The California Bearing Ratio test, almost always shortened to the CBR test, compares the bearing capacity of a compacted soil with that of a standard crushed stone. The California Division of Highways developed the method around 1930, and state agencies, federal bodies, and national standards organisations around the world soon adopted it. The test measures the resistance of a material to penetration by a standard plunger under controlled density and moisture conditions, and the result is used to evaluate the subgrade soil for roads and pavements. Every pavement structure rests on soil that must carry the traffic load, so the bearing strength of that soil has to be known before the layers above it are designed. Deep foundations are verified with their own field checks, such as the load test on piles, but subgrade evaluation depends on the bearing ratio.
This article explains what the CBR test measures, the apparatus it needs, the step-by-step procedure, typical values for different soils, and how the results are interpreted and reported for pavement design.
What the CBR Test Measures
The California bearing ratio is the proportion, expressed as a percentage, of the force per unit area required to penetrate a soil mass with a standard circular plunger of 50 mm diameter, driven at 1.25 mm per minute, to the force required for the same penetration in a standard crushed stone. The standard stone is assigned a CBR of 100 percent, and every soil is rated against it.
How the Bearing Ratio Is Calculated
The ratio is determined for penetrations of 2.5 mm and 5 mm. The test load at each penetration is divided by the standard load for the same penetration, and the result is multiplied by 100. If the ratio at 5 mm is consistently higher than the ratio at 2.5 mm, the 5 mm value is reported.
Standard Load Values
The reference values come from a well-graded crushed stone. The standard load at 2.5 mm penetration is about 13.7 kN, and the standard load at 5 mm penetration is about 20.5 kN. A soil that needs 3.4 kN to reach 2.5 mm of penetration therefore has a CBR of roughly 3.4 divided by 13.7, or about 25 percent.
Materials the Test Covers
The test is intended for cohesive materials with the largest particle size below 19 mm, about 0.75 inches. Specimens can be compacted in the laboratory at a chosen density and moisture content, or trimmed from undisturbed samples, and the test can be run on soaked or unsoaked specimens depending on the design condition being simulated.
The CBR appears in highway engineering and soil mechanics coursework, and together with structural analysis, surveying, and construction management, these subjects make up the core civil engineering subjects that a pavement engineer draws on every day.
Apparatus Required for the CBR Test
Loading Machine and Plunger
The loading machine must have a minimum capacity of 5000 kg and a movable head or base so the plunger can penetrate the specimen at a steady 1.25 mm per minute. The plunger is a standard circular cylinder of 50 mm diameter, long enough to penetrate the full depth of the mould.
Mould, Base Plate, and Accessories
The cylindrical mould has an inside diameter of 150 mm and a height of 175 mm, with a separable perforated base plate 10 mm thick and 235 mm in diameter, giving a net capacity of about 2250 ml. A detachable extension collar 60 mm high fits on top for compaction, and a spacer disc 148 mm in diameter and 47.7 mm high sits on the base plate during compaction so the specimen ends up at the right height. Surcharge is applied with slotted weights of 2.5 kg each and one annular metal weight 147 mm in diameter with a 53 mm centre hole.
Compaction Rammer
Compaction uses a rammer of 4.89 kg dropped from a height of 450 mm, which reproduces the standard compactive effort. The number of layers and blows follows the test specification, typically five layers with 55 blows each for the standard effort.
A complete walkthrough of the CBR test procedure, including specimen preparation, soaking, and penetration readings, is available in this dailycivil article on the California Bearing Ratio test.
CBR Test Procedure Step by Step
The procedure runs from specimen preparation to the final load readings, and each step is specified so results from different laboratories stay comparable.
- Lubricate the inside of the mould so the soil can be removed without friction after the test.
- Weigh the mould with its base plate, and place the spacer disc over the base plate.
- Compact the soil in layers with the 4.89 kg rammer dropped from 450 mm, using the specified number of blows per layer.
- Remove the collar and spacer disc, trim the excess soil flush with the top, and weigh the mould with the compacted specimen.
- Place the perforated plate and surcharge weights, then soak the specimen, normally for four days, measuring any swelling with a dial gauge.
- Transfer the mould to the loading machine, seat the plunger with a small initial load, and penetrate at 1.25 mm per minute.
- Record the load at penetration intervals of 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10.0, and 12.5 mm.
- Plot load against penetration, correct the curve if the start is irregular, and read the loads at 2.5 mm and 5 mm penetration.
