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

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.

16. References & Applicable Standards

1. 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).
2. IRC:37-2018 – Guidelines for the Design of Flexible Pavements, published by the Indian Roads Congress (IRC).
3. 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.
4. ASTM D1883 – Standard Test Method for California Bearing Ratio (CBR) of Laboratory-Compacted Soils, published by ASTM International.
5. 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

(D) Compaction Equipment

Standard Proctor Compaction

• Rammer Weight: 2.6 kg
• Drop Height: 310 mm
• Number of Layers: 3
• Blows per Layer: 56

Modified Proctor Compaction (If Required)

• Rammer Weight: 4.9 kg
• Drop Height: 450 mm
• Number of Layers: 5
• Blows per Layer: 55

Note: The same 150 mm diameter mould is used for both compaction and CBR testing.

(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)

7.1 Preparation of Test Specimen

1. Sample Selection

• Obtain a representative soil sample passing 19 mm IS sieve.
• Determine Optimum Moisture Content (OMC) and Maximum Dry Density (MDD) using Proctor Compaction Test as per IS 2720 (Part 7 or Part 8).

2. Mixing

• Thoroughly mix the soil with water at OMC to ensure uniform moisture distribution.

3. Compaction in CBR Mould

• Place spacer disc at the bottom of the mould.
• Compact soil in:
  • 3 equal layers (Standard Proctor), or
  • 5 equal layers (Modified Proctor).
• Apply specified number of blows per layer.
• Remove spacer disc and trim the specimen flush with the top and bottom.

4. Measurement

• Record weight of mould with compacted specimen.
• Calculate bulk density and dry density.

5. Soaking of Specimen

• Place filter papers at top and bottom of specimen.
• Apply surcharge weight (generally 5 kg for subgrade simulation).
• Submerge mould in water for 96 hours (4 days).
• For coarse granular materials, soaking of 72 hours may be sufficient.
• Measure swell reading during soaking using dial gauge.

7.2 Significance of 72–96 Hours Soaking

As per IS 2720 (Part 16):1987 and IRC 37-2018, soaked CBR represents the worst moisture condition of subgrade during pavement service life.

• Ensures complete saturation and swelling stabilization.
• Simulates monsoon or flood conditions.
• Provides true equilibrium moisture condition.
• Prevents unsafe pavement design due to artificially high CBR values.

IS 2720 (Part 16):1987, Clause 4.3:
“The specimen in the mould shall be soaked in water for 96 hours before testing.”

IRC 37-2018, Clause 6.2.1:
“Design CBR value shall correspond to the soaked condition of subgrade representing the weakest moisture regime likely during service life.”

7.3 Testing Procedure

1. Setup

• Remove specimen from soaking tank.
• Drain excess surface water.
• Place mould on loading frame.
• Apply required surcharge weight.

2. Penetration

• Position 50 mm diameter piston centrally on specimen surface.
• Apply load at uniform penetration rate of 1.25 mm/min.

3. Observation

Record load readings corresponding to penetrations of:

0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10.0, and 12.5 mm

4. Load–Penetration Curve

• Plot load (kN) versus penetration (mm).
• Correct initial concave portion due to surface seating.
• Determine corrected load at 2.5 mm and 5.0 mm penetration.

8. Calculation of CBR

8.1 CBR Formula

CBR (%) = (Test Load at Specific Penetration / Standard Load at Same Penetration) × 100

8.2 Standard Loads

• 2.5 mm penetration → 1370 kg (13.24 kN)
• 5.0 mm penetration → 2055 kg (20.09 kN)

8.3 Sample Calculation

Calculation at 2.5 mm:
CBR = (480 / 1370) × 100 = 35.0%

Calculation at 5.0 mm:
CBR = (600 / 2055) × 100 = 29.2%

8.4 Selection of Final CBR Value

Selection of Final CBR Value

Use the CBR value at 2.5 mm penetration by default. If the 5 mm value is higher, repeat the test. Only if the repeat confirms that the 5 mm value is consistently higher may it be considered.

As per IS 2720 (Part 16):1987, 2.5 mm penetration value is generally taken as the representative CBR for pavement design.

9. Interpretation of CBR Results

9.1 Subgrade Classification Based on CBR Value

Higher CBR value indicates better load-carrying capacity of soil and reduced pavement thickness requirement.

9.2 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%.

10. CBR Limits for Pavement Design (As per IRC 37-2018 & MoRTH)

11. 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.

12. 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.

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

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

14. Who Should Read This Guide?

• Highway site engineers
• QA / QC engineers
• Site laboratory technicians
• Civil engineering students
• EPC contractors and consultants

15. 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.