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| CBR Testing Apparatus |
The
California Bearing Ratio (CBR) test is a penetration test
developed by the
California State Highway Department and is used to determine
the mechanical
strength of a soil. The test can be carried out on site or
in the laboratory. The CBR test is carried out using a cylindrical
plunger which
penetrates the soil at a given rate. The relationship
between the force and
penetration can then be used to calculate the CBR value
of the soil being
tested.
The California Bearing Ratio test, described in ASTM D 1883, is a penetration test commonly used to evaluate the potential strength of subgrade, subbase and base course material. To perform the test, a technician uses a cylindrical piston with a 3-square-inch cross section to penetrate the soil at a rate of 0.05 inch per minute. At each 0.1 inch penetration up to 0.5 inch, the technician records the stress needed to push the piston into the soil. The CBR value is the ratio of this stress at different penetration levels to the bearing value of a standard crushed rock. In most cases, CBR decreases as the penetration increases, so the ratio at 0.1-inch penetration is used as the recorded CBR value. Sometimes designers use this value to choose an appropriate slab thickness for anticipated loadings.
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| Field CBR Test |
The test is carried out off the back of a 4WD vehicle or and
excavator either at the ground surface or in pits up to around 1.0 m deep. The
test area is leveled and any unrepresentative material is removed. Sufficient
surcharge discs are then placed around the test area and plunger to correspond
with the over burden pressure for the test. The test is carried out by forcing
the plunger to penetrate the soil over a given time. The results of the
penetration are used to provide a CBR value for the soild horizon.
However, the test is only appropriate to use on material with a
particle size that does not exceed 20 mm. An alternative technique to gain CBR values is to carry out a
hand held Dynamic Cone Penetrometer (DCP) test.
CBR% = 100 (P/Ps)
P = Measured Pressure for Site Soil(N/mm²=Mpa
Ps = Pressure to achieve equal penetration on standard soil(N/mm²)
Ps = 6.9 Mpa(1000psi) for 0.1"(2.54mm) ; 10.3 Mpa(1500psi) for 0.2"(5.08mm)
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| Sample CBR Graph for various soil samples |