The Crumb test is a simple indicator test that is recommended for initial field identification of dispersive soils. The crumb test provides a simple, quick method for field or laboratory identification of a dispersive clayey soil. Dispersive clay soils are identified by observing the behaviour of a few crumbs of soil placed in distilled water. The dispersion is classified depending on the degree of muddiness of the water.
• a 100 ml glass beaker
• distilled water
- Prepare a few crumbs, each about 6 mm to 10 mm diameter, from representative portions of the soil at the natural moisture content.
- Drop the crumbs in the beaker about one-third full of distilled water.
- Observe the reaction after allowing to stand for 5 to 10 min.
Observe the behaviour of the crumbs, and classify in accordance with the following guidelines:
Grade 1: No reaction. Crumbs may slake or run out to form a shallow heap on the bottom of the beaker, but there is no sign of cloudiness caused by colloids in suspension.
Grade 2: Slight reaction. A very slight cloudiness can be seen in the water at the surface of a crumb.
Grade 3: Moderate reaction. There is an easily recognizable cloud of colloids in suspension, usually spreading out in thin streaks at the bottom of the beaker.
Grade 4: Strong reaction. A colloidal cloud covers most of the bottom of the beaker, usually as a thin skin. In extreme cases all the water becomes cloudy.
Grades 1 and 2 represent a non-dispersive reaction, and grades 3 and 4 a dispersive reaction.