WHAT IS SIEVE ANALYSIS OF THE SOIL. WHAT WE GET AFTER TEST?

Concept of Sieve Analysis of Soils

Sieve analysis is a method used to determine the particle size distribution of granular materials, particularly soils. It is an essential test in geotechnical engineering and soil science that helps classify soils and determine their suitability for construction, agricultural, or other uses.

The primary purpose of sieve analysis is to separate soil particles into different size ranges and then analyze the proportion of each size. The test provides valuable information about the grain size distribution, which is crucial for understanding soil behavior, such as permeability, compaction, shear strength, and stability.

Importance of Sieve Analysis

1. Classification of Soil: Sieve analysis helps in the classification of soils into groups like sand, silt, gravel, and clay.
2. Soil Structure: It provides insights into the soil's mechanical properties, such as its strength and compressibility.
3. Permeability: Soils with larger particles (sand and gravel) tend to have higher permeability than soils with smaller particles (silt and clay).
4. Construction Suitability: It helps determine the suitability of soil for foundations, roadbeds, and embankments.

Equipment Used

1. Sieves: A series of sieves with progressively smaller mesh sizes (in millimeters or microns) is used. Standard sieve sizes are typically specified by ASTM or ISO standards.
2. Sieve Shaker: A mechanical shaker that vibrates or shakes the sieves, ensuring the soil sample is efficiently sieved.
3. Balance: A precision balance is used to weigh the soil before and after sieving.
4. Sample Containers: Containers for holding the soil sample before sieving and for collecting the material retained on each sieve.

Procedure for Sieve Analysis

1. Soil Sampling: A representative soil sample is collected and prepared by removing large debris, stones, or organic matter.
2. Drying the Soil: The soil sample is dried in an oven to remove moisture content.
3. Weighing the Soil: The dried soil is weighed before the analysis.
4. Sieving: The soil is placed on the top sieve (with the largest opening) and mechanically shaken for a specific period (usually 10-15 minutes).
5. Retained Soil Weighing: After shaking, the material retained on each sieve is weighed separately.
6. Percentage Calculation: The weight of soil retained on each sieve is expressed as a percentage of the total weight. This data is used to determine the particle size distribution.

Sieves and Standard Mesh Sizes

The sieves are arranged in descending order of mesh size, with the largest opening at the top. Common sieve sizes used in soil analysis (based on ASTM E11 standard) include:

• 4.75 mm (No. 4 sieve)
• 2.00 mm (No. 10 sieve)
• 0.85 mm (No. 20 sieve)
• 0.425 mm (No. 40 sieve)
• 0.250 mm (No. 60 sieve)
• 0.150 mm (No. 100 sieve)
• 0.075 mm (No. 200 sieve)

SIEVE ANALYSIS OF THE SOIL

Grain Size Distribution Curve

The results of sieve analysis are often plotted on a semi-logarithmic graph known as the grain size distribution curve (or gradation curve). The x-axis represents particle size on a logarithmic scale, and the y-axis represents the cumulative percentage passing (or retained) on a linear scale. The curve helps interpret the type and gradation of the soil, such as:

• Well-graded soil: A wide range of particle sizes with a smooth curve.
• Poorly graded soil: Dominance of certain particle sizes, leading to gaps in the curve.

Key Soil Parameters Derived from Sieve Analysis

1. Effective Size (D10): The particle size for which 10% of the soil sample is finer by weight. It is crucial for assessing permeability.
2. Uniformity Coefficient (Cu): Defined as 
   $Cu = \frac{D60}{D10}$
3. Coefficient of Gradation (Cc): Defined as 
   $Cc = \frac{(D30)^2}{(D10 \times D60)}$

Types of Soils Based on Sieve Analysis

• Coarse-Grained Soils (Gravel and Sand): These soils have particles predominantly retained on sieves with larger mesh sizes (e.g., No. 4 and No. 10).
• Fine-Grained Soils (Silt and Clay): These soils pass through finer sieves (e.g., No. 200 sieve) and often require further analysis using a hydrometer test.

Limitations of Sieve Analysis

1. Limited for Fine Soils: Sieve analysis is not very effective for particles smaller than 0.075 mm (silt and clay), so other methods like hydrometer analysis or laser diffraction are used for fine-grained soils.
2. Time-Consuming: The test can be time-consuming, especially when sieving large amounts of soil.
3. Mechanical Issues: Some particles may stick to the sieves or form lumps, leading to inaccurate results.

