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.
