Purpose and Importance of CBR Test | Significance of CBR Value

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Update: December 19, 2022

What is this post about?

Dear readers, this blog post gives knowledge about the purpose of CBR Test, importance of CBR Test and significance of CBR Value.

Definition of CBR Test

California Bearing Ratio aka CBR Test is a penetration test developed by California State Highway Department, USA. This is one of the most important geotechnical tests which is used for evaluating the bearing capacity of sub grade soil for design of flexible pavements and roads.

Purpose and Importance of CBR Test | Significance of CBR Value

Table of Contents   (toc)

CBR test may be performed on undisturbed specimens or remoulded specimens which may be compacted either statically or dynamically.
CBR is the ratio expressed in percentage of force per unit area required to penetrate a soil mass with a standard circular plunger of 50 mm diameter at the rate of 1.25 mm/min to that required for corresponding penetration in a standard material. The ratio is usually determined for penetration of 2.5 mm and 5 mm. When the ratio at 5 mm is consistently higher than that at 2.5 mm, the ratio at 5 mm is used.

Importance of CBR Test

Purpose of CBR Test

The main purpose of CBR Test is to determine the the bearing capacity of sub-grade soil, sub-base & base course material or the bitumen pavement for the design of thickness of highways and housing estate roads.

Importance of CBR Test

  1. The CBR test is used for determining the sub-grade strength value of pavements and roads.
  2. The results obtained from the CBR test are used with the Imperial curves for the determination of the thickness of pavement and various component layers of pavement.
  3. California bearing ratio value used as an index of soil strength and its bearing capacity.
  4. CBR testing is undertaken for the design of highways and housing estate roads to determine the strength of the subgrade soil and enable appropriate selection of suitable pavement thickness for the anticipated traffic density together with any precautions for frost heave.

Significance of CBR Value

In flexible pavements, different kinds of layers are used to withstand the traffic load. CBR value is required to determine the thickness of those layers, especially for base and subgrade materials.

For lower CBR, the thickness of layers will be higher and vice versa.
If the CBR value of a material is too low, the layer thickness will be much higher. It would not be cost-effective to work with that material.


What is the significance of CBR test in pavement design?

The california bearing ratio test is a geotechnical penetration test used for the evaluation of subgrade strength of roads and pavements. The results obtained by CBR tests are used with the empirical curves to determine the thickness of pavement and its component layers.

Why CBR test is important in the road construction?

The CBR test is important to evaluate the strength of subgrade soil, subbase and base course material of roads and pavements. The CBR value commonly used with the empirical curves is to design the thickness of road pavement and component layers on the road.

What does CBR value indicate?

The California Bearing Ratio (CBR) is a measure of the strength of the subgrade of a road or other paved area, and of the materials used in its construction. The ratio is measured using a standardized penetration test first developed by the California Division of Highways for highway engineering.

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