Not usual behavior of sleeve friction during PCPTU
Not usual behavior of sleeve friction during PCPTU
(OP)
Short description of behavior:
"Often a noticeable decrease in friction (ratio) occurs at 1 metre
intervals. This happens when the force on the rods/cone is reduced in
order to add another length of pushrod (1m). When the test is continued,
the friction has to "build up" to attain the previous level (end of
previous push). Full clipping op this data will result in data
loss. A similar effect occurs with the cone resistance, but the
"recovery" is much faster and barely noticeable after clipping"
Could it be "usual" behavior of silty sand, or it could be connected with water content and type of soil particles?
Any comments are welcome. I could send a sample by e-mail.
Best Regards,
Maxim
"Often a noticeable decrease in friction (ratio) occurs at 1 metre
intervals. This happens when the force on the rods/cone is reduced in
order to add another length of pushrod (1m). When the test is continued,
the friction has to "build up" to attain the previous level (end of
previous push). Full clipping op this data will result in data
loss. A similar effect occurs with the cone resistance, but the
"recovery" is much faster and barely noticeable after clipping"
Could it be "usual" behavior of silty sand, or it could be connected with water content and type of soil particles?
Any comments are welcome. I could send a sample by e-mail.
Best Regards,
Maxim





RE: Not usual behavior of sleeve friction during PCPTU
What do you mean by unusual behaviour? Is the friction value increased or decreased, or there are too many spikes?
RE: Not usual behavior of sleeve friction during PCPTU
As i understand it depends from the tested soil but i dont understand the process of this.
Unusual - friction require more time for increase up to level before break
Show me your e-mail and i will send you a sample.
Best Regards and thanks for reply
MaksimR
RE: Not usual behavior of sleeve friction during PCPTU
RE: Not usual behavior of sleeve friction during PCPTU
Why did you say they were unusual?
RE: Not usual behavior of sleeve friction during PCPTU
I asked the question to find out this. Maybe someone more experience then me will say me......Hey, don`t worry. It`s normal. It is always take place in silty sand.
Thank you for your reply.
Merry Christmas for you, Robert118
RE: Not usual behavior of sleeve friction during PCPTU
We use a Geotech wireless piezocone ( http://www.geotech.se/ )pushed by standard auger rigs. With this system, a "stringpot" is used for measuring depth and push speed. The stringpot is interfaced with the microphone for the cone and a computer to record the resistance parameters at user-defined depth intervals (we use 2-cm). The string from the stringpot is attached to the drill head, so the string moves in and out with the movement of the drill head. On a hard push, the drill will lift slightly off the truck due to some play in the skid. This lift will cause the stringpot to move, even though the push rods did not, and will result in the data spike.
On a CPT rig, or a different CPT unit, this may not be the case. I would take a close look at the equipment when it is operating to see if you can see anything happening which may cause this. If you've done this, then maybe there is some soi-related phenomena......I'd be interested to find out!
RE: Not usual behavior of sleeve friction during PCPTU
Please allow me to chip in, although a bit late here. I believe this phenomenon is not unreal and is a part of mechanics that occurs during pile or cone drive in.
When the cone is driven into the ground, a pressure bulb is generated in front of the cone tip. This causes an over expansion of the soil ahead of the cone. And as the soil is passed by the cone (or the pile tip), the soil collapses back to the shaft; that is, there is a reduction in the radial stress (radial stress relates to the sleeve friction). If the pushing is slowed down or stopped altogether this phenomenon is apparent. Furthermore, if the test is stopped for a period, you may notice that it may be difficult to push further (sometimes?). This occurs because of the increase in strength of the disturbed soil around the shaft (called Thixotropy or something) and reconsolidation...but that’s another story.
Back to this thread (sorry) during the continuous pushing (or driving in), this "reduction" is probably not apparent because the cone passes through the next layer of the soil and the sleeve friction is continuously been added.
This is true for all kinds of normally consolidated clays and loose to medium sands. I do not recall what happens in over consolidated clays and dense sands (your point BigRedGeo); I need to check that. But if you look at some of the papers by Vesic, Randolph, Puolos and Baligh, you may find much literature on the subject.
Regards
RE: Not usual behavior of sleeve friction during PCPTU
Regards
RE: Not usual behavior of sleeve friction during PCPTU
This issue could possibly show its self is if the CPT operator starts the data collection computer too quickly at each rod addition.....this is an operator error. I think most CPT operator review the data and correct these little error before the data is given to the client. On other occasions I have had the CPT operator tell me where he had to correct for a rod break.
A final note: The vast majority of the cone data I have reviewed has been seamless, that is you could not tell the depth at which the rod connection was made. In some soils the pore pressures can indicate the rod addition depths....but that is usefull info regarding the nature of the soil.
Coneboy
RE: Not usual behavior of sleeve friction during PCPTU
RE: Not usual behavior of sleeve friction during PCPTU
If the spikes do result from the change of rods, which repeat at equal intervals, the erractic figures should be removed from graph and calculation.
I had a look at the graphs sent by MaksimR, the spikes, as I interpret, should be the result of the build-break mechanism (Spikes occur between the change of rods).