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Abnormal CBR curve

Abnormal CBR curve

(OP)
thread261-314949: Abnormal CBR vs %compaction?

I was following the referenced thread because i am facing pretty similar issues with my CBR curves. I have 2 set of Standard CBR results (Both GM soil, approx. 18% fines on both). In both tests, the dry density at the maximum blows (65 blows) are slightly less than the Maximum Dry Density, So i had to extrapolate the curve to get the CBR value at 100% of max dry density. And also, the CBR values at lower blows are higher than CBR values at higher blows.

Test 1 (Max Dry Density from Std Proctor = 132.1 pcf)
20 blows----Dry Unit Wt. = 129.1 pcf------CBR at 0.1 in penetration = 13.35%
35 blows----Dry Unit Wt. = 129.7 pcf------CBR at 0.1 in penetration = 10.52%
65 blows----Dry Unit Wt. = 131.2 pcf------CBR at 0.1 in penetration = 10.68%

Final recommended CBR value (@100% max. dry density) = 15.6% (Which i don't think is unusual). Similar results on Test 2 as well. Both similar materials. The Stress vs Penetration curves are also abnormal for the three points (curves crossing each other multiple times and not widely spaced).

Any insights will be greatly appreciated.

RE: Abnormal CBR curve

You should look at how your sample has been handled and if any segregation has occurred. This can cause significant variation in your results for CBR. You mentioned that all 3 CBR curves cross each other which also makes me believe that your samples are segregated, assuming that you should see a very similar curve pattern on all 3 CBRs, each curve higher than the last respective to the number of blows for that sample.

Any significant change in the material, for example one of your samples contains more gravel than the others, the proctor for that sub-sample could be different.

Also investigate the moisture saturation for your CBR samples as inadequate mixing of water will also lead to strange results.

Hope this helps!

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