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ashjun (Geotechnical)
23 Mar 03 6:29
Hi,

Has anyone come across abnormally high CBR values especially on cement stabilised soils?  Here I have a soil sample mixed with 8% OPC that yields CBR of about 150.  I don’t think standard literature ever talks on CBR on stabilised soils.
Also, the unconfined compressive strength of the stabilised material is also high to the order of 24 kg/cm2.  Is there a relation which I can derive based on the CBR and UCS test results.  Furthermore, some of the engineers related to pavement designs quote UCS values instead of CBR values which using the stabilised soils.  Can someone please throw some light on this?

Any help on the matter shall deeply be appreciated.

Regards

Helpful Member!  VAD (Geotechnical)
23 Mar 03 11:03
Hello Asjun:

Nothing is unusual about the values obtained. Check the following references - "Soil Mechanics in Highway Enginering" by Rodriguez, Castillo and Sowers Transtech Publications 1988 for UCS versus cement content for various soil types. Values of plus 40 kg/cm2 have been reported. The other reference is "Soil Stabilization in Pavement Structures" A Users Manual - Volume 2 "Mixture Design Considerations" USDOT FHWA publication FHWA-IP-80-2. This publication gives a chart on the correlation between CBR of soil cement mixes and UCS . The relationship is CBR =.055(UC)times exponent 1.431.

With regard to the Pavement engineers using UCS. They are probably using the AASHTO method of pavement design which assigns a layer coefficient to soil cement mixes in order to derive a thickness for the layer.

The CBR method does not provide thickness of material above the soil cement layer if your CBR is 150 as the charts go to 100 max or lesser.

Assume that your subgrade requires by the CBR method a thickness of 24 inches overtop then if you place a 6 inch soil cement above this layer, you will still need 18 inches of pavement. However, overtop the soil cement you would not be able to determine the thickness. You would most likely make up the difference by material of lower quality and use the soil cement as  a base course. Hence your structure would be subgrade, subbase, and soil cement and a wearing course of asphalt concrete.

The asphalt concrete thickness can be determined from empirical relations which could give say 3 inches for medium heavy traffic. Yor final design can look like this -subgrade,15 inches of subbase material,6 inches of soil cement and 3 inches of asphalt pavement. Others may use an equivalency factor for the soil cement say 1.5 and reduce the subbase to 12 inches and one for the asphalt concrete say 2. Hence, you can further modify your thickness of pavement component materials.

It is worthwhile to note that the CBR test was premised on materials exhibiting shear deformation and in reality more applicable to unbound pavement materials. However, this method preceeded the AASHTO approach and hence the reason why the CBR test is still used by many especially in the developing countries. The CBR test was often done on soil cement mixes to assess their relative qualities i.e higher the CBR the better the material. This concept is not entirely true for cold climates.

It is always worthwhile to study the performance of pavements in your locale. Such information can be obtained from your local highway department as they often have a wealth of data of that sort. Also, take a look at their approach to design as well and remember traffic types and intensity are very important factors as well as environmental conditions. This latter aspect is important when attempting to use design approaches established by jurisdictions outside that of your own. Pavement design is still largely empirical or semi empirical.  

If you cannot locate these references I can provide you a copy of two relevant pages if you provide me with contact information.

I hope the above helps. Others respondents may have further information and insights.

 
   
Ron (Structural)
24 Mar 03 12:59
ashjun...as VAD alluded, the CBR is typically associated with unbound materials.  The UCS is commonly used for cement stabilized materials or for naturally cemented materials.

The values you referenced do not surprise me.  
Focht3 (Geotechnical)
24 Mar 03 14:26
VAD has given you very good comments and discussion.  To add to his discussion:  CBR values above 100 have no practical use from a design perspective.

Also be aware that cemented materials may not do well in expansive soil environments - high CBR values do not necessarily equate to good pavement performance when high PI subgrade is present.
Helpful Member!  AZdinak (Geotechnical)
31 Mar 03 14:40
Ashjun,

As has been said by the other posters here, your value is not atypical. In fact, if you did not get a number greater than 100 it would be suspect. I'm sure you are aware of this, but if you are not, the CBR value is a ratio of a measured resistance compared to the reference resistance on an unbound aggregate material. The reference resistance equals a CBR of 100, or 100 percent of the reference value. Thus, a CBR value greater than 100 is very likely for bound materials or coarse materials compacted to modified Proctor efforts. Just some backgound if you are not in the lab often.

Zdinak

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