What is CBR testing and how is it done?

CBR in full means California Bearing Ratio. The ratio came to light in 1938 and was introduced by the California Division of Highways. The test evaluates subgrades and base courses’ mechanical strengths. To do this test, one takes the needed pressure for penetrating through the soil in a soil that has been sampled out. You will need a plunger to do this. The plunger should have a piston that has the shape of a sphere, and the process is done at a speed of 1.25 mm per minute.

After this measuring, a division is done that involves the used pressure and the needed pressure. The standard of the proportion is one equal to a rock material. For full info see www.cbrtesting.com who provide the service throughout the UK.

What to use when testing a CBR

The soil to be used for testing should be natural or compacted in either soaked or un-soaked state. The results taken from the performance are compared to curves from standard tests so as to tell the strength the soil possesses. The apparatus that are used for this performance include spacer discs, moulds, surcharge weight, cutting collars for steel, dial gauges, IS sieves, penetration plungers, and loading machines

The process taken to test the CBR

In most cases, there are three specimens which are of about 7kgs. These specimens are compressed to about a density of 95-100% with blows of 10, 30 and 65. The weight of the empty mould is then taken. Afterward, the specialist adds water to the first specimen and compacts it to about five layers in 10 blows of every sheet. After this is done on the first specimen, the used collar is taken away followed by levelling off the surface. The content of the moisture is then measured.

This compacted specimen is added on the mould’s weight. After this is done, the mould is put in a tank for soaking for about four days. For the other samples, the same process is applied once blows that are different from them are applied to the samples. After the fourth day in the soaking tank, the soil has swollen, and these readings are used to determine a four-day percentage swell. After this, the mould is taken out of the water to dry. A 10lb surcharge load is placed on top of a piston that has been placed under piston for penetration.

An analysis of the values of the load from penetration is taken, and graphs are drawn between the load penetration and the penetration to get the value for the CBR. In conclusion, there is a graph drawn between the Dry Density and the CBR’s percentage hence getting the compact degree that is needed. This process has to be followed to the latter in order to get the right CRB. Also, the right instruments must be used in the whole process.

Why do a CBR test?

A CBR test is popularly used as a method of evaluating a subgrade soil’s strength, base course material and sub-base for the determination of the thickness of highways and pavements for airfields. The results from these tests are used together with empirical curves in order to get the thickness needed for pavements and the layers of its component. It is the most popular method for designing flexible pavements.

The bearing capacity and soil strength use the CBR figure as an index of both. The Indian Roads Congress standardized guidelines to design flexible pavements in accordance with the CBR figure. However, CBR testing cannot be used for soil properties evaluation like that for the angle of internal friction, shearing resistance, and cohesion.

Causes of inaccurate data

There are many instances that can lead to the wrong data. One of the cases is when the gauge reading keeps differentiating in every second. In such a case, how a human being reacts may lead to results that have been mistaken. In a scenario where the soil does not have a smooth surface, it is also possible to get a reading that is inaccurate. This is because the plunger will need to penetrate through the soil in a manner that is perpendicular. If the soil is coarse, the plunger might not be able to make an accurate perpendicular penetration thus affecting the data collected.

It is not possible for one to neglect the possibility of having air spaces in the soil. Even if the compact is made to be very firm, there is still a good chance that air spaces will always be present. Also, aggregates have different sizes, and the gap between these aggregates are empty. Such voids cannot be effectively determined which in line affects the results collected.

Engineers should keep in mind that soils in different parts of the world have different chemical compositions. Since the standard CBR was done in a British environment, it cannot be accurate to say that the standard is actually a standard CBR. This is because soils have different minerals and compositions and this too needs to be taken into account.

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