Civil CBR Test: Measuring Soil Strength for Safe Roads & Foundations

The California Bearing Ratio (CBR) test evaluates the load-bearing capacity of subgrade, subbase, and base course materials. It�s a core input for pavement thickness design and ground improvement decisions.

What is the CBR Test?

The California Bearing Ratio (CBR) test compares the penetration resistance of a soil to that of a well-graded crushed stone under standard conditions. Results are expressed as a percentage; a higher CBR indicates a stronger material suitable for heavier loads.

Where is CBR Used?

Highways & Roads: Determines pavement layer thickness and subgrade treatment.
Airfields & Parking: Verifies support capacity under heavy wheel loads.
Industrial Floors & Yards: Reduces settlement and cracking risk.
Value Engineering: Prevents over/under-design and optimizes cost.

How the CBR Test is Performed

Sample Preparation: Soil is compacted in a mold at target moisture and density.
Soaking (if specified): Specimen is soaked for 4 days to assess worst-case wet conditions.
Penetration: A 50 mm plunger penetrates the specimen at a fixed rate while load is recorded.
Calculation: Load at 2.5 mm and 5.0 mm penetration is compared with standard loads.

CBR Formula

CBR (%) = (Measured Load � Standard Load) � 100

Typical CBR Values

Material	CBR (%)
Crushed stone / high-quality base	80 � 100
Well-graded granular soil	30 � 60
Sandy / silty soil	10 � 20
Poor clayey soil	< 5

Rule of thumb: higher CBR = stronger support + thinner pavement layers (subject to design codes).

INAT�s CBR Testing Services

INAT Group performs laboratory and field CBR tests in compliance with ASTM and Aramco standards. Our reports include moisture�density data, penetration curves, and design-ready CBR values for your pavement or platform design.

Rapid turnaround with traceable QA/QC
Support for highways, airports, logistics yards, and industrial sites
Recommendations for subgrade improvement where needed

CBR FAQs

Do I always need soaked CBR?

Use soaked CBR when the subgrade may reach high moisture conditions (e.g., poor drainage, seasonal rise). Unsoaked CBR is used for dry conditions or stabilized layers as specified by the design brief.

Which value should I use�2.5 mm or 5 mm?

Designers commonly adopt the higher of the two (2.5 mm or 5 mm), unless a project spec or code states otherwise.

How many samples should we test?

It depends on project size and variability. For linear projects (roads/pipelines), test by chainage/soil change; for platforms/yards, grid-based sampling is typical.