DCP vs Bearing capasity 4

[jmirish]

I have completed a DCP test and have the results. How do I correlate the results from this test to the bearing capasity of the soil? I dont want to use CBR.

 

[MRM]

Here are my thoughts, jmirish: I sometimes use the DCP to evaluate allowable bearing pressure on sites that meet the criteria for the test by first getting blow count values beginning at the subgrade level to the required depth below proposed footing level, as you have done. Second, I take the values and estimate the modulus or compressibility similar to how you would handle CPT or SPT information. You may then estimate settlements and an allowable bearing pressure for the foundation in a way similar to how you would if you had other "conventional" soils information. The number of tests is a site-specific decision also, just like a conventional exploration.

The DCP is a usable test under the right site conditions and the properly evaluated proposed structure. The following applies only to estimation of allowable bearing pressure for footings. If other information for special engineering projects is needed, a conventional soils exploration should be designed with the proper equipment. The "right" conditions for this discussion include:

1. Groundwater table is deep enough so that an auger hole can be made without caving in. You can't test the soils unless you can auger down to them.
2. The building or structure is of lightweight construction, or the proposed footings are small enough so that you can test/observe the soils to a depth of about twice the footing width for column footings. I like to go a little deeper for wall footing since the stress influence is deeper. Obviously, the bigger the footing, the more cumbersome a hand test like the DCP becomes. Say a proposed footing is 10 feet wide, you're not going to hand auger and test at intervals to 20 feet below grade. I've gone as deep as 10 feet with a DCP and it gets heavy and dangerous to handle even at that depth. In those cases only a conventional soils exploration will do.
3. No additional fill will be placed on the site. Ex. if 8 feet of fill will be placed adjacent to the structure after you're gone, deeper soils out of the range of the hand auger and DCP can not be evaluated. The imported fill could drag the foundation down with it if compressible soil is present at depth. A deeper soil boring would be required in this case to check deeper soils.

If even one of these requirements is not met, then a conventional soil boring exploration is required.

Speaking to the DCP test itself, I have the following comments:
The DCP test is just as usable as the SPT provided you are working with a site meeting the conditions above and you have a suitable correlation between DCP blow counts and soil density, behavior, strength and other properties you'd ordinarily estimate based on the SPT. The correlation should be one that is based on your own experience. If someone else gives you their correlation, there is a good chance it won’t apply to your soils or situation. It is important to develop your own correlation based on the soils you're working with and your experience. For example, I've got several years worth of information regarding the DCP that I've collected. I feel that if the SPT is adequate to develop recommendations, the DCP provides information that's at at least as usable and perhaps more reliable. Some researchers have even suggested that the cone shape of the DCP provides a more consistent measure of the soil than an irregularly shaped SPT sampler. I would agree with that. Remember that the DCP configurations vary widely and again, correlations with another DCP may represent values for a totally different device unless you can be sure the DCP’s are both identical.

The DCP itself has many of the same caveats to its use as the SPT or any other dynamic test. A saturated dense silt or silty sand will give blow values greater than static, a saturated loose silt or silty sand with give blow values less than the static just like the SPT. The DCP is also allergic to gravel and cobbles, just like the SPT. In short, the DCP values must have the same amount of engineering judgment applied to get the best estimate of the behavior of the soil, i.e., those blindly using the DCP values to "plug into an equation" will run into problems, just like with the SPT.

To summarize, be aware that there are only certain applications (sites) the DCP should be used, and a "feel" for the DCP results must be established based on experience and testing. If the site is suitable, and you have a comfortable "feel” for the results and what they mean, the DCP is just as usable as the SPT. I know the usefulness of the SPT is often in question, but in the real world, it's sometimes all the cost the owner is willing to justify, and it has been used for 50+ years with "good" results in most cases when a healthy dose of engineering judgement is the main ingredient of the soils exploration.

 

[DBNodurf]

The DCP was invented by George Sowers for checking footings IN RESIDUAL SOIL when you had also explored the site with SPT. Anything beyond that is extrapolation; it amazes me what some folks are doing with the lowly DCP such as evaluating slopes and dams... arrrrgggghhhh!!!

 

[MRM]

Hi DBNodurf. Slopes and dams? No, I don't recommend you use the DCP for projects like those. The DCP should only be used under very specific conditions (project type, soil type, etc).

 

[Focht3]

[blue]DBNodurf[/blue]:

I agree. Although I didn't spend a lot of time with George, I spent enough to know that he NEVER intended for the DCP to be used as many are doing so today. Since he was one of my Dad's classmates at Harvard, I had many opportunities to talk with him before he died.

The misapplication of tools and techniques is all too common in our business; it is often a matter of convenience or cost, but occasionally a matter of laziness or ignorance. And it can be hard to tell the difference.

