CBR Test – IS 2720 Procedure, Formula & Result Interpretation (2026)
Author: Kishor Kumar · Updated: January 2026 · Read time: ~10 minutes
1. Introduction
The California Bearing Ratio (CBR) Test evaluates the strength of subgrade and base layers for pavements. This comprehensive guide, based on IS 2720, covers sample preparation, soaking, testing, calculation, and field interpretation, making it site-ready for engineers and QA/QC teams to ensure durable and long-lasting pavement design.
- Pavement layer thickness design: Optimize base and sub-base layer thickness based on CBR values.
- Subgrade improvement decisions: Identify whether soil stabilization or improvement is needed.
- Selection of suitable borrow soil: Choose the right soil type for highway subgrade and embankment construction.
2. Engineering Significance
The CBR test is a key parameter in flexible pavement design. It helps determine the
strength of subgrade, sub-base, and base layers. Pavement thickness under IRC and MoRTH
guidelines depends on soaked CBR values. Lower CBR values indicate weaker soils requiring
thicker pavement layers or stabilization.
3. Why CBR Test is Required
- Assess subgrade soil strength
- Comply with IRC:37 pavement design guidelines
- Verify MoRTH acceptance criteria
- Plan soil stabilization or replacement
Lower CBR values indicate weaker soil and require thicker pavement layers.
4. Applications in Highway Construction
- Subgrade approval before GSB and WMM layers
- Borrow area soil approval
- Pavement design stage as per IRC:37
- Weak soil investigation and improvement planning
CBR testing is mandatory for NH, SH, PMGSY, and EPC road projects.
5. Applicable Standards
- IS:2720 (Part 16) – Laboratory CBR Test
- IRC:37 – Guidelines for Pavement Design
- MoRTH Specifications – Subgrade & pavement layers
6. Apparatus Used
- CBR mould with collar and base plate
- Surcharge weights
- Penetration piston (50 mm diameter)
- Loading frame with dial gauges
- Soaking tank
7. Test Procedure
- Compact soil at OMC in the CBR mould
- Place surcharge weights
- Soak specimen for 96 hours (for soaked CBR)
- Apply load at 1.25 mm/min penetration rate
- Record load readings at standard penetrations
8. CBR Calculation
Formula: CBR (%) = (Test Load / Standard Load) × 100
| Penetration (mm) | Test Load (kg) | Standard Load (kg) | CBR (%) |
|---|
| 2.5 | 480 | 1370 | 35.0 |
| 5.0 | 600 | 2055 | 29.2 |
Final CBR value is taken as the higher of 2.5 mm or 5 mm penetration.
9. Result Interpretation
| CBR Value | Subgrade Quality |
|---|
| < 3% | Very Poor |
| 3 – 5% | Poor |
| 5 – 10% | Fair |
| 10 – 20% | Good |
| > 20% | Very Good |
10. Field Engineer Notes
- Always conduct soaked CBR for design and approval
- Ensure surcharge weights are placed before soaking
- Record swelling percentage during soaking period
- Check loading frame calibration before testing
- Reject results if penetration curve is abnormal
11. Common Mistakes
- Incorrect compaction energy
- Improper moisture content
- Missing surcharge weights
- Using unsoaked CBR for design
Always perform soaked CBR for MoRTH and IRC pavement design.
12. Reasons for Low CBR Values
- High plasticity clay content
- Excessive moisture in subgrade
- Poor compaction control
- Organic or unsuitable soil
- Improper drainage conditions
Soil stabilization or replacement is recommended for low CBR soils.
13. CBR Calculation Excel Sheet
⬇ Download CBR Excel Sheet
14. Who Should Read This Guide?
- Highway site engineers
- QA / QC engineers
- Site laboratory technicians
- Civil engineering students
- EPC contractors and consultants
15. Conclusion
CBR testing is a critical quality control and design tool in highway engineering.
Accurate testing ensures durable, economical, and compliant pavements.
Frequently Asked Questions
What is the minimum CBR value required for subgrade in highway projects?
Minimum CBR values depend on MoRTH & IRC:37; typically 8–10% for subgrade, but check project specs.
Why is soaked CBR preferred over unsoaked CBR?
Soaked CBR simulates worst-case moisture and is generally required for pavement design acceptance.
At what penetrations is CBR measured?
CBR is measured at 2.5 mm and 5.0 mm penetration; the higher value is reported.
How often should CBR testing be done in the field?
Frequency depends on quality assurance plan and project requirements.
Does oversize material affect CBR test results?
Yes — oversize particles must be handled correctly to avoid incorrect CBR values.
Why do CBR results sometimes vary?
Variations arise from inconsistent moisture, compaction effort, particle gradation, or sample prep issues.
