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The Casagrande Plasticity Chart

Atterberg Limits, Soil Classification, Liquid Limit, Plasticity Index, Fine-Grained Soils

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The Casagrande Plasticity Chart is widely used by geotechnical engineers for fine-grained soil classification. Try out our tool in the link below:

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What is the Casagrande Plasticity Chart?

The Casagrande Plasticity Chart is a tool used by geotechnical engineers for the classification of fine-grained soils based on their plasticity characteristics. The chart was developed by civil engineer Arthur Casagrande (1948), who argued that rather than grain size, plasticity was the most important characteristic for defining fine-grained soils. This formed the basis for a new soil classification system that later became the Unified Soil Classification System (USCS).

Key Components of the Casagrande Plasticity Chart

The Axes:

The chart extents are defined by the Liquid Limit (LL) on its x-axis and Plasticity Index (PI) on its y-axis. These two soil characteristics are laboratory-derived values, where:

  • Liquid Limit (LL): is the water content at which soil transitions from plastic to liquid state.

  • Plasticity Index (PI): is the range of water content over which soil exhibits plastic behaviour.

The PI is calulated using the following expression:

[ PI = LL - PL ], where:

  • Plastic Limit (PL): water content at which soil transitions from semi-solid to plastic state

Liquid Limit and Plastic Limit are two components of the Atterberg Limits.

Chart Lines:

The chart itself contains two important lines:

  • The A-line is an empirical boundary separating silts from clays by the following logic:

    • Soils plotting below the A-line → silts
    • Soils plotting above the A-line → clays

The A-Line is given by the following equation:

[ PI = 0.73(LL - 20) ]

  • The U-line represents the approximate upper bound for naturally occuring soils:

The U-Line is given by the following equation:

[ PI = 0.9(LL - 8) ]

Another vertical line shown on the graph represents the seperation of low and high plasticity soils at:

  • LL = 50%

This line is used to ditinguish between:

  • Low plasticity soils (LL < 50%)
  • High plasticity soils (LL > 50%)

What are Atterberg Limits?

Atterberg Limits were introduced by Albert Atterberg (1911) and later refined by Arthur Casagrande.

They define the critical water contents at which fine-grained soils transition through one of its four states, which are:

  • Solid state
  • Semi-solid state
  • Plastic state
  • Liquid state

Each state the consistency and behaviour of the soil, and thus its engineering properties are different. The transitions between each state are defined by:

  • Shrinkage Limit: the water content where further moisture loss does not reduce volume
  • Plastic Limit: the water content at which the soil transitions from semi-solid to plastic state
  • Liquid Limit: the water content at which the soil transition from plastic to liquid state

How are Atterberg imits used with the Casagrande Plasticity Chart

Atterberg Limits and the Casagrande Plasticity Chart can be used to classify fine-grained soils and distinguish between Silt-like and Clay-lile behaving soils. As higher plasticty soils tend to exhibit plasticity over a wider range of moisture contents than do low plasticity soils, this helps the differation between the two soil types where:

  • Silts: display lower plasticity and dilatant behaviour
  • Clays: display higher plasticity and a strong LL–PI correlation

The A-line represents this empirical separation.

Plasticity Classification

Although initially plasticity has been simply split between low and high plasticity by the vertical LL=50% line, further work has been done to refine the plasticity desription of soils. These are plasticity relating to Plasticity Index (PI) and relating to Liquid Limit (LL) as given below.

Based on Plasticity Index (PI)

  • 0 → Non-plastic
  • < 7 → Slightly plastic
  • 7–17 → Medium plastic
  • > 17 → Highly plastic

Based on Liquid Limit (LL)

  • < 35% → Low plasticity
  • 35–50% → Intermediate plasticity
  • 50–70% → High plasticity
  • 70–90% → Very high plasticity
  • > 90% → Extremely high plasticity

References

  1. AS 1726-2017. Geotechnical Site Investigations.
  2. ASTM D2487. Unified Soil Classification System.
  3. ASTM D2488. Visual-Manual Soil Description.
  4. BS 5930:2015. Code of Practice for Site Investigations.
  5. Casagrande, A. (1958). Notes on the Design of the Liquid Limit Device, Géotechnique, 8, 84–91.
  6. Casagrande, A. Classification and identification of soils. Transactions ASCE, 113, 901–930.
  7. Das, B.M. (2006). Principles of Geotechnical Engineering.
  8. New Zealand Geotechnical Society (2005). Field description of soil and rock.
  9. Seed, H.B. (1967). Fundamental Aspects of the Atterberg Limits. Journal of Soil Mechanics and Foundations Div.
  10. Sowers, G.F. (1979). Introductory Soil Mechanics and Foundations. Macmillan.