California Bearing Ratio

The California Bearing Ratio (CBR) test is a simple strength test that compares the bearing capacity of a material with that of a well-graded crushed stone (thus, a high-quality crushed stone material should have a CBR @ 100%).

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.

CBR (%) = ( x / y ) x 100


x = material resistance or the unit load on the piston (pressure)
      for 2.54 mm (0.1″) or 5.08 mm (0.2″) of penetration

y =  standard unit load (pressure) for well-graded crushed stone

  =   for 2.54 mm (0.1″) penetration = 6.9 MPa (1000 psi)

  =   for 5.08 mm (0.2″) penetration = 10.3 MPa (1500 psi)