What Is a CBR Test and When Is It Used?
A penetration test used to rate subgrade strength for pavement design
The core question it answers
Before designing how thick a pavement, driveway or parking lot structure needs to be, engineers need to know how strong the underlying (subgrade) soil actually is. The CBR test provides a standardized measure of that strength, expressed as a percentage relative to a reference crushed-stone material.
Typical situations where it's run
CBR testing is common during the geotechnical investigation phase for roads, highways, airport pavements, parking lots, and sometimes larger driveways — anywhere a flexible pavement structure's thickness needs to be justified against the actual subgrade conditions rather than assumed.
Lab vs. field testing
CBR can be measured in a laboratory on a compacted, sometimes soaked, sample (simulating expected field moisture conditions), or in some cases in the field directly on in-place soil. Lab testing is more common for design purposes since it allows controlled, repeatable conditions; field (in-situ) testing is used in certain quality-control or verification contexts.
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Sources
ASTM D1883
Standard Test Method for California Bearing Ratio (CBR) of Laboratory-Compacted Soils.