The California bearing ratio (CBR) test has been widely used in pavement design since mid 1940's. It is a relative measure of sub-grade soil or base/sub-base aggregate strength. Nine samples, about 50 kg are taken from different test pits dig to 1.5m from natural ground surface, the soil is fine grained either silt or clay. The nine (CBR) specimens were compacted at optimum moisture content and at 95% of the maximum dry density of the modified compaction test were prepared.
All specimens were soaked for periods of 96hr with more than 4.5kg surcharge load. Penetration test was done for both two faces (top and bottom) of the specimen. The tests denoted that most CBR curves are convex upwards so no correction is needed. CBR number is less than 20 so is very poor to fair, where the best using for sub grade due to its fine grained soil. Mostly CBR number that is calculated from bottom face is grater than top face, as they are less wetting than top face.
The paper reveals that most CBR values are small, depended from 5mm penetration of the bottom face of specimens, which no correction method is needed for curves.