A CBR test of soil, which stands for California Bearing Ratio test, is a toolkit to assess soil’s lead-bearing potential to determine whether the land is suitable for construction projects.
Although it may not seem like it, soil is much more than some meaningless dirt.
Soil provides the foundation for structures and roads to be built, and without analyzing its strength and stability, a flurry of potential problems could plague construction endeavors.
CBR tests act as a tool for engineers to do a thorough investigation of the land and make well-informed decisions to ensure new structures will pass the test of time.
In this article, you’ll learn about the nitty-gritty details of a CBR test of soil, from what it can detect to its limitations to much more.
So, let’s dive into everything you need to know about one of the most important tools in civil engineering.
1. What is a CBR Test of Soil?
A California Bearing Ratio (CBR) test of soil refers to a specific type of assessment to identify soil’s strength.
The CBR test was developed by the California Division of Highways at the end of the 1920s when there was a demand for better-quality roads.
By using a penetrating mechanism to test soil’s resistance levels, the engineers could confidently determine whether or not the soil could carry the load of the roads and the weight of vehicles.
The CBR test of soil has become an industry standard, and it’s used by the Federal Highway Administration (FHWA), Federal Aviation Administration (FAA), state Departments of Transportation (DOT), and countries across the world.
Although the test was designed about 100 years ago, its process has remained about the same.
So, let’s look at how the CBR test is performed and discuss its variation.
2. How is the CBR Test Performed?
The process of the CBR test of soil is uncomplicated.
Before the test can begin, engineers need to collect soil samples from various parts of the construction site.
The size of the project, testing objectives, and desired level of accuracy will determine how many samples are needed in the CBR test.
Here are the three standard stages of a CBR test of soil:
Sample Preparation
The sample preparation stage of the CBR test includes air-drying the soil to ensure excess moisture doesn’t influence results.
Then, the soil has to pass through a sieve to capture the finer particles and remove any coarse materials.
The standard sieve size in a CBR test is 20mm.
Lastly, the soil goes through a compaction process.
Various layers are compacted with a rammer, and each layer receives a specific number of blows to achieve the desired density.
Soaking
In some cases, engineers require a soaked CBR test to determine how soil’s durability reacts to certain moisture saturation levels.
Once the soil is compacted, the water soaks for about four days (96 hours) to ensure it is fully saturated, as it would be in a real-world scenario.
Engineers will measure the height and weight of the sample before and after the saturation process to determine the swell percentage.
Load Testing
During the load testing stage, the CBR value of the soil sample is determined.
The soaked or unsoaked soil sample is inserted under the compressing machine’s piston.
The cylindrical plunger then applies a constant penetration rate.
The apparatus has a dial gauge and proving ring to identify the amount of load applied to reach the level of penetration.
The ratio is then multiplied by 100 to find the CBR value in percentage form.
3. What Does a High CBR Value Indicate?
A higher CBR value indicates that the soil has a strong, natural load-bearing capacity–hard surfaces usually fall within this category.
Civil engineers are looking for high CBR values to ensure the land can handle the weight of the construction and its applications.
Roads that will see high levels of traffic or heavy construction projects will require higher CBR levels.
Here are some of the characteristics of soil with a high CBR value:
Ideal compaction and density.
Significant load-bearing capability.
Better shear resilience–resistance to sliding.
Less sensitive to moisture level variations.
Less susceptible to sinking or shifting.
4. What Does a Low CBR Value Indicate?
A lower CBR value indicates that the soil has a weak load-bearing capacity–softer surfaces usually fall within this category.
When levels are low enough, engineers may choose to change the location of the construction site if there are doubts about the soil.
A low CBR value doesn’t always mean that the project can’t take place on the land.
However, the land may require costly, time-consuming alterations, and it may only be suitable for roads that will have minimal traffic, such as in a residential area.
Here are the characteristics of soil with a low CBR value:
Weak compaction and density.
Limited load-bearing capability.
Poor shear resilience–resistance to sliding.
More sensitive to moisture level variations.
More susceptible to sinking or shifting.
5. What are the Factors Affecting CBR Test Results?
During a CBR test of soil, there are several factors that determine the results.
Knowing the determinants of the CBR value is crucial to verify the validity of the test.
Here are the factors affecting CBR test results:
Type of soil: What type of soil being tested greatly impacts the CBR values.
For example, cohesive soils that have more clay-like properties usually have lower CBR values compared to granular soils, like sands, which usually have higher CBR values.
Size and shape of soil particles: The size and shape of particles play a big role in determining the CBR value.
Well-graded soil (soil that has an equal mix of particle sizes) typically has a higher CBR value than poor-graded soil (soil without an equal mix of particle sizes).
Having a wide range of particle sizes allows for a better distribution of the load.
Moisture saturation levels: The amount of moisture in the soil greatly influences the CBR levels.
It’s especially important for projects taking place in wet environments to compare CBR results at maximum saturation levels.
Typically, the higher the moisture levels, the lower the CBR value.
Density levels: Higher density levels are associated with higher CBR values.
When particles are tightly packed and begin to interlock, the soil is able to resist heavier loads.
6. What is the Difference Between Soaked and Unsoaked CBR Tests?
There are two kinds of CBR tests: soaked and unsoaked tests.
But what’s the difference between the two?
Well, a soaked test involves moisture, and an unsoaked test does not.
Let’s go a bit deeper into their differences and talk about why only one or both tests would be used for a project.
Soaked CBR Tests
The purpose of a soaked CBR test is to see how soil’s load-bearing capacity changes at the maximum saturation point.
These tests are crucial for projects located in areas with significant levels of moisture.
Adding and testing various levels of moisture will replicate real-world scenarios, such as moisture increase caused by groundwater and rainfall.
