Experiment: Determination of Liquid Limit and Plastic Limit of Soil
Objective
To determine the Liquid Limit (LL) and Plastic Limit (PL) of a given soil sample and to calculate its Plasticity Index (PI).
Apparatus Required
- Casagrande Liquid Limit Device – With brass cup and cam mechanism
- Spatula – For mixing and placing soil
- Flat Glass Plate – For mixing and preparation
- Oven
- Weighing Balance – Sensitive up to 0.01 g
- Water
- Sieve (425 µm) – To prepare soil sample
Theory
Soil consistency is the relative ease with which a soil can be deformed. The consistency limits define the boundaries between different states of soil (solid, semi-solid, plastic, and liquid). These are known as Atterberg Limits:
- Liquid Limit (ωL): The minimum water content at which soil begins to behave as a liquid. It is determined when a soil paste flows under a standard groove closure for 25 blows in a Casagrande’s apparatus.
- Plastic Limit (ωP): The minimum water content at which the soil can be rolled into 3 mm diameter threads without crumbling.
- Plasticity Index (PI): Range of water content over which the soil remains plastic.
Plasticity Index Formula:
\[ PI = \omega_L – \omega_P \]
These limits help identify soil classification and predict engineering behavior, especially related to bearing capacity and settlement.
Procedure
Liquid Limit (Casagrande’s Method):
- Soil passing through 425 µm sieve was taken.
- Water was added and thoroughly mixed to prepare a paste.
- The paste was placed in the Casagrande’s cup and a groove was made using the standard grooving tool.
- The cup was repeatedly dropped at a rate of two drops per second until the groove closed over a distance of 12 mm.
- The number of blows required to close the groove was recorded.
- A portion of the soil was taken immediately for water content determination.
- The test was repeated for different moisture contents to obtain at least four readings in the range of 10 to 40 blows.
- A flow curve (log N vs. ω) was plotted, and the water content corresponding to 25 blows was taken as the Liquid Limit.
Plastic Limit:
- A portion of soil passing through the 425 µm sieve was taken and mixed with water to form a plastic mass.
- The soil was rolled on a glass plate to form threads.
- Rolling continued until the threads crumbled at approximately 3 mm diameter.
- The crumbled soil was collected and its water content was determined.
- The average of at least three tests was taken as the Plastic Limit.
Observation and Calculations
| Observation |
Liquid Limit |
Plastic Limit |
| 1 |
2 |
3 |
4 |
1 |
2 |
| Number of blows (N) |
38 |
33 |
23 |
12 |
– |
– |
| Can Number |
208 |
4 |
6 |
2 |
10 |
3 |
| Wt. of can (w₁) (g) |
9.288 |
9.507 |
9.288 |
9.571 |
7.445 |
6.937 |
| Wt. of can + wet soil (w₂) (g) |
24.126 |
28.506 |
30.748 |
29.64 |
9.602 |
9.381 |
| Wt. of can + dry soil (w₃) (g) |
21.168 |
24.516 |
25.785 |
24.423 |
9.267 |
8.97 |
| Wt. of water (wₐ = w₂ − w₃) (g) |
2.958 |
3.99 |
4.963 |
5.217 |
0.335 |
0.411 |
| Wt. of dry soil (wd = w₃ − w₁) (g) |
11.88 |
15.009 |
16.497 |
14.852 |
1.822 |
2.033 |
| Water content (ω = (wₐ / wd) × 100) |
24.90% |
26.58% |
30.08% |
35.13% |
18.39% |
20.22% |
Formulas:
Water Content (ω):
\[ \omega = \frac{w_a}{w_d} \times 100 = \frac{w_2 – w_3}{w_3 – w_1} \times 100 \]
Plastic Limit (PL):
\[ \omega_P = \frac{w_2 – w_3}{w_3 – w_1} \times 100 \]
Result
- Liquid Limit (LL): 28.5%
- Plastic Limit (PL): 19.26%
- Plasticity Index (PI): 9.24%
Discussion
The liquid limit of 28.5% indicates the water content at which the soil changes from plastic to liquid state. This value is moderate, suggesting the soil is neither too dry nor overly plastic.
The plastic limit of 19.26% defines the lower bound of the plastic state, indicating the water content at which soil just begins to crumble when rolled.
The Plasticity Index (PI = 9.24%) represents the range of water content where the soil remains plastic. According to soil classification standards:
- A PI between 7 and 17 is classified as moderately plastic.
- The soil is likely to be clayey silt or silty clay.
This moderate plasticity suggests moderate shrink-swell potential, acceptable workability, and good bearing capacity if well-compacted.
Slight variations in manual thread rolling or groove closing can cause minor deviations, but overall the results appear consistent and within expected ranges.
Conclusion
The Atterberg limits of the given soil sample were successfully determined:
- Liquid Limit (LL): 28.5%
- Plastic Limit (PL): 19.26%
- Plasticity Index (PI): 9.24%
These results classify the soil as moderately plastic, suggesting it contains both clay and silt components. The soil has moderate compressibility and fair stability under structural loading. It is generally suitable for engineering applications like subgrade and foundation work, provided proper compaction and drainage are ensured.
