Any pavement design method would have to consider 4 parameters:

1- traffic loads; 2- Environmental conditions ; 3- pavement materials' properties; and 4- Properties of the supporting subgrade.

Therefore, for the subgrade, early design method rely on the CBR test to get its bearing capacity. Then, AASHTO with its major experimental road test performed in the late 50s, switched from using CBR to resilient modulus for characterizing the subgrade. The resilient modulus is a weighted average based on the climatic conditions during the whole year. Therefore, for those who still use CBR as a Test and wanted to use the AASHTO statistical method needed a way to correlate between the two properties and many equations came out. With its recent mechanistic-empirical pavement design guide, AASHTO still uses the resilient modulus property, but use more complex equations that relate this property with moisture content and stress conditions.

In short, simple design method use simple easy to get properties, but people attack these methods because of their simplicity. More complex methods (such as the new AASHTO design guide) use sophisticated models in order to simulate the real conditions, but the average user finds himself/herself trying to input properties that are not easy to get and in most case use default values.

So now depending on which school you want to be, you can use simple design method or use more complex methods that are supposed to achieve more reliable designs. Of course, if you work in an engineering firm, the choice on which method to use comes from the client and not you.

CBR method is an empirical method of designing flexible pavements, it is argued that it does not have a full representation of the practical situation.

It is a empirical method which is performed in lab. as well as field. Many countries are adopted this method. Manjunatha Hagarnur also suggest DCP test. It is good method, by using empirical relation you can obtain CBR value of soil.

You can read this article.

Article On-Field Assessment of Sub-Grade Uniformity Using DCP

All the best

interesting question

Thanks for the answers and the article

but the question remains the same that, why are we still using CBR in flexible pavement design even it is empirical and does not simulate the exact field condition, why it is not be replaced with an alternative

And if there is an alternative which can be used in place of CBR for sub grade strength incorporation in pavement design then kindly mention it.

The UK design manual for roads and bridges interim advice note IAN73 lays out the design process using subgrade stiffness moduli instead of using CBR directly (the document is freely available for download on www.standardsforhighways.co.uk. However the field test undertaken to estimate this modulus is usually a CBR test, plate load test or DCP.

Within the note are expanations of some alternative dynamic plate tests to directly determine the surface modulus.

I suspect the main reason why the CBR method is still undertaken is that it is a well established and understood method. The field tests are also relatively quick, simple, cheap and require no specialist equipment.

@David Stone

Thank you Sir.

Any pavement design method would have to consider 4 parameters:

1- traffic loads; 2- Environmental conditions ; 3- pavement materials' properties; and 4- Properties of the supporting subgrade.

Therefore, for the subgrade, early design method rely on the CBR test to get its bearing capacity. Then, AASHTO with its major experimental road test performed in the late 50s, switched from using CBR to resilient modulus for characterizing the subgrade. The resilient modulus is a weighted average based on the climatic conditions during the whole year. Therefore, for those who still use CBR as a Test and wanted to use the AASHTO statistical method needed a way to correlate between the two properties and many equations came out. With its recent mechanistic-empirical pavement design guide, AASHTO still uses the resilient modulus property, but use more complex equations that relate this property with moisture content and stress conditions.

In short, simple design method use simple easy to get properties, but people attack these methods because of their simplicity. More complex methods (such as the new AASHTO design guide) use sophisticated models in order to simulate the real conditions, but the average user finds himself/herself trying to input properties that are not easy to get and in most case use default values.

So now depending on which school you want to be, you can use simple design method or use more complex methods that are supposed to achieve more reliable designs. Of course, if you work in an engineering firm, the choice on which method to use comes from the client and not you.

@Amara Loulizi Sir For very comprehensive answer to my question. Hence we can conclude that Cbr is good for simple and light traffic roads where in depth investigation of materials and subgrade is not very important.

Hence Cbr based design can be declared as Reliable for light traffic roads.

But for heavy traffic loading we must consult MEPDG approach being more sophisticated and shows better field conditions picture.

Dynamic Cone Penetrometer (DCP) test and Plate Load Test(PLT) are the best alternatives to CBR, which are more elaborate than the later. The reason of still following CBR test is that it is simple with little calculations, requiring no specific apparatus, can be easily performed by unskilled persons thereby having least chances of error both instrumental and human error. Also a great thing is that the sub-grade parameters determined by any of the above methods(CBR/DCPT/PLT) is far far enough for our justification that whether the subgrade is strong enough or not and what is the requirements of others layers(type of material) and their thickness. So there is no need of using any uneconomical methods like the former if we can arrive to the results by using a simple and method of CBR.

DCP test is an alternative method for cbr

No, it's good for quality control and empirical designs bit it still used by practitioners. But if you used a calibrated correlation between CBR and Mr, it will give better results.

Hi, I think you already have your answer. However, I would like to add something herein. Resilient modulus has been used instead of CBR for last two decades. However, MR testing is time consuming, complex, and difficult to perform if we compare to CBR testing. Therefore, transportation agencies are still determine CBR value for low volume roads, and use correlation equations to estimate MR from that. Number of correlations equations are developed and validated by different researchers for various types of soils which seems pretty reliable to use. Thanks!