The California Bearing Ratio (CBR) test is a penetration test used to evaluate the strength of subgrade soil, sub-base, and base course materials for road and pavement design. It is a widely used method in transportation engineering to estimate the bearing capacity of soil.
📌 Purpose of the CBR Test
- To determine the load-bearing capacity of soils.
- Helps in pavement thickness design (especially for flexible pavements).
- Indicates the shear strength and supporting power of the soil under road traffic.
🧱 Apparatus Required
- CBR mould with collar and base plate (150 mm dia × 175 mm height)
- Spacer disc
- Loading machine (with a capacity of 50 kN)
- Penetration piston (50 mm diameter)
- Dial gauges
- Soaking tank (for soaked test)
- Weighing balance, compaction rammer
⚙️ Test Procedure
A. Sample Preparation
- Take a representative soil sample and sieve through a 19 mm IS sieve.
- Mix the soil with optimum moisture content (OMC).
- Compact the soil in CBR mould in three equal layers, using a standard or modified Proctor method.
- Remove the spacer disc and level the surface.
B. Soaking (for soaked CBR)
- Place a surcharge weight to simulate overburden pressure.
- Submerge the specimen in water for 4 days.
C. Penetration Test
- Place the mould in the loading frame.
- Apply load through the plunger (50 mm dia) at a constant rate of 1.25 mm/min.
- Record the load readings at penetrations of 0.5 mm, 1.0 mm, 1.5 mm, up to 12.5 mm.
D. Calculation
- Determine the load values at 2.5 mm and 5.0 mm penetration.
- Compare with standard load values:
- 2.5 mm → 1370 kg
- 5.0 mm → 2055 kg
CBR=(Test LoadStandard Load)×100\text{CBR} = \left( \frac{\text{Test Load}}{\text{Standard Load}} \right) × 100
- Take the higher value of CBR from 2.5 mm or 5.0 mm as the final result.
📊 Typical CBR Values
| Soil Type | CBR (%) |
|---|---|
| Clay (soft) | < 5 |
| Sandy clay | 5 – 10 |
| Gravelly soil | 25 – 30 |
| Crushed rock | > 80 |
✅ Applications
- Designing the thickness of pavement layers.
- Assessing soil suitability for subgrade and sub-base.
- Estimating the compaction quality of soil layers.
📘 Test Standards
- IS 2720 (Part 16): 1987 – Indian Standard for CBR
- ASTM D1883 – Standard Test Method (USA)
📝 Conclusion
The CBR test is an essential field and laboratory test to determine the suitability of soils for pavement design. By analyzing how much pressure the soil can resist, engineers can design roadways that last longer and perform better under traffic loads.
Here’s a CBR Test Report Format along with a Checklist for field or lab use. These are based on IS 2720 (Part 16):1987 and standard engineering practices.
📄 CBR Test Report Format
California Bearing Ratio (CBR) Test Report
Project Name: ____________________________
Location: _________________________________
Client: _________________________________
Test Conducted By: ______________________
Date of Test: ____ / ____ / ______
Lab Reference No.: _______________________
🧱 Sample Details
| Parameter | Details |
|---|---|
| Sample Identification No. | _______________________________ |
| Soil Type | _______________________________ |
| Test Method | Soaked / Unsoaked |
| Mould Size | 150 mm dia × 175 mm height |
| Maximum Dry Density (MDD) | ___________ g/cc |
| Optimum Moisture Content (OMC) | ___________ % |
| Compaction Method | Standard / Modified Proctor |
| Surcharge Weight Applied | ___________ kg |
| Soaking Period (if applicable) | 4 Days |
⚙️ Load vs Penetration Data
| Penetration (mm) | Load (kg) |
|---|---|
| 0.0 | |
| 0.5 | |
| 1.0 | |
| 1.5 | |
| 2.0 | |
| 2.5 | |
| 3.0 | |
| 4.0 | |
| 5.0 | |
| 7.5 | |
| 10.0 | |
| 12.5 |
📐 CBR Calculation
- Standard Load at 2.5 mm = 1370 kg
- Standard Load at 5.0 mm = 2055 kg
CBR2.5mm=(Test Load at 2.5 mm1370)×100\text{CBR}_{2.5mm} = \left( \frac{\text{Test Load at 2.5 mm}}{1370} \right) \times 100 CBR5.0mm=(Test Load at 5.0 mm2055)×100\text{CBR}_{5.0mm} = \left( \frac{\text{Test Load at 5.0 mm}}{2055} \right) \times 100
| Penetration | Load (kg) | Standard Load (kg) | CBR (%) |
|---|---|---|---|
| 2.5 mm | __________ | 1370 | __________ |
| 5.0 mm | __________ | 2055 | __________ |
- Final CBR Value = Higher of the two
- CBR (%) = __________
✅ Remarks / Observations
Tested By: ____________________
Approved By: ____________________
✅ CBR Test Checklist for Technicians / Engineers
| S. No. | Activity | Check/Remarks |
|---|---|---|
| 1 | Sample collected and identified properly | |
| 2 | Soil sieved through 19 mm IS sieve | |
| 3 | OMC and MDD determined | |
| 4 | Compacted in 3 equal layers in mould | |
| 5 | Spacer disc and surcharge applied | |
| 6 | Soaking carried out for 4 days (if soaked test) | |
| 7 | CBR test done on soaked/unsoaked sample | |
| 8 | Loading rate maintained at 1.25 mm/min | |
| 9 | Load readings recorded at specified penetrations | |
| 10 | Dial gauge zeroed and calibrated | |
| 11 | Plunger centrally aligned during test | |
| 12 | Load vs penetration curve plotted | |
| 13 | CBR values calculated correctly | |
| 14 | Report prepared with proper details and signature |