Geotechnical ASTM 2017 Edition

ASTM D4318 — Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils

Standard test methods for determining the Atterberg limits of soils — the liquid limit, plastic limit, and plasticity index — which govern the consistency and engineering behaviour of fine-grained soils.

Scope

ASTM D4318-17 covers the laboratory determination of the liquid limit (LL), plastic limit (PL), and plasticity index (PI) of soils, collectively known as the Atterberg limits. These index tests are fundamental for classifying fine-grained soils under the Unified Soil Classification System (USCS, ASTM D2487) and the AASHTO soil classification system. The standard provides two methods for liquid limit determination: Method A (Casagrande cup, multipoint method with flow curve), Method B (Casagrande cup, one-point method), and Method C (fall cone penetrometer). The plastic limit is determined by the thread-rolling method in which a 3.2 mm diameter thread is rolled until it crumbles. The plasticity index is calculated as PI = LL − PL. The standard applies to soils that pass the 425 μm (No. 40) sieve and exhibit measurable plasticity.

Purpose

The Atterberg limits provide a quantitative measure of the critical water contents at which a soil transitions between four consistency states: solid, semi-solid, plastic, and liquid. The plastic limit is the boundary between semi-solid and plastic states, while the liquid limit is the boundary between plastic and liquid states. These limits are not fundamental soil properties but empirical indices that have been correlated extensively with engineering behaviour including compressibility, shear strength, permeability, shrink-swell potential, and compaction characteristics. The plasticity index quantifies the range of water content over which a soil remains plastic — a higher PI indicates greater plasticity and generally higher compressibility and shrink-swell potential.

[FIGURE — Casagrande cup apparatus showing the brass cup, grooving tool, and crank mechanism used for the multipoint liquid limit test with the flow curve plot of water content vs log number of blows]

Engineering Applications

  • USCS soil classification — the plasticity chart (PI vs LL) separates clays from silts and low-plasticity from high-plasticity soils per ASTM D2487
  • Shrinkage limit estimation — the shrinkage limit can be estimated from LL and clay content using empirical correlations
  • Soil activity — activity A = PI / (% clay fraction < 2 μm) classifies clays as inactive (A < 0.75), normal (0.75 ≤ A ≤ 1.25), or active (A > 1.25)
  • Liquidity index — LI = (w − PL) / PI indicates the in-situ consistency state relative to the Atterberg limits
  • Compaction control — correlations between PI and optimum moisture content / maximum dry density for fine-grained soils
  • Expansive soil identification — PI and LL are primary criteria for identifying potentially expansive clays
  • Lime and cement stabilization — PI reduction after additive treatment is used to evaluate stabilisation effectiveness

Design Philosophy

The Atterberg limits are based on the observation that fine-grained soils exhibit dramatically different mechanical behaviour depending on their water content. At low water content, soil behaves as a brittle solid; as water is added, it becomes plastic (mouldable), and eventually flows as a viscous liquid. The boundaries between these states are defined by standardised test procedures that have been empirically calibrated to produce consistent, repeatable results. The flow curve for the Casagrande cup method plots water content against the logarithm of the number of blows, producing an approximately linear relationship. The liquid limit is defined as the water content corresponding to 25 blows, a value that Casagrande selected to provide a convenient reference for distinguishing clays from silts.

Plasticity Index:  PI = LL − PL
Liquidity Index:  LI = (w − PL) / PI
Flow Index:  IF = (w1 − w2) / log(N2/N1)
Toughness Index:  IT = IF / PI
Soil Activity:  A = PI / (% passing 2 μm)

A-line (USCS):  PI = 0.73 × (LL − 20)
U-line (upper bound):  PI = 0.9 × (LL − 8)

Important Requirements

The liquid limit test requires that the soil sample be thoroughly mixed with distilled water to a uniform paste and allowed to temper (typically 16-24 hours for high-plasticity soils). For the Casagrande cup method, the grooving tool must cut a clean groove 2 mm wide at the base. The cup is lifted and dropped at 1.9 ± 0.1 drops per second. The test is performed at three to five different water contents, with the number of blows required to close the 13 mm groove recorded. For the plastic limit test, the soil thread is rolled to 3.2 mm diameter on a glass plate until it crumbles and cannot be rolled further. The water content of the crumbled thread segments is measured. The plasticity index is not reported if the plastic limit cannot be determined (non-plastic soil). The shrinkage limit can also be determined per ASTM D427 or D4943.

Warning:

The one-point liquid limit method (Method B) is less accurate than the multipoint method (Method A) and should be used only for routine classification or when sample quantity is limited. The multipoint flow curve method is the referee method for all specification and compliance testing and is required for classification per ASTM D2487.

