Granular Sub Base (GSB) is a crucial component in road construction, serving as the foundation layer that sits between the subgrade (natural soil) and the base course. This layer plays a vital role in load distribution, drainage, and the overall durability of the pavement structure.
The quality of GSB materials directly impacts the performance and longevity of roads. Poor quality GSB can lead to premature pavement failures, including rutting, cracking, and potholes, resulting in increased maintenance costs and reduced service life.
According to MORTH (Ministry of Road Transport and Highways) 5th Revision specifications, GSB materials are classified into six grades (Grade I to Grade VI) based on their particle size distribution and quality requirements. This procedure focuses on the most commonly used grades: I, II, and III.
Sieve analysis of GSB materials is performed to determine the particle size distribution (gradation) of the aggregate used in road construction. This test is fundamental for several critical reasons:
Ensures the GSB material meets the specified gradation requirements before it's used in construction, preventing future pavement failures.
Well-graded GSB materials achieve higher density when compacted, leading to better load-bearing capacity and stability.
The right gradation ensures proper drainage while maintaining stability, preventing water-related damage to the pavement structure.
Proper gradation helps distribute traffic loads evenly to the underlying subgrade, preventing localized failures.
The gradation of GSB materials is directly linked to the performance characteristics of the pavement. Non-compliance with specified gradation requirements can lead to reduced service life, increased maintenance needs, and premature failure of the pavement structure.
The Ministry of Road Transport and Highways (MORTH) 5th Revision provides detailed specifications for different grades of GSB materials. The gradation requirements for Grades I, II, and III are as follows:
| IS Sieve Size (mm) | Percentage Passing by Weight |
|---|---|
| 75 | 100 |
| 53 | 100 |
| 26.5 | 55-75 |
| 9.5 | 30-55 |
| 4.75 | 25-45 |
| 2.36 | 20-35 |
| 0.425 | 10-25 |
| 0.075 | 3-10 |
| IS Sieve Size (mm) | Percentage Passing by Weight |
|---|---|
| 75 | 100 |
| 53 | 80-100 |
| 26.5 | 55-80 |
| 9.5 | 35-65 |
| 4.75 | 25-50 |
| 2.36 | 20-40 |
| 0.425 | 10-25 |
| 0.075 | 3-10 |
| IS Sieve Size (mm) | Percentage Passing by Weight |
|---|---|
| 75 | 100 |
| 53 | 100 |
| 26.5 | 65-95 |
| 9.5 | 50-80 |
| 4.75 | 40-65 |
| 2.36 | 30-50 |
| 0.425 | 15-30 |
| 0.075 | 3-10 |
The choice of GSB grade depends on factors such as traffic volume, environmental conditions, availability of materials, and design requirements. Grade I is typically used for highways with high traffic volume, while Grades II and III may be used for roads with lower traffic loads.
Before beginning the GSB sieve analysis procedure, ensure you have the following equipment ready:
A precision balance with capacity of at least 10kg and accuracy of 1g for weighing large samples.
Standard test sieves of sizes 75mm, 53mm, 26.5mm, 9.5mm, 4.75mm, 2.36mm, 0.425mm, 0.075mm, and pan.
For consistent agitation of sieves (essential for large samples of GSB material).
For drying samples at 110±5°C to constant weight.
Large trays, brushes, and containers for handling aggregates.
For obtaining representative samples from larger stockpiles.
Follow these steps to conduct a proper sieve analysis for GSB materials:
Collect a representative sample from the GSB stockpile using proper sampling techniques. For GSB materials, use at least 25kg of material to ensure adequate representation of larger particles. The sample should be collected from different locations and depths of the stockpile.
If the material has excessive moisture, dry it in an oven at 110±5°C until it reaches a constant weight. After drying, use a riffle box sampler to obtain a representative test sample of approximately 5-10kg depending on the maximum particle size.
Accurately weigh the prepared test sample and record this as the total sample weight (W). This weight serves as the basis for all percentage calculations and should be recorded to the nearest gram.
Select and clean all the required sieves according to the GSB grade being tested. Arrange the sieves in descending order of aperture size (largest at the top): 75mm, 53mm, 26.5mm, 9.5mm, 4.75mm, 2.36mm, 0.425mm, 0.075mm, with a pan at the bottom.
Pour the weighed sample onto the top sieve. For larger particles (>9.5mm), hand sieving may be necessary first. Then place the stack of sieves in the mechanical shaker and secure properly. Run the shaker for 10-15 minutes. Ensure that not more than a single layer of particles is retained on each sieve to prevent clogging.
Carefully remove each sieve from the stack, starting from the top. Transfer the material retained on each sieve to separate containers and weigh to the nearest gram. Record these weights in a tabular format. Use a brush to ensure all particles are removed from the sieves.
Calculate the following for each sieve:
- Weight retained on each sieve
- Percentage retained = (Weight retained / Total weight) × 100
- Cumulative percentage retained = Sum of percentage retained on all coarser sieves
- Percentage passing = 100 - Cumulative percentage retained
Plot a gradation curve (semi-log graph) with sieve size on the x-axis (logarithmic scale) and percentage passing on the y-axis. Compare this curve with the specified limits for the intended GSB grade to evaluate compliance.
Compare the calculated percentage passing for each sieve size with the specified requirements for the target GSB grade. The material passes the test if all percentage passing values fall within the specified limits for each sieve size.
