Accurate determination of fine soil plasticity is crucial for predicting the mechanical behavior of foundations, embankments, and pavement layers. Systematic errors in sample preparation or Casagrande apparatus manipulation alter HRB/USCS classification and compromise project progress. This article analyzes standardized procedures, frequent sources of experimental deviation, and laboratory quality assurance protocols.
1. What is the test and what problem does it solve?
Atterberg Limits define the moisture boundaries that delineate the consistency states of fine-grained soil: liquid, plastic, semi-solid, and solid. The transition between these states depends on the water content present in the fraction passing the 0.425 mm (No. 40) sieve.
In geotechnical practice, soils with high plasticity exhibit significant volumetric variation (shrinkage and expansion) in the presence of water, as well as lower bearing capacity when saturated. Determining the Liquid Limit (LL) and Plastic Limit (PL) solves the problem of characterizing and selecting materials for earthworks, allowing the rejection of expansive or unsuitable soils before their application in construction.
2. What is it used for and when should it be used?
The determination of LL and PL is mandatory in the following phases and applications:
• Geotechnical classification of soils by AASHTO/HRB and USCS systems (ASTM D2487).
• Evaluation of soil suitability for the construction of embankments, capping layers, and road sub-bases.
• Verification of the Plasticity Index (PI = LL - PL) and Consistency Index (CI) for slope stability and foundation studies.
• Quality control upon receipt of borrow materials and fine soil deposits.
• Monitoring of mineralogical changes or contamination along a road alignment.
3. Reference standards
The execution of the test must strictly comply with the technical specifications in force for the project. The main reference standards are:
• NP 143: Soils - Determination of consistency limits (Liquid limit and plastic limit).
• ASTM D4318: Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils.
• EN ISO 17892-12: Geotechnical investigation and testing - Laboratory testing of soil - Part 12: Determination of Liquid and Plastic Limits.
• LNEC E 198: Soils - Determination of the liquid limit by the Casagrande apparatus.
• DNIT 122/2010 - ME: Soils - Determination of the plastic limit.
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4. Sample preparation
Proper sample preparation is crucial to ensure the reproducibility of tests. The standard procedure involves:
- Pre-drying the sample in air or in an oven at a controlled temperature (not exceeding 60 °C to avoid altering the mineralogical structure of organically active clays).
- Carefully disaggregating lumps with a mortar and pestle or rubber pestle, without fracturing individual mineral grains.
- Sieving the representative fraction through the ASTM No. 40 (0.425 mm) sieve, collecting all material passing through.
- Initial moistening of the soil with distilled or demineralized water and homogenization with a spatula until a uniform paste is obtained.
- Curing the soil paste in an airtight container for a minimum period of 12 to 24 hours (prolonged curing is required for montmorillonitic or high-plasticity clays), allowing complete hydration of the mineral particles.
5. Equipment and materials
The equipment must be properly calibrated and in good condition:
• Casagrande apparatus, manually or electrically operated (frequency of 2 drops per second).
• Standardized brass cup (120 g to 130 g) and hard rubber base with calibrated resilience.
• Standard grooving tool, ASTM type (curved) or Casagrande type (straight), made of hardened metal or calibrated plastic.
• Ground glass plate or marble slab for rolling plastic limit threads.
• Flexible stainless steel spatula with a blade approximately 75 mm to 100 mm.
• Moisture cans or airtight metal containers with lids for moisture content determination.
• Forced-air drying oven, maintained at 110 °C ± 5 °C.
• Analytical or semi-analytical balance with 0.01 g precision.
• Wash bottle with distilled water.
6. Step-by-step procedure
The experimental determination must follow the described technical sequence:
Liquid Limit (LL) Determination:
- Place a portion of the prepared soil in the Casagrande cup, leveling the surface with the spatula parallel to the base to a maximum thickness of 10 mm at the deepest point.
- Cut a groove in the center of the sample along the axis of symmetry of the cup, using the calibrated grooving tool held strictly perpendicular to the inner surface of the cup.
