Cara Menghitung CBR

>Hello Sohib EditorOnline, if you are in the civil engineering or road construction field, you must know about CBR. CBR or California Bearing Ratio is a test to measure the strength of soil or other materials used as subgrade for road construction. In this article, we will discuss how to calculate CBR in a detailed and comprehensive way.

1. What is CBR?

CBR is a test that measures the ability of soil or other materials to bear the pressure from traffic load. The test involves applying a load on a small sample of soil and measuring the amount of deformation that occurs. The result is then compared to a standard value to determine the CBR value. The CBR value ranges from 0 to 100, where 0 represents the weakest soil and 100 represents the strongest soil.

1.1. The Importance of CBR

CBR is an important parameter in road construction as it helps in designing the thickness of the pavement layers. The CBR value also determines the type of soil that can be used in road construction. The higher the CBR value, the better the soil quality and the more suitable it is for road construction.

1.2. Applications of CBR

The CBR test is widely used in the following applications:

1. Design of flexible pavements
2. Design of airport runways
3. Design of embankments
4. Selection of materials for subgrade and subbase

2. How to Perform CBR Test?

CBR test involves the following steps:

1. Take a soil sample from the field
2. Dry the soil sample and sieve it through a 19mm sieve
3. Weigh around 4 kg of soil and place it in a mould with a diameter of 152mm and a height of 127mm
4. Compact the soil in the mould with 56 blows of a hammer, each blow having a weight of 4.5 kg and falling from a height of 457mm
5. Weigh the mould with the compacted soil and note down the weight
6. Soak the sample in water for 96 hours
7. Place the sample in a CBR apparatus and apply a load at a rate of 1.25mm/minute
8. Measure the load and penetration for different values of penetration and note down the results
9. Calculate the CBR value using the formula CBR = (P/Ps) x 100, where P is the load required to cause a penetration of 2.5mm and Ps is the load required to cause the same penetration in a standard sample of crushed stone

3. Factors Affecting CBR Value

The CBR value of soil is affected by a number of factors including:

• Type of soil
• Moisture content
• Density
• Compaction method
• Size of the sample
• Type of loading device used

3.1. The Effect of Moisture Content on CBR Value

Moisture content plays a crucial role in determining the CBR value of soil. As the moisture content increases, the CBR value decreases. This is because water acts as a lubricant and reduces the friction between soil particles, thus decreasing the soil strength.

3.2. The Effect of Density on CBR Value

The CBR value of soil is also affected by the density of the soil. The higher the density, the higher the CBR value. This is because the soil particles are more tightly packed, providing greater resistance to the load.

3.3. The Effect of Compaction on CBR Value

The method of compaction also affects the CBR value of soil. A soil sample that is inadequately compacted will have a lower CBR value compared to a sample that is well-compacted. This is because the well-compacted sample has more tightly packed soil particles, providing greater resistance to the load.

3.4. The Effect of Sample Size on CBR Value

The size of the soil sample used for CBR testing also affects the CBR value. A smaller sample size may not be representative of the entire soil mass, leading to incorrect CBR values.

4. How to Calculate CBR Value?

To calculate the CBR value, we need to follow the following formula:

CBR = (P/Ps) x 100

Where:

• P is the load required to cause a penetration of 2.5mm
• Ps is the load required to cause the same penetration in a standard sample of crushed stone

4.1. Preparation of Graph

The CBR value can also be calculated by preparing a graph of load vs penetration, as shown below:

Penetration (mm)	2.54		5.08		7.62		10.16		12.70		15.24		20.32		25.40
Load (kg)	Standard	Tested Soil	Standard	Tested Soil	Standard	Tested Soil	Standard	Tested Soil	Standard	Tested Soil	Standard	Tested Soil	Standard	Tested Soil
0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
2.5	1370	Load 1	2055	Load 2	3215	Load 3	4535	Load 4	5075	Load 5	5765	Load 6	7340	Load 7
5.0	2440	Load 8	3425	Load 9	5350	Load 10	9075	Load 11	10670	Load 12	12400	Load 13	14950	Load 14
7.5	3505	Load 15	4975	Load 16	7750	Load 17	13605	Load 18	15995	Load 19	19020	Load 20	–	–
10.0	4535	Load 21	6570	Load 22	10205	Load 23	19250	Load 24	23390	Load 25	–	–	–	–

From the table, we can prepare the following graph:

The CBR value is then determined from the graph by dividing the load at 2.5mm penetration of the tested soil by the load at 2.5mm penetration of the standard crushed stone sample and multiplying the result by 100. For example, if the load at 2.5mm penetration of the tested soil is 3505 kg and the load at 2.5mm penetration of the standard crushed stone sample is 4535 kg, the CBR value is:

CBR = (3505/4535) x 100 = 77.19%

5. Conclusion

CBR is an important parameter in road construction and civil engineering. The CBR test helps in determining the soil strength and suitability for road construction. The CBR value is affected by a number of factors including soil type, moisture content, compaction method, density, sample size, and loading device. By following the steps mentioned in this article, you can easily calculate the CBR value.

FAQ

1. How is CBR test performed?

The CBR test is performed by taking a soil sample, drying it, compacting it in a mould, soaking it in water, and then applying a load at a rate of 1.25mm/minute. The load and penetration values are noted and used to calculate the CBR value.

2. What is the importance of CBR in road construction?

CBR is important in road construction as it helps in designing the thickness of the pavement layers. The CBR value also determines the type of soil that can be used in road construction.

3. What factors affect the CBR value?

The CBR value of soil is affected by a number of factors including soil type, moisture content, compaction method, density, sample size, and loading device.

Cara Menghitung CBR