CALIFORNIA BEARING RATIO (CBR) METHOD Typical Testing Machine

Introduction The 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 TEST Specimen may be Mould Prepared In-situ 150mm diameter 127.3mm height Separate base plate A 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 Equipment Soaking Samples Typical Testing Machine

CBR Equipment CBR Test – Sample Preparation CBR Test – Load-Penetration Measurement

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 penetration y 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 Type USC Soil Type CBR Range Clean gravels GW 40 – 80 GP 30 – 60 Gravels with fines GM 20 – 60 GC 20 – 40 Clean sands SW SP 10 – 40 Sands with fines SM SC 5 – 20 Silts and clays ML 15 or less CL OL 5 or less MH 10 or less CH (LL>50%) OH USC = unified soil classification Kind of futile giving typical values since they are not directly physically related. It’s like asking what color sports cars are. Typical CBR Ranges

California Bearing Ratio Vs Moisture content California Bearing Ratio Equipment

CBR DESIGN METHOD In 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.

Flexible Pavement Design Using CBR (Road Note 31)