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What is CBR Testing?

The CBR test is conducted to determine the ratio of the power per unit area required to penetrate a specific soil mass.

It is accomplished with a standard cylinder at a speed of 1.25mm/min. The CBR test is the abbreviation for “California Bearing Ratio Test”. The California State Highway Department created the test to assess the soil support limit for the construction of proposed road structures.

As a result, the test has become an industry staple.

How are CBR Tests Carried Out?

A CBR test is achieved by measuring the pressure necessary to penetrate a soil sample with a standard surface cylinder at a uniform rate. Then, the pressure measured is divided by the pressure required to evenly penetrate the standard rock material.

In general, depending on the permeability of the surface, the harder it is, the higher the CBR value. Generally, the tests are performed on the proposed construction sites at intervals of 20 to 30 m at ground level or at a depth of 500 to 1,000 mm. Furthermore, at least three tests are performed at each site. On a typical site, assuming the surface is ready, an operator can perform up to 8-10 tests per day and get preliminary results at that site.

These tests are conducted on normal or compacted soil on both wet and dry environs. Results obtained are compared with standard test bends to approximate the quality of the soil.

The CBR test is an undoubtedly accurate proportion of soil quality. It is commonly used on subgrade soils to approximate the thickness of the materials used for the construction of road structures and development projects. The more solid the material tested, the higher the CBR value leading to the use of thinner layers of material used lowering the cost of construction.

However, the CBR tests do not indicate the sheer strength, soil resistance, or the suitability of materials to be used. This is where a plate bearing test comes in.

CBR Testing Using the Plate Bearing Test (PBR)

Due to the direct ratio of raw materials, rapid methods and the ability to quickly obtain test results, it is generally preferable to use the plate bearing test technique for CBR field tests. The Plate Bearing Test or the Plate Load Test is a soil load test used to determine the final loading capacity of the soil and the possibility of settlement in any given load.

It mainly consists of applying a load on a steel plate of a certain diameter and taking records with each increment of load. The test load is gradually increased until the plate begins to stabilize quickly. The division of the ultimate value of the load on the plate by the area of the surface of the steel plate gives the final bearing capacity of the soil.

This test is performed using the same equipment and the same procedures as the plate bearing test.

Additionally, it is particularly appropriate for testing in the following situations:

  • The material tested is granular in nature.
  • Other parameters such as the stress under the plate must be measured.
  • Availability of construction plant acting as a reactive load for testing

In-Situ CBR Testing Using the Standard Method

The CBR In-Situ test is a method for approximating the CBR value of terrain materials. The hardware equipment is self-sufficient in our vehicle. Therefore, there is no need to set up a construction plant to act as a reactive load.

It works swiftly and can provide CBR values immediately after the test.

It is particularly suitable for testing in the following situations:

  • The test material contains no particles larger than 20 mm.
  • No construction plant can be placed as a load or reactive space.

Why Use the CBR Test?

The CBR test is undoubtedly one of the most commonly used methods to assess the quality of underground soils, underground foundations and course materials intended for the design of the thickness of airport roads and sidewalks.

It is required to assess the quality of these key construction structures. The thickness of the asphalt is obtained and the layers are separated. In general, it’s the most commonly used method in pavement planning.

Please contact us to discuss your site investigation requirements. We are highly qualified professionals and we offer site testing services throughout Reading.

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