16/08/2025
California Bearing Ratio (CBR) Test – Full Details
The California Bearing Ratio (CBR) Test is a pe*******on test used to determine the bearing capacity of subgrade soil, sub-base, and base courses in road and pavement construction.
It measures the strength of soil or granular material by comparing it to a standard high-quality crushed stone.
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Purpose of the CBR Test
The CBR test is conducted to evaluate the load-supporting capacity of soils for:
Highways
Airfields
Pavements
Railway tracks
It plays a crucial role in pavement thickness design and ensures the structure can withstand expected loads.
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Test Principle
A standard plunger (50 mm diameter) is penetrated into the soil at a rate of 1.25 mm/min.
The resistance offered by the soil is measured and compared to standard values.
Standard Load Values:
2.5 mm pe*******on → 1370 kg
5.0 mm pe*******on → 2055 kg
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Types of CBR Test
1. Laboratory CBR Test – Conducted on prepared, remoulded soil samples in controlled conditions.
2. Field CBR Test – Performed directly at the construction site using CBR apparatus and a load frame.
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Apparatus Required
CBR mould with collar and base plate (150 mm dia, 175 mm height)
Pe*******on plunger (50 mm dia)
Loading frame with proving ring
Dial gauges (for load and pe*******on measurement)
Soaking tank (for soaked CBR tests)
Weighing balance
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Laboratory Test Procedure
1. Sample Preparation
Prepare the soil at Optimum Moisture Content (OMC).
Compact in three equal layers inside the CBR mould.
2. Soaking (Optional)
If required, soak the sample for 4 days to simulate water table or wet field conditions.
3. Pe*******on Test
Place the mould in the loading machine.
Apply load using the plunger at 1.25 mm/min.
Record load readings at pe*******ons: 0.5 mm, 1.0 mm, 1.5 mm, … up to 12.5 mm.
4. Calculation
Determine the CBR value for 2.5 mm and 5 mm pe*******on.
The higher value is generally taken for design purposes.
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Typical CBR Values
