Hi S. Imtiyaz,
Thank you for your question regarding soil resistivity calculations, it is our pleasure to help.
It is important to understand that there are two (2) types of resistivity that people use today. The first one is called Apparent Resistivity, and the second one is Actual Resistivity.
Apparent Resistivity is a simple formula that only provides an average resistivity reading from the surface of the earth to the probe spacing distance. It is NOT a real soil resistivity number. For example, let’s say you have a Wenner test with a probe spacing of 5, and the apparent resistivity is 100 ohm meters. You also have a Wenner test with a probe spacing of 10 that tells you the apparent resistivity is 75 ohm meters. What is the resistivity between 5 and 10? The answer is 50 ohm meters. If the 0-5 ft layer is 100 ohm meters, and the 5-10 ft layer is 50 ohm meters, then the 0-10 ft apparent resistivity would read 75 ohm meters. Please see the link below, because it is very important to understand the difference between apparent resistivity and actual resistivity. You will also find the formula for apparent resistivity in this link:
What you really need however, is actual resistivity values. Unfortunately, there is really no way to properly hand-calculate actual soil resistivity, the formulas are simply to complex and numerous to do without the aid of a computer. We recommend and use the RESAP module from the CDEGS engineering software program. http://www.sestech.com/default.htm
The computer algorithms require a lot of data, and the data must be collected so that there are not to many “gaps” between the spacing’s. The maximum interval between spacing’s is a 1.5 ratio, with a 1.33 ratio preferred. So a measurement taken at a 20-ft spacing, would need to be followed up by a maximum 30-ft spacing, and preceded by at least a 14-ft spacing, in order to keep the 1.5 rule. In other words, if you have data for a 40-ft spacing, and then jump to an 80-ft spacing, the distance between spacing’s is a factor of two (2) which is to great and will cause errors in the math. You would need a 60-ft spacing between the 40-ft and 80-ft readings.
We recommend the following spacings for ground grids out to 240-ft maximum diagonal distances: 0.5, 1, 1.5, 2, 2.5, 3, 4, 5, 6, 7.5, 8, 9, 10, 12, 15, 20, 22.5, 30, 40, 45, 60, 80, 90, 120, 160, 180, and 240 feet. That’s a total of 26 measurements for a single test traverse. You should conduct several traverses depending on the project (substation, chemical factory, simple 5-ohm electrode, etc.).
E&S Grounding Solutions would be happy to help you with your project, but if not us, please get someone to assist you in this critical process.
The Engineering Team at E&S Grounding Solutions
Photo credit: E&S Grounding Solutions