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 Post subject: Pile Detection using GPR
PostPosted: Tue Apr 16, 2013 4:57 pm 
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Joined: Mon Mar 11, 2013 7:42 am
Posts: 7
I have a project to find out the exact location of the piles,space between piles and the piles length itself (Picture). But the problem is above the piles some structure already built,so I cannot put the antenna above the structure (ESF-1). Is there any possibility to conduct the GPR survey with certain angle (not perpendicular to the ground)?. Any advise would be a great help.

Best Regards,


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Pile.jpg
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PostPosted: Tue Apr 16, 2013 8:22 pm 
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Joined: Fri Apr 17, 2009 7:52 am
Posts: 379
Location: Boden, Sweden
Our partner in the UK has been doing lots and lots of that kind of jobs.
I'll see if I can convince him to share some experience with you here.
Regards,

_________________
Reinaldo Alvarez Cabrera
Geoscanners AB
Sweden
http://www.geoscanners.com


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PostPosted: Wed Apr 17, 2013 8:25 am 
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Joined: Tue Nov 30, 2010 3:21 pm
Posts: 30
Hi Bimos,
the other thing that can be done in this case would be to use the borehole antennas.
You would need to drill a vertical borehole outside of the object and then use the borehole response to locate the begining/ending of the piles.If they are not rebar reinforced you have a good chance of measuring the gap between successive piles as well. I do not know what are the exact conditions of the piles and soil so keep in mind that it may be so that you will not be able to gather the info about the piles in the middle with reasonable precision (depending on how much the outer piles "mask" the responses after them).
Here is an example of the borehole survey with a goal to map the positions of two foundation piles.


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File comment: Two piles underground
Piles.png
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PostPosted: Wed Apr 17, 2013 8:06 pm 
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Joined: Mon Mar 11, 2013 7:42 am
Posts: 7
Hi danne,
Thank you for your advise.Since there is no blueprint for civil drawing so,I think I should make a guess about the pile itself.For sure,the pile have rebar reinforced in it and unfortunately this project is in the building so it should be a NDT (Non Destructive Test).
But,if I am using borehole survey do I have to make holes surrounding outside the piles?. And is it possible to have a cross section between the boreholes to get a precise location?.

Best Regards,


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PostPosted: Thu Apr 18, 2013 12:13 pm 
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Joined: Tue Nov 30, 2010 3:21 pm
Posts: 30
Hi bimos,
having a lot of rebars in the piles will make for a good reflector, but at the same time it will "mask" the successive piles - so reflections after the first or second pile may be so small that you will not be able to make any conclusion about third,fourth.... pile.

Since the available borehole antennas in the market are not directional antennas - they are not perfect 360 degrees omni directional, but close - the borehole for this kind of survey could be on the sides or in the middle of the piles. However if you put the borehole in the middle and because the antenna is gathering from a 360 degrees around itself, it may be harder to interpret the data because of the over-lapping reflections coming from two directions.
Cross hole borehole survey can be done, but compared to the single hole survey it is much more complex to gather the data and process it. With a single borehole you can collect and observe your data like you would do with any other GPR antenna (with regard to non directional nature of the antenna) while for the cross hole you need:
a) two antennas (one transmitter and one receiver)
b) single transmitter position has multiple receiver positions
c) you have to be extremely precise in positioning the transmitter and the receiver antenna - errors in the positioning will distort the geometry of the processed data
d) collected data has to be processed since you will only be interested in the arrival times of the direct pulse between the antennas
e) the processed data will tend to "homogenize" the material the signal is traveling through between the antennas, so the thickness of the individual piles and gaps between them can become unclear.
Since you will still be using the EM wave there is a good chance that the successive rebars are going to affect the pulse, so depending on the distance between the boreholes you may not get the signal reaching from one borehole to the other.

I guess that in your particular case a single borehole survey can be considered precise - you can see the endings of the piles (like in the posted image) and since you know the general direction where the piles have to be is "antenna facing the building" you would be pretty sure about the position of the piles and gaps between them. With less information available, you could always try to drill 2 (or 3) boreholes and triangulate the position of the same reflections.


Attachments:
File comment: Use two or more boreholes to identify the direction
triangulate.png
triangulate.png [ 35.88 KiB | Viewed 6924 times ]
File comment: Borehole antennas are non - directional
omnidirectional.png
omnidirectional.png [ 20.18 KiB | Viewed 6924 times ]
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