The Darbandikhan Dam reservoir in NE Iraq is located on soft sedimentary rocks which sporadically were affected by erosion. The basement rocks include sandstone, siltstone, and claystone, and mineralogically comprise quartz, and feldspar with some lithic fragments that are distributed in a soft matrix. The scanning electron microscope (SEM) images show sheet layers with clay structures for the matrix which is confirm by Semi-quantitative analyses of the EDAX data. The results demonstrate that samples categorize as dispersive soil based on the crumb test (ASTM D 6572–21). The Crumb tests were done in six hours, and the short duration can be considered as a defect. In this research the test time was extended and the behavior of the samples was investigated up to 7 days in intervals of two minutes, one hour, 6, 24, 72 hours, and 7 days. The results indicated that some samples which didn’t show any sign of dispersion in standard duration underwent some kind of egg-shaped erosion after 72 hours, which completely were separated in 7 days, so it would perhaps be better to make modifications to the crumb test. Correspondingly, the Sodium Absorption Ratio (SAR) and Exchangeable Sodium Percentage (ESP) results of soft grain units demonstrated that these units are mainly dispersive.
