The Atterberg limit tests are used to identify the liquid limit, plastic limit and shrinkage limit of soil. These limits are important for soil classification and are used to establish correlations related to soil strength, plasticity, and behaviour, particularly for fine-grained fractions such as clay and silt. The results help engineers assess the suitability of soil for construction, foundation design, and geotechnical applications.
In this blog, we break down what Atterberg limit tests are, the procedures for conducting the test, and the standard codes governing it.
What Are Atterberg Limits?
Atterberg’s limits, or consistency limits, are water content indicators in fine-grained soils, such as silt and clay.
These tests mainly involve three different water levels in soil:
- Liquid Limit (LL): The moisture content at which soil transforms from a plastic to a liquid state. This indicates that the soil begins to flow and lose its shear strength.
- Plastic Limit (PL): The moisture content at which soil transforms from a semi-solid to a plastic state. It is identified by rolling the soil into thin threads until it breaks apart into crumbles.
- Shrinkage Limit (SL): The moisture content beyond which further drying of soil does not decrease soil volume anymore, showing maximum shrinkage due to moisture loss.
With the help of these limits, one can easily identify the differences between soil types.
What is the Atterberg Limits Test?
The Atterberg Limits Test refers to a set of three standardised laboratory tests conducted to determine the Liquid Limit, Plastic Limit, and Shrinkage Limit of fine-grained soils. These tests assess soil plasticity, consistency, and behaviour under varying moisture conditions, providing essential data for soil classification and engineering design.
The Indian Standards specify the Atterberg limits tests under IS 2720 (Part 5): 1985 for liquid and plastic limits, and IS 2720 (Part 6): 1972 for shrinkage limit.
Importance and Applications
Atterberg limit tests are vital in geotechnical engineering, giving major characteristics and behaviour of soil; based on the test results, engineers decide its suitability for various construction purposes.
Major applications of these soil tests are:
- Soil Classification: These tests help find important properties of soil like plasticity, strength, compressibility, swelling, and permeability. This information is used to classify the soil and understand how it will behave under different loads. Based on the results, and using systems like the IS Soil Classification and the Unified Soil Classification System (USCS), engineers can choose the right type of soil for construction work.
- Foundation Design: Assists in evaluating soil shear strength and stability to design safe and durable foundations.
- Earthworks and Embankments: Predicts settlement, shrinkage, and swelling behaviour, ensuring structural reliability and long-term performance.
- Pavement Design: Aids in understanding soil deformation under traffic loads to design stable and resilient pavements.
- Engineering Interpretation: Soils with high liquid limits are typically more compressible and have lower bearing capacity, while those with low plasticity are generally more stable and less prone to deformation.
Test Procedures involved in the Atterberg Limits Test
Liquid Limit Test
Liquid Limit Test Apparatus
The Casagrande apparatus features a brass cup attached to a cam-and-crank mechanism, which allows it to drop from a fixed height onto a hard rubber base. It comes with accessories such as grooving tools for shaping the soil sample and a height gauge to ensure accurate adjustment of the drop height.
Components of the Casagrande apparatus:
- Brass Cup: A semi-spherical cup for placing the soil sample, designed to be repeatedly dropped onto the base.
- Hard Rubber Base: Surface for dropping a brass cup repeatedly from a fixed height.
- Cam and Crank Mechanism: A handle-operated system that lifts the brass cup a specific height and allows it to drop onto the base.
- Grooving Tool: Used for cutting a groove in the soil paste within the cup.
- Counter: A device, usually attached to the apparatus, used to record the number of blows or drops during the test.
Stepwise Liquid Limit Test Procedure:
- Prepare a soil sample of approximately 120 g by air-drying soil from the portion that passes through a 425-micron IS sieve (No. 40 sieve).
- Mix the soil with distilled water in a mixing dish to form a uniform paste. The consistency should be such that it requires 30–35 blows to close the standard groove over a length of 12 mm.
- For clayey soils, keep the prepared paste aside for 24 hours to allow even moisture distribution.
- Remix the soil thoroughly before testing. Place a portion of the paste in the cup of the Casagrande apparatus and spread it evenly with minimal strokes of a spatula.
- Trim the soil to a depth of about 1 cm at the thickest point and remove the excess.
- Cut a clean, sharp groove along the diameter using the appropriate grooving tool, Casagrande’s tool for clayey soils and ASTM’s for sandy soils.
- Operate the crank to lift and drop the cup at a rate of two revolutions per second until the two halves of the soil come together over a 12 mm length by flow.
- Record the number of blows required for the groove to close.
- Take a small sample from the cup to determine the water content.
- Repeat the test at least three more times with varying moisture contents so that the number of blows ranges between 15 and 35.
Calculation for the Atterberg Limits Test
A flow curve is plotted on a semilogarithmic graph, where the water content is shown on the arithmetic (x) scale and the number of blows on the logarithmic (y) scale. Draw a straight line that best fits the four or more plotted points. The water content corresponding to 25 blows, as read from the curve, rounded to the nearest whole number, is reported as the liquid limit of the soil.
