DCP to SPT Correlation

[eric1037]

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!

 

[BigH]

eric1037 - you are on course to develop locally applicable correlations - a lot depends on the type of SPT hammer you use, the number of wraps the drillers use, etc. You might even wish to explore correlations using different drillers, too. The Canadian Geotechnical Manual has some information but as I've indicated in previous threads, this is for cones of 60deg apex angle, 2inch dia, 140lb hammer dropping 30 inches - in other words - similar to SPT work.
I note that you have mentioned the use of this to "verify design bearing capacity." If you have read some other threads, especially ones that Focht3, VAD, myself and a few others have responded too, you must make a distinction between design bearing capacity and allowable (net) bearing pressures. Capacity is based on shear; but, allowable bearing pressures are nearly always based on permissible settlements. E.g., you might have a design bearing capacity of 8ksf but estimates of settlement under this pressure would be 1.5 inches. But, you have, say, a permissible settlement of only 0.5 inches, or 1 inch - then you would have to modify the bearing capacity to a permissible bearing pressure (net).

 

[eric1037]

BigH:

Thanks for the input. I will note that when we provide a maximum net allowable bearing pressure, it is most often governed by settlement. The only time there are actual bearing issues is when the footing is very narrow. Therefore, we recommend a minimum foundation width.

What our field representatives do is verify that the actual bearing capacity of the soil (based on both bearing capacity and settlement) is at least the recommended bearing pressure with the appropriate factor of safety.

Therefore, there will need to be correlations on a variety of soil gradations. My thought is to just send a technician out with the driller for a period of time and do the tests next to each other on a variety of sites.

Over time, we will develop local correlations. But what do we do in the meantime?

At this point, we are using correlations developed for soil that isn't even in our region. I am not entirely comfortable with this practice so we have been very conservative. This may lead to soil improvements that are not really needed.

Is there any other proven technology to perform verification during foundation construction? We need something that meets the following criteria:

1) It needs to be portable and easily operated by one person.

2) It needs to be able to probe an absolute minimum of 1 footing width below the bottom of the foundation.

3) It needs to be fast and easy enough to use to be able to perform multiple tests at the pace that contractors work.

4) There needs to be established correlations to verify design bearing capacity.

5) The results need to be repeatable.

6) There needs to be sufficient documentation to be able to defend the test technique in the case of litigation.

Does anyone know of a device that fits this bill? Currently, I am leaning toward the DCP, but we are open to other technology. I have corresponded with Dr. Paul Mayne from Georgia Tech and he recommended a device called a helical probe.

Does anyone have experience with this?

 

[BigH]

WIll mull over your points - say hi to Paul for me when you next talk - I went to Cornell with him.
Howard Fries

 

[TrimbleUser]

Eric:

I understand your problem fully!
One quick evaluation which is cost effective and proportionately representative of design bearing capacity and allowable (net) bearing pressures that can be operated by just one manin the hills or wherever, is simply to use the heel of your shoe!

Yes thats right, elevate your front left or right (optional) leg and bring it down under its self weight. (obviously you have to weigh your leg first.) therefore Weight of leg multiplied by the force of gravity 9.81m/s. Measure the actual penetration of the soil and there you have it design bearing capacity and allowable (net) bearing pressures right under your feet! You can then apply scale factors to scale up the loads/penetration values to suit!

Please feel free to use this method as and when you please. The Choice of shoe obviously plays a determining factor, training shoes do tend to absorb a large percentage of the stress due to the spongevity and needs to be correlated for. Casual evening footwear can be a little too delicate. I myself prefer a dutch wooden clog as this gives optimum results although slightly heavier.

This method is a tried and tested result of british engineering and at the forefront of our technology. Large Damns, runways, channel tunnel and that huge french bridge (just been built) have all been constructed using this method.

Glad to be of help, if you have any other problems of civil engineering nature that you wish me to solve for you please feel free to ask as I have much experience in this field.

iain

 

[eric1037]

TrimbleUser:

LOL! Unfortunately, you are very close to the truth. What we have is not much better. The only difference is that we can assign numbers that give everyone a warm and fuzzy feeling.

 

[BigH]

TimbleUser - the ol' heel test - of course!!!! Likely, though you need to hire a tech that weighs 20 stone rather than 12 stone in order to gain proper impact depth, eh? - and then determine width of heel to ascertain to some precise certainty the impact pressure!

 

[MRM]

Eric,
I've done some work in correlating DCP values in Michigan. Please see the following threads:
261-59796
261-39397
Enter those in the search engine on this site for "threads" near the top of this page.

One point of caution in looking at my correlations and info given and applying them to your soils: Carefully check your sample for physical properties especially particle shape, (i.e. roundness/angularity, bulkiness/sphericity, roughness), mineral type, and gradation. Variations of these properties can lead to variations in the values obtained using the DCP. These variations in properties also affect SPT AND CPT results in similar ways as we all know. Hope those threads help you.

 

[fabbdc]

I have used a DCP (with hand augers) in several location across the USA and Canada (glacial till) county in various different soils. To calibrate my equipment, I generally use three field tools to get general soil properties - DCP, Torvane and a Pocket Penetrometer. I use all three to get the general feeling of the soil. If you are looking for more accurate results, I would not rely on field testing, you should use laboratory results (CBR, proctor and Atterberg…) and SPT field testing (using a drill rig).

Even with laborator results, I always stay conservative with soil properties. It is common to see variations in soil properties in the range of 10 to 25 percent in materials that appear the same.