Key Parameters

Parameter Symbol Definition Typical Range
Liquid limit LL Water content at 25 blows (Casagrande cup) 15–150%
Plastic limit PL Water content at 3.2 mm thread crumbling 10–60%
Plasticity index PI LL − PL 0–100+
Liquidity index LI (w − PL) / PI <0 to >1
Flow index IF Slope of flow curve 10–60
Toughness index IT PI / IF 0.3–2.0
Soil activity A PI / (% clay < 2 μm) 0.3–7.0+
Shrinkage limit SL Water content at no further volume change 5–35%

Plasticity Classification

Plasticity Description PI Range USCS Groups Typical Engineering Behaviour
Non-plastic PI = 0 GW, GP, SW, SP Free-draining, low compressibility, excellent subgrade
Low plasticity 1 ≤ PI ≤ 10 ML, CL-ML Slight frost susceptibility, fair subgrade, low shrink-swell
Medium plasticity 10 < PI ≤ 20 CL, OL Moderate frost susceptibility, fair subgrade
High plasticity 20 < PI ≤ 40 CH, MH, OH High shrink-swell potential, low permeability
Very high plasticity 40 < PI ≤ 70 CH, MH Very high shrink-swell, problematic for foundations
Extreme plasticity PI > 70 CH, MH Extreme shrink-swell, requires special foundation design

Practical Engineering Notes

Note:

The liquidity index LI = (w − PL)/PI is a useful field indicator of soil consistency. LI < 0 indicates soil moisture below the plastic limit (brittle/semi-solid), LI = 0 to 1 indicates plastic consistency, and LI > 1 indicates the soil is at moisture content above the liquid limit and will behave as a liquid. Natural soils with LI > 1.0 are highly unstable and require special handling.

Field Tip:

A quick field estimate of plasticity can be made using the thread test: roll a moist soil sample into a 3 mm thread. If it cannot be rolled, the soil is non-plastic. The longer the thread that can be rolled without crumbling, the higher the plasticity. The toughness and shine of the rolled surface also correlate with plasticity — clays produce a shiny surface, silts produce a matte surface.

Typical Workflow

  1. Air-dry and process the soil sample through a No. 40 (425 μm) sieve.
  2. Mix the prepared soil with distilled water to form a uniform paste; allow to temper.
  3. For the liquid limit: adjust the Casagrande cup height to 10 mm drop, prepare three to five water content increments, and record the number of blows for each.
  4. Plot the flow curve (water content vs. log blows) and read the liquid limit at 25 blows.
  5. For the plastic limit: roll 8-10 g of soil into a 3.2 mm thread on a glass plate until it crumbles.
  6. Measure the water content of the crumbled thread to obtain the plastic limit.
  7. Calculate the plasticity index: PI = LL − PL.
  8. Classify the soil on the plasticity chart and report all three parameters.

Common Mistakes

  • Insufficient tempering time — high-plasticity clays require 16-24 hours for moisture equilibration; testing too early gives inaccurate LL values.
  • Inconsistent groove closure — the groove must close over a distance of 13 mm (about ½ inch) by flow of the soil, not by sliding of the cup.
  • Plastic limit rolling technique — pressing too hard flattens the thread instead of reducing its diameter; the correct pressure produces a uniform diameter reduction.
  • Drying during testing — moisture loss during the multipoint LL test shifts the flow curve; use covered containers between tests.
  • Using the wrong grooving tool — the Casagrande curved tool is for sandy soils; the flat tool is for clayey soils (ASTM D4318 Section 6.2.1).

Best Practices

  • Always use the multipoint method (Method A) for liquid limit determination on projects where the results will be used for design or specification compliance.
  • Record the flow curve slope (flow index IF) as it provides additional quality control — an unusually steep or flat slope may indicate testing errors.
  • When reporting PI, note whether the soil is non-plastic (NP) if the plastic limit cannot be determined.
  • Perform duplicate plastic limit determinations and report the average if values agree within 1% water content.
  • Use the Atterberg Limits Calculator to automatically compute PI, flow index, toughness index, and liquidity index from your test data.

Limitations

  • The Casagrande cup method has inherent variability due to operator technique; the fall cone penetrometer (Method C) is generally more repeatable and is preferred in many European jurisdictions.
  • The plastic limit thread-rolling test is highly operator-dependent; inter-laboratory variability can be ±2-5% water content for high-plasticity soils.
  • Atterberg limits are empirical index properties, not fundamental soil parameters; correlations with engineering properties have significant uncertainty.
  • Organic soils and soils containing significant mica or diatomaceous earth may give misleading Atterberg limits due to particle shape and water absorption characteristics.

Related CivilFlow Calculators

Related Formulas

See the Geotechnical Formulas section for Atterberg limits, soil classification, compaction, and permeability equations.

Related Handbook Chapters

Refer to the Engineering Handbook for geotechnical design guidance including soil classification, bearing capacity, and earth pressure theory.

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Related Learn Pages

Explore the Geotechnical Engineering learn page for foundational concepts in soil mechanics and foundation engineering.

Related Glossary Terms

Visit the Glossary for definitions of Atterberg limits, plasticity index, liquidity index, and other geotechnical terms.

References

  • ASTM D4318-17, "Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils," ASTM International, 2017.
  • ASTM D2487-17, "Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System)," ASTM International, 2017.
  • Casagrande, A., "Research on the Atterberg Limits of Soils," Public Roads, Vol. 13, No. 8, 1932, pp. 121–136.
  • Holtz, R.D., Kovacs, W.D., and Sheahan, T.C., "An Introduction to Geotechnical Engineering," 2nd Ed., Pearson, 2011.
  • Sridharan, A. and Prakash, K., "Mechanisms Controlling the Liquid Limit of Clays," Geotechnical Testing Journal, Vol. 22, No. 3, 1999.