Let's walk through a complete sample calculation for GSB Grade I sieve analysis:
Total sample weight = 5000g
| Sieve Size (mm) | Weight Retained (g) | % Retained | Cum. % Retained | % Passing | Specification (Grade I) | Status |
|---|---|---|---|---|---|---|
| 75 | 0 | 0.00 | 0.00 | 100.00 | 100 | ✓ PASS |
| 53 | 0 | 0.00 | 0.00 | 100.00 | 100 | ✓ PASS |
| 26.5 | 1300 | 26.00 | 26.00 | 74.00 | 55-75 | ✓ PASS |
| 9.5 | 1000 | 20.00 | 46.00 | 54.00 | 30-55 | ✓ PASS |
| 4.75 | 500 | 10.00 | 56.00 | 44.00 | 25-45 | ✓ PASS |
| 2.36 | 500 | 10.00 | 66.00 | 34.00 | 20-35 | ✓ PASS |
| 0.425 | 800 | 16.00 | 82.00 | 18.00 | 10-25 | ✓ PASS |
| 0.075 | 400 | 8.00 | 90.00 | 10.00 | 3-10 | ✓ PASS |
| Pan | 500 | 10.00 | 100.00 | 0.00 | - | - |
| Total | 5000 | 100.00 | - | - | - | - |
% Retained = (Weight retained / Total weight) × 100
Example for 26.5mm sieve: % Retained = (1300g / 5000g) × 100 = 26.00%
Cumulative % Retained = Sum of % retained up to that sieve
Example for 9.5mm sieve: Cumulative % Retained = 0 + 0 + 26.00 + 20.00 = 46.00%
% Passing = 100 - Cumulative % Retained
Example for 9.5mm sieve: % Passing = 100 - 46.00 = 54.00%
Compare each "% Passing" value with the specification limits for the target GSB grade.
For example, at 9.5mm sieve, the % passing is 54.00%, which falls within the specified range of 30-55% for Grade I GSB.
The GSB sample meets all MORTH 5th Revision specifications for Grade I GSB. It has a well-distributed gradation within the specified limits for all sieve sizes, making it suitable for use in high-traffic highway construction.
Our GSB Sieve Analysis Calculator simplifies the entire process by automating all calculations and providing instant compliance checks. Here's how to use it:
Interactive calculator for GSB gradation analysis
The calculator features an intuitive interface that allows you to:
Choose from Grade I through Grade VI based on your project requirements and MORTH specifications.
Input the total weight of your GSB sample in grams.
Enter the sieve sizes and the weight retained on each sieve. The calculator comes pre-loaded with the standard sieve sizes for GSB analysis.
Click the "Calculate" button to instantly process your data. The calculator will:
- Compute percentages retained, cumulative percentages, and passing percentages
- Generate a particle size distribution graph
- Check compliance with MORTH specifications
- Provide pass/fail status for each sieve size
Generate a comprehensive PDF report of your analysis or save the results for future reference.
Understanding your GSB sieve analysis results is crucial for ensuring proper material selection and construction quality. Here are key aspects to consider when interpreting results:
Contains particles across all size ranges in balanced proportions. Provides optimal compaction, density, and stability. Ideal for GSB applications.
Missing certain particle sizes. May lead to inadequate interlocking, reduced density, and potential settlement issues after construction.
Contains particles of similar size. Results in higher void content, reduced bearing capacity, and potential instability under traffic loads.
| Issue | Potential Cause | Solution |
|---|---|---|
| Excess fines (passing 0.075mm) | Poor material selection or contamination during processing/storage | Blend with coarser material or screen out excess fines |
| Deficiency of fines (passing 0.075mm) | Washing during processing or segregation | Blend with finer material or adjust crushing process |
| Excess coarse fraction (>26.5mm) | Inadequate crushing or screening | Further crushing or screening of oversized material |
| Gap gradation | Natural material characteristics or improper blending | Blend with material containing the missing fraction |
| Segregation during sampling | Improper sampling technique | Use proper sampling methods (quartering or riffle box) |
| Inconsistent results between tests | Non-homogeneous stockpile or sampling error | Increase sample size and number of sample locations |
When GSB material fails to meet specifications, field adjustments like blending with other materials can be considered. However, any modification must be properly documented and the modified material should be retested to ensure compliance before use in construction.
Granular Sub Base (GSB) plays a crucial role in various types of road pavements. Understanding its applications helps in selecting the appropriate grade and construction method:
GSB serves as a foundation layer between the subgrade and base course, distributing traffic loads and preventing subgrade intrusion into the pavement layers.
In concrete roads, GSB provides uniform support to the concrete slab, prevents pumping of fine materials, and enhances drainage capabilities.
Well-graded GSB facilitates proper drainage while maintaining structural stability, preventing water accumulation beneath the pavement.
For industrial areas, ports, and container terminals, thicker GSB layers with higher quality materials (typically Grade I) provide enhanced load-bearing capacity.
Always conduct sieve analysis tests on GSB materials before placement to ensure compliance with specifications. Field testing during construction is essential to account for potential segregation during transport and placement.
GSB should be placed in layers not exceeding 150mm compacted thickness for effective compaction. Multiple layers may be necessary for thicker GSB sections.
Maintaining optimum moisture content (OMC ± 2%) during compaction is crucial for achieving maximum dry density. Both too wet and too dry conditions can lead to inadequate compaction.
Use appropriate compaction equipment (vibratory rollers for coarse GSB) and ensure adequate passes to achieve the specified density (typically 98% of modified Proctor density).
For further information on GSB sieve analysis and specifications, refer to the following standards and resources: