Atterberg Limits
Concept
The Atterberg Limits test is a fundamental geotechnical procedure used to characterize the plasticity and behavior of fine-grained soils, particularly clays and silts. The test defines three key limits: the Liquid Limit (LL), Plastic Limit (PL), and Shrinkage Limit (SL), which together describe the soil’s transition between different states of consistency. The goal of the Atterberg Limits test is to provide a quantitative measure of the soil’s moisture content at which it changes from a liquid to a plastic state, from a plastic to a semi-solid state, and from a semi-solid to a solid state. These parameters are crucial for soil classification and predicting the soil's performance in construction projects. Standards governing the Atterberg Limits test include ASTM D4318 and ISO 17892-12.
Descriptions and Procedure
Liquid Limit (LL)
Objective: Determine the moisture content at which soil changes from a plastic to a liquid state.
Apparatus: Casagrande cup or cone penetrometer, grooving tool, balance, mixing dishes.
Procedure:
- 1. Sample Preparation: Dry the soil sample and pass it through a No. 40 sieve.
- 2. Mixing: Add distilled water to the soil to create a uniform paste.
- 3. Testing with Casagrande Cup
- - Place a portion of the soil paste in the cup and form a groove using the grooving tool.
- - Drop the cup at a rate of 2 drops per second until the groove closes over a distance of 12.7 mm (0.5 inches).
- - Record the number of drops and the corresponding moisture content.
- - Repeat the process for multiple moisture contents to establish a flow curve.
- 4. Calculations: Plot the number of drops against the moisture content and determine the moisture content at 25 drops from the flow curve, which is the Liquid Limit.
Plastic Limit (PL)
Objective: Determine the moisture content at which soil changes from a plastic to a semi-solid state.
Apparatus: Rolling surface, spatula, balance.
Procedure:
- 1. Sample Preparation: Use the soil from the Liquid Limit test.
- 2. Rolling Test:
- - Take a small portion of soil and roll it into a thread on a glass plate or rolling surface.
- - Continue rolling until the thread crumbles at a diameter of 3.2 mm (1/8 inch).
- - Collect the crumbled soil and determine its moisture content.
- 3. Calculations: The moisture content at which the thread crumbles is the Plastic Limit.
Shrinkage Limit (SL)
Objective: Determine the moisture content at which soil transitions from a semi-solid to a solid state with no further volume change.
Apparatus: Shrinkage dish, mercury, glass plate, oven, balance.
Procedure:
- 1. Sample Preparation: Prepare a soil paste similar to the Liquid Limit test.
- 2. Molding: Fill the shrinkage dish with the soil paste, ensuring no air pockets.
- 3. Drying: Allow the soil to air-dry and then oven-dry to a constant weight.
- 4. Volume Measurement: Measure the volume of the dried soil using mercury displacement.
- 5. Calculations: Calculate the Shrinkage Limit using the initial volume, dry volume, and corresponding moisture contents.
Data Interpretation
Flow Curve (Liquid Limit): The flow curve is plotted with the logarithm of the number of drops on the x-axis and the moisture content on the y-axis. The moisture content corresponding to 25 drops is determined from this curve.
Plasticity Index (PI): PI is calculated as the difference between the Liquid Limit and the Plastic Limit (PI = LL - PL). This index is used to classify the soil's plasticity.
Plasticity Chart: The Atterberg Limits are plotted on a plasticity chart to classify the soil according to the Unified Soil Classification System (USCS) or AASHTO soil classification system.
The Atterberg Limits test is essential for understanding the mechanical behavior of fine-grained soils and is widely used in geotechnical engineering for soil classification, foundation design, and assessing soil suitability for construction projects.