CBR (California Bearing Ratio) Test of Soil: Procedure and Standards As Per IS 2720 Part 16

The California Bearing Ratio (CBR) test is one of the most widely used methods for evaluating the strength of subgrade soil and base course materials in pavement design. Developed by the California Division of Highways, this test measures the resistance of a soil specimen to penetration under controlled moisture and density conditions. The test procedure is standardized under IS 2720 Part 16, which outlines the laboratory method for determining the CBR value of both undisturbed and remoulded soil samples. Engineers rely on CBR values to design flexible and rigid pavements, as the test provides a direct measure of the soil shear strength under simulated field conditions. For a broader overview of how CBR testing relates to subgrade evaluation, refer to our article on California Bearing Ratio Test On Subgrade Soil Procedure And Values.

Understanding the California Bearing Ratio Test

The CBR test determines the bearing ratio of soil by measuring the load required to push a cylindrical plunger of 50 mm diameter into a soil specimen at a standard rate of penetration. The ratio is expressed as a percentage of the unit load required to penetrate the soil to the unit load required to penetrate a standard crushed stone material at the same penetration depth. This percentage value is what engineers refer to as the CBR value.

The primary objective of performing the CBR test is to assess the strength of subgrade soil for pavement design. The test can be carried out on either undisturbed soil specimens collected directly from the field or on remoulded specimens prepared in the laboratory at the desired density and moisture content. The choice between these two approaches depends on the project requirements and whether the soil is already in place or being compacted as part of new construction. For projects that also require field verification of bearing capacity, the Plate Load Test To Calculate Bearing Capacity And Settlement Of Soil provides complementary data for foundation design.

The CBR value serves as a critical input parameter in pavement thickness design charts developed by organizations such as the American Association of State Highway and Transportation Officials (AASHTO) and the Indian Roads Congress (IRC). Higher CBR values indicate stronger subgrade material that requires a thinner pavement section, while lower values point to weaker soil needing thicker pavement layers to distribute traffic loads safely.

Equipment Required and Sample Preparation for CBR Testing

Essential Apparatus and Equipment

The CBR test requires a specific set of equipment to ensure accurate and repeatable results. The following items are essential for conducting the test as per IS 2720 Part 16:

• Compression testing machine capable of applying load at a constant rate of 1.25 mm per minute
• Proving ring with dial gauge to measure the applied load accurately
• Penetration plunger of 50 mm diameter and at least 100 mm length
• Split mould of 150 mm internal diameter and 175 mm height with a base plate and extension collar
• Surcharge weights including annular weights of 2.5 kg each
• Dial gauges for measuring penetration and expansion during soaking
• Sampling tubes for collecting undisturbed specimens
• Vernier caliper and weighing balance for dimensional and mass measurements
• Timer for controlling the penetration rate

Additional information on the complete testing procedure can be found at Cbr Test Procedure California Bearing Ratio Test, which offers further guidance on equipment setup and test execution.

Sample Preparation for Undisturbed Specimens

When testing undisturbed soil, the specimen is obtained by fitting a steel cutting edge of 150 mm internal diameter to the mould and gently pushing the assembly into the ground. Once the mould is sufficiently filled, it is removed by under-digging around it. The top and bottom surfaces of the extracted specimen are trimmed flat to produce a sample with the exact length required for testing. This method preserves the natural structure and fabric of the soil, providing a realistic representation of in-situ conditions.

Sample Preparation for Remoulded Specimens

For remoulded specimens, the dry density used for compaction should match either the field density or, if the subgrade is to be compacted during construction, the maximum dry density obtained from the Proctor compaction test. The moisture content for preparation depends on whether the specimen will be tested in an unsoaked or soaked condition:

• Unsoaked specimens should be prepared at the equilibrium moisture content the soil is expected to reach after road construction
• Soaked specimens should be prepared at optimum moisture content and then immersed in water for 96 hours before testing

All material used for remoulded specimens must pass through a 19 mm IS sieve. Material larger than 19 mm is replaced by an equal amount of material passing the 19 mm sieve but retained on a 4.75 mm sieve. Compaction can be achieved through either the static method, where the calculated wet soil mass is pressed into the mould using a displacer disc, or the dynamic method, where the soil-water mixture is compacted using a standard compaction hammer in accordance with light or heavy compaction procedures.

Step-by-Step CBR Test Procedure

The CBR test procedure follows a systematic sequence designed to ensure consistent and repeatable results. The steps below describe the standard laboratory procedure as outlined in IS 2720 Part 16. For projects dealing with weak subgrade conditions, understanding how How To Use Preloading To Improve Soil Bearing Capacity can complement soil strength data obtained from CBR testing.

1. Mount the specimen – Place the mould containing the prepared specimen with the base plate attached onto the lower plate of the compression testing machine. The top face of the specimen remains exposed.
2. Apply surcharge weights – Place surcharge weights on the specimen to simulate the overburden pressure from the base material and pavement layers that the soil would experience in the field.
3. Prevent soil upheaval – Place a 2.5 kg annular weight on the soil surface prior to seating the penetration plunger. This prevents soil from moving upward into the hole of the surcharge weights. The remaining surcharge weights are added after the annular weight is in place.
4. Seat the plunger – Lower the penetration plunger onto the soil surface under a seating load of 4 kg to ensure full contact between the plunger face and the specimen surface.
5. Set gauges to zero – Reset both the stress gauge (proving ring dial) and the strain gauge (penetration dial) to their zero positions before beginning the test.
6. Apply the load – Activate the compression machine to drive the plunger into the soil at a constant rate of approximately 1.25 mm per minute.
7. Record readings – Take load readings at specified penetration intervals. The standard penetration points are 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 4.0, 5.0, 7.5, 10.0, and 12.5 mm.
8. Complete the test – Once the maximum penetration of 12.5 mm is reached, raise the plunger and detach the mould from the loading equipment.

