What is the difference between the CBR and Plate Bearing test?
In order to determine the strength of the subgrade natural soils and engineered fill (e.g. piling mats), two tests are commonly used in the UK. Namely, the California Bearing Ratio (CBR) test and the Plate Load Bearing Test. Before commencing your construction project, it is vital to select the most suitable test for your requirements. However, due to the similar nature of their investigative procedures, it can be difficult to decide which is best for your intentions. Whilst they are used in similar contexts, their purposes and applications can vary widely.
Within cohesive or clayey soils, we can also derive the Equilibrium Suction Indices CBRs (ESI CBRs) for the long-term characterisation of sub-grade condition and performance based on the guidance given in the Transport and Road Research Laboratory Report (LR1132) entitled ‘The Structural Design of Bituminous Roads’ (1984) which is commonly adopted by the local authorities in the UK.
Within this article, we aim to present a clear understanding of the two tests so that you can make an informed decision for your next project. IBEX CE is a team of chartered and experienced geotechnical engineers. Let us help you choose the right test before offering a competitively priced quotation for our professional services.
What are their purposes?
Both tests offer semi-empirical measurements of the composition and strength of the subgrade layers. The tests should also both be completed before the commencement of any construction projects due to the influence of their results. Despite this, they offer varying information that is used with different intentions. Find out more below.
The California Bearing Ratio test offers an assessment of the subgrade natural soil’s strengths by analysing its sub-grade materials.
This information is most frequently utilised in the construction of roads and pavements and occasionally foundations for light structures.
Based on the determined CBR value, you can determine the thickness of subbase material that is required to offset the hardness of the subgrade natural soil and the predicted traffic for that area.
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Plate Load Bearing Test
The plate load-bearing test is a comprehensive evaluation of the bearing capacity of a larger proportion of the ground.
It is most commonly used to determine whether the subgrade natural soil can take the load of certain construction plans or equipment such as piling mats and crane mats.
The plate load-bearing test offers a much more detailed account of the actual strength of the subgrade natural soil. It can also measure a wider surface area, offering increased utility for the assessment of larger construction projects.
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The Test Procedures
A CBR test can be completed in-situ or within a laboratory. Whilst the results and procedures mildly differ between the two variations, they provide information for the same purposes. Across the two versions of the test, a common procedure is followed. A sample section of the subgrade is selected and sampled. In the laboratory, moisture is added to the sample and then dried and compacted into a mould. In both tests, a cylindrical plunger is then used to apply pressure to the ground. The pressure required to penetrate the subgrade natural soil is recorded. Whether this information has been recorded on-site or in the lab, the results are mathematically divided by the pressure required to penetrate a standardised rock material using the same procedure.
In-situ CBR tests are repeated at least 3 times for each site in order to determine an accurate reading along the proposed access roads and car parking areas. Results are given in a percentage and indicate the hardness of the material.
Plate Bearing procedure
Unlike CBR tests which can be completed in-situ or within a laboratory, plate bearing tests are exclusively conducted on-site. In preparation for the load-bearing test, a flat section of ground should be used. Either a light layer of the topsoil can be removed or bedding sand can be applied to ensure a complete connection between the plate and the ground. After the equipment and measuring dials are set up, a reaction load is connected to the apparatus. Typically, the minimum kentledge for a reaction load is around 15 tonnes. This weight can be provided by construction vehicles such as excavators. A measurement of the plate settlement should be taken before any loads are applied. This will provide a standard measurement to compare the following results to.
After the successful establishment of the testing apparatus, the first increment of the required load can be applied to the plate. This load is created by increasing the pressure of the hydraulic jack to the reaction load. This will increase the weight on the plate, pushing it into the subgrade natural soil. After settling the plate into each load increment, the appropriate measurements are recorded. Typically, 5 increments of load application will occur for each test. The maximum load is specified by the client’s requirements.
The subsequent results are inserted into a graph, plotting pressure against deformation. This establishes the load-bearing capacity and settlement characteristics of the subgrade natural soils. These results are used to inform decisions regarding the planning and budgeting of the following construction task.
CBR test results within the Plate Bearing Test
One of the additional applications of the plate bearing test is that a fairly accurate CBR reading can be obtained from the results. This is fortunate due to the limitations of the traditional plunger method of CBR testing. The typical method of testing sub-grade materials is only applicable for particles less than 20mm in diameter. With its larger surface area, the plate bearing method allows for the testing of more granular material.
A CBR value can be obtained from the plate bearing test by using a mathematically derived formula. During the typical plate load procedure, readings are taken for the 5 increments of displacement between 0mm and 1.5mm. These readings are plotted on a graph against their load. The equivalent CBR can then be derived from the load required to reach 1.25mm displacement by calculating the modulus of subgrade reaction.
For more information on how to determine CBR results from the plate load test, see our in-depth article.
The California Bearing Ratio and Plate Load Bearing tests are both geotechnical investigative procedures that analyse the strength of the subgrade natural deposits as well as man-made temporary fills such as piling mats. Whilst the CBR test aims to inform the thickness of materials for the proposed roads and pavements, the plate bearing tests could also provide confirmation of the bearing capacity of newly installed piling mats or engineered fill for high loads (crane mats).
Within this article, we have aimed to provide information regarding the uses and aims of each method of testing. If you are in need of CBR testing, plate load bearing testing or expert guidance, get in touch today. We can help you to decide the best test for your needs. Contact IBEX CE today for professional geotechnical investigations and advice.