TRANSPORTATION ENGINEERING IICALIFORNIA BEARING RATIO (CBR) METHODTypical Testing Machine

IntroductionThe California Bearing Ratio devised by engineers of the California Division of Highways in nine years period to 1938.Most universally accepted pavement design methods.A standard penetration-type load-deformation test is carried out, and using the values obtained from the test from an empirical design chart, the pavement thickness are calculated.

CBR LABORATORY TESTSpecimen may bePreparedIn-situMould 150mm diameter127.3mm heightSeparate base plateA collar

The specimen is soaked and the expansion is measured.Load is applied by the loading frame through a cylinder plunger of 50mm diameter and penetration is measured.Rate of penetration is maintained at 1.25mm/minute.Loads are recorded for 2.5 and 5.0mm.Load is expressed as a percentage of the standard load at the respective deformation level, and is known as the CBR value.

CBR EquipmentTypical Testing MachineSoaking Samples

CBR Test Load-Penetration MeasurementCBR Test Sample PreparationCBR Equipment

California Bearing Ratio (CBR)Load a piston at a constant rate

See what load it takes to make it penetrate a known amount

CBR Equation

x=material resistance or the unit load on the piston (pressure) for 2.5 mm or 5 mm of penetrationy=standard unit load (pressure) for well graded crushed stone. For 2.5 mm of penetration = 1000 psi For 5.0 mm of penetration = 1500 psi

Typical CBR Ranges

General Soil TypeUSC Soil TypeCBR RangeClean gravelsGW40 80GP30 60Gravels with finesGM20 60GC20 40Clean sandsSW20 40SP10 40Sands with finesSM10 40SC5 20Silts and claysML15 or lessCL15 or lessOL5 or lessMH10 or lessCH (LL>50%)15 or lessOH5 or less

California Bearing Ratio Equipment California Bearing Ratio Vs Moisture content

CBR DESIGN METHODIn this method, the CBR values are used to determine the total thickness of the flexible pavement and its various layers.Following are the graphs used in the design of the pavement thickness.

*USC = unified soil classificationKind of futile giving typical values since they are not directly physically related.Its like asking what color sports cars are.