Identification and Nature of Dispersive Soils

Some fine-grained soils, called dispersive soils, with higher content of dissolved pore-water sodium than ordinary soils, rapidly erode forming tunnels and deep gullies by a process in which the individual clay particles go into suspension in slow-moving water (colloidal erosion), damaging earth dams, canals, and other structures. Dispersive soils cannot be differentiated from ordinary soils by conventional soil mechanics tests. An investigation in which four different laboratory tests for dispersion were performed on a considerable number of soils of diverse origins and properties has provided improved understanding of the properties of dispersive soil and strengthened identification criteria. High pore-water sodium is confirmed to be the main factor causing a soil to be dispersive, although there are a few exceptional low sodium dispersive soils. The newly developed pinhole test, in which erosion is measured directly by causing water to flow through a small hole in a compacted specimen, is the most reliable single test.