Converting to Two Layer Soil Resistivity Model

[ThePunisher]

HI all,

I received a multi-layer soil resistivity data derived from 4 pin Wenner test method based on equal probe spacing (spacing not given) and graphical curve fitting from one area and wishes to use it as preliminary basis only. The soil resistivity values are as follows:

a. 220 ohm-meter at 1.2 meter soil thickness (1.2 meter depth below grade)
b. 175 ohm-meter at next 6 meter soil thickness (7.2 meter depth below grade)
c. 500 ohm-meter at next 20 meter soil thickness (27.2 meter depth below grade)
d. 100 ohm-meter at infinite depth below 27.2 meter depth

ETAP ground grid software uses 2-layer soil model: top and bottom soil (as per IEEE 80-2000). I would use 100 ohm-meter for the bottom soil model (item d). This leaves me to come with an equivalent top soil (not surface soil)resistivity value based on items a~c.

I. On method I am thinking of is to apply weighted percentage on each soil resistivity based on percentage depth and take the sum. Hence,

(220 x 1.2/27.2) + (175 x 6/27.2) + (500 x 20/27.2) = 415.96 Ohm-meter for soil thickness of 27.2 meters

II. Another method is to simply get the average of items a-c disregarding their different depths. Hence,
(220 + 175 + 500)/3 = 298.33 ohm-meter

By inspection, for approximation, method I will give me more conservative result than method II.

My question to experienced folks, is method I reasonable enough to approximate conversion of all three layers (a-c) to one top soil layer? Any alternatives?

Regards,

 

[jghrist]

The best alternative is to get the Wenner test measurements and probe spacings and calculate the best fit 2-layer soil model. Does ETAP have a module for calculating soil layer models from test results?

The best fit 2-layer soil model lower layer will not be the same as the bottom layer of the 4-layer soil model. It will have a different depth and resistivity.

Another option, probably not feasible in your situation, is to use a grounding analysis program that can use a 4-layer soil model (like CDEGS from SES).