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Atterberg Limits Calculator

Determine liquid limit, plastic limit, plasticity index, liquidity index, flow index, toughness index, and USCS soil classification from Atterberg limits test data.

Soil Mechanics Geotechnical engineers, lab technicians Commercial Intent: HIGH
Flow Curve Data Points

Engineering Formulas

Liquid Limit (Flow Curve)

LL = w × (N/25)^(0.121) LL from flow curve: w = a + b × log(N) where a, b are regression coefficients
LL: Liquid limit (%)
w: Water content (%)
N: Number of blows

Plasticity Index

PI = LL - PL
PI: Plasticity index (%)
LL: Liquid limit (%)
PL: Plastic limit (%)

Liquidity Index

LI = (wₙ - PL) / (LL - PL)
LI: Liquidity index
wₙ: Natural water content (%)
PL: Plastic limit (%)
LL: Liquid limit (%)

Flow Index

FI = (w₁ - w₂) / log(N₂/N₁)
FI: Flow index
w₁, w₂: Water contents at N₁, N₂ blows

Toughness Index

TI = PI / FI
TI: Toughness index
PI: Plasticity index (%)
FI: Flow index

Activity

A = PI / (% clay fraction)
A: Activity
PI: Plasticity index (%)

Worked Example

Atterberg Limits for CL Soil

soilTypePreset: CLm1: 45n1: 15m2: 38n2: 25m3: 32n3: 35plasticLimit: 22naturalMoisture: 28clayFraction: 0.45
Flow Curve
Points: (15, 45%), (25, 38%), (35, 32%) → log-linear regression
LL at 25 blows
From regression: LL = 38.0%
PI
PI = 38.0 - 22.0 = 16.0%
LI
LI = (28 - 22) / (38 - 22) = 0.38
Classification
LL=38% (<50), PI=16% (>7, above A-line) → CL (Low plasticity clay)
Result: LL = 38.0%, PL = 22.0%, PI = 16.0%. Soil classified as CL — Low Plasticity Clay. Consistency: Very Stiff (LI = 0.38). Expansivity: Low (PI = 16).

Engineering Notes

Always plot the flow curve on a semi-log graph (log blows on x-axis) to verify linear relationship.
For highly organic soils, oven-dry the sample first and retest to distinguish organic from inorganic.
Liquid limit by Casagrande method is sensitive to operator technique — ensure consistent grooving tool and drop height.
The one-point method (LL from single test at any blow count) is less accurate than multi-point method.
Activity correlates with clay mineral type: kaolinite (0.3–0.5), illite (0.5–1.2), montmorillonite (1.5–7+).

Assumptions

• Flow curve follows linear log(N) vs water content relationship
• Casagrande cup method (percussion) used for liquid limit determination
• Plastic limit determined by thread rolling method (3 mm diameter)
• Natural water content represents in-situ conditions
• Casagrande plasticity chart used for USCS classification

Common Mistakes

✕ Using too few data points (minimum 3 required for reliable flow curve)
✕ Plotting blows on linear scale instead of log scale for flow curve
✕ Confusing liquid limit with natural water content
✕ Not oven-drying threads properly for plastic limit determination
✕ Ignoring soil type effects on typical Atterberg ranges

Frequently Asked Questions

What are Atterberg limits?

Atterberg limits are water content thresholds that define the consistency states of fine-grained soils: liquid limit (LL), plastic limit (PL), and shrinkage limit (SL). They are fundamental for soil classification.

How is the liquid limit determined?

The liquid limit is determined using the Casagrande cup method (ASTM D4318). A soil sample is placed in the cup, grooved, and the cup is dropped until the groove closes over 13 mm. Water content at 25 blows defines the LL.

What is the plastic limit test?

The plastic limit is the water content at which soil begins to crumble when rolled into 3 mm diameter threads. It represents the boundary between plastic and semi-solid states.

What is a typical plasticity index for clay?

Low plasticity clays (CL) have PI of 7–17%, medium plasticity (CI) 17–35%, and high plasticity (CH) >35%. Very high plasticity clays can have PI over 50%.

How is the liquidity index used?

The liquidity index (LI) relates natural water content to Atterberg limits. LI < 0 indicates semi-solid state, LI = 0–1 indicates plastic state, and LI > 1 indicates liquid state. It helps assess soil consistency in situ.

What is soil activity?

Activity (A) is the ratio of plasticity index to clay fraction (< 2µm). Inactive clays (A < 0.75), normal clays (0.75 < A < 1.25), and active clays (A > 1.25). Highly active clays like bentonite can have A > 6.

What is the USCS classification system?

The Unified Soil Classification System (USCS) classifies soils based on grain size and Atterberg limits. Fine-grained soils are classified as CL, CH, ML, MH, etc. based on LL and PI on the Casagrande plasticity chart.

What is expansivity potential?

Expansivity indicates the potential for volume change with moisture variation. Based on PI: Low (<20), Medium (20–35), High (35–55), Very High (>55). High expansivity soils require special foundation treatment.

References & Standards

ASTM D4318IS 2720 Part 5BS 1377-2
ASTM D4318
Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils
IS 2720 Part 5
Determination of Liquid and Plastic Limit
BS 1377-2
Classification Tests and Determination of Atterberg Limits
Casagrande (1932)
Research on the Atterberg Limits of Soils — original flow curve method

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