of this study lies in establishing a direct connection between field-scale dam incidents and laboratory-scale suffusion mechanisms. The results demonstrate that Bar dam foundation materials are susceptible to suffusion-induced deformation and sinkhole formation, highlighting the need for protective schemes such as partially impervious clay blankets.</p>