CBR value for subgrade

The CBR (California bearing ratio) value is a penetration test that evaluates the mechanical strength of subgrades and basecourses that lay beneath new road constructions or pavements.

Typical Values for subgrades are;

Crushed stone CBR 20 to 100
Sandy Soils CBR 5 to 40
Silty Soils CBR 3 to 15
Clay Soils CBR 3 to 10
Tilled farmland CBR of 3
Turf CBR of 4.75
Moist sand CBR of 10

CBR Testing Frequency or Distance

The frequency or distance of sampling depends on the level of investigation, uniformity of soils, and the potential for detrimental reaction from chemical stabilisation.

General recommendations for various soil conditions are:

Uniform                                         0.5 to 1.0 mile
Non-uniform                                  0.25 to 0.5 mile
Highly variable                              1,000 ft. to 0.25 mile
The above is a general guide and should cover most situations.

The Need for CBR Testing
You need to perform cbr testing in order to evaluate the strength of the subgrade which is beneath the road or pavement in order to determine the thickness of the road and its sub component layers, in order to carry the required loads.

On British roads the maximum weights aloud are 44000KGS which includes the cargo weight, container weight and truck and trailer.

This is a considerable load so it is vital to perform CBR testing in order to calculate the thickness of subgrade and road surfaces in order to cope with these loads. In practice roads are constructed to carry considerably more as the transport of Abnormal loads often occurs on British roads.

Poor subgrade should be avoided when possible, but if you have to build over weak soils there are several methods used to improved subgrade performance.

  • You can use additional base layers which spread loads over a larger subgrade area.
  • You can also use stabilisation with an appropriate binder, which can increase subgrade stiffness.
  • The last option is the removal and replacement of poor subgrade soil which can be removed and replaced with a higher quality fill. This method however can be extremely expensive.

You should always carry out site investigation to evaluate CBR during construction and during lifecycle of road or pavement.

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