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. References & Applicable Standards
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IS 2720 (Part 16):1987 – Methods of Test for Soils: Laboratory Determination of California Bearing Ratio (CBR),
published by the Bureau of Indian Standards (BIS).
-
IRC:37-2018 – Guidelines for the Design of Flexible Pavements,
published by the Indian Roads Congress (IRC).
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MoRTH Specifications for Road and Bridge Works (5th Revision) –
Section 300 (Subgrade, GSB) and Section 400 (Base & Surface Courses),
issued by the Ministry of Road Transport & Highways.
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ASTM D1883 – Standard Test Method for California Bearing Ratio (CBR) of Laboratory-Compacted Soils,
published by ASTM International.
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IRC:SP:72-2015 – Guidelines for the Design of Flexible Pavements for Low Volume Rural Roads,
Indian Roads Congress.
These standards collectively govern laboratory testing procedures, pavement design methodology, and construction quality control practices for highway projects in India.
6. Apparatus and Equipment for CBR Test (As per IS 2720 Part 16:1987)
All apparatus used in the California Bearing Ratio (CBR) Test shall conform to the specifications given in IS: 2720 (Part 16): 1987.
(A) CBR Mould Assembly
- Inside Diameter: 150 mm
- Height: 175 mm
- Volume: 2250 cm³
- Material: Mild steel with detachable base plate and 50 mm height extension collar
- Minimum Sidewall Thickness: 5 mm
- Accessories:
- Spacer disc (148 mm diameter × 47.7 mm height)
- Two filter papers per test
(B) Loading Machine
- Capable of applying load at a uniform penetration rate of 1.25 ± 0.05 mm per minute
- Equipped with:
- Proving ring or load cell (capacity up to 50 kN)
- Dial gauge for penetration measurement (least count 0.01 mm)
(C) Penetration Piston
- Diameter: 50 mm (±0.25 mm)
- Cross-sectional Area: 19.35 cm²
- Material: Hardened steel with smooth and flat surface
(E) Soaking and Surcharge Arrangement
- Soaking Tank: Large enough to accommodate moulds and keep specimens fully submerged during soaking (96 hours for soaked CBR).
- Surcharge Weights: Annular discs of 147 mm diameter with central hole for piston passage.
- Typically 5 kg for subgrade simulation
- Additional 2.5 kg discs may be added for higher surcharge conditions
(F) Miscellaneous Equipment
- Straight edge
- Mixing tray
- Spatula
- Weighing balance (accuracy ±1 g)
- Hot air oven (temperature 105–110°C)
- Moisture content containers (tins)
7. CBR Test Procedure (As per IS 2720 Part 16:1987)
CBR Test Procedure (Laboratory – Soaked Condition)
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About 45 kg of material is dried and sieved through 19mm sieve. If there is not worthy proportion of materials retained on 19mm sieve, allowance for larger size materials is made by replacing it by an equal weight of material passing 19mm sieve and retained on 4.75mm sieve.
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Determine the Optimum Moisture Content (OMC) and Maximum Dry Density (MDD) of the soil by conducting compaction test using either light compaction or heavy compaction (Modified Proctor), as per project requirements and relevant IS specifications.
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Each batch of soil (of at least 5.5 kg weight for granular soil and 4.5 to 5.0 kg weight for fine grained soils) is mixed with water up to the optimum moisture content or the field moisture content if specified so.
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The spacer disc (approx. 47.7 mm height) is placed at the bottom of the mould over the base plate, and a coarse filter paper (matching mould diameter ~150 mm) is placed over the spacer disc.
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For IS heavy compaction or the modified Proctor compaction, the soil is divided into five equal parts; the soil is compacted in five equal layers, each of compacted thickness about 26.5mm by applying 56 evenly distributed blows of the 4.89 kg rammer.
The rammer shall be allowed to fall freely from a height of 450 mm, and blows shall be applied uniformly over the entire surface to ensure uniform compaction energy throughout each layer.
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After compacting the last layer, the collar is removed and the excess soil above the top of the mould is evenly trimmed off by means of the straight edge.
Care shall be taken to ensure that the top surface is level, smooth, and flush with the top of the mould without disturbing the compacted soil.
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Repeat the same procedure to prepare another two moulds and collect about 100g of soil sample from each mould for moisture content determination.
