CBR Testing Services

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CBR Testing 101

CBR Testing is the proportion of power per unit area that is needed to enter a certain soil mass. This is with a standard round cylinder at 1.25 mm/min rate needed for the comparing entrance of a specific standard material. CBR Test is the short form for California Bearing Ratio Test. It is a test created by the California Highway Department in the U.S.A. for assessing the bearing limit of subgrade soil for the structure of flexible pavement.

Tests are done on regular or compacted soils, both wet and dry conditions and the outcomes acquired are contrasted with the bends of a standard test to estimate the soil quality. We do these everywhere throughout the UK. We can do it under short notice generally the reports are ready on that day.

The CBR test is an exact proportion of soil quality. It is regularly utilised on subgrade for street structures like roads and development to estimate the thicknesses of the materials to be employed. The more grounded the equipment being tried, the higher the value of the CBR, which may prompt more slender layers of content being utilised, which can lessen the development cost. The CBR test cant doesn’t show the shear strength, soil bearing limit or appropriateness of material to be used for example heaping mat or a crane base. These will require a plate bearing testing.

CBR Testing utilizing the Plate Bearing Test PBR

With its direct proportion of the material in-situ, it’s a snappy technique, and it has brief accessibility of test outcomes. The testing for CBR on the location by this strategy is regularly the favoured methodology. The test is done using a similar gear and procedure as the plate bearing test method and is especially fit when testing when:

  • The material is granular.
  • The extra parameters need estimation as well, for example, the stress under the plate.
  • There is a development plant accessible to go about as the reaction load.

InSitu CBR Testing utilizing the Standard Method

This testing is the sound strategy for deciding the CBR estimation of materials on the location. The hardware is joined to our vehicles, and it is independent, requiring no development plant to be given to go about as the reaction load. It completes fast, and the CBR values can be given right after testing. It is especially suitable for testing when:-

  • The material doesn’t have particles over 20 mm.
  • Tests are needed in little unearthed preliminary gaps.
  • There is no development plant accessible to go about as the response load or there is no space to put it.

If the surface is hard, the CBR value goes high. Typically, an estimation of 2% compares to clay, while a few grains of sand could have a CBR estimation of 10%. A suitable sub-base will have an estimate of 80% and above in the most extreme cases. A CBR test is done on soils which have a molecule’s size of 20mm.

The use and significance of the CBR test

This test is among the most utilised techniques to do assessments on the quality of a sub-level soil, the sub-base, and course material for the plan of thickness for the roadways and pavements in airfields.

The California bearing proportion test is needed for the assessment of the subgrade quality of these essential constructions. The thickness of the asphalt acquires the outcomes, and it’s segmenting layers. It is the most utilised technique for pavement plans. Hiring professionals with a good reputation is the first step if you want good results.

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Uses of CBR Test

The California Bearing Ratio Test also known as CBR test, is a basic strength test that compares the bearing capability of a material against that of a crushed stone that is well-graded. A crushed stone material of high quality should have a CBR of 100%. CBR can also be referred to as a penetration test that intends to evaluate the subgrade test of roads and pavements.

California Bearing Ratio can be defined as a ratio test of the load that is needed to force a cylindrical plunger that has a cross-sectional area of 19.355cm2 into a mass of soil at the rate of 0.25cm/min to that of the load needed to force a plunger to penetrate a sample of crushed stone. This load is referred to as the standard load.

CBR is the measurement of a material’s resistance to penetration of a normal plunger that is operated under controlled conditions of density and moisture. The California Division of Highways developed the CBR test in 1929 to classify and scrutinize soil-sub grade as well as base coarse materials that make a flexible pavement.

Uses of CBR test

  • The results of the CBR tests are used together with empirical curves to find out the pavement’s thickness and its component layers. This method is widely used to come up with designs of flexible pavements.
  • The tests are used to measure the material’s strength. But, it is not an actual representation of the resilient modulus.
  •  it is used to approximate the bearing value and mechanical ability of highway subgrades and subbases.

Carrying out of California Bearing Ratio test

CBR test is a penetration test that can be carried out in a laboratory. They can be known as small scale bearing tests whereby the penetration ratio to that of the size of the loaded area is greater compared to bearing tests.

For penetration tests, CBR test and cone penetration tests are the tests that are commonly used.

Equipment for CBR test

The apparatus needed for a laboratory CBR test includes a mould that has an internal diameter of 15cm with a base plate and a collar. For the mould to be effective, it needs to have an internal height of 17.7cm.

While preparing the specimen, a displacer disc that has a 5cm depth will be placed in the mould that enables obtaining a specimen with a depth of 12.7cm. You will also require a loading frame with a cylindrical plunger that has a diameter of 5cm. Dial gauges will also be needed to measure the specimen’s expansion after being soaked in water. Penetration measurements will also be necessary.

