CBR Testing and Plate Load Testing Service in UK

What is CBR Testing?

CBR Testing also known as The California bearing ratio is a test to ascertain the mechanical strength of a piece of natural ground.

The strength of ground or soil is very important. Source

Usually, the CBR Testing is performed before a new carriageway is laid down on that stretch of ground. CBR tests test the pressure that is needed to break through this piece of ground, and use this as an indicator of how strong the ground is. CBR tests can also be used to test the subgrades and the basecourses of ground beneath existing carriageways. The results of a CBR test are expressed as a ‘CBR rating’: the higher a stretch of soil’s CBR rating is, the harder that soil will be. The ‘CBR’ in ‘CBR Testing’ stands for ‘California Bearing Ratio’ because the test was developed in the early 20th century by California’s Transport Department.

cbr testing
CBR testing on road

How CBR Tests are performed

The test is performed using a soil plunger, which is moved downwards through the soil at a constant rate of 1mm per minute. The plunger used for CBR tests has a standardized area, enabling different samples of soil to be compared with ease. First, the tester will measure the pressure that is needed to break through the soil being tested. Then, they will compare this to the pressure that is required to break through soil or crushed rock of a standard hardness. The standard hardness used here is that of California limestone, which has a CBR value of exactly 100. The two pressure values are then compared and expressed as a ratio. The pressure value of the soil being tested is divided by the pressure value associated with standard soil, and the resulting number is then multiplied by 100 to give the final CBR value.

CBR value for subgrade

CBR tests on subgrades ought to be done before new pavements and carriageways are constructed in order to establish whether or not the subgrade is strong enough to support construction work. In addition, CBR tests ought to be performed on the subgrade at regular intervals during the life span of the carriageway in order to make sure that the carriageway is not causing any distress to the subgrade, and the subgrade continues to be able to support it. In general, soil with a CBR value of 80 or more is suitable for building carriageways on. Softer, more unreliable soils will tend to have very low CBR values: the average CBR value of wet sand is 20, for example. It is very rare to find soil that has a CBR value higher than that of California limestone, which is precisely why this material is used as the measure of comparison.

Plate bearing testing

The Plate Bearing Test is another type of test that is performed on soils to examine whether or not they are suitable for building on. This test is usually performed at foundation level. In contrast to the standardised plungers that are used for CBR tests, there is no single standard plate width used for these tests, though the maximum diameter is usually 7.2 cm. This test is performed using a loading plate and loads that increase incrementally. The loading plate is placed in a suitable position on the soil to be tested, and then it is connected up to a reaction load. Usually, a hydraulic jack is deployed to load the plate up with increasing weights. With each load increment, the settlement of the plate is measured. This provides an indication of how well the foundations of the carriageway, pavement, or building would settle in to the soil.

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