Field identification of fine-grained soils

Field identification tests are quick hand tests on a small moist sample, used to tell fine-grained soils apart without laboratory equipment. They use the dry strength, dilatancy, toughness, plasticity and appearance of the soil, and are carried out on the fraction finer than 0.425 mm (No. 40 sieve).

1. Dilatancy (shaking) test

Make a pat of moist soil (soft putty consistency) and shake it in the palm, then squeeze it.

• Silt: water quickly rises to the surface and gives it a shiny look; it disappears on squeezing (rapid reaction).
• Clay: no change (no reaction).

2. Dry strength (crushing) test

Dry a moulded pat and crush it between the fingers.

• Clay: high dry strength; it cannot be crushed easily.
• Silt: low dry strength; it crumbles easily to powder.

3. Toughness (thread) test

Roll the soil into a thread about 3 mm in diameter, then fold and re-roll it repeatedly until it crumbles at the plastic limit.

• Clay: strong, stiff thread; tough lump after crumbling.
• Silt: weak thread and soft lump; it loses coherence quickly.

4. Plasticity / stickiness

Clay is plastic and sticky in the hand and cannot be washed off easily; silt feels gritty or floury and washes off easily.

5. Other tests

• Visual and touch: clay feels smooth, while silt feels slightly gritty between the teeth.
• Shine test: a cut or scratched surface of clay looks shiny; silt stays dull.
• Sedimentation test: shaken in a jar of water, sand settles in 30 - 60 seconds and silt in about 15 - 60 minutes, while clay stays in suspension for hours or even days.