IS 2720:1985 Part 5 is the Indian Standard (BIS) for methods of test for soils - determination of liquid and plastic limits. This standard prescribes the methods for determining the liquid limit and plastic limit (Atterberg limits) of soils. These limits are essential for classifying fine-grained soils and evaluating their engineering properties such as compressibility, shear strength, and shrink-swell potential.
Specifies methods for determining the liquid limit and plastic limit of soils, which are key Atterberg limits.
Quick Reference — IS 2720 Part 5:1985 Atterberg Limits
Casagrande and cone-penetration LL, plastic limit, plasticity index, A-line / U-line classification per IS 1498.
| Reference | Value | Clause |
|---|
| Test | Determination of liquid limit (LL) and plastic limit (PL) | Cl. 1 |
| Soil fraction tested | Passing 425 µm sieve | Cl. 3.1 |
| Casagrande LL — apparatus drop | 10 mm (standardised) | Cl. 4.2 |
| Casagrande LL — closure length | 13 mm (½ inch) at base of groove | Cl. 4.4 |
| Casagrande LL — blow count target | Plot vs blows; LL = w at 25 blows | Cl. 4.5 |
| Casagrande LL — number of trials | ≥ 4 (in 15–35 blow range) | Cl. 4.5 |
| Cone-penetration LL — cone mass | 80 g, apex angle 30° | Cl. 5.1 |
| Cone-penetration LL — penetration depth | 20 mm in 5 seconds | Cl. 5.4 |
| Cone-penetration LL — preferred method | More repeatable; primary in many labs | Cl. 5 |
| Plastic limit — thread diameter | 3 mm (just crumbles) | Cl. 6.4 |
| Plasticity index (PI / IP) | PI = LL − PL | Cl. 7 |
| Liquidity index | LI = (w − PL) / PI | Cl. 7 |
| Plasticity classification — A-line | PI = 0.73 (LL − 20) | Cl. 7 / IS 1498 |
| Plasticity classification — U-line | PI = 0.9 (LL − 8) (upper bound) | Cl. 7 / IS 1498 |
| Soil class — CL (low-plasticity clay) | LL < 35, above A-line | IS 1498 |
| Soil class — CI (intermediate) | 35 ≤ LL < 50 | IS 1498 |
| Soil class — CH (high-plasticity) | LL ≥ 50, above A-line | IS 1498 |
| Shrinkage limit (SL) — see Part 6 | Refer IS 2720 Part 6:1972 | cross-ref |
| Reporting precision | Whole number for LL/PL (%) | Cl. 8 |
⚠ Verify cone-penetration depth (20 mm) in latest BIS reaffirmation; some labs default to Casagrande for legacy comparability.
Overview
- Status
- Current
- Usage level
- Essential
- Domain
- Geotechnical — Soil and Foundation
- Type
- Testing Method
- Amendments
- Amendment 1 (1995)
Also on InfraLens for IS 2720
Practical Notes
! Ensure the soil paste is allowed to mature (cure) for at least 24 hours before testing, particularly for highly plastic clays, to ensure uniform moisture distribution.
! The Casagrande device must be mounted on a firm, unyielding surface to prevent loss of blow energy which can skew results.
! When rolling the soil thread for the plastic limit test, apply uniform pressure to prevent premature crumbling before the exact 3 mm diameter is reached.
Frequently referenced clauses
Updates & Amendments1 amendment
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 sample preparation425-micron IS Sieve
standard blows for liquid limit25 blows
blow rate for casagrande device2 revolutions per second
thread diameter for plastic limit3 mm
cone penetration for liquid limit20 mm
oven drying temperature105 to 110 °C
Key Formulas
Plasticity Index (PI) = Liquid Limit (LL) - Plastic Limit (PL)
Flow Index (If) = (w1 - w2) / log10(N2 / N1) — slope of the flow curve
Tables & Referenced Sections
Key Clauses
Clause 3 - Preparation of Test Sample
Clause 4 - Determination of Liquid Limit
Clause 5 - Determination of Plastic Limit
Frequently Asked Questions4
What sieve size is required for preparing soil samples for Atterberg limits?+
The portion of soil passing the 425-micron IS Sieve is used (Clause 3.1).
How is the liquid limit determined using the Casagrande method?+
It is the moisture content corresponding to exactly 25 blows on the plotted flow curve (Clause 4.1.4).
What defines the plastic limit of a soil?+
The moisture content at which a soil thread just begins to crumble when rolled to a diameter of 3 mm (Clause 5.2.2).
What is the alternative to the Casagrande apparatus for finding the liquid limit?+
The static cone penetrometer method, where the liquid limit is defined as the moisture content corresponding to a cone penetration of 20 mm (Clause 4.3).