OSHA 500 Trainer Course in Occupational Safety and Health Standards for the Construction Industry Practice Test

Cohesive soil is primarily characterized by its high clay content, which gives it unique properties such as the ability to stick together and hold water. Clay particles are very fine and have a plate-like shape, which allows them to adhere to one another effectively. This cohesive nature allows clay to expand when wet and shrink when dry, making it quite distinct from other soil types.

In contrast, soil types like sand and gravel consist of larger particles that do not adhere to one another in the same way. Sand, being coarse, does not have the cohesive properties that clay exhibits; it drains quickly and lacks the ability to hold together. Gravel has even larger particles and is primarily used for drainage, lacking cohesion altogether. Silt is finer than sand but does not exhibit the significant cohesive qualities of clay, thus making it less effective in binding together. Therefore, the defining characteristic of cohesive soil is indeed the predominance of clay.