Automatic SPT Hammer

[Grouser]

I'm curious to know if much has been done to research or specify an 'automatic' hammer for SPT sampling. There was some talk of this in the late 70's. Anyone know of developements? Maybe the CPT has supplanted the need for added uniformity in the SPT? It was my understanding that a device similar to a PDA could be afixed to the rods and an 'exact' energy value delivered to the soil could be recorded/obtained. Any news of this either?

TIA to all responders.

Grouser

 

[Focht3]

Some work was done at UC Berkeley under Harry Seed's direction; I don't have the citations handy. I think the work continued into the 1990's, but that's just an impression. (I haven't had a reason to concern myself with this since I moved from San Juan Capistrano to San Antonio in 1991.) As I recall, the papers I read indicated that the auto-hammer was more consistent than "manual" methods, but didn't deliver as much energy. (I always thought that was an odd finding.)

Different PDAs were tried - some at the top of the pipe string, some behind the SPT. No surprise - the one behind the SPT gave better results.

If you need some help, contact Prof. Don Coduto at Cal Poly Pomona. He's smart, and has a good perspective on the practicing engineer's needs. And he's up to speed on the liquefaction-related stuff like SPT corrections, etc.





Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MRM]

There is a paper entitled “Reproducible SPT Hammer Impact Force with an Automatic Free Fall SPT Hammer System,” authored by Charles O. Riggs, Norbert O. Schmidt, and Charles L. Rassieur, published in ‘Geotechnical Testing Journal’ December 1983. Two of the three authors being from CME. It appears that a goal of the paper was to sell the automatic hammer on its consistent operation by backing up the claim with some technical info…seems reasonable.

The research found that a manual safety hammer (with 2 wraps around the cathead) delivered practically the same energy to the rods as the automatic hammer. Allow me to qualify that; most of the drill rig operators consistently “overpulled” the rope resulting in a slightly higher drop height. This partially explains why the energies may have measured out to be near the same.

The COV of energy delivered by the automatic hammer was indeed found to be low, on the order of 1 to 2%, while the manual hammer COV was higher as you’d expect. The Authors State that the raw N values obtained with the automatic hammer can normally be taken as the lower limit of the values you’d obtain by using a manual safety hammer, which makes sense. It appears, however, that they had some problems with some of the electronics used for energy measurement for some of the tests. All in all, they give some good information on the subject.

 

[Grouser]

Thank you Focht3 and MRM for your prompt and informative responses.

 

[BigRedGeo]

Certain state DOTs and cities do require calibration testing for SPT equipment. My company has had numerous automatic hammers checked by Goble Rausche Likins (GRL) to meet these requirements. Out of seven rigs from different manufacturers, hammer efficiencies ranged from a little over 50 to a little over 80 percent. I'm not sure where on the string the energy measurements were taken, but I can look into that.

Not a bad idea to do some checking to see if what you think is N60 really is N60.

 

[BigH]

MRM hit it on the head about the drillers overpulling - After so many years (in early career) on drill-rig, I could almost (at least in my own mind) tell if a pull was more or less than the driller's standard one - the "hit" didn't ping right.

By the way - maybe in some areas of US and Europe the CPT has supplanted the SPT, but not in India or other places I've been overseas.

 

[coneboy]

Hello Grouser

My understanding of "Hammers" is that the Auto trip hammer is by far the most reliable and repeatable as the rope/ cathead and drill operator variable is taken out of the equation.

SPT hammer Energy calibrations that I have observed have shown that the auto trip hammers to have a energy transfer of 60 - 110% energy. I also remember a couple of rope/cathead hammers that were only operating at 45% energy. An even bigger problem is that rope/cathead hammer is highly unrepeatable.

Regarding your inquiry about the use of PDA: Yes, PDAs are used to determine the hammer energy, but I believe they are more commonly used for pile analysis. Another method of hammer energy calculation is with the use of an accelorometer and the force-time calculation method.

Recently, transportaion officials from the US looked at some of the SPT energy testing research that was being conducted at UBC in Vancouver. One very interesting item which arose from the testing program is the importance of keeping the rods tight during the test. On a few of the research tests the rods were intentionally loosened and the amount of energy transfered dropped dramatically. I think the research also showed that the accelorometer used to record the hammer energy must be not be too small. I believe a paper is coming out very shortly which will summarize the findings.

One other item...the world wide SPT data base was exceeded by data from the electronic cone a few years ago.

Very interesting topic!

Coneboy