Energy Correction For Automatic Hammer 1

[cbosy]

This is more a poll then anything else. The automatic hammer delivers about 90% of the theoretical enegy while the old cathead driven hammer delivers about 60%. Several papers now say to correct the N Values by multipying them by anywhere from 1.25 to 1.5. I personally use 1.4. What are are you using?

 

[jdmm]

cbosy,

I use the CPT. I am suprised at the number of companies that still use SPT. I recognize that much of our design is based on SPT but really... You can't get two drill rigs to give the same value at the same depth so why would you continue to use it.

Two CPT rigs would give the same value of N(1)60 at the same depth within a blow or two and you could get SPT values at 2 inch intervals to depths of 100 feet within 2 hours.

There was controversy in the 80's about the correlations...confirmation in the 90's...please, this is the 21st century...... dump the cat head vs. auto hammer controversy and advance a repeatable test, the CPT.

 

[cbosy]

Thank you for quite uselessly not answering my question. This is glacial till country up here and the cone is rarely used because of inpenetratable layers and cobbles. Anybody else have some CONSTRUCTIVE comments?

 

[Focht3]

I very rarely use SPT correlations for design; but then, my geology permits me the luxury of avoiding the use of SPT - &[ignore]phi[/ignore]; correlations...



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[MRM]

Sorry about the long post.
Some time ago, I had the same question for myself and I sat down and attempted to determine what the correction is for the raw SPT values. First, the SPT correction question is one big mess in general! Here's my best effort to make sense of it.

Here's some background first:
When I use the SPT to help me determine the compactness of the sandy soils, I typically use the Gibbs and Holtz chart just as most people do. Now through my research I found that the N value shown on the Gibbs and Holtz chart was not necessarily the N60 value. This is because they didn't correct for certain items in their work (as far as I can tell). If they did correct, they didn't make it known. Anyway, the values I've got below reflect the corrections I believe are necessary to get to the N value used by Gibbs and Holtz (Ngh). I call it "Ngh." This helps me distinguish it from N60, since Ngh is not necessarily the same as N60. This avoids my own confusion mostly.

There is no overburden correction included in the values below because overburden is a variable in the Gibbs and Holtz chart. The corrections attempt to normalize the differences between the hammers, linings, borehole diameters, etc. that we use compared to how Gibbs and Holtz did their work.

CME-45 rig (manual safety hammer)

depth correction factor
0-10 feet 1.0
10-20 feet 1.1
20 and down 1.2

CME-55 rig (automatic hammer)

depth correction factor
0-10 feet 1.1
10-20 feet 1.2
20 and down 1.3

Now typically, I do not use these factors directly at all. I only did the research to see what the correction factors might be. I do use the factors indirectly in that I can have some kind of a feeling that the raw N values are on the conservative side (i.e., they'll lead to more conservative estimated of Dr, phi, or whatever). That's usually a good thing, right? Also, when I estimate relative density of a soil mass, only qualitative descriptions are usually appropriate. The sand is medium dense, not 60% Dr. Or I would give a range, say 55 to 70% Dr. Afterall, a qualitative idea of where the soil is at (and your experience) is all we really need to evaluate the allowable bearing pressure in most cases. If you get more "fine tuned" than that, you may be setting yourself up for problems.

One more note on the subject though...
As I've mentioned in past threads, I've had the opportunity to perform SPT's on a site and then later evaluate the same levels and areas with a nuke gauge during excavation with a toothless backhoe. I've been surprised with the level of agreement between my Dr estimates based on SPT's and the actual relative density of the sand tested! I've done it enough to know that it's not just coincidence too. Note that the sites I've done this on though were as uniform as you could expect a soil mass to be. In our area, we have several areas that contain normally consolidated (yet somewhat aged) fine to medium sands that exhibit textbook SPT behavior. By that I mean the N values increase at a steady rate with overburden such that after doing a few holes you can almost predict what the next SPT count will be 5 feet down from where you’re at. These are typically the sites I've done tests like that on. It's always nice to know that some things can possibly be somewhat predictable in the geotech world! That's how I felt anyway! Now the sites that are heterogeneous I can’t do that, but it "ain't no thang" because now I'm back to using the SPT as just a simple indication of soil conditions along with collecting my sample to play with.

I've gone on long enough now! It really is a subject that interests me though.

 

[DRC1]

I never have had information from an automatic hammer. I guess I live in the stone age. I know GRL has developed methods to measure energy in a drill rod with technology similar to the PDA. I would imagine they could provide a lot of comparative data between the two systems.
I correct for overburden and ussually that is it. I also don't rate an N=6 as a better soil than N=4.The tests are subject to such variability that increased precsion doe not lead to increased accuracy. JDMM has some good points about the SPT, but as MRM has pointed out we all come to our seperate peace with the test.

The SPT kind of reminds of the drunk looking for his wallet at the street corner one night. A kind soul stops to help him and asks after several minutes were exactly he lost it. The drunk replies "about halfway up the block." "Then why are you looking here?" the man asks. And the drunk replies "Because the light is better here!"

Okay I'm going back to wait for the dawn of the bronze age.

 

[MRM]

I think you're right, DRC1. You have to make your own peace with the tests you use, and have an idea of the real accuracy you can expect in order to use them responsibly-you said it well.

You know, I came across a paper written or sponsored by CME regarding what the efficiency is of their automatic hammers. They also compare the repeatability of the automatic hammer with the old manual safety hammer. The paper had interesting results in it, and it was worthwhile to read, but there were a few things that didn't add up to me. For example, the actual efficiency they measured for their automatic hammers with trip release approached 90-100% in some cases, if I remember right. Now while they were measuring the energy ratio delivered to the anvil rather than to the spoon, these values approaching 100% still didn't make sense to me. The paper also address repeatability of the results.

Using some common sense, first of all, the noise created by the pounding is energy lost. Anyone who knows what the test sounds like knows that you'll lose part of your hearing if you don't have protection, stand right by it, and do for years. To me, that's a lot of energy lost already in the noise created. Secondly, I think a lot of the vibration is lost in vibration-you can definitely feel the vibration created-there's a lot of it. That's energy lost too. Thirdly, there's lots of heat buildup-things get hot. Fourthly(?), field conditions probably have other variations that produce energy sinks too compared to the conditions during their research. On top of this, I know of very few mechanical systems that have an efficiency of 70% or above. Heck, most cars have a real efficiency of what, 30-40%? (My car is probably more in the 10% range with everything that's wrong with it)

After considering all these things, I used a raw efficiency of hammer to anvil of about 70% of the theoretical free fall energy, which is accepted by most for an auto hammer with trip release. That 70% was used as the basis in my estimates given above. Just some extra info. Let me know if you're interested in knowing the authors, date, etc. of that paper-I'll get that info to you if anyone's interested. Or if you contact CME, they were good enough to send me a copy for free. Like I said there are some interesting things in it...

Does anyone else have an opinion about the real efficiency of hammer to anvil? I'd be interested to hear.

 

[MRM]

One other thought too: I originally tried using the 90% efficiency value in for my correction factor estimates. Using this value, the correction factors I shared above were much higher. Having high correction values (and then using them) leads to unconservative estimates of N60 (or Ngh), Dr, phi, etc. The correction values obtained using a 70% efficiency seemed more realistic and more conservative.