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[TowerTalk] Soil resistivity test done: Some questions

Subject: [TowerTalk] Soil resistivity test done: Some questions
From: Beat Meier <>
Date: Fri, 08 Sep 2006 17:03:34 -0300
List-post: <>
Hi all

I'm still a grounding dumy but I've just bought a GEO test and have done 
some soil resistivity test
(for just 800US$ I can do 4 pole methods, cheapest I have found and 
still has serial port
to colect data ...)
I'm still in putting my new tower of 200ft ...

Now I have some questions.
In a place where I have 2 ground roads (each 2.5m) I have a resistivity 
of 120.
4m away I have a resistivity of 500.
The test was done with the 4 pole method.
I have not measured the ground of the house, only the soil just above 
where the
roads are with several depth and in the deps of 3 m I have this 120

Now my questions:

1. This low resistivity comes from the 2 ground roads already there
I have read that 2 ground roads in parallel will half the resistivity 
i..e 500/2 250 but I have  120
If you use 4 you get Resistividad/4 i.e. 500/4=125
Is this formula right?
The other idea is that the fundament of the house is very near (about 
1m) could this also
have such influence to lower the resistivity because of concret??

2. The resistivity of 500 is not very good, isn't it?

I have calculated that you must put 36 ground roads in parallel to 
achive a resitance of 13.8
which is still pretty bad (of course only if the formula above is right)

BTW: The tests have showed what someone already told me that the soil 
here is not good
to go in depth ... I have the following:

a=2m -> 486
a=3m -> 546
a=4M ->535
a=6m -> 429

so it seems that going in deep does not make any difference (apart from 
the cables you must use
to connect the ground roads) in contrast to use parallel ground roads.

And last question:
How many copper have good ground roads. The ground roads I can buy here 
have only 200um of copper
(5/8''-2m or 5/8''-3m). I have seen that they peel quickly if driven 
into earth and there is any obstacle ...

What I can get also is plate of copper 30cmx30cm but this costs about 8 
times more than one ground road
I don't know how they lower the resistividad of the soil ... I have read 
in an article about
0.8*Resitivida/circumference  is this right?

What about putting using the fundament of the house i.e. drive a whole 
in the concrete and connect a
copper or what ever cable ot the iron and put again concret?

Thanks for any ideas or comments


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