SPT x DCPT Correlation 1

[ONENGINEER]

I have seen the following in another thread.

Typical correlation between DCP and SPT values:
DCP value in mm/blow SPT value blow/300mm
5 50
6 44
7 38
8 33
9 28
10 24
12 22
14 18
16 16
18 15
20 14

How are these values derived? Seem a bit aggressive as they say DCPT=1.5 SPT upto 4 m and DCPT=1.75 SPT from 4m to 9 m.

Is there a recent correlation factor? Thanks.

 

[BigH]

As almost every time questions regarding the DCP come up, I wished that one would identify the details of the DCP they are using . . . is it the Canadian Pentest which is a 2 inch cone 60 deg apex angle driven with a 140# (65 kg) hammer? Is it the miniature one developed by the TTRL for pavement evaluations? Is it Sowers? Is it the Mackintosh Probe which is used extensively in places like Malaysia? Please advise. If the Pentest - the original (first edition) of the Canadian Foundation manual had correlations.

 

[ONENGINEER]

BigH, thanks for your comments and sorry to have missed to mention the energy specs. The same equipment used for Standard Penetration Testing was used for DCPT, i.e. 76 cm freefall of a hammer of 63.5 kg and the DCPT had 2 inch cone with 60 degree apex.

I do not have CFM first version but will look for it. The numbers above are based on another Eng-Tip posting and indicate DCPT = 1.1 to 1.2 SPT.

 

[BigH]

I figured you were in Canada - normally we have traditionally called your DCPT version as "pentest" for penetration test. I don't have the first edition with me; in deep storage in Toronto. I do remember that there is an adjustment - i.e., to a certain depth it is one correlation; below a different correlation. I found them to be quite useful. I had one site that gave weird results due to gravel in the SPT - but the pentest proved to be a much better indication of the "denseness". I had wanted to use this test in New Jersey for finding thin sand layers within organic deposits that were going to be preloaded and the owner said no - even though he had worked for Geocon at one time; then when I moved back to Toronto, he called me up and begged for the Toronto area manufacturer - he had a need of this more robust testing for deep piling in Atlantic City where one pile load test had an unexpected "failure" due to a very localized clay pocket below the tip - so this was a way to determine if the piling needed to go deeper. The benefit of the pentest (for a dynamic cone test) is that is much more robust than the other pentest types which are for much shallower usage.

 

[BigH]

BTW - have you tried getting in contact with the Canadian Geotechnical Society? - or you might try to contact Becker at Golders - he is, I believe, a large driving force in the newer manuals and might have the old one.

 

[Melzar]

For the Sowers DCP, I have seen the average number of DCP blows for 3 intervals equal to the N-value, where each interval is 2 inches.