What are the typical CBR values?
The harder the material, the higher the CBR value. A CBR value of 2% is usually found for clay, high-quality sub-base will have CBR values between 80% and 100%, and some sands may have values around 10%.
The CBR testing can be applied to soils with a maximum particle size of 20 mm. For soils with bigger particles, other types of bearing capacity can be used like the Plate Bearing Test.
Since the coarse-grained soils and fine-grained soils have particles with size smaller than 20 mm, they can be evaluated by the CBR testing.
The following definitions were established by the Unified Soil Classification System:
- Coarse-grained soils are soils where 50% of the particles are retained in a 0.075 sieve. This means that more than half of the individual grains are visible to the naked eye.
- Fine-grained soils are soils where 50% or more of the particles pass through a 0.075 sieve.
The coarse-grained soils can reach higher CBR values when compared with those of the fine-grained soils. As expected, with bigger particles the soil may have bigger CBR values.
But before looking at the typical values of each type, let’s quickly see how coarse-grained soils can be classified.
So the coarse-grained soils can be divided into gravelly soils and sandy soils.
In the gravelly soils, more than 50% of the coarse fraction has a particle size larger than 4.75 mm. The CBR values of gravelly soils range from 20% to 80%.
In the sandy soils, more than 50% of the coarse fraction has a particle size smaller than 4.75 mm. And their CBR values range from 5% to 40%.
And what about the fine-grained soils? Their CBR values are lower, ranging from 5% to 15%.
|General Soil Type||USC Soil Type||CBR Range|
|Coarse-grained soils||GW||40 – 80|
|GP||30 – 60|
|GM||20 – 60|
|GC||20 – 40|
|SW||20 – 40|
|SM||10 – 40|
|SP||10 – 40|
|SC||5 – 20|
|Fine-grained soils||ML||15 or less|
|CL LL < 50%||15 or less|
|OL||5 or less|
|MH||10 or less|
|CH LL > 50%||15 or less|
|OH||5 or less|
How is the CBR value calculated?
The unit load (pressure) on the plunger for 2.54 mm or 5.08 mm of penetration is divided by the unit load of the standard material and the result is multiplied by 100.
The bearing capacity of the standard material is considered as a reference value for this test. That’s why the CBR values are percentages, indicating the strength of a granular material in relation to the standard material.