CBR values and why they matter

The California Bearing Ratio (CBR) test is used to measure soil strength and stiffness as part of road construction projects. First developed by the California Division of Highways in the 1930s, this type of test remains relevant and is still used around the world today. For example, in Britain, BS 1377-9 specifies CBR tests for assessing the strength of subgrade and subbase materials in civil engineering.

This guide explores everything you need to know, from the CBR test procedure through to the calculations involved and an explanation of CBR values. There’s plenty of ground to cover. Use the links below to navigate to individual sections:

In this blog, we’ll cover:

• What is a CBR test?
• Aren't we beyond CBR testing now?
• CBR testing procedure
• CBR values explained
• How are CBR values calculated?
• Using CBR values for road design
• Why do we still use CBR values for road design?
• How is CBR normally tested on site?
• Does CD225 use CBR values for design?
• Alternative CBR testing methods
• Minimum CBR value for road subgrade
• How we can help.

 

Looking for design support on a road construction project? Speak to our team today for a free consultation.

What is the California Bearing Ratio (CBR) test?

The California Bearing Ratio test (CBR test) is a penetration test designed to evaluate the strength of subgrade soil in the design of roads, foundations and flexible pavements other paved areas and their used materials. Developed by the California State Highway Department, the CBR test has since been widely adopted internationally. 

So, what is CBR? Ultimately, it’s a measure of soil strength and stiffness crucial for road foundation design.

Aren't we beyond CBR testing now?

Why do I still need a CBR value when my road foundation design is based on modulus?

Forget soil for a moment. Instead, imagine that you’re baking a cake, and think about how sweet the cake will be. and this will depend on the amount of sugar you put in. Instead of measuring sweetness directly, you follow a recipe that lists sugar in cups. Since you don't have a standard cup, you convert cups to grams using a chart and weighing scales. This way, you end up with the right amount of sugar for the desired sweetness.

Similarly, when working with the California Bearing Ratio (CBR) test, we estimate soil strength using CBR values, even though it's challenging to measure directly. Just as you trust the recipe and conversion chart for the cake, we rely on CBR values to understand soil strength for road design. Keep reading to learn more about how this works!

CBR test procedure

Prepare the test material:

• Place the soil sample in a cylindrical mould and compact it to the specified density and moisture content.

 

Conduct penetration testing:

• Insert a 50mm cylindrical plunger into the test material.
• Apply pressure at a constant rate of 1.25mm per minute.

 

Record the load:

• Measure the load required to achieve penetrations of 2.5mm and 5mm into the material.

 

Calculate the CBR value:

• Express the recorded loads as a percentage of a standard load obtained from testing a high-quality granular material. This percentage is the CBR value.

 

Determine the appropriate CBR value:

• Typically, the CBR value at 2.5mm penetration is higher than that at 5mm, so the 2.5mm value is used. If the 5mm value is higher, repeat the test and use the 5mm value if it remains higher.

 

Evaluate subgrade stabilisation needs:

• Based on the CBR value, assess whether the subgrade needs stabilisation. If the CBR value is low, consider stabilisation methods, such as mechanical stabilisation or adding materials to working platforms to improve soil strength.

 

Conduct tests:

Perform the California Bearing Ratio test either in-situ, accounting for seasonal moisture variations, or in a lab using undisturbed or remoulded samples. Laboratory tests should simulate the subgrade moisture content and expected site disturbances.

CBR values of soil explained

The CBR value of soil is the ratio of force required to penetrate a soil mass with a standard circular piston to a specific depth at the rate of 1.25 mm/min., to that required for the corresponding penetration of a standard material, expressed as a percentage. 

The CBR value increases with the hardness of the surface. For example, the CBR value of clay soils generally has a CBR of 2%, while sands may reach 10%. High-quality sub-base materials typically range between 80% and 100%, making them ideal for providing strong support for the structures above.

CBR values provide a quantified estimate of soil strength and surface stiffness suitable for use in road foundation design. Understanding CBR values helps in determining the CBR value of soil and its appropriateness for construction purposes.

To enhance the performance of road foundations, geosynthetics such as Tensar Basetex can be integrated into the design to improve soil stability and load bearing capacity.

How are CBR values calculated?

CBR test results for soil are expressed as a percentage, e.g., CBR 2.5%. The CBR test compares the penetration resistance of the tested soil to that of a standard high quality granular material. Typically, a high CBR percentage indicates a harder surface. The result is a ratio which is then multiplied by 100, to generate a percentage value representing the CBR value of soil.

Using CBR values for road design

Road and highway construction rely heavily on accurate road foundation design methods, such as those outlined in CD225 ‘Design for New Pavement Foundations’. These methods require subgrade soil conditions to be classified by subgrade surface modulus. Since measuring this modulus directly is challenging, it is estimated based on CBR values. CD225 provides a CBR formula to convert the California Bearing Ratio to subgrade surface modulus.

