Soil electrical resistivity (or ER) is a significant indicator that indirectly determines both soil physical and chemical properties, including moisture, salinity, porosity, organic matter level, bulk density, and soil texture.
As such, it is an important part of determining soil quality.
In this blog, we’ll take a closer look at soil resistivity and everything you should know about it when it comes to your land.
Let’s get started!
1. What is soil resistivity?
Soil resistivity is defined as the “measure of how much the soil resists or conducts electric current.”
Soil resistivity is expressed in ohm-meter or ohm-centimeter.
This is a critical factor in the design of systems that rely on passing current through the Earth’s surface, and numerous factors impact soil resistivity including composition, temperature, and moisture content.
Soil resistivity varies because it isn’t a homogenous substance.
Depending on the depth of the soil, its resistivity will change.
Lower soil layers have a greater moisture content, which equates to lower resistivity.
Harder and rockier soil has increased resistivity.
Understanding and measuring soil resistivity is important in trenchless technology as well as in the identification and location of ore, bedrock depth, and geological characteristics.
Soil resistivity also affects the degree of corrosion in buried pipelines and the design of grounding systems.
2. What are some basic definitions you should know to understand soil resistivity?
Soil resistivity has its roots in physics.
Let’s define resistance and resistivity, so you can understand the difference between these complex topics.
Resistance is the measure of opposition to the flow of current in an electrical circuit.
The measurement of resistance is calculated in ohms, which is symbolized with the Greek letter omega (Ω).
Resistance is useful in sensors, fuses, heaters, etc.
Resistivity is the measure of the resistance of a specific material of a specific size to electrical conduction.
You can also say that resistivity refers to the volume resistivity or specific electrical resistance.
The unit used for resistivity is an ohms-meter.
Resistivity is effective as a quality control test.
3. Why determine the soil resistivity?
Soil resistivity is an important element when determining the design of the grounding system for new installations.
You want to ensure that they meet the ground resistance requirements.
The ideal location would have the lowest possible resistance.
That said, poor soil conditions can be overcome with more elaborate grounding.
4. Is soil resistivity the only type of geophysical testing?
No! There are numerous forms of geophysical testing.
Here are some other types.
Seismic Refraction (ReMi)
Ground Penetrating Radar (GPR)
Electrical Resistivity (ER)
Gravity
Electro-magnetics
Magnetics
Reflection and Refraction Seismic Methods
5. Why is testing soil resistivity important in construction?
Just like an environmental site assessment allows you to evaluate the threat of dangerous and expensive pollutants above the surface, soil resistivity can help you evaluate the subsurface conditions.
Here is the role of soil resistivity testing:
It helps to determine where to ground electrical systems without creating a fire hazard.
It provides values that can be used to perform geographical surveys, which assist in finding the depth of bedrock, core locations, and other geological phenomena in order to properly design foundations
It can identify the cause of damage to existing structures
It tests the stability of the soil on the construction site to determine whether it is suitable to bear the weight loads produced by the new construction (i.e., roadway or high-rise building)
It helps to determine the degree of corrosion in underground pipelines
6. What should you keep in mind when looking into a grounding system?
When looking at a grounding system, you want a location with the lowest possible resistance.
That said, poor soil conditions can be overcome with a more elaborate grounding system.
The two primary factors to consider are cost and available space.
The two leading causes of equipment malfunction include 1) improper wiring and 2) poor grounding.
As such, the need for low-resistance grounding systems is crucial.
Note: At sites where specific grounding systems performance goals are not met, equipment warranties may be invalidated.
If you’re planning to build, DO NOT skip over a soil resistivity test as this will impact your whole grounding system.
7. What are the two main methods of soil resistivity testing?
The two methods of soil testing are the Wenner method and the Schlumberger method.
The Wenner method is the four-point test method.
It’s the most common and reliable.
That said, both the Wenner and the Schlumberger methods are very similar.
One difference between the two of them, however, is that they use different formulas to calculate resistivity values.