Conclusion

Sieve analysis is a fundamental test in soil mechanics that helps determine the particle size distribution of soils. This test provides crucial data for the classification and characterization of soils, which is essential for engineering applications.

After conducting a sieve analysis test, the following results and data are obtained:

1. Soil Particle Size Distribution

• The primary outcome of the sieve analysis is the particle size distribution of the soil sample. The test separates the soil into different fractions based on particle size, allowing you to quantify the proportion of various particle sizes (e.g., gravel, sand, silt, and clay).

2. Weight Retained on Each Sieve

• The weight of soil particles retained on each sieve is recorded. This data shows how much soil of a particular particle size is in the sample.

• For example, you might have:

• 20% of soil retained on a 4.75 mm sieve (gravel).

• 30% retained on a 0.425 mm sieve (sand).

• A small percentage passing through the 0.075 mm sieve (silt and clay).

3. Cumulative Percentage Retained

• The cumulative weight retained across the sieves is calculated. This is important for assessing the overall distribution of particle sizes.

• For each sieve, you add the weight retained on that sieve and all the sieves above it to get the cumulative weight retained.

4. Cumulative Percentage Passing

• The cumulative percentage passing each sieve is determined. This indicates the proportion of the soil sample that passes through a particular sieve.

• For instance, if 80% of the soil passes through the 0.425 mm sieve, this means that 80% of the sample consists of particles smaller than 0.425 mm.

• The percentage passing is calculated as:

$$\text{Cumulative Percentage Passing}$$$$=$$$$100$$$$\mathrm{\%}$$$$-$$$$\text{Cumulative Percentage Retained}$$5. Grain Size Distribution Curve

• The results are plotted on a grain size distribution curve, also known as the gradation curve. The x-axis represents particle size (in logarithmic scale), and the y-axis represents the cumulative percentage passing (in linear scale).

• This curve helps visually assess the gradation of the soil.

• Well-graded soil: Shows a smooth, continuous curve indicating a wide range of particle sizes.

• Poorly graded soil: Shows gaps or sharp changes in the curve, indicating dominance of certain sizes.

6. Key Soil Parameters$C$$u$$=$$\frac{D}{}$$\frac{60}{}$$\frac{D}{}$$\frac{10}{}$Cu=D10D60​, which gives a measure of how uniform or well-graded the soil is. Higher values indicate a wider range of particle sizes.

• Cu > 4: Indicates well-graded soils (for gravel).

• Cu > 6: Indicates well-graded soils (for sand).

$C$$c$$=$$\frac{(}{}$$\frac{D}{}$$\frac{30}{}$$\frac{{)}^{}}{}$$\frac{2^{}}{}$$\frac{(}{}$$\frac{D}{}$$\frac{10}{}$$\frac{\times }{}$$\frac{D}{}$$\frac{60}{}$$\frac{)}{}$Cc=(D10×D60)(D30)2​. It helps further classify the soil gradation.

• A value between 1 and 3 indicates a well-graded soil.

7. Soil Classification

• Based on the particle size distribution and other derived parameters, the soil can be classified according to systems like the Unified Soil Classification System (USCS) or the American Association of State Highway and Transportation Officials (AASHTO) system.

• Gravel: Soil with larger particles, mainly retained on sieves with mesh sizes like No. 4 (4.75 mm).

• Sand: Soil particles passing through the No. 4 sieve but retained on finer sieves (like No. 200 sieve).

• Silt and Clay: Particles passing through the No. 200 sieve (0.075 mm) are considered fine-grained.

Summary of What You Get After the Test

• Particle size distribution (percentage of soil retained and passing through different sieve sizes).

• Grain size distribution curve for visual representation of soil gradation.
• Key parameters like effective size (D10), uniformity coefficient (Cu), and coefficient of gradation (Cc).
• Soil classification (gravel, sand, silt, clay) based on the particle sizes.
• Information on soil suitability for engineering applications, like foundation construction, roadwork, and permeability studies.

This data helps engineers and soil scientists understand the mechanical behavior of the soil, such as its strength, compressibility, and drainage properties.