<P> The California bearing ratio (CBR) is a penetration test for evaluation of the mechanical strength of natural ground, subgrades and basecourses beneath new carriageway construction . It was developed by the California Department of Transportation before World War II . </P> <P> The basic site test is performed by measuring the pressure required to penetrate soil or aggregate 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, and in Part 9: In - situ tests . </P> <P> 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, meaning that it is not unusual to see CBR values of over 100 in well compacted areas . </P> <P> C B R = p p s ⋅ 100 (\ displaystyle CBR = (\ frac (p) (p_ (s))) \ cdot 100 \ quad) </P>

Why cbr test need to be carried out
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