California Bearing Ratio (CBR) Test of Soil

The California Bearing Ratio (CBR) Test is a penetration test used to evaluate the strength of subgrade soil and base materials for road and pavement design.

Objective of CBR Test

· Determine strength of subgrade soil

· Design thickness of pavement

· Evaluate suitability of soil for roads

· Compare load-bearing capacity of soils

Types of CBR Test

1. Laboratory CBR Test

Conducted on prepared soil sample in laboratory.

2. Field CBR Test

Conducted directly at construction site.

Apparatus Required

· CBR mold

· Loading machine

· Penetration piston

· Dial gauges

· Weighing balance

· Surcharge weights

Principle of Test

A standard piston penetrates the soil sample at a constant rate. The load required for penetration is compared with standard load values.

Test Procedure

1. Prepare soil sample at OMC and MDD

2. Compact soil in CBR mold

3. Place surcharge weights

4. Soak sample if required

5. Mount mold in loading machine

6. Apply load through penetration piston

7. Record load at different penetrations

8. Calculate CBR value

Formula

CBR = (Test Load / Standard Load) × 100

Standard Loads

Penetration	Standard Load
2.5 mm	1370 kg
5.0 mm	2055 kg

Example Calculation

Test load at 2.5 mm penetration = 820 kg

Step 1: Calculate CBR

CBR = (820 / 1370) × 100

CBR = 59.85%

Interpretation of CBR Value

CBR Value	Soil Quality
< 3%	Poor
3 – 7%	Fair
7 – 20%	Good
> 20%	Excellent

Final Result

CBR Value = 59.85%

Soil Quality = Excellent

Advantages

· Simple and reliable test

· Widely used for pavement design

· Suitable for road construction projects

Limitations

· Time-consuming for soaked CBR

· Results affected by moisture condition

Standard Codes

IS 2720 Part 16 – Laboratory Determination of CBR

ASTM D1883

Applications

· Highway pavement design

· Airfield pavement construction

· Subgrade evaluation

· Road embankment projects