Engineers cannot confidently move forward building structures and roads that will be exposed to water without conducting soaked CBR tests.
Unsoaked CBR Tests
The purpose of an unsoaked CBR test is to see the soil’s load-bearing capacity in dry conditions.
These tests should be conducted for projects located in dry climates or climates that experience seasonal changes.
In many cases, engineers working on a project in areas with significant levels of moisture will still want to perform unsoaked CBR tests to get a comprehensive understanding of the soil’s behavior.
7. How is Moisture Content Controlled in the CBR Test?
A soaked CBR test of soil is important if a construction project takes place in areas with varying moisture levels.
Engineers need to ensure that the soil will maintain an adequate load-bearing capacity no matter the weather conditions in the present and future.
When soil has higher levels of moisture, its compaction and strength suffer.
So, here is how moisture content is controlled in a CBR test of soil:
Initial Moisture Measurements
When the soil samples are first collected, the moisture levels are taken and typically used as a benchmark for moisture control levels during the testing.
Additionally, the optimum moisture content (OMC) will be determined via a Standard Proctor Compaction test.
These tests compact soil samples at various moisture levels and determine how the soil’s load-bearing abilities change.
Moisture Addition or Reduction
Depending on the original moisture levels of soil samples, water will either be gradually added or the soil will be air-dried.
Engineers want to test the soil at OMC, so they will carefully add or remove water to reach those levels for accurate testing.
Mixing
When water is added to the soil, it goes through a thorough mixing process.
The moisture has to be evenly distributed to ensure accurate testing.
Mixing is typically performed with a machine, but it can also be done by hand.
Weight and Height Measurements
The weight of the soil will be measured before and after the soaking process to get a clear idea of the moisture levels during the test.
Additionally, the soil’s height will be measured to check the swell percentage.
8. What Are the Limitations of the CBR Test of Soil?
CBR tests aren’t perfect.
No matter how the test is performed, there’s no way to take every potential factor into consideration that could impact the behavior of the soil.
So, engineers will need to perform additional assessments to get the most accurate predictions possible.
Here are the main limitations of the CBR test of soil:
Small Sample Sizes
CBR tests only use a relatively small sample size.
That leaves room for potential errors as the results may not depict an accurate portrayal of the entire construction site.
In many cases, the soil in a small piece of land can be vastly different in terms of structure and composition.
To overcome the obstacle of small sample sizes, project managers may have to conduct a large number of tests, which can be time-consuming and expensive.
Sensitivity to Moisture Variations
Moisture levels play a significant role in the results of a CBR test of soil.
Because it can be difficult to imitate the vast range of moisture levels in the environment of the project, the results of the test may not accurately reflect reality.
When the tests are performed, project managers need to consider potential factors, such as fluctuations in groundwater and rainfall levels, and test each circumstance as thoroughly as possible.
Compaction Control
The compaction of soil in the laboratory is unlikely to be an exact match to the soil compaction in the field.
Construction activities have the ability to alter the soil compaction.
Activities such as excavation, using heavy machinery, trenching, and soil mixing all have the ability to weaken the compaction (in some cases, construction activities could strengthen compaction).
So, the tests performed in controlled laboratory conditions may not produce results that are completely accurate.
Limited Shear Strength Data
CBR tests are excellent for uncovering penetration insights; however, they are less reliable for identifying soil’s shear strength.
Shear strength refers to soil’s defense against deformation caused by shear force.
Examples of shear force impacting soil are earthquakes, excess water, steep hillslopes, and heavy structures.
Understanding soil’s shear strength is crucial, but it’s not directly measured in a CBR test.
Lack of Dynamic Effects Testing
For construction projects like new roads, which CBR tests were originally designed for, there is an endless number of real-world conditions that will cause a fluctuation in the stress that the soil experiences.
CBR tests aren’t able to replicate these dynamic factors, weakening the validity of the results.
Engineers have to perform additional assessments outside of the CBR test to ensure dynamic effects are taken into account, allowing them to make more comprehensive predictions of the soil’s behavior.
9. What is the Difference Between CBR and R-Value in Soil Mechanics?
CBR and R-Value tests are both used to understand soil mechanics for construction sites.
Here are the key characteristics of CBR tests:
Measures how well soil handles various load sizes and determines whether the soil is suitable for heavy construction projects.
Uses penetration to measure soil’s resistance to loads.
Uses static loading simulations (single load sizes).
Helps determine ideal pavement designs.
Here are the key characteristics of R-Value tests:
Measures how well soil handles various loads for an extended period of time and determines soil’s ability to reshape itself after being compressed.
Tests for soil’s deformation response.
Uses dynamic loading simulations that mimic real-world scenarios (multiple load sizes).
Helps engineers predict the performance of pavement under various circumstances.
Both tests are important tools that are used to calculate different aspects of soil behavior.
CBR tests are more focused on soil’s load-bearing capacity, while R-Value tests are more focused on resilience in dynamic conditions.
Final Thoughts
CBR tests of soil have become an integral part of the construction of roads and structures.
Being aware of the soil’s load-bearing capacity is essential to ensure the success of large-scale projects.
By using the CBR test, engineers can more accurately design foundations, choose materials, and predict the feasibility of the endeavor.
Of course, these tests alone aren’t enough to get a comprehensive understanding of soil behavior. However, they have become an essential tool that helps lead to smooth, efficient projects.
As of now, the CBR test of soil will continue to play a vital role in various infrastructure projects, and new technologies will create the possibility for more refined testing and more accurate results.
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Disclaimer: we are not lawyers, accountants or financial advisors and the information in this article is for informational purposes only. This article is based on our own research and experience and we do our best to keep it accurate and up-to-date, but it may contain errors. Please be sure to consult a legal or financial professional before making any investment decisions.
