Email This Page

CBR Test (California Bearing Ratio) Explained

pdf icon. download how to guide

Plate Bearing Tests Explained

Originally developed by the California Division of Highways in the 1930’s, this the CBR Test has been developed and adapted to form the basis of most testing for sub grade quality and compaction.

CBR (California Bearing Ratio) TestLaboratory CBR Test

Why would I need to do a CBR test?

If you have excavated virgin land that you wish to construct on, and have found clay or sand and want to know if it’s strong enough to bear weight, a sample can be taken and used in a CBR test.

What National Standards does the CBR test cover?

It is covered by various National Standards including BS1377.

What is the process for this the CBR Test?

A sample of the material that needs testing is loaded into our mould, either on site or in our laboratory. Then a 3sq inch (approx. 50mm dia) plunger is loaded against the sample and the penetration into the sample is measured at various increments.

What will we find out from a CBR Test?

The results are expressed in relative terms, as a percentage of the determined value of 3000lbf to penetrate 0.1inch (2.5mm at 13.3KN in today’s terms) in the originals tests.

In modern construction, the required CBR value is specified for soils and fill from formation level up through granular fills to the sub-base immediately below a concrete slab or asphalt road.

How do I know what results I need from a CBR Test?

The actual requirements will be determined by the Design Engineer but typical values might be 3-5% for clay formation or 15-30% for Type 1 sub-base.

Advantages to the CBR Test

  • The tests are relatively quick and inexpensive.
  • They can be performed in the laboratory or on site on undisturbed or re-compacted samples.
  • Results can be obtained within a day.
  • We can perform these tests on a self-contained basis (although the assistance of an excavator on site is often helpful).

Limitations of the CBR Test

  • The actual CBR (California Bearing Ratio) Test test uses only a small diameter plunger of approx. 50mm diameter. This is clearly of little practical use on 6F2 containing half bricks!
  • These true CBR test is thus limited to the fine grained materials such as clays, sands, pulverised fuel ash etc or finely crushed stone.

I need to test a much coarser material, what do I do?

If you need CBR values for coarse granulated materials, these can easily be obtained by calculation from the results of a PLATE BEARING TESTS  Or if you’re still not sure, use the following needs analysis flow chart or please contact us and ask the question

If you would like to arrange for CBR (California Bearing Ratio) tests or enquire further, please follow the link or call us.


CBR (California Bearing Ratio) Test Test Enquiry Form


pdf icon. download how to CBR (California Bearing Ratio) Test guide

Plate Bearing Tests Explained
Contact us now

01405 860905