Plastic Limit of Soil Test (PL Test)

Objective: To determine the plastic limit of Soil (PL) of soil, which is the moisture content at which soil starts to exhibit plastic behavior.

Apparatus Required

• Evaporating dish
• Spatula
• Glass plate
• Moisture containers with lids
• Rolling rod (3 mm diameter)
• Sensitive balance (accuracy 0.01 g)
• Oven (temperature 105–110 °C)

Procedure for Plastic Limit Test

1. Take about 20 g of dry, pulverized soil passing through a 425 μm IS sieve.
2. Mix the soil thoroughly with distilled water until it forms a plastic paste.
3. Form a small ball of soil and roll it between fingers and glass plate to form a thread.
4. Use just enough pressure to roll without crushing, at a speed of 80–90 strokes per minute.
5. Continue rolling until the thread diameter is 3 mm.
6. Observe crumbling:
   • Diameter < 3 mm → moisture content is above PL
   • Diameter > 3 mm → moisture content is below PL
7. Repeat rolling and kneading until the thread crumbles at exactly 3 mm.
8. Transfer the crumbled soil immediately to a moisture container, cover, and weigh.
9. Dry the sample in an oven to determine dry weight and calculate moisture content.
10. Repeat at least three times to obtain consistent PL values.

Calculations

Plasticity Index (PI or Ip):

PI = Liquid Limit (LL) – Plastic Limit (PL)
PI = WL – WP

Toughness Index (TI or IT):

TI = IP / IF
(IF = flow index from liquid limit test)

Liquidity Index (LI or IL):

LI = (W – WP) / IP
W = natural moisture content of soil

Consistency Index (CI or IC):

CI = (WP – W) / IP

Notes:

• Maintain uniform rolling strokes for accurate results.
• Measure moisture content immediately after crumbling.
• Helps classify soil as low, medium, or high plasticity.