calculating the resistivity of two layer soil

[a.e.s]

Good evening!!

i have recently been looking into earth electrode systems, which got me onto reading BS 7430:2011. I am hoping that someone on here can clear a few things up for me.

First of all, is there a reason why BS 7430:2011 makes no reference at all to non homogeneous soil types? It seems crazy to assume that for the depth of an electrode, the electrode will only encounter one soil type.

Is it generally assumed that a comprehensive site survey including soil report shall be carried out??

Which leads me onto my next question, I have come across the following formula for calculating apparent soil resistivity;

p(a)=apparent resistivity (ohm metres)
p1=top layer soil resistivity
p2=bottom layer soil resistivity
K=reflection factor coefficient= (p2-p1)/(p2+p1)
n=number of electrodes
h=depth of the top layer of soil
s=distance apart of the rods

p(a)=p1(1+4*(sum of the following (between the limits of n-1 and infinity) (K^n/(sqrt1+(2n(h/s))^2)-(K^n/(sqrt4+(2n(h/s))^2)

I think it is taken directly from the IEEE, and playing around with it on excel was giving realistic values. However, I am curious as to why if the number and spacing of the rods is relevant to this formula, how come a variable for the length and diameter is not necessary?

Regards

 

[Engineer54]

You obviously have far too much time on your hands worrying about something that can be determined by relatively simple field resistance testing.

I think it is taken directly from the IEEE, and playing around with it on excel was giving realistic values.

Sorry, but after 40 odd years of providing project and system earth fields in just about every type of soil and condition, in a magnitude of country's, i've yet to see any formula for soil resistivity to give realistic values of how an earth field will actually perform!!!

The number and spacing of earth electrode rods as well as their length and diameter has nothing to do with determining soil resistivity, unless you are talking about the test equipments test spikes??

A full comprehensive site survey will generally include a 3 to 4 metre core being taken from various points of the survey site and analysed. Generally, someone that has a good amount of experience of testing soil resistivity and earth electrode systems as a whole, can generally determine (by the initial testing results obtained) with some degree of accuracy how that proposed site is going to perform as an earth field (without any fancy formulas...)