Procedure of the CBR test

The specimen is separated into four layers. For four days, the specimen soaks and swells in water after which the absorption values are recorded. A surcharge load is then placed on top of the specimen.

A load is then applied at the rate of 1.25mm/min. The loads are then noted down as per the values of penetration of 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 7.5, 12.0 mm. during this procedure, a plunger with a diameter of 5cm is penetrated into the component of the pavement at the rate of 1.25mm/min. The load penetration for either 2.5mm and 5mm is recorded.

To get the CBR value, this load is represented as a percentage of a standard load value at each deformation level. To obtain the standard load values, an average of several tests is done on different crushed stones. The data is then presented in a graph using empirical curves.

The California Bearing Ratio value is normally selected at the 2.5mm penetration. If the value of CBR at 5mm is higher than that at the 2.5mm, then the test needs to be carried out again. If the test results are the same, then the value for the 5mm penetration is used.

Overall, the CBR test is carried out for three test specimens after which their average value is recorded. The CBR value is obtained by dividing the load that the specimen sustained at the 2.5 or 5.0mm penetration by the load sustained by standard aggregates at the same penetration levels and multiplying by 100.

CBR is a test of strength and it cannot be used to carry out the tests for soil properties such as shearing resistance, cohesion and internal friction.

  • CBR value at the 2.5mm penetration=
  • CBR value at the 5.0mm penetration=
  • CBR of subgrade =

Calculating pavement thickness using CBR value

According to research, there is a relationship between CBR value and the wheel load and pavement thickness. Pavement thickness is expressed by:

  • t (cm) = √{P[(1.75÷CBR)-(1÷pπ]}
  •  t (cm) = √[(1.75p÷CBR)-(A÷π
  • Where p=P/A, P= wheel load (kg), t= pavement thickness (cm), CBR= CBR in percentage, P= tyre pressure (kg/cm2) and A= contact area (cm2).

The CBR value of clay is 2% while some sands tend to have a CBR value of 10%. Sub-base with high quality will have CBR value of between 80-100%. CBR tests are done on regular or compacted soils. The wet and dry conditions as well as the outcomes, are compared with standard tests to determine the quality of the soil. The CBR value tends to increase depending on the grounding level of the equipment being tried.

Advantages of a CBR test

  • The test can be carried out by personnel that have little training and experience.
  • The test is related to construction methods and has been successful for a long period of time.
  • Soil can be tested with portable equipment.
  • The CBR method adapts faster to airfield pavement design for instant use compared to other methods.
  • Tests can either be run in the fields of the laboratory for construction control or evaluation construction that is ongoing.
  • The tests are basically meant for subgrades but can also apply to other materials.
  • Tests can be done on samples to represent future water conditions.

Disadvantages of CBR test

  • The compaction methods of the laboratory and the field are very identical. However, further tests show that there are correlation results gotten from samples compacted from field materials and samples compacted in the laboratory.
  • Many procedures are arbitrary in nature. Therefore, tests should be done with high standards for the tables to be valid.

California bearing ratio (CBR) testing vs plate bearing ratio (PBR) testing

While coming up with foundation designs, you have to consider surface bearing strengths. The test performed on the ground is aimed at getting the soil properties. Plate bearing test and CBR tests are the main tests done to establish soil properties. The tests are done either in the laboratory or on the site ground.

Differences between the two tests

1. The main purpose

Plate bearing ratio aims at determining the real strength as well as the bearing capacity of the ground that is awaiting a massive construction. On the other hand, CBR testing is a penetration method that establishes the ability of the ground as well as the capability of its subgrades and base layers to hold a certain amount of load.

2. Condition of use

PBR testing involves particles of large sizes or soils with a hard texture. This is because the plates on the testing machine have a large surface area. CBR works well for areas that have small particles that are not more than 20mm.

The size of the small particle is for suitable usage in the CBR devices since they have a small cylindrical surface that is used to penetrate the ground. The obtained pressure is compared with the standard pressure that has been set for equal penetration. PBR tests are normally used for temporary structures. The results can be used while measuring the soil pressure on the ground where the pavement or the road is being constructed.

3. Aims of the test

PBR test is carried out to test the maximum weights that a surface can comfortably hold. This means the construction weights can be affected by the results of the PBR tests. In some situations, these results may limit building constructions or the size of the construction may reduce if the plans have to go ahead.

In other situations, the PBR tests may be a factor when it comes to determining the depth of the groundwork. On the other hand, CBR tests aim at testing the thickness that is required for all materials for construction purposes. The CBR tests are done to determine the strength of handling any pressures. The results of the CBR tests are crucial since they determine the materials to be used for construction. The test will influence natural properties such as size and density.

