California Bearing Ratio Introduction The California bearing ratio (CBR) is a penetration test for evaluation of the mechanical strength of road subgrades and basecourses. It was developed by the California Department of Transportation before World War II. In this test, a plunger of a satandard area is pushed into the soil at a fixed rate of penetration, and the force requied to maintain thath force is measured. The CBR value is then defined as the ratio of the measured force to that required for similar penetration into a standard sample of crushed California limestone rock: CBR=(F/F s ).100% Where F is the measured force and F s is the force required for similar penetration into a standard sample. Higher values of CBR indicate harder surface of material. The standard crush rock from California values are as follows: Standard Crushed Rock From California Load (kN) 13.24 19.96 Penetration (mm) 2.5 5.0 The test is performed by measuring the pressure required to penetrate a soil sample with a plunger of standard area. The measured pressure is then divided by the pressure required to achieve an equal penetration on a standard crushed rock material. The CBR test is described in ASTM Standards D1883-05 (for laboratory-prepared samples) and D4429 (for soils in place in field), and AASHTO T193. The CBR test is fully described in BS 1377 : Soils for civil engineering purposes : Part 4, Compaction related tests.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
California Bearing Ratio
Introduction
The California bearing ratio (CBR) is a penetration test for evaluation of the mechanical strength of
road subgrades and basecourses. It was developed by the California Department of Transportation
before World War II. In this test, a plunger of a satandard area is pushed into the soil at a fixed rate
of penetration, and the force requied to maintain thath force is measured. The CBR value is then
defined as the ratio of the measured force to that required for similar penetration into a standard
sample of crushed California limestone rock:
CBR=(F/Fs).100%
Where F is the measured force and Fs is the force required for similar penetration into a standard
sample. Higher values of CBR indicate harder surface of material. The standard crush rock from
California values are as follows:
Standard Crushed Rock From California
Load (kN) 13.24 19.96
Penetration (mm) 2.5 5.0
The test is performed by measuring the pressure required to penetrate a soil sample with a plunger
of standard area. The measured pressure is then divided by the pressure required to achieve an
equal penetration on a standard crushed rock material. The CBR test is described in ASTM Standards
D1883-05 (for laboratory-prepared samples) and D4429 (for soils in place in field), and AASHTO
T193. The CBR test is fully described in BS 1377 : Soils for civil engineering purposes : Part 4,
Compaction related tests.
The CBR rating was developed for measuring the load-bearing capacity of soils used for building
roads. The CBR can also be used for measuring the load-bearing capacity of unimproved airstrips or
for soils under paved airstrips. The harder the surface, the higher the CBR rating. A CBR of 3 equates
to tilled farmland, a CBR of 4.75 equates to turf or moist clay, while moist sand may have a CBR of
10. High quality crushed rock has a CBR over 80. The standard material for this test is crushed
California limestone which has a value of 100. The test remains in existence around the world due to
its low equipment requirements, easy of performance and history of use.