About CBR Testing

The overarching aim of the California Bearing Ratio (CBR) test is to determine the strength of the underlying subgrade natural soils. The data produced within this test has informative purposes within the design of pavements and roads. Depending on the strength of subgrade materials, the thickness of subbase layers may have to be altered in order to account for the results of the test as well anticipated surcharge traffic.

At IBEX CE, we offer rapid CBR testing to fast-track your project. To generate an assessment of the subgrade soils, either a TRL DCP test or plate bearing test can be implemented based on your requirements as well as Local Authority (LA) requirements. Within this article, you will discover the multifaceted use of the plate bearing test and how the data generated offers a comprehensive insight into the CBR design values of the subgrade soils.   

What is the Plate Load Bearing test? 

The plate load-bearing test is an investigative geotechnical procedure to determine temporary load-bearing capacity as well as CBR design values of the subgrade soils. Typically the results of this measure are utilised within the preliminary planning stages of construction projects. The data indicates the loading capacity for certain construction plans or the equipment to be used during the process. 

As a comprehensive assessment of the subgrade stiffness and bearing capacity of the underlying soils or fill. A CBR plate load-bearing test offers a substitute method for the calculation of subgrade soil’s bearing capacity. In a previous article, we discussed the differences between the CBR and plate load-bearing tests, take a look if you are interested in the procedures or applications of these two geotechnical assessment methods.

Why would you use the plate load-bearing test to obtain a CBR result?

Whilst the plate load bearings test’s intended use is for the determination of ground bearing capacity for temporary/permanent roads, there are also other beneficial applications of the test. As a comprehensive assessment of the subgrade stiffness, the results obtained from the plate load-bearing test can be used to calculate a CBR result. This is particularly useful due to the limitations of other CBR testing options. Especially, where the subgrade soils consist of granular fill. 

Under typical conditions for the construction of road and paving projects, a CBR design value needs to be determined. This is to establish the subgrade stiffness and frost susceptibility of a designated road corridor. The test is completed to establish a CBR design value to adjust the thickness of the subbase layers as well as to validate the bearing capacity of the fill where it is crucial for cranes and earthworks. For results that are less than this designated CBR value, the thickness of the construction material may need to be adjusted. The CBR can also be determined either through laboratory soil testing using the long term equilibrium suction index (ESI) method, soaked CBR testing or the in-situ plate load testing and in-situ DCP TRL method. The eligibility and results of each test will be determined by site conditions such as groundwater conditions or existing drainage plans 

Geotechnical engineers must also consider the particle size of the subgrade soils to be tested. Any subgrade particle that is larger than 20mm cannot be tested under typical CBR procedures, either within a laboratory or in-situ. This is due to the limitations in the apparatus used during the testing procedure. Plungers that are used to penetrate the ground aren’t compatible with larger particle sizes, preventing accurate readings. Alternatively, a CBR plate bearing test can be used to provide reading for particles of this size. This is possible due to the larger surface area that is covered by the bearing plate. 

CBR plate load test results

During the typical analysis of plate load-bearing test results, the raw data is plotted on a load-settlement graph. For each plotted curve, the point at which it curves down indicates the corresponding value of pressure that demonstrates a failing point. This equates to the ultimate bearing capacity of the sample of soil. The modulus of the subgrade reaction is also calculated during this procedure. This is a measurement of the reaction pressure that is sustained by a soil sample under a bearing plate. It is used to define the support conditions of footings and mat foundations. Within many construction projects, the coefficient of subgrade reaction (k) is used as the primary parameter in pavement and foundation planning and design. 

The data generated from plate bearing tests also allows for a fairly accurate calculation of CBR, derived from the subgrade reaction. This has advantages for the application of road and paving design as testing can occur over a greater surface area. It allows for the testing of cohesive and granular subgrade soil types. 

Each test will be carried out in accordance with BS1377 part 9 Method 4.1. Testing includes six increments of loading, at 25%, 50%, 75%, 100%, 125% and 150% of working load (100kN/m2), together with monitoring on release to 0%. The settlement of a 600mm diameter plate test shall be no greater than 5mm at a pressure of 150kN/m2 (150%).

The following relationship is used to determine the ground stiffness.

  • Settlement = ρ;
  • Plate Diameter = B;
  • Ground Stiffness = E;
  • Plate Pressure = q;
  • Poisson’s Ratio = v;
  • So that ρ = (((q x B x (1 – v2))/ E) x (π/ 4).

In situ CBR tests shall be carried along the route of the proposed access roads at the finished level, at a frequency of 1 test every 30m along the centreline of the proposed roads or car parking areas unless otherwise agreed by the designer. Tests need to be carried out in accordance with BS 1377 Part 9 Method 4.3. 

If CBR values less than 2% are recorded, then the material shall be investigated further by trial holes excavated by the appointed contractor at the direction of the designer. If necessary, the materials shall be subjected to further compaction and/or removal and replacement. 


The standard methods of determining a CBR design value (using estimated equilibrium suction indices and laboratory methods) don’t allow for the assessment of coarse subgrade materials. Fortunately, the plate load-bearing test offers an equivalent CBR value of the subgrade stiffness of the representative natural deposits. Due to the wider plates and increased reaction loads that are utilised within the plate load bearing test, a comprehensive account of the soil and its bearing capacity can be analysed. 

If you require professional geotechnical investigation to determine the subgrade ground strength for your road or paving projects, contact IBEX CE today for guaranteed expertise and cost-effective quotations. We offer BS 1377 regulated in-situ and laboratory CBR testing for fast and accurate results.