When and How to Use a California Bearing Ratio CBR Test for Site Investigations
Understanding the ground beneath your feet is essential for any civil engineering project. Before you commit to pavement construction, road construction, or foundation design, you must know how strong the subgrade soil is. Among the field tests available, the California Bearing Ratio CBR test remains one of the most trusted methods for assessing soil strength, bearing capacity, and how a site will perform under load. Professionals across the UK look to experts like Site Testing Services (National) LTD for their experienced team, efficient test procedure, and comprehensive approach to geotechnical testing.
This guide explains exactly when to use a CBR test during site investigations, how the test procedure works, and why this simple penetration test continues to be a standard reference across the construction industry.
What Is a California Bearing Ratio CBR Test?
The California Bearing Ratio test is a penetration test that originated with the California State Highway Department. The original idea was to figure out just how easy or hard it was to push a plunger into the ground and measure the penetration resistance. Today, you can see why the CBR test has become so widely used for a whole range of civil engineering jobs, pavement foundations, road subgrades, you name it.
The test itself is pretty straightforward. You take a standard plunger, push it into a soil sample at a certain depth and speed, and see how much force it takes. Then you compare that to a standard crushed rock material to come up with a bearing ratio CBR value.
A CBR value of 10%, for example, means the soil is about one-tenth as strong as the reference material. It’s this strength measurement that’s really key; it’s what engineers use to figure out how to design pavements, how safe they are, and how well they’ll hold up over time.
When Should You Use a CBR Test in Site Investigations?
A CBR California Bearing Ratio test is not needed for every project, but there are clear situations where it provides essential design data.
Pavement Design for Roads, Car Parks, and New Pavement Foundations
The most common use for the CBR test is pavement design. Engineers take the CBR value of the subgrade soil to determine layer thicknesses for:
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Sub-base
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Base course
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Asphalt layers
Lower bearing ratio values indicate weaker soil, meaning the pavement must be thicker to spread the applied load. Higher values reduce material requirements and cost.
Temporary Works and Heavy Load Platforms
If you’re building something like a haul road, a pile mat, a crane pad, or any other temporary works that need to support a lot of weight, a CBR test procedure is a no-brainer. It helps you make sure the ground can handle the weight without settling too much, which is a major safety issue.
Early Feasibility and Pre-Purchase Assessments
A field test early in the planning stage can highlight unsuitable soil types, weak subgrade strength, or areas requiring improvement. This data helps you budget realistically and avoid unexpected costs later.
Quality Control During Construction
Once you’ve laid down your sub-base materials and compacted them, a CBR test can check and see that they’ve achieved the right dry density, strength, and other physical properties before you move forward with construction.
There are other tests you might use in combination with CBR, like the Dynamic Cone Penetrometer (DCP test), the lightweight deflectometer (LWD test), and the plate bearing test.
How Is a California Bearing Ratio Test Performed?
CBR testing can be completed either on-site (in situ) or in a laboratory. Each approach has specific benefits.
In-Situ CBR Testing
On-site testing provides quick, practical information for live construction projects. A testing team positions the equipment directly on the ground or sub-base layer.
Typical in-situ test method:
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Prepare the test area
A small, level area is cleared and compacted. -
Apply surcharge weights
Circular weights simulate the pressure of pavement layers above the point being tested. -
Apply reaction load
A vehicle or frame provides resistance for the test equipment. -
Penetration of the soil
The standard piston pushes into the soil at a controlled rate. -
Measure load values and penetration
Gauges record the measured pressure, corresponding penetration, and load required to reach equal penetration depths (commonly 2.5 mm and 5 mm). -
Calculate the CBR value
Engineers compare the data to the standard material to produce provisional results and a certified report.
In some cases, multiple tests are carried out across a site for a more accurate assessment of soil types, moisture conditions, and seasonal moisture variations.
Laboratory CBR Testing
Laboratory testing offers a deeper analysis of:
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Moisture conditions
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Dry density
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Maximum particle size
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Physical properties
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Compaction levels
Soil is collected in bulk and tested under controlled conditions. The most important method is the soaked CBR test, where the soil is submerged for several days to simulate worst-case drainage or saturation.
These values are often more conservative and are used for long-term foundation design, pavement construction, and road subgrade calculations.
Related Tests Used in Site Investigations
While the CBR test is widely used, engineers often combine it with other testing methods:
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Incremental Plate Load Test
Uses a rigid plate to apply incremental loads and assess subgrade reaction and settlement behaviour. -
Plate Load Test / Plate Bearing Test / Plate Bearing
Ideal for coarse soils and materials with large particle sizes where CBR is not suitable. -
Dynamic Cone Penetrometer (DCP Test)
Uses a sliding hammer and metal cone to estimate strength and derive equivalent CBR values. -
Lightweight Deflectometer (LWD Tests)
Measures deflection to assess pavement foundations quickly.
These tests provide a fuller picture of the subgrade soil performance for civil engineering purposes.
Building on a Foundation of Certainty
The California Bearing Ratio Test has been a mainstay of geotechnical investigations for ages. It’s simple to use, the results are clear and easy to understand, and it’s just so useful for all sorts of civil engineering projects, road construction, pavement design, and the works.
The more you know about how it works and when to use it, the better. Because the last thing you want is to be second-guessing your test results down the line. You want to be able to design something safe and reliable, and that’s where this test comes in.