California Bearing Ratio Test

Reference

Subgrade strength expressed as a percentage of crushed rock: the CBR is how pavement design asks the ground a simple question and gets a usable number back.

most native subgrades sit between 3 and 15
StandardsASTM D1883, AASHTO T193
The measurementpiston resistance at 2.54 and 5.08 mm penetration
The referencestandard crushed rock stresses of 6.9 and 10.3 MPa equal CBR 100
Specimen preparationcompacted to the Proctor target density and moisture
Soaking96 hours under surcharge for the design condition
Soft claysCBR 2 to 5
Silty and sandy subgradesCBR 5 to 15
Granular subbaseCBR 20 to 50
Crushed base courseCBR 80 to 100

How the test works

Compact the soil into the mold at the density and moisture the field spec will demand, soak it under a surcharge that represents the pavement above, then press a 50 mm piston into it at a fixed rate. The stress at 2.54 mm penetration divided by what standard crushed rock resists at the same penetration, times 100, is the CBR. If the 5.08 mm reading comes out higher, rerun, and if it repeats, report the higher value: some soils genuinely stiffen with penetration.

Where it goes wrong

Unsoaked optimism is the standing temptation: an unsoaked CBR can read several times the soaked value, and unless the subgrade will provably never see water, the soaked number is the design number. The surcharge matters more than it looks, since it confines the specimen the way the real pavement would. And treat the CBR to modulus correlations as the rough company they are: they carry the test into mechanistic design, but a correlation is not a measurement, and stacking two of them is how paper pavements get thin.

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