DCP to SPT correlation 1

[eric1037]

I posted this on the Soil Testing Forum, but didn't receive a response. I thought I would try it here.

OK. Another Dynamic Cone Penetrometer (DCP) thread. I believe I have read all of the threads on this site concerning the use/misuse of this tool. I have also done extensive research on the web as well.

Unfortunately, there seems to be a significant lack of published information.

A little background:

Our company has been using this tool for verification of design bearing capacity. It is most often used on projects that we have done a geotechnical evaluation and we are verifying that the actual conditions match up with the geotech borings. We use it primarily in sand soils. The projects are primarily lightly loaded structures with relatively narrow foundations.

As the geotechncial manager, I want to be thoroughly educated on the use of the DCP.

Our DCP uses a 15lb weight falling 20 inches, advanced 1.75” at a time. The cone angle is 45 degrees. A seating loading increment is usually completed prior to counting. A paper was completed by G.F. Sowers and C.S. Hedges “Dynamic cone for Shallow In-Situ Penetration Testing” ASTM STP 399, Am. Soc. Testing Mats., 1966, p 29. This paper also has some correlations of the DCP blows to SPT blows for various soils including piedmont soils in the south.

Unfortunately, we are not in the south and do not have piedmont soils.

What I am wondering is if anyone out there has any correlations between this DCP and SPT N-values in post-glacial soils in the Michigan region. If not, does anyone have any sources where I may attain this information?

We have not gone to the effort of actually performing SPT right next to DCP tests. I am currently in the process of trying to convince the company president that this is a worthwhile endeavor. Unfortunately, the soils in this area a quite variable and it would take significant testing in variable conditions to get good correlations.

Any advice? Should we just use a different method to verify design bearing capacity? If so, is there a relatively cheap, quick, and easy method to do so?

Thanks for your input!

 

[Ron]

Check Sowers 3rd edition text. I believe it has something on it. Sowers also wrote a paper in ASCE on the DCP. I don't have a copy of either anymore.

 

[BigH]

Eric1037: check out your other post. Have patience!

 

[cbosy]

I always thought the dynamic cone was worthless. I suggest using a static cone instead. I have been using it for over 20 years from FL to the midwest and it hasn't failed me yet. You also get bearing capacity right on the spot.

 

[BigH]

Cbosy - you must have lots of money in your budgets. Sure the static cone is better (and the piezo-cone or seismic-cone even more so) - it really wasn't an issue when the DCPs were developed (what I call the pentest); but given budgetary constraints, other reasons, don't throw away good tried and tested methods (albethey somewhat "coarse"). Its like new engineers today who have no idea what an engineer's scale is.

 

[cbosy]

What are you talking about? It is a manual version made so one skinny person like me can handle it in the field (and even in low headroom basements)! My boss worked in the South for 30 years (he is not "new" and I wish I still was) and would throw a tech off a job if they showed up with the DCP!

 

[eric1037]

cbosy:

Is the static cone that you refer to have a proving ring at the top that indicates pressure? How welll does it work in sand?

 

[BigH]

cbosy - okay so you are talking portable. I am talking the drill-rig version. The DCP (pentest) that we use(d) in Canada (since I started in 1975)cannot be hand done - with 140lb hammer and 30 inch drop! (actually, it could, but only limited and with "big" boys!)
This points out a caution that I've made several times in other threads. When one says "DCP", there are a number of versions ranging from miniature TRRL-types to full rig set-ups. Similarly, in static cone - there is the reconnaisance style set-ups to full rig set-ups. We all need to be clear as to which variety we are using. As to throwing off the tech from site - probably should have thrown off his superior for sending him there with it - if you were unhappy with the selection. My mentor, one of Canada's leading geotechnical engineers, used pentests all the time as one of the toys in which he based his judgment.
In any event, whatever tools are used, one must know the limitations of such and how to apply the needed judgment in using the various tools. Miniature TRRL DCPs are used all the time and all over the world in road construction as a "measure" of CBR values - although I don't give much creedence to them, personally. But, in a Schmidt (rebound) hammer estimation, they do have some value.

 

[cbosy]

eric1037-it is hydraulic and reads out in pressure. It is really made for sand but can be used to back up Qp values in clay. It is made by Durham Geo-Enterprises 770-465-7557