CBR Testing is used if you are building a road or car park on granular material.

If you need to test for crane hard standing or pile mat you would need to use a plate bearing test.

CBR Testing is used to evaluate the bearing capacity of sub-grade soil.

A full description of What CBR testing is, and what it entails is available in a document titled BS1377: Soils for Engineering Purposes. The California State Highway Department is officially credited with developing the very first such document. The main purpose of developing it was to provide field experts with something that they could use to run arbitrary soil tests.

The strength test informed its design and eventual drafting. An arbitrary strength test refers to a method used to perform stress tests on different types of soils in an attempt to duplicate different sets of wheel loads. A CBR test, therefore, functions as an investigation method that is performed on-site.

Its aim is to help the road construction experts determine the kind of materials they need to use when constructing roads.

The CBR Test—How Is It Undertaken?

For the CBR test to be performed, the investigator needs to measure the total amount of pressure that is required to penetrate a certain soil section. A plunger is required to perform this particular test. Once the pressure gets determined, the investigator has to divide it with the pressure that would be needed in order to obtain the same type of penetration. But in this case, pressure being considered pertains to that of a normal rock material that has already been crushed.

Harder surfaces tend to have higher CBR values. Thus, as the pressure increases, you can expect the CBR value to continue increasing. Traditionally, clay has a value of 2 percent. Other readily available sands have a value of 10 percent. For high-quality sub-bases, you can expect them to retain a value of between 80-100 percent.

A CBR test cannot be carried out on just any soil. The investigator has to confirm that the soil being considered has an overall particle size of around 20mm. If the materials in question are greater than this, then it is recommended that one pays attention to the Plate Bearing Tests. It is a method that involves the use of cylindrical plunger into a ground surface.

A four-wheel drive is used for the Plate Bearing Tests to drive the small plunger into the ground surface at a uniform rate. Such tests can only get performed at depths of between 500-1000mm or at surface levels. An interval distance between 20-30m is observed by the researchers. Additionally, the researchers are required to make sure that they have performed at least three tests on each site they have come across.

When dealing with typical sites, the investigator and accompanying team can carry out as many as eight to ten tests on a day. As such, you can have access to hundreds of test results to go through depending on the number of investigators available. The site test results are typically available within a few minutes of the investigator testing the soil.

However, note that these are only provisional results. Detailed testing needs to occur using more advanced equipment for the investigator to provide accurate results.