Liquid Limit of Soil by Cone Penetration Method (IS 2720 Part 5)

The Liquid Limit (LL) of soil is the water content at which soil changes from a plastic state to a liquid state. The cone penetration method as per IS 2720 (Part 5): 1985 is a reliable and repeatable laboratory method used mainly for fine-grained soils.

Objective

• To determine the liquid limit of soil using cone penetration method
• To assess consistency and classification of fine-grained soils

Apparatus Required

• Cone Penetration Apparatus (30° cone, 80 g ± 0.1 g)
• Weighing balance (accuracy 0.01 g)
• Oven maintained at 105–110°C
• Distilled water
• Glass plate and spatula

Sample Preparation

1. Take soil passing 425 micron IS sieve
2. Mix with distilled water to form uniform paste
3. Allow soil paste to rest for 24 hours for moisture equilibrium

Test Procedure – Cone Penetration Method

1. Fill soil paste in the metal cup without entrapped air
2. Level the surface smoothly
3. Lower the cone gently to touch the soil surface
4. Release the cone and allow it to penetrate freely
5. Note the penetration depth in millimeters
6. Repeat test at different moisture contents
7. The Liquid Limit is the moisture content corresponding to 20 mm penetration

Calculation of Liquid Limit

Liquid Limit is determined by plotting a graph between moisture content (%) and penetration (mm). The moisture content corresponding to 20 mm penetration is reported as Liquid Limit.

Applications

• Soil classification as per IS and USCS
• Design of pavement subgrade and foundations
• Assessment of compressibility and swelling characteristics

Acceptance Criteria for Highway Works