California Bearing Ratio or CBR test is the ratio of the force per unit area required to penetrate a soil mass with a standard circular piston of 50 mm dia, at the rate of 1.25 mm/min to that of force required to penetrate sample of compacted stone having CBR of 100%.
The standard load corresponding to 2.5 mm and 5 mm penetration of the plunger into the standard sample is reported to be 1370 kg and 2055 kg respectively.
CBR test may be conducted in the laboratory either on remoulded or undisturbed soil specimens. CBR test can also be conducted in the field.
CBR test is done both on the soaked and unsoaked samples. Soaking of specimen simulate the worst field condition that a subgrade soil can be subjected to, similar to monsoon and post-monsoon conditions.
Both during soaking and penetration test, the specimen is covered with equal surcharge weights to simulate the effect of overlying pavement or the particular layer under construction. Each surcharge slotted weight, 147 mm in dia with a central hole of 53 mm in dia and weighing 2.5 kg is considered approximately equivalent to 6.5 cm of construction.
The initial portion of the load-penetration curve of CBR test is generally convex upward. But sometimes the initial portion becomes concave upward. This is due to one or more of the following reasons.
o Top layer of soaked soil is too soft or slushy after soaking in water
o The top surface of the soil specimen is not even
o The penetration plunger of the loading machine is not vertical resulting in the bottom surface of plunger not being horizontal and not fully in contact with top surface of the specimen.
The initial concavity in the curve indicates that during the initial application of load, the plunger penetrated at a more rapid rate and later further penetration values are consistent with respect to the load applied.
Generally the CBR value at 2.5 mm penetration is higher and this value is adopted as the CBR value of the soil sample. However if higher CBR value is obtained at 5.0 mm penetration, the CBR test is to be repeated to verify the result. If CBR value at 5.0 mm penetration is higher in the repeat test also, this higher value is adopted as the CBR value of the soil sample.
Presence of coarse grained particles would result in poor reproducibility of result. Therefore material passing 20 mm sieve is only used in the test.
CBR test is an empirical test method and cannot be related accurately with any fundamental or physical property of the soil or pavement material tested.
Indian Road Congress (IRC) has standardized the guidelines for the design of flexible pavements based on CBR test (vide IRC-37) and this method is being followed for the design of flexible pavements for all the categories of roads in India.
As per IRC guidelines whenever possible the remoulded specimens for CBR test should be prepared by static compaction otherwise by dynamic compaction so as to achieve the desired dry density.
Often it is required to determine the CBR value of a soil compacted at some other desired percentage of MDD, other than the MDD value as per heavy (modified) or light (standard) compaction. For example as per MORTH, the soil subgrades of highways are to be compacted in the field to 97% of MDD by heavy compaction obtained in the laboratory. In this case compaction test may be carried out by applying different number of blows per layer (say 25, 40 & 55 blows/layer) and a graph is plotted with number of blows/layer vs the dry density achieved. From this graph it is possible to determine the number of blows required to obtain any desired value or percentage of dry density by interpolation and then to conduct CBR test on specimen compacted accordingly.
According to IRC:37-2001, if the maximum variation in CBR values between the three specimens tested in the laboratory exceed the permissible variation in CBR values for different ranges (as given in the table below), the CBR test should be repeated on additional three specimens and the average value of six specimens is to be adopted as the CBR value.
CBR value (%)
Maximum permissible variation in CBR values between 3 individual test value (±, %)