Pocket Penetrometer, Torvane, and soil internal friction angle 1

[Mekkatronix]

Realizing the many and significant limitations in using the Pocket Penetrometer (PP) and the handy Torvane device, all of the PP literature provided by the manufacturers states that these two devices can be used "... for rapid estimation of the degree of internal friction of soil."

Can anyone explain how this could be done, even if only for a very rough estimation? Thank you for your help.

 

[oldestguy]

The ones I used to use were calibrated in Tons/s.f. All you need to do is read the instructions, very simple to use. Say out on a construction job just clear off disturbed soil and take your reading by pushing against the cleared zone. Do same for text boring samples in hand. Accuracy drops off at high soil strengths, but then usually it does not matter. Of course only good for clays or silty clays, not sands.

 

[GeoPaveTraffic]

PP and Torvanes give undrained shear strength, not phi angles.

Mike Lambert

 

[PEinc]

The pocket penetrometer, or hand penetrometer, can be used to make quick estimates of unconfined compressive strength of cohesive soils, especially in field applications. As Mike said, the two instruments do not give soil friction angles.

http://www.PeirceEngineering.com

 

[Mekkatronix]

Thanks for all of the replies. I agree with the consensus that these instruments provide a rough estimate of the unconfined compressive strength/undrained shear strength and is at best a strength index for field guidance, etc and not design. The manufacturer's product sheets all say that these two devices can be used "... for rapid estimation of the degree of internal friction of soil."

That is the question that I asked. I don't quite see how friction angle can be estimated from only the readings that these instruments provide. Lots of assumptions would have to be made, I am sure, and I do not know what "equation" the manufacturers are using. And apparently neither do they, as I have asked and they can not provide.

Without correlating with SPT, Direct Shear, Triaxial, etc., how are they estimating the friction of the soil? I am not a soils engineer or soils tech so I am definitely not seeing what is needed to make this connection work.

PP gives qu. Su = 1/2 qu (with assumptions). Can these be plotted on a graph and a line fitted, with the resulting slope of the line being the angle? That cant be right. Any ideas?

Thanks again. If anyone can help with this, you all are the ones to figure this out.

 

[MTNClimber]

As others have said, it cannot be done. The manufacturer has made a mistake.

 

[Mekkatronix]

MTNClimber, thanks for the rapid reply. It looks like one or two shops make all of the PPs on the market and the various brands just apply their labels (much like the original VHS recorders back in the day). This explains why all of the vendors advertise the same incorrect information. Many thanks.

 

[LRJ]

Internal friction is not necessarily the same as internal friction angle. The undrained shear strength may be described as internal friction, i.e. a soil-soil frictional (shear) resistance.

 

[morecoffeeplz]

It requires the use of a torsional vane shear tester in addition to the PP.

 

[Kevin9679]

Btw, I was shocked to find out that pocket penetrometers would give completely different results depending on whether you put the adapter foot on or not. And it's not a systematic difference that can be calibrated (I know you multiply by 16 and that's sort of a calibration). It's a random deviation by as much as 500-psf.

Someone recently gave me a torvane. Not sure how much better it is than the Pocket Penetrometer. People tell me often that it is much better.

 

[Mekkatronix]

morecoffeeplz, that is precisely what I asked for in my original post. Can you explain how this is done with any useful results?

 

[Mekkatronix]

Kevin9679, that is indeed unsettling that the adapter would introduce such changes.