So far so good, but CBR testing is a little difficult and the results vary greatly with moisture content. The design value should reflect the ‘equilibrium’ moisture content and the condition expected during the service life of the road foundation. It is going to require some calibration and estimation. Although the CBR formula to convert CBR to modulus seems precise, the input value of CBR may not be.

CBR testing is rarely carried out during a regular site investigation. Instead, a CBR value is usually obtained indirectly from another in-situ test, such as the plate load test (PLT), or the dynamic cone penetrometer (DCP) test. Calibration charts enable the conversion of penetration results to CBR value. Then from CBR we use the equation to estimate the subgrade surface modulus.

 

Plate load test in action to derive CBR values

Why do we still use CBR values for road design?

Despite most national road construction methods relying on subgrade surface modulus for foundation design, this parameter is rarely measured directly on site. Engineers typically use CBR values to estimate the modulus.

The physical properties of a soil control its strength and stiffness. The relationships between these properties are complex and not fully understood. The CBR test, first introduced in the 1930s, has stood the test of time and is generally accepted to be a practical and cost-effective measure that gives an indication of both strength and stiffness.

Highway engineers and geotechnical engineers have become familiar with CBR values as a measure of subgrade strength, to the point where many engineers can give a good initial estimate of CBR values on site by pressing a thumb or heel into the subgrade surface. This approach, while lacking precision, is widely accepted in geotechnical engineering and gives experienced engineers the confidence to estimate CBR values based on their on-site assessments.

In contrast, quoting a shear strength value in kPa implies a level of precision that may not reflect the reality. Charts are available to estimate CBR test results based upon a description of soil type, construction condition and moisture content. CD225 allows the use of Table C1 from LR1132 to estimate CBR value for foundation design where no CBR measure is available.
 

Table C1 reproduced from LR1132

Because of its longevity, the results from many other soils tests have been calibrated to equivalent CBR values and good engineers have a concept of this. California Bearing Ratio therefore provides a common scale, understood by all and adopted as a common input value for various design methods in road construction. This standardisation ensures road surfaces are built with the right strength and durability.

How is CBR normally tested on site?

CBR tests involving the penetration of a standard cylindrical plunger are rarely done during standard site investigations. Instead, alternative test methods are used and the CBR value obtained using correlation charts. As mentioned above the alternative test methods most commonly used are DCP or PLTs. Correlations between these test values and CBR test results are well established and widely accepted.

Does CD225 use CBR values for design?

The UK Design Manual for Roads and Bridges CD225 – ‘Design for new pavement foundations’ proposes a restricted design method based upon the design subgrade surface modulus. This modulus is taken as the lower value during construction or when equilibrium is reached in service.

However, CD225 recognises that direct measurement of subgrade surface modulus is impractical, and that design subgrade surface modulus will need to be estimated. CD225 suggests that the process outlined in TRL document LR1132, ‘The structural design of bituminous roads’, may be used when estimating the short and long-term subgrade surface modulus. LR1132 suggests CBR values provide the best practical means of estimating subgrade surface modulus.

CD225 then requires that the equation below is used to convert CBR value to subgrade surface modulus.

E = 17.6(CBR) 0.64

The equation is limited to CBR values between 2% and 12% and soils of maximum particle size of 20mm.

Although the use of CBR values is proposed, the use of a CBR test is not a requirement. Alternative in-situ test methods are accepted that can be correlated to CBR values. Cone penetrometer testing is specifically mentioned in LR1132.

Alternative CBR testing methods

There are also alternative in-situ tests for measuring modulus values, like the falling weight deflectometer (FWD) and light weight deflectometer (LWD) tests. The LWD equipment is handheld and can be correlated to the FWD equipment to improve accuracy. These dynamic tests are less common but provide a more direct measure of the modulus value of the subgrade, which aligns with the road foundations DMRB-CD225 standards.

Minimum CBR value for road subgrade

CD225 notes that subgrades with a design subgrade surface modulus lower than 30 MPa are unsuitable to support the construction of a pavement. That equates to a CBR value of approximately 2%. Below that, subgrade replacement or subgrade improvement is necessary. CD225 includes mechanical stabilisation using geogrids as one suitable option. If you're dealing with weak subgrades in your project, consider filling out the subgrade stabilisation form for expert support from Tensar.

How we can help

Tensar offers expert advice on the use of Tensar geogrids for mechanical stabilisation over weak and variable subgrade soils. We can calculate the surface modulus at the top of the subgrade improvement layer, ensuring it meets the required 30MPa for a stable pavement foundation. This calculation can be based on CBR test results, or the results of alternative testing methods such as FWD and LWD tests.

If you have any questions or need design support for your upcoming projects, don't hesitate to contact us. We're here to help.