What is the California Bearing Ratio?
California Bearing Ratio (CBR) BS 1377-9 is the definitive measurement for determining the thickness of roads and their load bearing strength. It is commonly used to assist with the designing and construction of roads, pavements, car parks, runways, and foundations. The ratio is measured using a standardised penetration test and was originally developed by the California State Highway Department in 1990.
GSTL maintains two types of California Bearing Ratio Test frames:
- Vehicle mounted
- JCB attachment
The California Bearing Ratio Test will be carried out by one of our trained technicians. They will arrange to go to the site using a 4-wheel drive or JBC to provide the force and transport the specialist equipment.
Once set up they will insert a cylindrical plunger into the previously prepared ground at regular intervals. This will measure the pressure needed to penetrate a standard area of soil. The test is repeated up to 10 times and should be completed within a day, for the rapid reporting of results.
What projects need a CBR Test?
Roads & Pavements
If your project involves constructing roads or pavements, you will need a California Bearing Ratio Test. The test determines the strength or the load bearing capacity of the highway subbase and subgrade soil. The test is designed to replicate the stresses placed on subgrade soils once they are subjected to vehicular traffic. It will also allow you to select the appropriate pavement thickness.
Foundations
In civil engineering you must consider the stability, strength, and characteristics of the ground you are building on.
Undertaking a California Bearing Ratio Test is the easiest way to determine if the soil is suitable for your project. It will identify if you need to incorporate additional elements into the design to increase stability of your planned construction.
Please visit our Plate Bearing Test section for testing on working platforms.
Frequently Asked Questions
How do you calculate the CBR ratio?
The CBR value is calculated by comparing the force required to penetrate a soil sample with a standard piston, to the force required to penetrate a similar piston into a standard crushed stone material.
The formula is:
CBR = ( Load required to penetrate soil / Load required to penetrate standard material ) ×100
What is a good CBR value?
A good CBR (California Bearing Ratio) value depends on the intended use of the soil and the requirements of the construction project, such as road design or pavement thickness. Generally, a higher CBR indicates better soil strength and load-bearing capacity, while a lower CBR indicate a weaker soil that may require additional design considerations.
To summarise
- Good CBR values for most construction projects (especially for road construction) typically start around 10% and above.
- For highways or heavy-load pavements, CBR values of 20% or higher are often considered ideal.
- Lower values (below 5%) usually require soil improvement methods like stabilization, grading, or adding higher-quality materials.
Thus, a good CBR value is one that meets the specific requirements of your project, but generally, a CBR value of 10% or higher is often considered acceptable for many types of construction, with values above 20% being considered strong and ideal for road foundations.
How do you test for California Bearing Ratio?
· Set up the CBR testing machine with the penetration piston and dial gauge.
· Apply load gradually and measure the load required to achieve specific penetration depths (2.5 mm and 5.0 mm).
· Calculate the CBR using the load data, comparing it to standard crushed stone values.
· Interpret the CBR values to determine the suitability of the soil for construction.
Is a CBR test the same as a plate bearing test?
No, a CBR (California Bearing Ratio) test and a Plate Bearing test are not the same, though both are used to assess the bearing capacity of soil for construction purposes. These two tests measure different properties and involve different methodologies. Here's a breakdown of the differences between the two:
· The CBR test is more focused on evaluating the load-bearing capacity of soil under penetration conditions, typically in a laboratory setting. It is primarily used in pavement design and road construction, particularly for estimating the strength of soil at various depths.
· The Plate Bearing test, on the other hand, provides a direct measure of the bearing capacity of the soil in the field. It involves applying a load to a steel plate and measuring the settlement under load to determine how the soil will behave under real-world conditions, which is useful for designing foundations and structures.
