The CBR test of soil (California Bearing Ratio test) is one of the most widely used methods in civil engineering to evaluate the strength of subgrade soil, sub-base, and base layers for roads, pavements, and airfields. It helps determine the soil’s load-bearing capacity and provides essential data for designing pavement thickness. In this comprehensive guide, you will learn the purpose, apparatus, procedure, formula, and significance of the CBR test in highway and geotechnical engineering.
What is the CBR Test of Soil?
The California Bearing Ratio (CBR) test is a penetration test developed by the California State Highway Department in 1929. It measures the resistance of a soil sample to penetration by a standard plunger under controlled conditions. The result is expressed as a percentage, comparing the test load to the standard load required to penetrate a crushed stone sample.
In simple terms, CBR = (Load carried by soil / Load carried by standard crushed stone) × 100.
Higher CBR values indicate stronger soil that can support heavier loads, while lower values suggest weaker subgrade requiring thicker pavement layers.
Purpose of the CBR Test
The main objectives of performing the CBR test on soil are:
- To determine the bearing capacity of soil for road and pavement design.
- To classify soil based on its load-bearing potential.
- To estimate the thickness of subgrade, sub-base, and base courses.
- To evaluate soil compaction efficiency in field and laboratory conditions.
Types of CBR Test
There are two types of CBR tests depending on the testing conditions:
1. Laboratory CBR Test
Conducted in a controlled environment using compacted soil samples. It provides accurate results for soil strength and pavement design.
2. Field CBR Test
Performed directly on site using in-situ soil conditions. It helps assess compaction quality and the suitability of subgrade materials.
Apparatus Required for CBR Test
The standard equipment used in the CBR test includes:
- CBR mould: 150 mm diameter and 175 mm height, with a detachable base plate and extension collar.
- Plunger: 50 mm diameter.
- Loading machine: Capable of applying loads at a rate of 1.25 mm/min.
- Dial gauges: For measuring penetration and load.
- Surcharge weights: To simulate pavement load (2.5 kg or 5 kg each).
- Soil compaction equipment: Proctor hammer, balance, straight edge, and sieve.
- Water tank: For soaking samples during the soaked CBR test.
Preparation of Sample for CBR Test
- Take a representative soil sample and dry it in an oven.
- Sieve the soil through a 20 mm IS sieve to remove oversized particles.
- Determine the Optimum Moisture Content (OMC) and Maximum Dry Density (MDD) using the Proctor Test.
- Compact the soil into the CBR mould in three layers (for coarse soil) or five layers (for fine soil) at OMC, using a standard compaction hammer.
- Trim the excess soil and attach the base plate and collar firmly.
Procedure for CBR Test of Soil
The CBR test can be conducted in two conditions – soaked and unsoaked.
A. Unsoaked CBR Test Procedure
- Sample Preparation: Compact the soil in the mould at OMC and MDD.
- Set Up the Plunger: Place the mould under the loading machine and center the plunger on the sample surface.
- Apply Load: Apply a vertical load through the plunger at a constant rate of 1.25 mm per minute.
- Record Data: Note down the load values corresponding to penetrations of 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 4.0, 5.0, 7.5, and 10.0 mm.
- Plot Load-Penetration Curve: On graph paper, plot penetration (mm) on the X-axis and load (kg) on the Y-axis.
- Determine CBR Value: Calculate the CBR at 2.5 mm and 5.0 mm penetrations using the standard loads.
B. Soaked CBR Test Procedure
- Soaking: After compaction, soak the specimen in water for 96 hours (4 days) to simulate worst-case wet conditions.
- Swelling Measurement: Measure any expansion using a dial gauge.
- Testing: After soaking, perform the test following the same steps as the unsoaked procedure.
Calculation of CBR Value
The CBR value is calculated using the formula: CBR (%)=Test Load (at specific penetration)Standard Load×100text{CBR (%)} = frac{text{Test Load (at specific penetration)}}{text{Standard Load}} times 100CBR (%)=Standard LoadTest Load (at specific penetration)×100
Standard loads for CBR test:
- At 2.5 mm penetration = 1370 kg
- At 5.0 mm penetration = 2055 kg
The higher CBR value from these two readings is taken as the final result.
Example Calculation:
If the load at 2.5 mm penetration = 650 kg, CBR=6501370×100=47.45%text{CBR} = frac{650}{1370} times 100 = 47.45%CBR=1370650×100=47.45%
Hence, the CBR value of the soil = 47.45%.
Typical CBR Values for Different Soils
| Type of Soil | Typical CBR Value (%) |
|---|---|
| Well-graded crushed stone | 80 – 100 |
| Granular soil (gravel/sand) | 30 – 60 |
| Silty sand | 10 – 20 |
| Clayey soil | 3 – 10 |
| Poorly graded clay | 2 – 5 |
Interpretation of Results
- A higher CBR value means the soil has good load-bearing capacity and requires a thinner pavement layer.
- A lower CBR value means weak subgrade soil that needs stabilization or a thicker pavement structure.
- Generally, for highway subgrades, a CBR value between 5% to 10% is considered adequate.
Advantages of CBR Test
- Simple and Reliable: Easy to perform with minimal equipment.
- Useful for Design: Directly applicable in flexible pavement design.
- Cost-Effective: Inexpensive compared to advanced soil testing methods.
- Applicable in Field and Lab: Can be conducted under real or controlled conditions.
Disadvantages of CBR Test
- Empirical Nature: Based on experience, not a theoretical model.
- Not Suitable for Cohesive Soils: Results vary with moisture content.
- Time-Consuming: Soaked tests require several days.
- Limited Accuracy: May not represent in-situ conditions for large areas.
Factors Affecting CBR Value
Several factors influence the outcome of the CBR test, including:
- Soil type and gradation
- Moisture content and degree of saturation
- Density and compaction effort
- Soaking duration
- Presence of fines or clay content
Properly controlling these parameters ensures accurate and reproducible results.
Applications of CBR Test
The CBR test of soil is extensively used in:
- Highway and road pavement design (to determine subgrade strength).
- Airport runway design (to assess soil bearing capacity).
- Railway track foundation analysis.
- Soil stabilization and improvement projects.
- Quality control during construction.
Comparison: Laboratory vs Field CBR Test
| Parameter | Laboratory CBR | Field CBR |
|---|---|---|
| Control | High (standard conditions) | Low (depends on site conditions) |
| Accuracy | More accurate | Moderate |
| Purpose | Design & classification | Quality control |
| Time | Longer (includes soaking) | Faster |
| Equipment | Standard lab setup | Portable testing kit |
Conclusion
The CBR test of soil is one of the most practical and efficient methods for evaluating the bearing capacity of subgrade materials. Its results are directly used in pavement design to determine layer thickness and material selection. Whether performed in the laboratory or the field, the CBR value provides valuable insight into soil strength and its suitability for road and highway construction.
