Question

How we can rate the different types of soils for different layers of the pavements on the basis of their CBR values?

Answer 1

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 CBRtesting can be applied to soils with a maximum particle size of 20 mm.

Subgrade soil CBR value :

A clayey soil generally has a low CBR value(less than 8). Sands are more granular and drain better and will generally have CBR valuesbetween 15 and 35. Gravel will have the bestCBR values, generally 25 and up. That is why it is suggested to save the best material to cap the subgrade. Min. CBR Value is 6. If the value for subgradeis less than 4, 300mm thick bolder solling is required (Base Corse). If the value for subgrade is less than 6, 75mm thick Jahama solling is required (Base Corse).

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 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

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