CBR Test: All you Need to Know

The characteristics of soil at the subgrade level are necessary for road construction projects. Common soil tests for constructing roads include soil classification, particle size distribution, determining the amount of moisture present, specific gravity, and liquid and plastic limit tests. For subgrade particles smaller than 20mm, one of the most reliable measures of soil strength is the CBR test. Its findings can be applied to the planning and creation of the materials needed to build pavements and roads. This article is a guide on the whole specifics of the CBR test and how it operates through this article.

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The CBR test

The main purpose of CBR Testing (California Bearing Ratio) is to gather information for paving road surfaces. The California State Highway Department invented it first. It is a permeation test that is primarily used to assess the subgrade strength of foundations, pavements, and roadways. The findings of these tests are combined with empirical curves to calculate the thickness of the layers that make up the pavement. The most popular technique for creating flexible pavement is this one.

The carrying capacity of the soil or bituminous pavement is assessed using the California Bearing Ratio test. The thickness of the materials required for the intended road construction is determined using the CBR test, which is a component of the site study.

CBR test: How does CBR Value work?

With increasing surface roughness, the CBR value goes up. Clay typically has a CBR value of 2%. However, other sands may have a CBR value of 10%. Values of high-quality sub-base will range from 80 to 100% of the maximum. A soil sample with a nominal maximum aggregate size of 20 mm is used for the CBR test.

CBR test: Objectives

• The primary goal of the California bearing ratio test is to gauge how far a soil’s resistance can be penetrated before shearing occurs at its highest possible level.
• Two California bearing ratio measurements are intended to be a ratio in the range of 0 to 100.
• Variations of distinct soil conditions and diverse types of buildings play a key role in the California bearing ratio test.
• Assessment of the subgrade durability of roads and pavement is done using the California bearing ratio test.
• California bearing ratio value is a measure of the stability and bearing capability of the soil.
• The Imperial curves and the CBR test data are used to determine the pavement’s width and the thickness of its various element layers.

CBR test: How are the tests run?

With a plunger of a standardised area, the CBR test is conducted by determining the pressure necessary to penetrate a soil sample. The pressure that must be applied to a standard shattered rock material to obtain an identical penetration is then subtracted from the measured pressure.

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A soil sample with a maximum particulate size of 20 mm is used for the CBR test. The method entails applying a constant force to a small, cylindrical plunger while employing a four-wheel drive vehicle as the response load to generate the force.

Tests are often conducted at the surface or depths of 500 to 1000 mm at intervals of 20 to 30 metres along the centerline of the proposed construction. At each site, at least three tests are typically run.

CBR calculation formula

CBR = 100 * (PT/PS)

where,

PS stands for “Standard Load” for the chosen thickness from the load penetration curve, and 

PT for “Corrected Test Load” for that same penetration.

Load Standards for CBR Test;

• The value of C.B.R. at 2.5 mm penetration is 3.
• The value of C.B.R. at 5.0 mm penetration is 4.

It is customary to calculate the C.B.R. results for penetrations of 2.5 mm and 5 mm. The C.B.R. value at 2.5 mm would typically be higher than at 5 mm, hence in this situation, the previous estimate will be used as the C.B.R. for design points.

The test should be repeated if the C.B.R. results on 5 mm are greater than those on 2.5 mm. If matched results persist, the design should use the C.B.R. value corresponding to 5 mm penetration.

CBR test: How is this test conducted?

To conduct a CBR Test, a standard area plunger must be pressed into a soil sample, and this pressure must be measured.

This value is multiplied by the amount of force required to penetrate a standard, premium-grade crushed rock material equally. 

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The CBR test can be run on soils with particles no larger than 20 mm. Tests are typically conducted at ground level or depths of 500 to 1000 m, most frequently at intervals of 20 to 30 mm along the centerline of the proposed building.

A single operator typically conducts many CBR tests in a day on a typical site with prepared surfaces. On-site preliminary results are frequently provided.

CBR test: What are the purposes of CBR test results?

A CBR design value is utilised as a benchmark for the subgrade’s strength while designing pavements. The CBR’s goal is to assess the subgrade soils’ strength to precisely calculate the thickness of the capping level and/or subgrades for the proposed highways, asphalt, and parking garages.

It is to be observed that the long-term stability value, which can be obtained once the roadway has been installed, is likely to be different from the in-situ CBR of the base course established at the time of a surface investigation. While many clayey sediments are susceptible to seasonal softening, compaction could benefit loose, coarse soils.

In this procedure, the thickness of the pavement and its layers are calculated using a blend of load penetration tests conducted in the lab or situ together with empirical design charts.

This is among the most used techniques for building flexible pavement. CBR values indicate the depth of the various components that make up a pavement

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