Abnormal CBR vs %compaction? 2

[calltorahat]

hi

We are performing CBR according to AASHTO T 193.
It has been happening for some time now that we are getting CBR vs Compaction curve different then typical curves. That is, for example:
CBR value at 10 blows is higher then the 30 & 65 blows value and CBR value at 65 blows is higher then 30 and less then 10 blows value.

Other example:
CBR value at 30 blows is greater then value at 30 blows and CBR at 65 blows is less then 30 blows (and greater then 10 blows value).

Is there any explanation or logic for that? (material's LL and PL values are given)


Any information and reference in this matter will be highly appreciated.

http://files.engineering.com/getfil...ad0a-5b4c1854cdbc&file=CBR_strange_curves.png

 

[Ron]

Assuming you are trying to assess the stability at various moisture contents, this implies that two of your specimens are past optimum and that optimum is likely occurring somewhere between the 10 and 30 blow points.

Run the CBR at optimum, with 25 blows per layer, and see what you get.

 

[calltorahat]

Thanks Ron for your answer.


My apologies, forgot to mention that all 3 specimens are compacted at optimum moisture content.

Optimum moisture content & maximum dry density is obtained using modified Proctor (AASHTO T 180 Method D)

CBR is being checked at 95% compaction of maximum dry density.

All three specimens are compacted at 10, 30 and 65 blows respectively with 4.54 kg rammer.


Regards

 

[Ron]

Where do your 10,30 and 65 blow densities land on the original Proctor curve?

 

[calltorahat]

density of 10, 30 and 65 blows are ~90%, ~95% and ~100% of max density respectively.

 

[Ron]

calltorahat....yes, but did they fall on the original proctor curve when they were plotted?

 

[calltorahat]

Hi Ron,

We're talking here the Design CBR for one water content only.

Now, since all three specimens are compacted using the same (optimum) water content, I am kinda confused that how to compare those 3 specimens with proctor curve.

Of course, we can see that the density at 65 blows is pretty near (almost identical) to our maximum dry density that we achieved during proctor, but for the rest of compaction efforts (10 and 30), I have no idea how to correlate those with our proctor of that material.

I know its too much to ask, but an example would be really helpful.

 

[motorbiketocrank]

I don't think you can say there is a "typical curve". It's quite common that once you reach a certain compaction level in cohesive soils, your CBR value will go down with additional compaction. It has to do with various issues such as the fact that higher compaction levels simply increase the pore pressure in the soils which results in lower strength. I would basically double check to make sure that you're running the tests correctly and consistently between samples........and if you are, then take the results at face value and don't assume they're incorrect.

 

[calltorahat]

Thank you geobdg,

That makes a whole lot of sense.
So, if re-testing produces similar results, then we are safe to conclude that the soil with 100% compaction is loosing strength due to access load caused by the plunger thus getting less CBR values and the material that is below 100% compaction is gaining more strength under the plunger and causing high CBR values.

 

[Ron]

Calltorahat...agree with geobdg....

As for the plotting, the point I was trying to make was you should plot on your Proctor curve and you should have a straight vertical line for moisture with the % compaction mark points as you noted...if not....

It's just another check.

Check the saturation on your Proctor samples to see if it makes sense as well. Post your Proctor curve and data.

 

[calltorahat]

Test reports of material is attached.

 

[motorbiketocrank]

Well, my discussion of sometimes getting lower CBR values with higher blows due to increased pore pressures is true. But, looking over the lab results you posted, I don't think that's the case for your particular test. You have a pretty granular soil with only 12% passing the #200....and densities consistently increase with increasing blows. So generally, I'd expect to see the CBR increase with increasing blows.

My best guess as to why this didn't happen for your sample would be due to factors such as sample variation. For example, maybe the 30 blow sample just had more gravel in it or the way a few large gravels lined up in the mold resulted in less penetration at the same pressure. It's really not unusual to have some significant data scatter due to these sorts of issues.

Not sure of how this data is being used. But hopefully the designer is looking at all the results and considering them in his/her interpretation (and not just saying.....oh, I have a 39.0% CBR). If it were me, I'd look at the data and think....there's a good chance I have a CBR somewhere around 20% at optimum moisture. And then would also consider what impact variations from optimum moisture would have (since there probably will be a moisture range such as plus or minus 2% for construction).

 

[calltorahat]

The data is being used in our project's routine testing. Design CBR requirement is min 15% for subgrade.

Thanks again very much for your valuable information and analysis, it was very helpful. I'll keep these points in mind for our future lab test reports.


regards