Care shall be taken to collect the sample immediately after compaction from different locations within the mould to ensure uniform and representative moisture content, and the determination shall be carried out as per relevant IS standards.
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The clamps are removed and the mould with the compacted soil is lifted leaving below the perforated base plate and the spacer disc, which is removed. The mould with the compacted soil is weighed. A filter paper is placed on the perforated base plate, the mould with compacted soil is inverted and placed in position over the base plate and the clamps of the base plate are tightened. Another filter paper is placed on the top surface of the sample and the perforated plate with adjustable stem is placed over it. Surcharge weights of 2.5 or 5.0 kg weight are placed over the perforated plate and the whole mould with the weights is placed in a water tank for soaking such that water can enter the specimen both from the top and bottom.
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The swell measuring device consisting of the tripod and the dial gauge are placed on the top edge of the mould and the spindle of the dial gauge is placed touching the top of the adjustable stem of the perforated plate. The initial dial gauge reading is recorded and the test set up is kept undisturbed in the water tank to allow soaking of the soil specimen for four full days or 96 hours. The final dial gauge reading is noted to measure the expansion or swelling of the soil specimen due to soaking.
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The swell measuring assembly is removed, the mould is taken out of the water tank and the sample is allowed to drain in a vertical position for 15 minutes. The surcharge weights, the perforated plate with stem and the filter paper are removed. The mould with the soil sample is removed from the base plate and is weighed again to determine the weight of water absorption.
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The mould with the specimen is clamped over the base plate and the same surcharge weights are placed on the specimen centrally such that the penetration test could be conducted. The mould with base plate is placed under the penetration plunger of the loading machine. The penetration plunger is seated at the center of the specimen and is brought in contact with the top surface of the soil sample by applying a seating load of 4.0 kg. The dial gauge for measuring the penetration values of the plunger is fitted in position. The dial gauge of the proving ring (for load readings) and the penetration dial gauge are set to zero. The load is applied through the penetration plunger at a uniform rate of 1.25 mm/min. The load readings are recorded at penetration readings of 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10.0 and 12.5 mm. In case the load readings start decreasing before 12.5mm penetration, the maximum load value and the corresponding penetration value are recorded. After the final reading, the load is released and the mould is removed from the loading machine. The proving ring calibration factor is noted so that the load dial values can be converted into load in kg. About 50g of soil is collected from the top three cm depth of the soil sample for the determination of moisture content.
8 Sample Calculation
| Penetration (mm) | Test Load (kg) | Standard Load (kg) | CBR (%) |
|---|
| 2.5 | 480 | 1370 | 35.0 |
| 5.0 | 600 | 2055 | 29.2 |
Calculation at 2.5 mm:
CBR = (480 / 1370) × 100 = 35.0%
Calculation at 5.0 mm:
CBR = (600 / 2055) × 100 = 29.2%
Calculations – Swell Ratio & CBR
The swelling or expansion ratio is calculated from the observations during the swelling test using the formula:
Expansion ratio or swelling = 100 (df − di) / h
- df = final dial gauge reading after soaking, mm
- di = initial dial gauge reading before soaking, mm
- h = initial height of the specimen (127.3 mm), mm
The load values noted for each penetration level are divided by the area of the loading plunger (19.635 cm2) to obtain the pressure or unit load values on the loading plunger.
The load-penetration curve is then plotted in natural scale for each specimen. If the curve is uniformly convex upwards, no correction is needed. In case there is a reverse curve or the initial portion of the curve is concave upwards, necessity of a correction is indicated. A tangent is drawn from the steepest point on the curve to intersect the base at a point, which is the corrected origin corresponding to zero penetration.
The unit load values corresponding to 2.5 mm and 5.0 mm penetration values are found from the graph.
CBR Calculation
The CBR value is calculated from the formula:
CBR (%) = (Unit load carried by soil sample at defined penetration level / Unit load carried by standard crushed stones) × 100
9. Selection of Final CBR Value
RESULTS
The CBR values at 2.5 mm and 5.0 mm penetrations are calculated for each specimen from the corresponding graphs.
Generally the CBR value at 2.5 mm penetration is higher and this value is adopted.
However, if higher CBR value is obtained at 5.0 mm penetration, the test is to be repeated to verify the results;
if the value at 5.0 mm is again higher, this is adopted as the CBR value of the soil sample.
