Plastic Limit of Soil Test (PL Test)

Objective: To determine the plastic limit of soil (PL), 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 a glass plate to form a thread.
4. Apply uniform pressure and roll at a speed of 80–90 strokes per minute.
5. Continue rolling until the thread diameter reaches 3 mm.
6. Observe crumbling:
   • Thread crumbles below 3 mm → moisture content is above PL
   • Thread crumbles above 3 mm → moisture content is below PL
7. Repeat rolling and kneading until the thread crumbles at exactly 3 mm diameter.
8. Collect the crumbled soil immediately in a moisture container, cover it, and weigh.
9. Dry the sample in an oven to determine the dry weight and calculate moisture content.
10. Repeat the test at least three times and report the average value as PL.

Calculations

Plasticity Index (PI or Ip):

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

Toughness Index (TI or IT):

TI = IP / IF
(Where 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.
• Determine moisture content immediately after crumbling.
• The Plastic Limit helps classify soil into low, medium, or high plasticity.