The CBR test (California Bearing Ratio test) is a field and laboratory test used in road construction to measure the strength of soil subgrade. Engineers use the CBR test to decide the thickness of pavement layers in flexible pavement. A higher CBR value means stronger soil and thinner pavement is required, while a lower CBR value means weak soil needing thicker road layers.
Why CBR Test Is Important
The entire pavement rests on soil, so knowing its bearing capacity is essential before construction. Without the CBR test, roads may settle, crack or develop potholes early. It helps engineers design economical and safe pavement structure according to traffic load and soil condition.
Principle of CBR Test
In the CBR test, a standard plunger penetrates the soil sample at a fixed rate and the resistance offered by the soil is measured. This resistance is compared with the resistance of standard crushed stone. The ratio obtained is called the CBR value and expressed in percentage.
Types of CBR Test
Laboratory CBR Test
Conducted on prepared soil sample in mould at controlled moisture and compaction. Used during road design stage.
Field CBR Test
Performed directly on site subgrade to verify actual ground strength before pavement construction.
Apparatus Used in CBR Test
- CBR mould
- Loading machine
- Penetration plunger (50 mm diameter)
- Dial gauges
- Soaking tank
- Compaction hammer
Proper calibration ensures accurate results.
Procedure of CBR Test
First the soil is compacted in mould at optimum moisture content. The sample is soaked in water for 4 days if soaked CBR is required. The mould is placed in testing machine and load is applied through plunger at constant penetration rate of 1.25 mm per minute. Load readings are recorded at standard penetrations and CBR value is calculated.
CBR Value Calculation
CBR value is calculated using formula:
CBR = (Test load / Standard load) × 100
Standard load values are predefined for 2.5 mm and 5 mm penetration. The higher value is taken as final CBR.
Typical CBR Values of Soil
| Soil Type | CBR Value (%) |
|---|---|
| Clay | 2 – 5 |
| Silty soil | 5 – 10 |
| Sand | 10 – 30 |
| Gravel | 30 – 80 |
| Higher values indicate stronger subgrade support. |
Factors Affecting CBR Value
- Moisture content
- Degree of compaction
- Soil type
- Density of soil
- Soaking condition
Proper drainage improves CBR strength significantly.
Use of CBR in Pavement Design
Engineers use CBR value to determine thickness of sub-base, base and surface layers. Weak soil requires thicker layers while strong soil needs less material, reducing construction cost.
FAQs About CBR Test
What is good CBR value for road
Generally above 8% is suitable for village roads and above 15% for highways.
Why soaked CBR is taken
Because soil becomes weaker when wet, giving safe design thickness.
At which penetration CBR is taken
Usually at 2.5 mm, but higher of 2.5 mm and 5 mm is considered.
Is CBR test compulsory
Yes, it is essential for flexible pavement design.
Conclusion
The CBR test is a simple but crucial soil strength test used in pavement design. It helps engineers decide correct road thickness and prevent early road failure. Proper testing and interpretation ensure durable, economical and safe road construction.