The average CBR values of three specimens are reported to the first decimal place.
The CBR values shall be determined from the load vs penetration curve by comparing the test load
with the standard load at respective penetrations (2.5 mm and 5.0 mm), ensuring accurate interpretation of results.
For highway pavement design under IRC 37-2018 and MoRTH specifications, the soaked CBR value at 2.5 mm penetration is generally used.
10. Interpretation of CBR Results
Subgrade Classification Based on CBR Value
| CBR Value (%) | Subgrade Quality |
|---|
| < 3 | Very Poor |
| 3 – 5 | Poor |
| 5 – 10 | Fair |
| 10 – 20 | Good |
| > 20 | Very Good |
Higher CBR value indicates better load-carrying capacity of soil and reduced pavement thickness requirement.
11. Engineering Interpretation of CBR Value
The CBR value obtained from laboratory testing represents the load-carrying capacity of the soil under soaked conditions. In pavement design, it directly influences the total thickness of granular and bituminous layers.
- Low CBR (< 5%) – Indicates weak subgrade requiring improvement through stabilization, replacement, or increased pavement thickness.
- Moderate CBR (5–10%) – Suitable for low to medium traffic roads with adequate granular layers.
- High CBR (> 10%) – Indicates good bearing capacity and allows economical pavement design.
Laboratory CBR vs Field Performance
- Laboratory soaked CBR represents worst moisture condition.
- Field CBR may vary depending on compaction quality and drainage condition.
- For design purposes, soaked laboratory CBR is always adopted as per IRC 37-2018.
Note: For expansive soils such as black cotton soil, stabilization using lime or cement is generally recommended if CBR is below 5%.
12. CBR Limits for Pavement Design (As per IRC 37-2018 & MoRTH)
| Pavement Layer | Minimum Laboratory CBR (%) | Reference |
|---|
| Natural Subgrade | ≥ 8 (Minimum 5 for low volume roads) | IRC 37-2018 Table 3 |
| Improved Subgrade | ≥ 10 | IRC 37-2018 Clause 6.2 |
| Granular Sub-Base (GSB) | 30 | MoRTH Section 400 |
| Base Layer (WMM/WBM) | 50 – 80 | MoRTH Section 400 |
| Bituminous Layers | Design Based | IRC 37 Traffic & Stiffness Criteria |
13. Application of CBR in Pavement Design (IRC 37-2018)
- The design CBR value is used to determine total pavement thickness (bituminous + granular layers) from IRC 37 design charts.
- Lower CBR → Higher pavement thickness.
Example:
- CBR = 8%, Traffic = 20 msa → Pavement thickness ≈ 580 mm
- CBR = 5%, Same traffic → Pavement thickness ≈ 700 mm
Conclusion: Accurate CBR determination directly impacts safety, durability, and economy of pavement design.
14. Precautions for Reliable CBR Results
- Ensure uniform compaction at correct Optimum Moisture Content (OMC).
- Avoid air voids during specimen preparation.
- Maintain consistent 72–96 hours soaking for soaked CBR.
- Calibrate proving ring and dial gauges before testing.
- Conduct minimum three specimens and adopt average value.
- For expansive or silty soils, test both soaked and unsoaked conditions to assess seasonal variation.
15. 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
16. 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.
17. 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.
18. CBR Calculation Excel Sheet
⬇ Download CBR Excel Sheet
19. Conclusion
The California Bearing Ratio (CBR) Test provides a reliable and standardized measure of subgrade and granular layer strength essential for flexible pavement design under Indian conditions.
Strict adherence to IS 2720 (Part 16):1987 for laboratory testing and IRC 37-2018 for pavement design ensures that highway structures are safe, durable, and economically optimized.
In professional practice, the 96-hour soaked CBR value, determined using the standard 150 mm mould and tested at a penetration rate of 1.25 mm/min, represents the most dependable indicator of worst-case field subgrade performance. This value forms the basis of pavement thickness design for NHAI and MoRTH projects across India.
Accurate specimen preparation, proper soaking, calibrated loading equipment, and correct interpretation of results are critical to obtaining dependable CBR values. Since pavement thickness is directly influenced by CBR, even minor testing inaccuracies may lead to unsafe or overdesigned structures.
Therefore, CBR testing remains one of the most important quality control and design verification tools in highway engineering practice.
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.