4. Performance

PBR is done at the foundation level. But it can also be done on the surface that is just below the ground. The method involves pressure application on the surface that is being tested. However, on CBR tests, penetration is the main factor in that the cylindrical plunger penetrates the ground at a controlled rate. A four-wheel-drive vehicle is used to generate production force since the test requires real weights.

PBR needs a maximum of six tests to conclude on the desired ground. CBR requires a maximum of eight tests to analyze and reach a conclusion. The differences in testing is because of the surface areas. It’s easier to conclude while carrying out a PBR test compared to a CBR test since CBR tests smaller areas in reference to surface penetration. PBR involves a larger surface area for testing.

For PBR, tests can be carried out on the ground or in a laboratory. Both can be reliable and effective. For CBR, lab and site tests can be done. CBR is recommended to be done on the ground site.

5. Testing process

PBR testing is more straightforward since the plates are subjected to many amounts of pressure. The pressure is usually exerted at a constant rate. The records are then presented on a graph to find the bearing capacity value of the ground that is being tested. CBR tests are done by measuring amounts of pressure that the plunger needs to penetrate on the different surfaces of the ground.

6. Additional strengths

On the possibility of defects, CBR is more effective compared to the PBR tests since identifying defects is easier during the procedure and analysis.

Similarities between PBR and CBR tests

  •  Both tests are done to promote stability of the pavements or construction. Both practices are investigative.
  • Both tests apply to temporal foundations for building purposes.
  • Both tests are done before any construction work starts. They are always done before commencing of construction works. The test results can affect the schedules of construction, budget and other factors.

What is subgrade?

Subgrade refers to the top surface of a cut section shaped to look exactly like a typical section on which the structure of the pavement and its shoulders will be built.

Importance of subgrade

Heavy trucks are always operating along the pavements. The loads are always relayed through the pavement and to the subgrade. The loads being applied on the surface of the pavement tend to deform the structure of the subgrade. It is of great concern on how the subgrade will react when traffic loads are applied to them.

If the subgrade is not prepared well or has low strength, then it cannot resist these stresses and that will lead to a lot of damages. If the pavement is constructed to be thick, then the load applied will distribute evenly, thereby having low stress on the subgrade. Reducing stress on the subgrade can be accomplished by building a thicker pavement.

Types of subgrade material

  • Material in place: Subgrade is always in the original ground whenever the road is cut in a section. Most soils have their density between 80 to 90 per cent of the standard proctor density when they are in their natural state. Soils that are in this condition will not have the strength to support the structure of the pavement. These soils have to be compacted to achieve the desired strength.
  • Imported material: Subgrade material that consists of material that is imported is known as ‘Borrow material’. This material can be gotten from excavations, commercial sources or quarries. Placing and compacting of borrow material should be done following the right procedure.
  • Treated material in place: Some soils, scarifying and compacting them will not yield the needed strength to support a pavement. It will be great to stabilize the subgrade with cement, lime or fly ash. This will provide a stable condition for the pavement.

Flexible pavement designs

A flexible pavement structure is made up of several layers of material. Each layer receives loads from the layer above, spread them equally then passes the loads to the next layer underneath it. This way, the stresses are minimal, the top layer has maximum stress but minimum on the upper part of the subgrade.

The layers are arranged in a descending load-bearing capacity manner with the top having the maximum load-bearing capacity material while the lowest having a low bearing capacity material.

Design procedures

For the pavements to be flexible, the design of the structure focuses mainly on determining the right thickness of the layer and its composition. The main factors of design are stresses because of the traffic load and temperature variations. Flexible pavements utilize two main structural designs:

1. Empirical design

Empirical designs focus on the results of the experiment. This design can be done with or without testing the strength of the soil. For instance, HRB soil classification system does not take into consideration test for soil strength. The classification system groups soils from A-1 to A-7 and a group index is included to differentiate soils.

2. Mechanistic-empirical design

This design focuses on the mechanics of materials that depend on input factors such as wheel load and output factors such as the response of the pavement. In the design of the pavement, the responses include stresses and deflections within the structure of the pavement. At the same time, the physical causes are the loads and properties of the materials used in the structure of the pavement.


California bearing ratio test is one of the most utilized techniques used for assessment of the quality of the sub-level soil, course material and the sub-base for roadways and pavements. The CBR test is necessary for determining the quality of the subgrade of constructions. This technique is used mostly in the construction of pavements.

CBR is expressed in form of a percentage of the real load that is causing penetrations of 2.5mm or 5.0mm to the standard loads.

CBR values will tend to go high if the surfaces are hard, clay is estimated to have a CBR value of 2% while sand particles have a CBR value estimation of 10%. A good sub-base will have an estimated CBR value of 80%. CBR tests are done mostly on soils that have a molecule size of 20mm.