1. CALIFORNIA BEARING RATIO (CBR) METHOD
Typical Testing Machine

2. 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.

3. CBR LABORATORY TEST Specimen may be Mould Prepared In-situ
150mm diameter
127.3mm height
Separate base plate
A collar

4. 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.

5. CBR Equipment
Soaking Samples
Typical Testing Machine

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

7. California Bearing Ratio (CBR)
Load a piston at a constant rate
See what load it takes to make it penetrate a known amount

8. 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

9. 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

10. California Bearing Ratio Vs Moisture content
California Bearing Ratio Equipment

11. 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.

12. Flexible Pavement Design Using CBR (Road Note 31)