field cbr vs. lab cbr

[civilQ]

hello everyone
i know this is a repeated question but i did not find a good answer , is there a correlation between in-situ cbr and lab cbr , because i'm trying to relate DCP TO CBR for a specific type of soil

 

[fattdad]

. . . I think there's a correlation between DCP and CBR. Question is, "Do you want to use it!" I don't! DCP and field CBR cannot replicated the same boundary condition of near saturation. As a result your results will be influenced by dry strength, which is a liberal variation.

I prefer saturated CBR values derived by the 48 hour soaking period of the ASTM method.

f-d

ípapß gordo ainÆt no madre flaca!

 

[civilQ]

the ASTM method states that the sample should be compacted with 56 blow if i do so the sample will have higher density than field density and the asmt correlate dcp with field cbr which is difficult for me to accomplish . so i'm trying to correlate with lab cbr . i already know that lab cbr is higher due to mold confinement but i'm trying to find if there a correlation between field and lab cbr .

 

[fattdad]

We (state DOT) requires CBR testing at 97.5 to 102.5 percent relative compaction. We also require the upper reaches of fill and the surface of cut subgrades be compacted to 100 percent relative compaction. So, yes, if you are not going to have quality control on your subgrade it may be of some value to know the CBR at less than typical compaction. I just wouldn't do that.

f-d

ípapß gordo ainÆt no madre flaca!

 

[BigH]

You can search for a paper by Tosovic & Vujanic "C.B.R. testing with Dynamic Conical Penetrometer in the process of road rehabilitation and construction control" - sorry but I didn't save the URL. Similarly search MNROAD User Guide to the Dynamic Cone Penetrometer.

You have, though, failed to identify the type of cone penetrometer you will be using. There are a number of different ones - the TRRL cone, the Mackintosh Probe, the Pentest (Canada's 140# hammer dropping 30"), etc. There have been a number of discussions on this.

Fattdad is correct in that the lab value is a soaked value and the CBR in the field and cone penetrometer in the field are relative to that particular moisture content - which in the summer (Northern hemisphere) can be "dry" and in the wetter months, nearly saturated. This will make a difference. I tried (and failed) to get a contractor in Laos to do a soaked test and a "dry" test of the same sample in order to get a correlation of the CBR saturated and CBR at or nearly at OMC. If one were to do many tests to develop a good correlation - one might be able to correlated DCP test results to the soaked values of CBR. We usually did our CBRs at 95% of the standard MDD (the requisite relative compaction level specified).

 

[civilQ]

the cone i'm using is 60 deg 20 mm dia with 8kg hammer falling from 575 mm .

so would it be OK to correlate dcp to lab (soaked) cbr if i prepared samples with same density as in field ?

 

[BigH]

The dry density is one thing - but you want to correlate with soaked specimen . . . Please reread the third paragraph . . . Your cone seems to be the TRRL one (see Tosovic's paper)