Atterberg Limits Test for Soil

 

Objective:

The Atterberg Limits Test determines the consistency limits of fine-grained soils, helping in soil classification and assessing its behavior under varying moisture conditions. The three limits are:

1. Liquid Limit (LL): The moisture content at which soil changes from a plastic to a liquid state.
2. Plastic Limit (PL): The moisture content at which soil changes from a semi-solid to a plastic state.
3. Shrinkage Limit (SL) (Optional): The moisture content below which soil no longer shrinks upon drying.

ASTM Standard Code:

• ASTM D4318 – Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils.

Apparatus Required:

For Liquid Limit Test:

1. Casagrande’s Liquid Limit Device – A brass cup mounted on a base with a standardized drop mechanism.
2. Grooving Tool – Used to create a standard groove in the soil sample.
3. Weighing Balance – Accuracy 0.01g.
4. Spatula – For mixing and handling soil.
5. Evaporating Dish – Used for sample preparation.
6. Water Content Containers – For moisture determination.

For Plastic Limit Test:

1. Glass Plate – For rolling the soil sample into threads.
2. Weighing Balance – To determine the plastic limit moisture content.
3. Spatula & Mixing Tools – For soil sample preparation.

Test Procedure:

1. Liquid Limit (LL) Test:

1. Prepare Soil Sample – Mix soil with distilled water until it reaches a soft consistency.
2. Place in Casagrande Cup – Fill the brass cup with 3 mm thickness of soil paste.
3. Cut a Groove – Use the grooving tool to create a 13 mm wide groove in the center.
4. Drop the Cup – Rotate the handle at 2 drops per second until the groove closes for 12 mm length.
5. Record the Number of Blows – Repeat the process with different moisture contents and plot a graph of moisture content vs. log(number of blows).
6. Determine LL – The moisture content at 25 blows is the Liquid Limit.

2. Plastic Limit (PL) Test:

1. Take a Small Soil Sample – Add water until the soil is plastic and moldable.
2. Roll into Threads – Roll the soil into 3 mm diameter threads on the glass plate.
3. Break the Threads – If the soil thread crumbles at 3 mm diameter, record the moisture content.
4. Repeat for Accuracy – Take multiple readings and calculate the Plastic Limit.

3. Plasticity Index (PI):

PI=LL−PLPI = LL - PL

Where:

• LL = Liquid Limit (%)
• PL = Plastic Limit (%)

The Plasticity Index determines the soil’s plasticity behavior. Higher PI values indicate more clay content and greater plasticity.

Results Interpretation:

Soil Type	Liquid Limit (LL%)	Plasticity Index (PI%)	Plasticity Classification
Sand	< 20	0	Non-Plastic (NP)
Silt	20 - 50	5 - 20	Low Plasticity
Clay (Low)	30 - 50	10 - 20	Medium Plasticity
Clay (High)	> 50	> 20	High Plasticity

 

Limitations of the Test:

• Operator Dependence – Results may vary due to human handling of the device and rolling technique.
• Not Suitable for Coarse-Grained Soils – The test is only applicable to fine-grained soils (clays and silts).
• Effect of Drying – Over-drying can alter soil properties, leading to incorrect Atterberg limits.
• Presence of Organic Matter – Organic soils exhibit unpredictable plasticity, affecting results.

Conclusion:

The Atterberg Limits Test (ASTM D4318) helps classify fine-grained soils and evaluate their behavior in construction projects. These limits are crucial for foundation design, road construction, and slope stability analysis.