Many embankment dams constructed with a core of nonplastic or very low plasticity silt–sand–gravel (typically of glacial, fluvioglacial or alluvial origin) have experienced internal erosion. This has often expressed itself with the development of sinkholes or with intermittent episodes of increased leakage, which then reduces. In this investigation 22 soil samples with gradations representing the range of the soils used in embankment dam cores have been tested in the laboratory. All 22 soils tested were shown to be internally unstable with particle movement within the soil after placement. Some soils exhibited global backward erosion (GBE), others suffusion, and some internal instability but with no erosion from the sample, indicating self-filtering. The internal erosion process was very rapid for suffusive soils, typically occurring within minutes of test commencement, and at a gradient of 1. For soils subject to GBE and no-erosion soils, the internal movement of particles continued for weeks and months, and re-activated when the overall gradient was increased. For GBE, the erosion process occurred over a range of gradients. A method for predicting the amount of erosion and the erosion mechanism based on the gradation of the soil has been developed that is related to the ability of the soil to self-filter.