What is the CBR (California Bearing Ratio)?

California bearing ratio connotes the ratio of the load required to reach a specified penetration to the standard load for the same penetration. The value of CBR can lie anywhere between 0 to 100 %. The value so obtained is an important indicator of the soil strength that can also be employed to approximate the pavement thickness.

CBR Test Apparatus

Fig 1: CBR apparatus

The apparatus comprises a loading machine that operates in a strain-controlled manner and exerts load at a constant rate such that the penetration is 1.25mm per minute. 

Cylindrical moulds of 150mm diameter and 175mm height are provided with a collar. The length of the collar is 50mm, and it also has a perforated base which is detachable. 

A Compaction rammer and surcharge annular of 2.5kg weight and 147mm diameter are also provided. The apparatus comprises of:

• Mould
• Cutting collar
• Surcharge weight
• Spacer Disc
• Dial gauges
• IS Sieves
• Penetration Plunger
• Loading Machine (CBR Testing machine)

Sometimes, the initial portion of the stress-penetration graph shows an upward concavity near the origin. This occurs due to surface irregularities or seating errors and must be corrected before calculation.

The corrected method includes:

• Drawing a tangent to the curve at its steepest slope.
• Shifting the origin to meet this tangent.
• Taking the load values at 2.5 mm and 5 mm penetration from the corrected curve only.

This correction ensures accuracy in the CBR calculation and aligns with IS: 2720 (Part 16) standards.

CBR Test Procedure

There are a few steps to get the CBR test done, which are discussed below in detail.

Specimen Preparation

• The soil sample is passed through a 19 mm sieve. The soil passing the sieve can be employed for the test. The soil retained on the sieve is replaced with the same quantity of new soil.
• Upon sieving the sample, 3 samples of 6.8 kg are considered for the test.
• Specimens 1,2 and 3 are compacted to 10, 30, and 56 blows, respectively. This leads to a variation in the maximum dry densities.
• Samples are mixed with a sufficient quantity of water to maintain an adequate water content (OMC).
• Subsequently, the mould is connected to the base plate by means of an extension collar. 
• After that, the weight is measured.
• A spacer disc is placed on top of the mould along with filter paper.
• Soil is transferred in the mould in 3 layers. 
• Each layer is well compacted. As a precedent, Specimen 1 is subjected to 10 rammer blows so as to ensure sufficient compaction.
• The water content of soil before and after compaction is measured.
• The extension collar is then taken off, and the top of the mould is trimmed with a straightedge to make the surface smooth.
• Following the same steps as above, the other two samples are compacted with an adequate number of blows.
• Subsequently, both the base plate and spacer disc are removed. 
• The weight of the mould and that of the compacted soil are measured.

• The mould is placed in an inverted position, and the base plate is attached to the mould by means of a course filter paper.

Soaking

• A surcharge load of 4.54 kg is applied on top of the base plate.
• The specimen is then soaked in water for the next 4 days.
• The weight of the specimen before and after soaking is determined so as to estimate the swelling percentage. In this context, the equipment for measuring expansion can be attached.
• After 4 days of water soaking, the mould is taken off from the water. 
• Also, the base plate and the filter paper are removed.
• The surcharge load is also removed, and the mass of the mould and the soil is measured.

Load Test

• The mould is placed under the penetration piston in the CTM (Compression Testing Machine), also called the CBR Testing Machine.
• A surcharge load of 4.54 kg is again placed on top of the mould.
• Soon after, the CTM is started at a strain-controlled rate such that the penetration is 1.25mm/minute. The piston shall penetrate through the soil.
• For measurement purposes, the CTM is accompanied by the dial gauge and the proving ring. While the dial gauge measures the penetration, the proving ring depicts the load applied to the sample.
• For arriving at the piston load, the readings obtained from the Proving ring are multiplied by the machine constant.
• Subsequently, the load determined to reach different levels of penetration is noted and then divided by the standard load for the given penetration.

Field CBR Test vs Laboratory CBR Test

The CBR test can be conducted either in the field or laboratory. However, there are key differences in procedure and application. Understanding them helps in choosing the right method.

California Bearing Ratio Calculation from the Test Values

Stress-Strain Curve

• From the test values, a Stress vs. strain curve is plotted. 
• If the curve shows concavity upwards near the origin, the values are adjusted as per the code guidelines.

CBR Calculation

Fig 2: Stress Penetration Graph

• From the aforementioned curve, stress is determined corresponding to 2.5 mm and 5 mm penetrations. 
• Generally, CBR at 2.5 mm penetration is considered as the CBR value, and its value is generally greater than the CBR value obtained at 5 mm penetration.
• If the CBR value at 5 mm exceeds the value obtained at 2.5 mm penetration, the test is repeated. However, if the value at 5 mm again exceeds the CBR value for 2.5 mm penetration, then this value is adopted.

Design Bearing Ratio for Specified Dry Density

• The dry density of each specimen subjected to different blows (10, 30, and 56 blows) is determined. 
• Subsequently, a graph is plotted between the dry density and the corresponding CBR values of each specimen.
• The curve so obtained is used for ascertaining the CBR values corresponding to the specified dry density.

Fig 3: CBR vs Dry Density Curve

Standard CBR Values for Common Soils

The CBR value varies significantly with soil type, compaction, and moisture content. It’s useful to know standard ranges of CBR values for different soil types when planning pavement thickness or comparing test results.Here is a table showing typical CBR values of commonly found soils:

Advantages and Disadvantages Of The CBR Test

Advantages

• The test can be performed without any technical proficiency or pre-mandated training.
• CBR test is a reliable test, and the results obtained from it are considered pretty much accurate. 
• The CBR method can be used right away on the airfield pavement as against other methods.
• The apparatus for the test is easy to handle and transport.
• It can be employed both in the field as well as in the laboratory. 
• The CBR value can also be used to ascertain the pavement thickness.
• Although this test is mostly used for subgrades, it can be used for the sub-base and base course as well.

Disadvantages

• The methods for compaction used in the lab and on the field are different.
• The added strength of highly stabilized surfaces like asphaltic concrete is not incorporated in the CBR test.
• The assumption of the subgrade being completely saturated can lead to a low factor of safety.
• Many of the test procedures are conducted on the spot. As such, strict supervision and control need to be ensured.

Conclusion

In this article, we have briefed our readers on the CBR test, its purpose, apparatus, procedure, and the disadvantages and disadvantages associated with the test. To gain command over such topics, we recommend our readers enrol in the AE/JE Civil Coaching on the Testbook App.

Students should also keep up with the habit of appearing in various SSC JE Civil Mock Tests and SSC JE Civil Previous Year Papers.