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.
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 |