Definition of California Bearing Ratio Value, what it measures and why it’s used?

California Bearing Ratio Value is defined as force per unit area needed to penetrate a given sample of soil or cohesive material with a standard circular plunger of 50 mm diameter at a rate of 1.25 mm/min to that is required for corresponding penetration in a standard material. Often, this standard material is graded as a crushed stone. This methodology in particular refers to the lab standard of testing. In the case of in-situ measurements done via proxy testing, different testing equipment is used to arrive at the CBR value such as plate load test or the dynamic cone penetrometer (DCP) test.

The California Bearing Ratio Value assesses soil strength under specific conditions, expressed as a percentage ranging from 0 to 100%. This test involves applying pressure to a soil sample and comparing it to the pressure needed to compress a well-graded crushed stone. The CBR value indicates the soil or material’s ability to withstand compression and pressure, with higher values indicating better resistance.

This is a standardised measure, done either in-situ or in lab conditions, to check the ability of a soil mass to carry load, and what pressure is likely to cause compression. The CBR Value helps civil engineers decide what kind of subgrade they need to build, and what kind of layers of reinforcement are needed for different applications: be it a paved road, slope wall, retaining wall or other similar applications. Based on CBR values, engineers use this data to evaluate the soil’s strength and load-bearing capacity, especially in pavement design. This information allows them to plan the thickness of the subgrade and type of pavement layers that will be needed, depending on the anticipated traffic loads.

CBR testing serves as an intermediary step in road foundation design, requiring careful consideration and calibration due to its sensitivity to moisture content. Calibration charts are then used to convert these results into CBR values, which are essential for estimating the subgrade surface modulus.

What is a good CBR value for a subgrade and why is it important?

CBR values for different types of soils vary vastly. As a rule of thumb, the higher the CBR value, the better the quality of the subgrade. However, the CBR value is dependent on the soil classification which takes into account the density, moisture, composition of the soil itself and compaction. For example, is the soil composed of clay, sand, peat and so on. For each soil type, the CBR values tend to vary. Typically, a value of 2% equates to clay, while some sands may have a CBR value of 10%. A high quality sub-base will have a value of between 80-100% (maximum). The CBR test is carried out on soils with a maximum particle size of 20mm.

Today, despite its direct limitations, CBR value is used as a common metric understood by engineers since other direct and accurate measures of shear strength are not possible. Therefore, while most pavements are designed with subgrade surface modulus measurements, the convention is to arrive at the CBR value and apply appropriate conversions to represent the final subgrade surface modulus values. As per the Highways and Transport Authority of most countries, a design of subgrade surface modulus lower than 30 MPA is considered unsuitable to support construction of a pavement. In such scenarios the use of geogrids is recommended to improve structural stability.

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