Throughout the test, it is important to maintain a steady penetration rate and record each load reading promptly at the specified penetration depth. Any deviation in the loading rate can affect the accuracy of the CBR calculation.

Calculating the CBR Value from Test Data

Once the test data has been collected, the next step is to determine the CBR value through graphical analysis and computation. The process begins with plotting the load-penetration curve. This curve is essential for obtaining corrected load values before performing the final CBR calculation. For additional methods of improving subgrade performance, our article on Improving Bearing Capacity Of Soil covers various ground improvement techniques that are often used alongside CBR testing.

Plotting the Load-Penetration Curve

The load-penetration curve is constructed by plotting penetration values on the x-axis (in mm) and the corresponding load values on the y-axis (in kg or kN). The resulting curve typically shows a rising trend that may exhibit an initial concave-upward shape in some soils due to the seating of the plunger. If the curve is concave upward near the origin, a correction is applied by drawing a tangent to the steepest portion of the curve and extending it back to intersect the x-axis. This intersection point becomes the corrected zero penetration point.

Standard Load Values for CBR Calculation

The CBR value is calculated using the following formula:

CBR = (P_T / P_S) x 100

Where:

• P_T = Corrected test load corresponding to the chosen penetration depth
• P_S = Standard load for the same penetration depth, taken from the standard code

The standard load values for crushed stone, as defined in IS 2720 Part 16, are shown in the table below:

The CBR value is typically calculated at both 2.5 mm and 5.0 mm penetration depths. In most cases, the CBR calculated at 2.5 mm penetration is higher than that at 5.0 mm, and the higher value is reported as the CBR of the soil. However, if the CBR at 5.0 mm penetration exceeds that at 2.5 mm, the test should be repeated to confirm the results. The selected CBR value is reported to the first decimal place.

Reporting CBR Results and Engineering Significance

The final CBR value provides engineers with a quantitative measure of soil strength that directly influences pavement thickness design. A well-structured understanding of Bearing Capacity Of Soil Types And Calculations is essential when interpreting CBR test results in the broader context of foundation and pavement engineering.

CBR values for different soil types typically fall within the following ranges:

• Excellent subgrade – CBR greater than 20% (well-graded gravel, crushed stone)
• Good subgrade – CBR between 10% and 20% (sandy soils, gravelly soils)
• Fair subgrade – CBR between 5% and 10% (silty soils, clayey sands)
• Poor subgrade – CBR between 3% and 5% (soft clays, organic soils)
• Very poor subgrade – CBR less than 3% (highly organic soils, peat)

It is important to note that CBR is a ratio expressed as a percentage and has no units. The test is considered a strength index rather than a fundamental soil property, meaning its primary value lies in empirical correlations developed through decades of pavement performance observations. The soaked CBR value is typically lower than the unsoaked value because water saturation reduces soil suction and weakens the soil structure, making the soaked condition the more conservative and commonly used design parameter.

Safety Precautions and Quality Control During CBR Testing

Accurate and reliable CBR test results depend not only on following the correct procedure but also on maintaining proper laboratory practices and safety measures. The following precautions should be observed during testing:

• Clean sieves thoroughly with a brush after each sieving operation to prevent cross-contamination between different soil samples
• Center the sieve properly on the balance when weighing samples to ensure accurate mass measurements
• Inspect electrical connections of the sieve shaker and compression machine before starting the test to avoid electrical hazards
• Verify the proving ring calibration regularly using a calibrated proving ring calibration chart, as the conversion factor varies for each proving ring assembly
• Check the penetration rate at the beginning of each test to confirm the machine is delivering the required 1.25 mm per minute
• Inspect the plunger face for wear or damage that could affect the contact area and introduce errors in the load readings

Quality control measures also include preparing duplicate specimens whenever possible and averaging the results to account for natural soil variability. The CBR test, while simple in principle, requires careful attention to detail at every stage from sample preparation through data analysis.

Conclusion

The California Bearing Ratio test remains a cornerstone of geotechnical investigation for pavement design and subgrade evaluation. Its straightforward methodology, standardized through IS 2720 Part 16, provides engineers with reliable data for determining the appropriate thickness of pavement layers based on the strength characteristics of the underlying soil. While the test itself measures penetration resistance under controlled conditions, the CBR value ultimately informs decisions about load distribution across pavement structures. Understanding how soil strength relates to structural performance is fundamental to safe and economical design, which is why topics such as Load Bearing Structures remain closely connected to the geotechnical data obtained from tests like the CBR. Proper execution of the test, careful sample preparation, accurate data recording, and correct interpretation of the load-penetration curve all contribute to reliable CBR values that form the basis of durable and cost-effective pavement designs.