Atterberg limits and plasticity index in plain words

A fine-grained soil changes state as its water content changes: from liquid, to plastic, to semi-solid, to solid. The water contents at the boundaries are the Atterberg limits.

The three limits

• Liquid limit (LL): the water content at which soil starts to flow. In the Casagrande test it is the water content at which a standard groove closes over 12.7 mm in 25 blows.
• Plastic limit (PL): the water content at which a soil thread just crumbles when rolled to 3 mm diameter.
• Shrinkage limit (SL): below this water content, losing more water does not reduce the volume.

Indices

Plasticity index: I_p = LL − PL

Liquidity index: I_L = w − PLI_p Consistency index: I_c = LL − wI_p

I_c + I_L = 1. A soil with I_L above 1 is at a water content beyond its liquid limit and behaves like a thick liquid when disturbed.

Activity

A = I_p% finer than 2 μm

Activity above 1.25 points to active clays such as montmorillonite (black cotton soil), which swell and shrink a lot.

The plasticity chart and A-line

IS 1498 classifies fine soils on a chart of I_p against LL. The A-line is

I_p = 0.73 (LL − 20)

Soils plotting above it are clays (C), below it silts (M) or organic soils (O). Compressibility depends on LL: low (L) below 35, intermediate (I) 35 to 50, high (H) above 50.

Example: LL = 45, PL = 25, so I_p = 20. On the A-line, I_p = 0.73 × 25 = 18.25. The soil plots above it with LL between 35 and 50, so it is CI, clay of intermediate compressibility.