The discussion and suggestions that followed yesterday from Siavash
Ghabezloo's inquiry about plugged standpipe piezometers may have activated a
part of my brain that would better be left dormant, but I now have a
question that may be a silly one, but I will ask it anyway.
By the way, in my humble opinion, Siavash's question was a very good one,
arising from a problem all of us have encountered at some time or another
and would love to have a simple solution for.
Here goes:
A double acting steam or air pile hammer can be turned over and used to
extract a pile (steel H-pile).
I am able to model the driving of a pile with this hammer, making
(hopefully) appropriate assumptions wrt soil properties, resistance
distribution, hammer performance, etc., and come up with some kind of
correlation between the set per blow and the mobilized static resistance,
using wave equation methods. I can go a little further and measure and
process the compression wave's velocity and magnitude as it travels up and
down the pile, using dynamic measurements, and improve my estimate. I can
even (horrors) make a kind of crude correlation between the set per blow and
the capacity, and completely ignore the pile and the soil.
Is there a good reason why this could not also be done for an extracted
pile, using the same hammer, turned over, giving me some kind of idea of the
mobilized uplift resistance?
Has anyone tried this? How did you deal with it differently from the
compressive case? I gather you would have to do this for just at the
beginning of the extraction operation.
Some of the problems I see are:
1. Stiffness of the connection between the hammer and pile (redundant
if the wave speed and magnitude are measured).
2. What happens to the wave when it reaches the lower end, is it any
different from the greatly decayed second return of a compressive wave from
the head when driving the pile?
3. Is the soil-pile shear resistance the same in both directions? What
about damping?
4. Is the effect of a reduced pile size, under tension, different
enough from the static load case to worry about it. Many of our uplift
cases for bridges are due to earthquake or ice (or vessel or vehicle)
impact, which are dynamic anyway. Or is the uplift capacity under a dynamic
load significantly higher than for a static load?
I would appreciate anyone's thoughts or experiences with this, regardless of
how speculative or off-the-wall they may seem.
Thanks and regards, Mike
PS: I read recently that there are no stupid questions, just a surfeit of
inquisitive idiots. I hope I have not just included myself in the sample
population, providing more evidence to support this hypothesis.
Michael R. Bleakney, P. Eng.
Geotechnical Engineer
New Brunswick Department of Transportation
Design Branch
PO Box 6000
Fredericton, NB
E3B 5H1
Canada
Phone: (506)453-2674
Fax: (506)457-6714
E-mail: [log in to unmask] <mailto:[log in to unmask]>
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