- Turn the crank of the apparatus at a constant rate of 2 drops per second until the two halves of the soil come together over a continuous length of 13 mm (approximately 1/2 inch) at the bottom of the groove.
- Record the number of drops (N) required for the groove to close. The test is valid only if closure occurs between 15 and 35 drops.
- Remove an aliquot of soil from the area where the closure occurred, place it in an identified moisture can, and determine the moisture content (w) by oven drying at 110 °C ± 5 °C for a minimum of 16 hours.
- Repeat the procedure for at least four points with increasing/decreasing water contents distributed between 15-25, 20-30, and 25-35 drops.
Plastic Limit (PL) Determination:
- Mold a small portion of the moistened soil into a sphere and roll it manually on the ground glass plate with the palm of the hand.
- Apply sufficient pressure to form a uniform cylinder (thread).
- Continue rolling until the thread reaches a diameter of 3.0 mm (using the comparison rod as a reference) and begins to crack, crumble, or fragment longitudinally and transversely.
- Immediately collect the fragments of the cracked thread, place them in a moisture can with a lid, and determine the water content by oven drying.
- Perform the procedure until at least three valid determinations are obtained, whose moisture content does not differ by more than 1.0% to 1.5% from each other.
7. Calculations and results processing
The mathematical processing of laboratory data follows these steps:
1. Calculation of Moisture Content (w):
w = ((M1 - M2) / (M2 - M3)) * 100 (%)
Where M1 is the mass of the moisture can with wet soil (g), M2 is the mass of the moisture can with dry soil (g), and M3 is the mass of the empty moisture can (g).
2. Obtaining the Liquid Limit (LL):
Graphically plot the points on a semi-logarithmic diagram (horizontal axis logarithmic: number of drops N; vertical axis linear: moisture content w in %). Fit the flow curve to the experimental points. The Liquid Limit (LL) is the moisture content corresponding to the intersection of the curve with the 25-drop line, expressed as an integer (rounded without decimal places).
Alternatively, when applicable by the standard and for a single-point test:
LL = w * (N / 25)^0.121 (for N between 20 and 30 drops).
3. Obtaining the Plastic Limit (PL):
The PL is the arithmetic mean of the moisture contents of the 3 mm threads that cracked, expressed as an integer.
4. Calculation of the Plasticity Index (PI):
PI = LL - PL (%)
If PL is greater than or equal to LL, or if it is not possible to roll 3 mm threads at any moisture content, the soil is classified as Non-Plastic (NP).
8. Interpretation of results
The combination of LL and PI allows the soil to be placed on the Casagrande Plasticity Chart and its geotechnical behavior determined:
• Soils below the A-Line (PI = 0.73 * (LL - 20)): Predominance of organic or inorganic silts of medium to high compressibility.
• Soils above the A-Line: Predominance of inorganic clays of low plasticity (CL) or high plasticity (CH).
• Hatched region (PI between 4 and 7 and LL between 12 and 25): Borderline soils with dual classification (CL-ML).
• PI < 7: Low plasticity soil, poorly cohesive, susceptible to rapid erosion, and with predominantly granular behavior.
• 7 <= PI <= 15: Medium plasticity, suitable for embankments and sub-bases provided it is compacted at optimum moisture content.
• PI > 15: High plasticity soil, highly expansive, and susceptible to large volume changes with changes in water content.
9. Common errors and how to avoid them
In daily laboratory practice, the following operational errors are common:
• Uncalibrated cup drop height: If the drop height is different from 10.0 mm ± 0.2 mm, the transmitted energy alters the groove closure. Solution: Check daily with the grooving tool calibrator before the test.
• Wear of the rubber base or cup: Cavities formed at the impact point alter resilience. Solution: Inspect and replace worn components.
• Poorly executed groove: Using worn or inclined grooving tools creates an irregular gap at the bottom. Solution: Keep the grooving tool strictly perpendicular and replace it when the bevel dimension loses tolerance.
• Insufficient paste curing time: Dry clays with added water without resting time provide a falsely lower LL. Solution: Observe the minimum 12-hour curing in a humid chamber.