Sample Preparation Notes
The soil should be compacted at its optimum moisture content, or at the field moisture content specified for the design, and material retained on the 19 mm sieve is removed before compaction. The compactive effort should match the density expected in the field, because CBR rises steeply as density increases.
Calculation During the Experiment
For each penetration of interest, the CBR is the test load divided by the standard load at that penetration, multiplied by 100. The same load-versus-deformation logic appears in bituminous mix testing, where the Marshall stability test measures the resistance of compacted asphalt specimens to failure and is used to tune the binder content of the mix.
Typical CBR Values and Interpretation
CBR values span a wide range depending on the soil type and its density and moisture state. The table below lists typical ranges for common subgrade and pavement materials; a value below 5 indicates a poor subgrade that needs a thick granular layer, while a value above 80 suits a base course.
| Material | Typical CBR range | Usual pavement use |
|---|---|---|
| Soft clay (poor subgrade) | 2-5 | Needs thick granular overlay |
| Silty clay | 5-8 | Subgrade, moderate overlay |
| Sandy clay and silt | 8-15 | Subgrade, thinner overlay |
| Well-graded sand | 15-30 | Select fill or subbase |
| Gravelly sand | 30-60 | Subbase or base |
| Crushed stone | 80-100 | Base course |
| Standard crushed stone (reference) | 100 | Reference material |
What the Value Means for Pavement Design
Pavement design charts relate the subgrade CBR to the thickness of the layers above it. A subgrade with a CBR of 3 percent may need 300 mm or more of granular material, while a subgrade at 30 percent needs only a thin layer. The relationship is why the test is run before the design is finalised.
Soaked Versus Unsoaked Values
Specimens are often soaked for four days before penetration to simulate the worst seasonal condition, and the swelling measured during soaking is reported alongside the CBR. Unsoaked tests are run when the pavement will sit above the water table year-round. The choice of condition matters as much as the value itself.
Method selection follows the same logic used for other field checks. Pipeline contractors compare the air test and the water test for gravity pipeline leakage to pick the method that matches the pipe material and site constraints, and pavement engineers likewise choose the soak regime and compaction effort that match the field condition they are designing for.
Applications of the CBR Test
Subgrade and Pavement Design
The primary use of the CBR test is the evaluation of subgrade soil and base materials for roads, airport pavements, and parking areas. It is also used to compare borrow materials during design, to accept or reject compacted layers during construction, and to check the effect of moisture and density changes on a given soil.
Related Strength and Verification Tests
CBR is a penetration test, and it sits alongside other geotechnical measurements that each answer a different question. When a structure is supported on piles rather than spread footings, the bearing capacity of the soil at the surface is less relevant than the capacity of the pile itself, and dynamic pile tests are sometimes proposed as a faster substitute for static loading. The question of whether the PDA test can replace a pile load test for driven piles is discussed in this engineeringcivil article, and the same caution about matching the test to the question applies to CBR.
Limits of the CBR Test
The CBR test does not measure every property a pavement needs. Expansive soils, frost-susceptible materials, and very soft clays need additional tests, and the CBR value alone does not predict settlement. It is a strength index for design purposes, not a complete soil characterisation.
Standards, Cost, and Reporting
Specifications and Standards
The laboratory method for finding the CBR of compacted soil specimens in a wet state is set out in IS 2720 Part 16 in India, ASTM D1883 in the United States, and AASHTO T 193 for highway agencies. The standards agree on the plunger size, penetration rate, and soak procedure, which keeps results comparable across projects and countries.
Report Format and Interpretation
A proper report states the source and description of the soil, the compactive effort, the moisture content and dry density, the soaking period and swelling, and the CBR at 2.5 mm and 5 mm penetration with the reported value clearly identified. The load-penetration curve should be attached so the reader can see how the value was derived. A complete report should include:
- Source and description of the soil tested.
- Compactive effort, moisture content, and dry density.
- Soaking period and measured swelling.
- CBR at 2.5 mm and 5 mm penetration, with the reported value identified.
- The load-penetration curve.
For engineers who specify and review these tests regularly, the technical notes on the CBR test of soil summarise the points that most often trip up pavement design engineers.
The acceptance criteria, curve corrections, and reporting requirements are collected in the CBR test procedure and standards as per IS 2720 Part 16, which also covers the corrections applied when the load-penetration curve has an irregular start. When every laboratory follows the same standard from sampling to reporting, CBR results stay comparable across projects and years, and pavement thickness decisions rest on numbers that mean the same thing everywhere.