DCP on sand? 2

[Rjeffery]

Is the DCP (dynamic cone penetrometer) appropreate for checking (verifying) the bearing of poorly graded or even well graded sand? The top size of the sand in question is less than #10 seive. Silt or clay fraction is small to non-existant?

 

[jdmm]

Dynamic cones vary widely in design and energy delivery. Consequently you have to either have the benefit of some local experience or you have to determine the suitability of the particular equipment relative to SPT values by carrying out SPT in holes adjacent to the DCP holes. Personally I only use DCP to determine the depth to hard ground through soft ground to estimate pile lengths for example. For the soil you describe the CPT is far superior because of the standardized design, repeatability and design procedures available.

 

[BigH]

Agree with jmdd. Still, in my practice, our cones were "standardized" - same machine shop made them for all the drillers and engineers. 2" diam 60deg apex. We drove with 140lb. hammer at 30" drop. Canadian Foundation Manual has relationship of Ndcp vs Nspt. Unless recently, didn't include any energy corrections for N60.

I find them useful. Was working in NJ and they have coastal organic soils interspersed with sand lenses. I had suggested that the DCP be used to "verify" the locations of sand lenses as these were imporant in preload/surcharge estimations. Didn't meet with much success until, after I had returned to Canada, a pile load test didn't do "so well." Seems there was a soft clayey zone just below the Franki pile and it reduced the capacity. Engineer ended up using the local machine shop to get several hundred cone tips (disposable - they are left down the hole) for his use.

Bottom line - all tools have some merit but you must decide on how the correct context for a particular method and, when you have the results, the correct interpretation.

jmdd - wouldn't the piezocone be better than the cpt?

 

[Ron]

In my area the DCP is commonly used on sands. Use the Sowers style DCP with 30 lb. ring. It can be correlated with some degree of warm fuzzy to SPT. Check Sowers (3rd Edition) or other Geot. Text for parameters. I'll try to remember to look up my info in my office and post later.

 

[Rawand]

As fara as I know DSP is good what ever the grade of soil is, Check Soil Mechanics Mcgraw Hill since the soil is not cohesive or it is neglectable

 

[coneboy]

As mentioned above, the SPT blow counts should be corrected for hammer energy. An older donut or safety hammmer with a cathead have erratic energies so correcting your SPT values could be extremely challenging. Auto trip hammers generally have between 60-110% energy.

I realize budgets don't always permit electronic cone testing, but if a high degree accuracy was required then CPT testing might be the best option.

One other brief comment: To the best of knowledge, most of contractors in N. America use a pore pressure sensor on the electronic cone.

Coneboy

 

[MRM]

This is part of a response I've just posted in the Soil Testing Section under "Machintosh, DCP, CPT." It sort of relates to this thread too:


I’ve done some work with our DCP in our area to provide us with a correlation of the DCP blow counts with the soil density, and as a comparison to the SPT values. Our soils in Northern Michigan are generally water deposited, fine to medium quartz sands at many locations. Other organic deposits (Lake Marl, Muck, Peat) and clay deposits exist, but much of the area is the sand described above.

I’m not sure what the difference between our DCP and the Mackintosh probe are, but our DCP uses a 15lb weight falling 20 inches, advanced 1.75” at a time. The cone angle is 45 degrees, slightly rounded at the end. 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.

The testing I’ve done has included DCP’s performed adjacent to SPT’s, and DCP’s performed at various levels below the surface on a given site with excavation afterwards to those levels, and actual density testing with a nuclear gauge at the various levels. I’ve also obtained some bulk samples to determine limit densities and use the nuke gauge to determine the estimated relative density of the soil in-situ in some cases. In this way, I’ve been able to establish a few good correlations. The sites I chose were uniform in soil type and relatively uniform in soil density. Here are some of my main conclusions to date:

Overburden: approximately 1’ to 6’ below subgrade level:
Average DCP blows per 1.75” after seating increment Relative density
1-3 Very loose to loose
4-7 Loose to Medium dense
8-12 Medium dense to dense
>13 Dense to very dense

If blows are less than say 2, it is usually a good indication that there may be loosely placed imported fill, since most of the soils usually have a in-place RD of 40% or greater (meaning it is sometimes difficult to find natural soils with an RD less than 30-40% (in my experience) unless some major particle bulking is still occurring since deposition. As the relative density approaches 90-100%, the DCP blows tend to be very high (>30/increment or more). Additionally, the DCP is affected the same as the SPT when it comes to silty and very fine sands in that unrealistic values may result from dilative tendencies. Gravel influences the DCP blow counts by raising the values, similar to the SPT. The values are also influenced by the amount of overburden as with the SPT. I’ve found that if you’re 1’ to 6’ below subgrade, the effects are not very pronounced. If deeper DCP’s are made, you should think about an overburden correction similar to the SPT to arrive at a better indication of the in-place relative density. DCP’s below the groundwater table are usually not possible since an auger hole will not stay open as they would with drilling mud or hollow stem augers-this is one of the main limitations of the DCP in my opinion.

Please share your own findings too. I’d like to hear them. I hope some of this is useful to someone.

 

[Rjeffery]

MRM

I have read the ASTM STP 399 and have found it to be an asset when doing geotechnical investigations at shallow (hand auger) depths. In my area, the Patomic river Basin, we often run into water deposited sand (and clay) lenses.

I was wondering if the use of the DCP to verify (for quality control purposes) bearing in the area where the footing has been excavated, prior to placement of reinforcing steel and concrete, if the material is clean sand is appropriate?

