Thank you for your excellent question regarding soil resistivity measurements in wet soil (Monsoon) conditions. It is our pleasure to help.
Often when we describe a Soil Resistivity test, such as the Wenner 4-pin (or 4-point) Method, we correlate the spacing’s between the probes as a depth or sounding reading. In other words, the distance between the pins equates to the depth being measured, or so it is commonly believed. In many cases this is in fact true, but it is not exactly true either. There are many factors that relate to the measurements read by the meter and what actual depths in the earth are being measured.
When a resistivity meter takes a measurement, that individual number means nothing in itself, without first doing a little math to determine resistivity. The simplified formula is to take the reading from the meter, multiply by the probe spacing (in feet), and then multiply it again by 1.915. The result is a resistivity number. When we combine a series of measurements taken at different spacing’s, we can begin to determine what the characteristics (resistivity) of the earth are like at various depths. This process of comparing numerous individual soil resistivity measurements (at differing spacing’s) is called, developing a “Soil Model”.
The soil model will show changes in resistivity of the earth at various depths. What is the resistivity of the soil at 1-meter? What is it at 10-meters? A good soil model will answer these questions. Of course, there are many rules as to how many measurements must be taken and at what spacing’s are required in order to get an accurate model, but that is a different topic. The concern in this case is what happens when drastic changes in resistivity occur from one layer to the next.
When we conduct a soil resistivity test, we are trying to inject a test signal (electrical energy) into the surface of the earth, down through the soil to various depths, and record the loss of energy as a resistance. As the electrical test signal passes from one layer to the next, the test signal will degrade (refraction, diffraction, scatter, etc.) in proportion to the changes in resistance that it encounters. This is especially true when the signal must try to move from a very conductive layer of soil, to a very resistive layer of soil. The test signal will simply try to stay in the most conductive material.
If you have ever seen a submarine war movie, the sub commander will move his submarine below a colder layer of ocean water to avoid being detected by sonar. The cold layer of water will “bounce’ the sonar signal up and away from the submarine, hiding it from the enemy. This is similar to what happens when conducting soil resistivity tests; the test signal may in fact not penetrate the layers as well as we might hope.
These changes in layer resistivity affect the signal in a predictable way, and as such can be calculated and its effects corrected. This is why good engineers prefer soil resistivity models calculated using computer modeling programs, instead of simple hand-calculations (good computer modeling programs perform thousands, if not hundreds of thousands, of calculations). Today’s sophisticated algorithms’ take into account most of the variables and provide vastly superior and more accurate soil resistivity models.
That said, computer algorithms’ can only help correct the math. A good soil resistivity technician will know how to improve the original signal itself. The first step is to always use true Direct-Current (DC) meters. E&S Grounding Solutions uses 800-volt p-p DC meters which require an additional car-battery to generate the needed power. The next step is to have lots and lots of readings starting at 6-inch spacing’s, with spacing intervals increasing at a factor of no greater than 1.5, with 1.33 preferred. You would also need to keep taking readings and increasing your spacing’s at those intervals until your spacing’s are at least as great as the depth you are trying to read, if not 2x or 3x. This would certainly mean many dozens of measurements and a lot of work.
In your case, you have very wet and conductive soil at the surface, with potentially very resistive soil at depth. This is indeed a very difficult scenario to take measurements, but it is not impossible. All of the items above of course apply, but one of the best things you can do is to try and bypass the wet soil. Often, the rain water will only penetrate down a few inches or a few feet at most. Start by figuring out how deep the water has penetrated into the earth.
One option requiring a lot of man power is to excavate the wet material from around the area where the probes are to be installed so that you can get to the dry soil. Another option is to use very long probes that are capable of getting into the dry soil. Once you determine how deep the wet soil is, you can use electrical tape to help isolate the probe away from the wet soil, thereby only allowing the tip of the probe that is in the dry soil to inject the test signal.
Unfortunately, Monsoon weather doesn’t make anything easier, soil resistivity testing included. We wish we had better news for you, but that is what you are dealing with in this case. You are certainly looking at a very large project involving many man-hours.
E&S Grounding Solutions is of course more than happy to help you with this project. We are often called upon to consult on these big projects, to not only provide guidance for the preparations, but to be present during the test to help ensure that test is conducted properly. Please feel free to contact one of our engineers directly and they will be glad to discuss your issues free of charge.
The Engineering Team at E&S Grounding Solutions
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