Flow Index (If):
The flow index is calculated using the formula:
If = (W1-W2) ÷ log10(N2/N1)
where,
- If = Flow Index
- W₁ = Moisture content (%) corresponding to N₁ blows
- W₂ = Moisture content (%) corresponding to N₂ blows
Plastic Limit Test of Soil
The Plastic Limit (PL) of soil defines the moisture content at which soil changes from a plastic to a brittle or semi-solid state.
Plastic Limit Test Apparatus
The apparatus required for performing the Plastic Limit test includes:
- Porcelain evaporating dish or plate (flat, non-porous surface)
- Glass plate (minimum thickness 10 mm and around 45 cm square)
- Brass rod (3 mm diameter and about 10 cm long) for shaping soil threads
- Palette knife or spatula for soil handling
- Weighing balance
- Oven for drying soil samples
Stepwise Plastic Limit Test Procedure
- Collect a representative soil sample and take about 10 grams of the air-dried soil portion.
- Moisten the soil sample gradually until it becomes plastic enough to be rolled.
- Roll the soil sample between the fingers and the glass plate to form a thread of uniform diameter, applying just enough pressure.
- Continue rolling until the soil thread crumbles at a diameter of 3 mm.
- Collect the pieces of the crumbled soil thread and determine their moisture content by oven drying.
- Repeat the test at least three times and calculate the average moisture content at which the thread breaks—the Plastic Limit.
- If the soil cannot be rolled into a 3 mm thread at any moisture content, classify it as non-plastic.
Calculation for the Plastic Limits Test
The Plastic Limit is calculated as the moisture content percentage by weight of the oven-dry soil where the soil changes from plastic to semi-solid state. The general formula for plastic limit moisture content PL is:
PL=[(Weight of water)/(Weight of dry soil)]×100
Where:
- Weight of water = initial wet soil weight – oven-dried soil weight
- Weight of dry soil = oven-dried soil weight
The plastic limit is the average of the moisture contents determined from the repeated tests. Plasticity Index (PI), an important parameter in soil classification, is calculated as:
PI=LL−PL
Where LL is the Liquid Limit of the soil.
Shrinkage Limit Test in India
The Shrinkage Limit (SL) of soil is the moisture content at which further loss of water does not result in any reduction in volume of the soil mass. This test is crucial for soils that experience large volume changes due to wet-dry cycles, such as in earth dams, pavements, and expansive clay regions.
Shrinkage Limit Test Apparatus
The apparatus required for the Shrinkage Limit test includes:
- Porcelain or non-corroding metal shrinkage dish (typically 45 mm diameter and 15 mm depth)
- Glass plate (with three metal prongs for measuring volume)
- Spatula and straight edge
- Thermostatically controlled oven
- Balance (sensitive to 0.01 g)
- Mercury or water displacement setup for volume measurement
- Wash bottle with distilled water
- Graduated glass cylinder (25 ml capacity)
Stepwise Shrinkage Limit Test Procedure
- Take about 100 grams of soil passing through a 425-micron sieve and prepare a creamy paste by adding water above the liquid limit.
- Fill the shrinkage dish in three layers, each layer being compacted by tapping gently until the soil flows over the edges.
- Strike off the excess soil with a straight edge to ensure a smooth top surface.
- Weigh the wet soil in the dish and record its initial weight and volume.
- Dry the soil sample in an oven at 105–110°C until constant weight is achieved.
- After drying, determine the volume of the dried soil pat using mercury displacement or water displacement method.
- Calculate the shrinkage limit using the recorded weights and volumes.
Calculation for the Shrinkage Limits Test
Wt. of container in gm, W1
Wt. of container + wet soil pat in gm, W2
Wt. of container + dry soil pat in gm, W3
Wt. of oven-dry soil pat, W0 in gm = (W3-W1)
Wt. of water in gm = (W2-W3)
Moisture content (%), W = (W2-W3)/ (W3-W1)*100
Volume of wet soil pat (V), in cm
Volume of dry soil pat (Vd) in cm³ = (Wm)/ (Gm)
By the mercury displacement method,
a. Weight of displaced mercury in gm (Wm)
b. Specific gravity of the mercury (Gm)
The Shrinkage Limit (SL) = [W – {(V-Vd)*( γw /Wo)}] x 100
Shrinkage ratio (R) = {(V-Vd)/Vd}*100/(W-Ws)
Results of the Shrinkage Limits Test
| Shrinkage Limit (%) | Degree of Expansion |
| >15 | Low |
| 10-16 | Medium |
| 7-12 | High |
| <11 | Very high |
On a final note, the Atterberg limits tests are an effective method for evaluating the liquid limit, plastic limit and shrinkage limit of fine-grained soil. This test is mainly designed for clay and silt soils, which are prone to significant volume changes with moisture variation. These tests are crucial in construction to assess the suitability of site soil for foundations, roads, and other structures, helping to classify soils and determine their engineering behaviour. Leading construction firms such as Brick & Bolt utilize the Atterberg limits tests alongside other evaluation methods to ensure compliance with IS codes and to design foundations appropriately.
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