DCP Help 1

[stevie714]

Need some help on a basic calculation for determining bearing capacity from a dynamic cone penetrometer test. Test was done counting blows number of blows for 1.75in w/a 15lb. weight dropped 20in. Material is fine compacted sand w/some to trace silt (fill). Blows for the two holes were 12/10/9 and 12/21/22. Need to have capacity of 2000 lbs/ft2.

 

[PSlem]

You should just seat it the first 2", although 1.75" is enough to bury the cone, then read the second 1.75". There is no third reading. I assume you want to do the physics. 1.75"/10 blows=.175"/blow. You have a 15# weight falling 20" so the force is the ratio of the g forces, or 20x15/.175=1714#. The tip is 1-1/2" in diameter, or .01227SF=139,690 PSF. There are confining pressures, etc that will greatly drop this number when aplied to the soil. I haven't seen any papers that attempted to do this mathematically. In my neck of the woods 10 blow would be 2500, but we have silt, no sand.

 

[Ron]

stevie714...find a copy of George Sowers textbook either 2nd or 3rd edition and this should help. There is also a paper written by Sowers and published in the ASCE Geotech Journal that covers this.

Assuming you have reasonable confinement (12-18 inches of adjacent overburden), you are likely OK with the values you are showing, but you should check it.

 

[MRM]

It sounds like we use the same instrument, so hopefully this helps.

I can't tell you what the allowable bearing pressure is of the soils under consideration because that value depends on several other items not given; structure type/purpose, proposed footing/foundation element size or width, soft underlying soils, and level of risk considered acceptable, to name a few items. With the DCP test, you get blow counts that can be correlated with relative density, which is just one component to properly evaluating allowable bearing pressure.

However, based on the numbers you've given to us, and assuming that these readings were obtained in the upper say 2-3 feet, and the soil is relatively uniform gradation with generally SA to SR particle shapes, I'd estimate the "10/9" hole to be 60-70% relative density, and the "21/22" hole to be above 80%.

Now for a more pressing concern: The first set of numbers appears to decrease. This could be a sign of a problem. Since you said this is a compacted fill, there is a chance that the soils below that are much looser. I don’t know if you or someone from your firm observed the compaction activities, but in my experience, some earthwork contractors like to put thick fill lifts in when no one is looking and then beat the top as hard as they can before you show up to test compaction. Little do they know, even a large vibratory roller can only improve the density of soils down about 3 feet below grade at the most. Below that, it remains loose. I don’t know that that’s the case here, but it may be.

Here’s what I would do, unless you know for sure that I'm way off base: Go back to the site with a hand auger and do a few more DCP tests. Perform DCP tests in the first 2 feet below grade or so, then 4 feet, then 6 feet (the approximate reach of most hand augers without extension), all in the same boring. Or you can also perform DCP tests continuously if you periodically jiggle the DCP to open up the hole to keep the side friction low. I’ve been on many sites that the contractor did just as I described and compacted the upper soils only. It can be hard as a rock at the surface. On my DCP it typically has looked like the following from the upper compacted soils to lower: 10/12/13/13/12/10/8/5/4/2/2/2/2, for example. After you see 3's and 2's, you can usually push the DCP in by hand with ease. My concern is that you may have noticed a continued decrease in blow counts if you had advanced the DCP further. Do you think this could be the case based on your knowledge of the compaction activities?

 

[BigH]

just a pet peeve - we need to be careful as to accurately describing the DCP - there are miniature versions to 2inch ones driven with 140lb hammers at 30inches. Post had the hammer weight, the drop but not the cone dia. While you all seem to be in agreement as to the type of cone, there isn't a universal standard version of a DCP.
- and MRM is right - I've seen a problem in a large site fill where large settlements in housing development occured because the top of the layer was okay but the bottom was not compacted at all - sad.

 

[MRM]

Very true about the DCP "version." My DCP fits the original description of 15# weight, 20 inch drop, and 1.75 inch increments. My DCP is 1.5 inches wide at the widest part of the cone and the shaft is slightly narrower at 1.375 inches. The cone has an angle of 45 degrees. I believe the DCP was manufactured by Boart Longyear. Stevie714, please check to see if mine still matches yours.

 

[stevie714]

Thanks for the help everyone. The DCP is the one that was manufactured by Boart. Basically, this is one of those jobs where they don't want to pay to adaquetly test the materials so I was trying to give an absolute "snap-shot" of the bearing the material where the test was conducted w/o any inference to the materials below it. Aparently, the site was already approved based on earlier geotech testing but they dug the footers one foot deeper and needed the bearing of the material at the base of the footer. Thanks again to everyone for the help. This is not my speciality but everyone else in the office was swamped when this came through the door. Thankfully, I was able to pass this off on one of our geotech engineers to finish up the project.

 

[eric1037]

I have recently done some study on this exact DCP. Sowers 3rd edition book says that the DCP per 1-3/4" should be approximately equal to the N-value. However, in the ASTM STP 399, Sowers and Hedges provide graphs that seem to contradict this.

There is a great paper from 24th Annual Symposium on Engineering Geology & Soils Engineering called "Dynamic CPT for Test-Pit Field Investigations: Experiences with Sowers' Cone Penetrometer" by L.E. Robinson that provides several equations to correlate Sowers' DCP blows to SPT.

If you search Google, you will find a couple of other papers on the subject.

After reviewing all of the available info that I could find, I have come to the determination that you have to develop your own correlations based on project experience with your local soils. The correlations that are available are all over the board. It is very difficult to come to any conclusion by comparing them to each other.

I would hope that such a common tool would have much better research, but that is not the case.

I am currently researching other methods of testing that are better researched.