Keep in mind that [blue]Eng-Tips[/blue] is not exclusive to degreed engineers - many of the members have no degree at all. So the theoretical basis for many common tools and techniques may not be clearly understood by all who participate in these threads. And very few complete the personal profile, so there is usually no way of judging someone's credentials. (In one thread, I found myself arguing consolidation properties with a lab technician...)

It is occasionally a burden that we must bear while on these fora -

 

[MRM]

I agree with both of you in that the vast majority of geotechs misuse the tools available to estimate soil behavior-that includes testing methods and equipment, published charts and tables, and even the advice of others. As I stressed earlier, solid engineering judgement should always play a major role in the final verdict.

I would recommend re-reading the George Sowers and Charles Hedges technical publication related to the DCP (Dynamic Cone for Shallow In-Situ Penetration Testing, ASTM STP#399) that you have referred to. On page 4, Sowers and Hedges state, “A secondary use (of the DCP) is the field investigation of subsurface conditions for lightly loaded structures where local experience from previous field investigations and laboratory analysis have established narrow limits of the strength parameters and consistencies; here again it is a verification tool to be used for an economical foundation analysis.” The authors stress that it should only be used “…where local experience from previous field investigations and laboratory analysis have established narrow limits of the strength parameters…” I couldn’t agree with them more. If you don’t know the soils, or the soils aren’t conducive to the use of the DCP, don’t use the DCP. It’s that simple! You’ll have to use another method in that case. If you don’t know what the results of the DCP mean and you are unable to interpret them because of a lack of knowledge of the DCP or soil mechanics in general, don’t use it!

On page 9 the authors go on to state in the conclusions, “…the dynamic portable penetrometer is a useful tool for construction control and field exploration for lightweight structures where value does not justify the cost of a drilling rig or where access prohibits a drilling machine.” Again, I say know the soils or do not rely of the DCP solely (see my initial post for an abridged summary of other caveats).

Another word of caution to anyone involved in this discussion; read the article, but “x” out the chart presented in the paper, or at least ignore it to some extent and do not use it in practice! There is only a small chance they would ever apply to your soil conditions where you intend to use the DCP. I would recommend undertaking an on-going 2.5-year personal study, like I did, on the correlation of the DCP with the SPT and relative compaction/density estimates with a nuke gauge for your own local soils conditions where you plan to employ the DCP. This will help identify the limitations associated with its use.

The notion that George Sowers did not intend for the DCP to be used for the application I described in my initial post doesn’t hold water with me.

 

[BigH]

I'd like to wade in - for a thought or two.

(1) In MRM's original post, he intimated (thought I'd throw in that term for the ol' India hands) something very basic and one that I find missing in any thread on DCP. What is the DCP you are talking about? There are many kinds - we used a 2inch 60deg apex cone attached to A-rods and driven with 140 pound hammer. This is the variety in Canadian Foundation Manual. Others use smaller hammers, smaller cones, or the miniature DCP from TRRL (for roadway CBR correlations). When we talk of the DCP, we should always remember that there are many varieties and we should indicate which one of which we are talking.

(2) MRM did clearly state that he developed site specific correlations in his area of work for "his" DCP and the SPT - this is important. He cautioned about this - and the need for engineering judgment. All the practicing engineers in these forums (nearly all) understand the importance of engineering judgment. I think that this is one of the big differences in our field to others - we have to, I believe, use a lot more of it than others - and we still get boxed into . . .

(3) As in any field or lab test, one should appreciate the reason for the test and understand the limitations.

(4) I've use the DCP noted above (we called it a pentest) ever since I moved to Canada. It is a good test - given other information. I have used it to delineate soft clayey zones in sand deposits. I suggested its use as such in NJ one time for identifying sandy zones in organic deposits as such are important in preloading situations (faster consolidation if sandy zones are present); and it was not well received as a suggestion - why, likely due to its 'archaic' history. Still, within a couple of years, I had the firm come back to me to see where they could buy one - why? They had a pile load test come up "short" and later found that the sand deposits had, occasional pockets/thin layers of clay. So, they wanted a cheap throw-away test (using disposable cones) that they could drive at the tip of the Franki-pile before they did the concreting so that they could get an idea if such zones were present at reasonable thickness below the pile tip.

So, in the end - like any tool - if you know the limitations for your use and understand how you wish to use it, it is a fine tool. If you try to get more out of it than you can, it can be devastating - but isn't that true of most investigative methods?

to all. Thanks MRM for several good posts.

 

[MRM]

BigH,
Regarding your post, and especially your final paragraph...BINGO. I was beginning to worry that I was the only one who understood this fundamental concept.