About the California bearing ratio test 

Prior to the placement of the capping layer and/or subbase materials, pavement and paved car parking areas, the California Bearing Ratio (CBR) test need to be undertaken in order to determine the strength of the subgrade natural deposits to satisfy the structural requirements both during the construction (the short term) and the post-construction phase (the long term).

CBR test results can be gathered either in-situ or within the accredited soil laboratories using samples of the subgrade natural soils to ensure that thickness of the capping layer and/or subbase materials are correct and the underlying subgrade can carry the required loads. The CBR tests and associated sampling will be repeated across various locations along the proposed roads or car parking areas at regular intervals to determine a representative CBR value. The long term CBR or Stiffness Modulus (Ms) of the subgrade soils at equilibrium suction indices (ESI) must be undertaken in the laboratory. 

This article outlines different types of the determination of CBR values. Read on to discover how the CBR  test result is calculated and how its results can influence the outcome of road and paving design projects as well as to confirm the allowable settlements of heavy loaded cranes which is generally 10-15 of the plate diameters used during the plate load testing. 


What are CBR test results used for?

For the purpose of pavement design, a CBR design value is used as a measure of the strength of the subgrade. The purpose of the CBR is to identify the strength of subgrade soils in order to determine the thickness of the capping layer and/or subbase accurately for the proposed roads, pavement and car parking areas. 

It should be noted that the in-situ CBR of the subgrade determined at the time of a ground investigation is likely to differ from the long-term equilibrium value which can be determined once the pavement has been constructed. Whilst many clayey soils are subject to seasonal softening, loose coarse soils could be improved by compaction. 


How to calculate the results

There are a number of CBR testing techniques in the UK which are commonly adopted by the local authorities. They are as follows:

In-situ CBR tests

  • Mexe Cone Penetrometer;
  • Dynamic Cone Penetrometer (DCP) in line with TRRL LR1132;
  • Clegg Hammer;
  • Pilcon Hand Vane; and
  • Plate Load Bearing Tests in accordance with BS1377 part 9 Method 4.1.

Laboratory CBR tests

  • Particle Size Distribution (PSD) tests to identify the soils (cohesive/granular) which is essential for determining CBR by correlation/classification after BS1377;
  • Plasticity properties after BS1377 which is essential for determining the long term equilibrium suction indices (ESI) by correlation with Plasticity Index (PI); and
  • Soaked CBR Tests in line with BS1377: Part 4: Clause 7: 1990.


What do CBR test results indicate?

The CBR value is used as a measure to indicate the stiffness of the subgrade natural soils. 

For CBR readings <2% a capping layer founding stratum might be required where the subgrade stiffness is not viable for the construction of roads and pavement. Recycled granular materials can also be used to protect the subgrade layer from unexpected traffic surcharge as well as from the excess stormwater for good drainage to prevent softening. Alternatively, it may be more economical in some cases to remove the soft layer of subgrade material where organic content is substantially high. The area can then be filled with a competent fill. 


Final Thoughts

The CBR test is one of the most effective indicators of soils strength for the subgrade particles smaller than 20mm. Its results can be utilised in the planning and design of materials required for the construction of roads and pavements. 

IBEX CE offers accurate and affordable CBR testing for construction projects throughout the UK. The test can be completed on-site or in an accredited laboratory environment in accordance with the best practice. With professional and expert assistance, IBEX CE can equip you with fast and accurate CBR results for your project.