I feel that it is not. Generally the DCP will give reliable reliable results in cohesive materials that are a few % points on either side of optimum moisture. but on sand or gravel the results are skewed to the weaker side of breaing capacity.

There has to be a better, quicker way of verifying that the breaing is acceptable in a footing that will not receive any (or little) dynamic load.

I am at a loss, hence the question.

 

[MRM]

RJeffery,
I've had good results using the DCP in sands (particularly without silt and gravel). Unless the gradation or minerology of the sand in your area is way different from the stuff I've tested in, I would think you'd have good results also. Is your sand especially silty and/or have substantial gravel content? Carbonate or mica content? I would think those things could increase the variability of the results obtained with the DCP.

The application you mentioned, of using the DCP in the footing subgrade prior to steel placement, is where I typically use the DCP to check local areas of the subgrade. Please note that it is important that the DCP FOLLOWS a more conventional soils exploration with SPT's or CPT's. This is where the DCP really shines; it tests localized areas of the subgrade (up to 10 or 15 feet below subgrade if you want to), providing a follow up to the more conventional drill rig work. In addition, during the DCP testing, you're also checking that soil types are consistent with the soil exploration because it is necessary to hand auger to the various testing levels (and visually examine the soils).

As BigH said before in another thread (or this one maybe), "you've got to use what you have." The DCP can be just as reliable as the other methods if engineering judgement is used during the testing and result interpretation. The results of the DCP should not be used to say, "the allowable bearing pressure here is 2140psf and over there it is 1342psf. The DCP gives general information that can be used to evaluate the general allowable bearing pressure for a footing.

I haven't had much experience using the DCP in clayey soils. I'm interested in your comment about its applicability to those soils. Sometimes we run across them and I'd like to start to use the DCP to check footing subgrades that are clayey too. If you would, please provide some more info on correlations and caveats of the DCP in clayey soils. I'd find that interesting and your tips might give me the ammo I need to convince others here in the office that there is a value in running the DCP even in clayey soils. Thanks.

 

[Rjeffery]

MRM

Please contact me at nh_yankee@hotmail.com

Thank you

 

[BigH]

This is a thread that piques my interest. First, we should use monikers like DCP with some caution - there are many cones out there in all shapes and sizes. My first comment was on the Canadian practice of using 140lb. hammer driving cone with 30inch drop - the cone is 2" 60deg apex angle. The tip is actually such that a cylindrical tip is added to a tip at end of A rod to form cone - when done, the outer shell is left down the hole.

A second cone that I am familiar with is the miniture cone developed by the TRRL of UK. This is a small diameter cone that is used to check CBR (at least that is the overall purpose) of subgrades and underlying soils. It is a small hammer with a drop and you basically drive 10 blows and measure the penetration (in mm). Drive another 10 blows and measure penetration. Continue. Is this the CPT of MRM and Rjeffery? Please advise. Enjoy the comments and lots of good info is given!

 

[MRM]

Hello BigH,
My DCP was described in my original message as:

...our DCP uses a 15lb weight falling 20 inches, advanced 1.75” at a time. The cone angle is 45 degrees, slightly rounded at the end. 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. (more info on the DCP is in my previous post)

I agree with you that in order for anyone to make sense of anyone else's DCP blow counts, the DCP used to obtain those numbers must be described. There are many different types around, and some people have even machined or created their own. There is very little standardization to this point, but maybe that will change with time.

 

[djeep]

Rjeffery,

If I am interpreting your queries correctly, you are really trying to characterize bearing conditions in sand in real time during foundation construction.

Under the right conditions you may want to consider the use of a plate load test(s) using Schmertmann's method. In a nutshell, you determine the compression modulus Es for a measured settlement during the plate test, and apply this modulus to the footing geometry to estimate the settlement of the actual footing.

Although you will not directly solve for bearing capacity (unless you load the plate to failure…. doubtful) you will be able to (perhaps more importantly) estimate the elastic settlement of the footing. You (or preferably the structural designer) may then decide if the structure can tolerate the estimated settlements.

I have found this approach very useful for on site footing evaluations on non-cohesive soils. It is fairly quick and requires a minimum of equipment. In order for the test to be meaningful a few conditions should be met as follows:

1) Plate diameter should be of a similar dimension to footing or pier width, applied at footing elevation. This is important because you want to at a minimum encompass the same zone of bearing influence as the footings. Therefore the test may loose its practical significance if the footings are large.

2) Test is intended for non-cohesive soils

3) Should have a good feel for overall soil conditions on the site and in the area.

4) Apply a significant pressure with the plate(s). I try for at least 2-3 times the presumptive allowable bearing pressure of the soil. (Which puts a lower bound on your factor of safety)

5) Don’t overlook more global issues such as liquefaction potential of loose sands, compressible layers at depth subject to increased stress, slope stability etc.

Good luck, if you need more info on the method let me know

 

[TJWATKINSPG]

My experience of using an electronic cone penetrometer is one that I am proud of. I have followed a name brand e-cone with STP's and found (in Florida and the surrounding Gulf Coast plains sediments) that the results stack well with each other. I have used the CPT to define where to best recover shelby tubes with excellent results. The depth/strata comparison is remarkable. I have been on a cone rig that avearged 700 feet of hole per day. When combined with one or two SPT borings I found the site evaluation unquestionable. The truth will set you free. Best of luck. Geodude