The California Bearing Ratio (CBR) test is a way of determining how well a material can resist penetration from a standard plunger under specific conditions of moisture and density. This test was initially developed by the California Division of Highways to evaluate and classify soil-subgrade and base course materials for flexible pavements.
The CBR test can be conducted using either remoulded or undisturbed samples of the material in question. During the test, a cylindrical plunger with a diameter of 50mm is gradually pushed into the pavement component material at a controlled rate of 1.25mm per minute. The load needed to reach deformations of 2.5mm and 5mm is recorded during the test.
To calculate the CBR value, the load required to achieve the desired deformation levels is expressed as a percentage of the standard load value. This value is specific to each deformation level and is used to determine the material’s overall resistance to penetration. The CBR value is an essential parameter that can help assess a material’s suitability for use in flexible pavements.
California Bearing Ratio Test on Subgrade Soil
The purpose of this test is to ascertain the California Bearing Ratio (CBR) of the subgrade soil. This value is a measure of the strength of the soil and its ability to bear the load of a structure or pavement. The CBR test involves subjecting the soil to a standard load and measuring the depth of penetration of a plunger into the soil. The ratio of the load required to penetrate the soil to a specified depth to the load required to achieve the same penetration in a standard material is the CBR value. This value is essential in determining the thickness of the pavement and designing the appropriate foundation for a structure.
Apparatus for CBR Test
A compression machine capable of operating at a constant rate of 1.25mm per minute can be utilized in the process. The cylindrical moulds to be used are those with a diameter of 150mm and a height of 175mm, which are equipped with a detachable perforated base and a collar of approximately 50mm in length. The compaction rammer and surcharge weight should also be prepared, with annular weights measuring 2.5kg each and having a diameter of 147mm. Additionally, a coarse filter paper, an IS sieve with a size of 20mm, and a balance are necessary tools in this procedure. These materials are essential to ensure the accuracy and effectiveness of the compression process. By following these guidelines and utilizing the appropriate equipment and materials, the desired outcome can be achieved.
Procedure of California Bearing Ratio Test
To perform a soil compaction test, a sample of soil specimen is taken and sieved through a 20mm IS sieve. Five kilograms of the sample is then mixed with water to reach the optimum moisture content or field moisture content. The soil and water are thoroughly mixed, and the prepared soil-water mix is divided into five parts. A spacer disc is placed on the base plate of a mould, and a coarse filter paper is placed over the spacer disc. The mould is cleaned, oiled, and then filled with one-fifth of the prepared soil mix. The layer is compacted by giving 56 evenly distributed blows using a hammer of weight 4.89kg. The process is repeated for the second and third layers, and after the third layer, a collar is attached to the mould, and the process is continued until the fifth layer. The collar is then removed, excess soil is struck off, and the base plate is removed. The mould is inverted and clamped to the base plate. Surcharge weights of 2.5kg are placed on the top surface of the soil, and the mould containing the specimen is placed in position on the testing machine.
The penetration plunger is brought into contact with the soil, and a seating load of 4kg is applied to establish contact between the soil and plunger. The dial readings are then adjusted to zero, and a load is applied such that the penetration rate is 1.25mm per minute. The load at penetrations of 0.5, 1, 1.5, 2, 2.5, 3, 4, 5, 7.5, 10, and 12.5mm are noted during the test.
Standard Load Values for CBR Test
| Penetration(mm) | Standard Load(kg) | Unit Standard Load(kg/cm2) |
| 2.5 | 1370 | 70 |
| 5 | 2055 | 105 |
| 7.5 | 2630 | 134 |
| 10.0 | 3180 | 162 |
| 12.5 | 3600 | 183 |
Observations during CBR Test
Weight of soil taken = Weight of surcharge = Area of plunger, A = Proving Ring Calibration Factor =
| Sl No. | Penetration(mm) | Proving dial reading | Load on plunger(kg) | Corrected load | Unit Load |
Result of California Bearing Ratio Test
The California Bearing Ratio (CBR) is a test used to evaluate the strength of a subgrade soil and its ability to withstand the load of traffic. The CBR test involves measuring the pressure required to penetrate a soil sample with a cylindrical plunger, at different depths of penetration.
In this context, it is given that the California Bearing Ratio (CBR) is being evaluated for a subgrade soil. Specifically, the CBR at two different depths of penetration, 2.5mm and 5.0mm, is being measured. The CBR value at each depth is a measure of the soil’s strength at that depth and its ability to resist deformation under load.
The CBR test is an important tool used in civil engineering and road construction, as it helps to determine the suitability of a soil for use as a subgrade material. A higher CBR value indicates a stronger soil and a greater ability to support heavy loads, which is desirable for road construction.