IS 2720:1972 Part 6 is the Indian Standard (BIS) for methods of test for soils - part 6: determination of liquid and plastic limits. This standard details the laboratory methods for determining the liquid limit and plastic limit (Atterberg limits) of fine-grained soils. These parameters are essential for soil classification, estimating the plasticity index, and predicting the engineering behavior of soils under varying moisture conditions.
Describes methods for determining the liquid limit and plastic limit of fine-grained soils.
Quick Reference — IS 2720 Part 6:1972 Atterberg Limits
Liquid limit (LL) and plastic limit (PL) for soils. Apparatus, procedure, plasticity index.
| Reference | Value | Clause |
|---|
| Liquid limit — apparatus | Casagrande LL device (cup + grooving tool) | Cl. 3 |
| LL device cup — drop height | 10 ± 0.2 mm | Cl. 3.2 |
| LL device cam rate | 2 revolutions per second | Cl. 3.4 |
| Sample preparation — passing sieve | 425 μm (No. 40) | Cl. 4.1 |
| Sample size | ≥ 120 g of fines | Cl. 4.2 |
| Soaking before test (clay) | ≥ 16 hours | Cl. 4.3 |
| Number of trials per LL test | minimum 4 (different moisture) | Cl. 5.4 |
| Closure groove length — at LL | 12 mm closure | Cl. 5.3 |
| Number of blows for LL— interpolate on flow curve at N = 25 | 25 blows | Cl. 5.5 |
| Cone penetrometer alternative — penetration— BS-style cone test alternative | 20 mm in 5 s | Annex A |
| Plastic limit — thread diameter | 3 mm just crumbles | Cl. 6.3 |
| Plastic limit — minimum 2 determinations | ≤ 2 % difference between trials | Cl. 6.4 |
| Plasticity index PI | = LL − PL | Cl. 7 |
| Non-plastic soil designation | NP (when PL cannot be determined) | Cl. 6.5 |
| Liquidity index LI— LI < 0 brittle; > 1 viscous | = (w − PL) / PI | Cl. 7 |
| Activity number — clay | = PI / clay fraction (%) | (IS 1498 reference) |
| Activity classification — inactive | A < 0.75 | (IS 1498 Annex) |
| Activity classification — normal | A 0.75–1.25 | (IS 1498 Annex) |
| Activity classification — active (BC, montmorillonite) | A > 1.25 | (IS 1498 Annex) |
| Test temperature — laboratory | 27 ± 2 °C | Cl. 5.1 |
⚠ Methods of Test for Soils — Part 6: Liquid and Plastic Limits. Reaffirmed; cross-referenced by IS 1498 classification.
Overview
- Status
- Current
- Usage level
- Essential
- Domain
- Geotechnical — Testing Methods and Quality Control
- Type
- Testing Method
- Amendments
- Amendment 1 (1978); Amendment 2 (1985)
Also on InfraLens for IS 2720
Practical Notes
! Soil must be thoroughly mixed with distilled water and allowed to mature (curry) for a sufficient time (up to 24 hours for highly plastic clays) to ensure uniform moisture distribution.
! When rolling threads for the plastic limit test, apply just enough pressure to reduce the soil thread diameter to exactly 3 mm before it crumbles.
! The Casagrande apparatus base and cup should be strictly calibrated and checked for wear, as variations can significantly alter the liquid limit blow count.
Frequently referenced clauses
Updates & Amendments2 amendments
1978Amendment 1 (1978)
1985Amendment 2 (1985)
Consolidated list per BIS. For the text of each amendment, refer to the BIS portal link above.
soilclaysiltfine-grained soils
International Equivalents
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Key Values6
Quick Reference Values
Sieve size for soil preparation425-micron IS Sieve
Casagrande apparatus drop rate2 revolutions per second
Liquid limit standard blows25 drops
Standard groove closure length12 mm
Plastic limit thread diameter3 mm
Oven drying temperature105 to 110 °C
Key Formulas
PI = LL - PL — Plasticity Index
If = (w1 - w2) / log10(N2/N1) — Flow Index
Tables & Referenced Sections
Key Clauses
Clause 3 - Preparation of Test Sample
Clause 4 - Liquid Limit Determination
Clause 5 - Plastic Limit Determination
Frequently Asked Questions4
What sieve size is used to prepare soil for Atterberg limits?+
Soil passing the 425-micron IS Sieve is used.
What defines the liquid limit in the Casagrande test?+
The moisture content at which a standard groove closes for a length of 12 mm under exactly 25 drops of the cup.
What is the standard diameter at which soil should crumble for the plastic limit test?+
3 mm.
What should be the rate of turning the Casagrande device handle?+
2 revolutions per second.
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