How Does CBR Testing Work?
If you’re planning a new road, car park, or foundation, understanding the strength of the ground beneath is crucial. That’s where CBR testing comes in. Short for California Bearing Ratio, this test helps determine how well the soil can support structural loads.
Let’s take a closer look at how CBR testing works, why it matters, and what the results really mean for your project.
What is the California Bearing Ratio Test?
The California Bearing Ratio test — or CBR test — is a type of penetration test used to assess the bearing capacity of subgrade soil, subbase, or subbase materials. It’s widely used in civil engineering for road construction, foundation design, and general earthworks.
The test measures how much pressure is required to push a standard circular piston into the ground. This measured pressure is then compared to the force needed to penetrate a standard crushed rock material. The result is the CBR value, expressed as a percentage. The stronger the soil, the higher the value.
The test was originally developed by the California State Highway Department to design more durable road surfaces — and it’s still widely used across the world today.
Where Is It Used?
You’ll typically find CBR tests used in:
- Road and pavement construction
- Car parks and driveways
- Airfields and runways
- Earthworks and embankments
- Structural foundations
It’s especially helpful when you’re working with variable soil types or uncertain physical properties in your test area.
CBR Test Procedure (Field Test)
The CBR test procedure can be carried out in a lab or on site. Here’s how it typically works on location:
1. Preparing the Soil Sample
The top layer of soil is removed to expose the layer being tested—usually the subgrade soil. The test is often conducted at a specific depth that reflects where the load will be applied.
2. Setting Up the Equipment
A standard piston is placed on the soil. If needed, surcharge weights are applied to simulate the load from construction layers above.
3. Applying the Load
The piston is pushed into the soil at a controlled rate. The force applied is recorded at regular intervals—typically every 0.25 mm of penetration.
4. Measuring the Results
The measured pressure is compared to a standard material, and the CBR value is calculated. This value shows how your soil stacks up against the resistance of crushed stone.
What Do the CBR Values Mean?
The CBR value tells you how strong the ground is. Here’s a simple breakdown:
2–5%: Very weak soil, often clay. Needs significant improvement or replacement.
5–10%: Weak to moderate. May require reinforcement such as geotextiles or chemical stabilisation.
10–30%: Moderate to strong soil, like sand or compacted fill. Suitable for most roads and pavements.
30%+: Very strong material, often crushed rock. Little to no treatment needed.
Higher values indicate greater penetration resistance and better support for structures.
Real-World Example (Theoretical)
Let’s say a contractor is building a lorry park. A CBR test reveals the subgrade soil has a value of just 4%. That’s too low to support heavy vehicle loads. Based on the results, the site is excavated to 600mm and backfilled with a well-compacted crushed stone subbase. A geogrid is added for extra stability. The improved base achieves a CBR value over 30%, making it suitable for the intended use.
This is a common scenario where early testing avoids future failures.
Common Questions (And Helpful Answers)
What if my CBR value is too low?
You have several options: improve the soil through compaction or stabilisation, replace it with stronger material, or increase the pavement thickness.
Can I test compacted fill instead of natural soil?
Yes. You can carry out a CBR test on compacted fill to check how well it’s performing after placement.
Is the test accurate in wet conditions?
CBR values can drop significantly when the soil is saturated. It’s important to test at the worst-case moisture content, especially in areas prone to seasonal moisture variations.
How long does a CBR test take?
The on-site test can be completed in a few hours, but lab testing for moisture content or dry density may take longer.
What to Do After Receiving Your CBR Results
Once your results come in, the next step is to interpret and act on them. Here’s what to consider:
- Low CBR value (under 10%): May require excavation, fill replacement, or stabilisation.
- Moderate CBR (10–30%): Suitable for lighter traffic or may need a moderate subbase thickness.
- High CBR (30%+): Typically ready to build on with minimal preparation.
A qualified geotechnical engineer can help you use these results to refine your design and reduce long-term risk.
A typical test report includes the CBR value, penetration readings, moisture content, dry density, and site conditions — all of which help inform construction decisions.
Lab vs Field Testing — Which Should You Use?
- Field testing is ideal when quick, on-site data is needed, especially for compacted fill or subgrade materials.
- Lab testing is useful for more controlled analysis, particularly when you need to test under specific moisture or compaction conditions.
Who Carries Out the Test?
CBR tests are normally carried out by trained geotechnical technicians or materials testing engineers using standardised equipment. Results are interpreted by professionals to guide construction planning.
Limitations of the CBR Test
The CBR test is widely used, but it’s not suitable for every situation:
- It’s not ideal for soils with particles larger than 20mm.
- It doesn’t show how soil responds to long-term or repeated loading.
- It’s less accurate in highly organic or coarse gravel soils.
- The results can vary depending on how compacted or saturated the soil is.
In these cases, you might also consider alternative methods like the plate load test or LWD tests.
Quick Definitions for Clarity
Subgrade soil: The natural soil layer beneath a road or structure.
CBR value: A percentage score showing how resistant the soil is to penetration.
Standard piston: The circular tool used to push into the soil during the test.
Surcharge weight: Extra weight added to simulate overlying materials.
Measured pressure: The actual force recorded during penetration.
Pressure required: The amount of load needed to achieve a certain penetration.
Standard material: A crushed stone baseline used for comparison.
Final Thoughts
The California bearing ratio test is simple, smart, and incredibly effective. It gives you clear data about the ground’s ability to support your structure. That data helps you design safer, longer-lasting infrastructure — without guesswork.
Understanding how CBR testing works, what the CBR value means, and how the measured pressure compares to industry standards gives you a real advantage in planning and building with confidence.
Before laying a single brick or pouring a drop of concrete, make sure your soil is up to the task — and consider CBR testing early in the design phase to avoid costly surprises later on.
