The Casagrande Plasticity Chart

Casagrande Plasticity Chart for soft soil​

The Casagrande Plasticity Chart is a graphical tool used in geotechnical engineering to classify fine-grained soils (such as clays and silts) based on their plasticity characteristics. It helps engineers assess the behavior and engineering properties of soils, particularly for construction and foundation purposes. It is used primarily for soil classification and to predict soil behavior under varying moisture conditions.

Check our on-line plasticity chart geotool!

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The chart plots Liquid Limit (LL) on the x-axis and Plasticity Index (PI) on the y-axis to determine soil classification.

Key components of the Casagrande Plasticity Chart:

1. Liquid Limit (LL): The water content at which soil transitions from a plastic to a liquid state. To define Liquid Limit we need to perform a Casagrande test using the Casagrande apparatus or cup.

2. Plasticity Index (PI): The range of water content over which the soil exhibits plastic behavior. It is calculated as:

PI=LL−PL

where

LL is the Liquid Limit (the water content at which soil transitions from a plastic to a liquid state

PL is the Plastic Limit (the water content at which soil transitions from a semi-solid to a plastic state)

There are two important lines in a Casagrande Chart:

1. The A-line: It is a boundary line on the chart that differentiates between clays and silts. Soils that plot above the A-line are classified as clays, while those below the A-line are classified as silts.

The A-line equation: PI = 0.73 ∙ (LL - 20)

1. The U-line: The upper boundary for most natural soils, providing a reference for extreme plasticity cases.

The U-line equation: PI = 0.9 ∙ (LL - 8)

Based on Casagrande chart soil can also be classified in terms of its plasticity from non-plastic or low plasticity to high plasticity soils.

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About Arthur Casagrande

Arthur Casagrande was an influential figure in geotechnical engineering, known for his pioneering work in soil mechanics. Born in Austria in 1902, he later moved to the U.S., where he made significant contributions to the field. Casagrande studied civil engineering at the Technical University of Vienna before joining Karl Terzaghi, the "father of soil mechanics," in the United States. He worked closely with Terzaghi at MIT, where they researched soil behavior under varying conditions.

Casagrande's legacy includes the development of the Casagrande Plasticity Chart, a widely used tool for classifying soils based on their liquid limit and plasticity index, crucial for understanding soil properties in construction. Additionally, he designed the Casagrande apparatus to measure soil consistency, providing engineers with a standardized method for soil classification. His innovations at MIT laid the foundation for modern geotechnical engineering practices, and his work continues to influence infrastructure and construction projects worldwide.

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