Soil testing is conducted by placing four test probes into the ground, spaced equal distances apart across the raw land where the grounding system is required.
Next, a ground meter is attached to the probes to collect the numbers necessary to calculate the soil resistance.
Soil values may range from 500 Ω cm with large amounts of electrolytes to over 1 million Ω cm in sandy dry soil.
Overall, five tests are conducted to provide acceptable results.
This includes four sides and a diagonal.
8. What is done during the testing of soil resistivity?
When testing soil resistivity, follow these steps:
Connect the ground tester to the four earth ground stakes
The four earth ground stakes should be positioned in the soil in a straight line and equidistant from one another
The distance between the earth ground stakes should be at least three times greater than the stake depth
The Fluke 1625 generates a known current through the two outer ground stakes and the drop in voltage potential is measured between these and the two inner ground stakes
Using Ohm’s Law (V=IR), the Fluke tester automatically calculates the soil resistance
Measurements are often distorted and invalidated by underground pieces of metal, underground aquifers, etc., and thus additional measurements where the stake’s axis are turned 90 degrees is always recommended
By changing the depth and distance several times, a profile is produced that can determine a suitable ground resistance system
Soil resistivity measurements can be corrupted by the existence of ground currents and their harmonics.
You can prevent this with the Fluke 1625, which uses an Automatic Frequency Control (AFC) System.
This system will select the testing frequency automatically with the least amount of noise enabling you to get a clear reading
9. Why is soil resistivity testing necessary? Isn’t all soil the same?
No! Soil types and quality vary greatly with depth and location.
Plus, soil classification can provide only a rough estimate of the soil properties as factors (i.e., moisture, temperature, type, and depth) change the resistivity measurements of soil.
Here’s more information on each of those factors.
Moisture: Moisture enhances soil conductivity.
When the moisture content is greater, the soil resistivity is lower, which is considered good.
Temperature: Soil temperature is important in areas with colder climates.
When temperatures are below freezing, it can have a detrimental effect on grounding systems that are not designed to accommodate freezing temperatures.
This is because clay or cement-based backfill materials that rely on water as their primary conductor will solidify and fail.
Soil types: Soil types with high organic contents are typically good conductors, and they will hold more moisture.
Sandy soils will lose moisture and have a lower number of electrolytes.
The worst soils are those that are rocky or volcanic ash.
These have almost no moisture at all.
Depth: The type of soil may change depending on its depth.
For instance, while the site may have a nice organic mix of soil on the top, the soil can get drier or more clay-like as you go deeper.
These variations will impact soil resistivity values.
10. What is the effect of moisture content on soil resistivity?
As noted above, moisture is one of the factors that affects and controls soil resistivity.
The moisture content reduces soil resistivity because it increases the conductivity of soil.
Moisture is expressed as a percentage of the weight that the soil has when it’s dry.
11. What is the effect of salt content on soil resistivity?
You aren’t able to achieve low resistivity with soil alone.
Your soil must also contain mineral salts as a way to form an electrolyte to conduct electricity.
Mineral salts can be natural (due to rainfall or chemical elements), or they can be introduced artificially into the soil to improve its conductivity.
Mineral salts have the most significant impact on reducing resistivity.
If you’re treating the soil to improve electrical characteristics, mineral salts are the first choice for treatment.
12. What is the effect of temperature on soil resistivity?
The temperature coefficient for the soil is negative.
This means that the lower temperature, then the higher the resistivity.
13. What are the two types of soil resistance meters?
There are two basic types of soil resistance meters: Low-Frequency and High-Frequency Models.
Both meter types can be used for 4-point and 3-point testing.
They can even be used as standard (2-point) voltmeter for measuring common soil resistivity.
You should always be mindful when selecting a soil resistance meter.
The Earth can be a “noisy” place in terms of electronics.
There are overhead power lines, electric substations, railroad tracks, various signal transmitters, and countless other sources that contribute to signal noise found in any given location.
Be sure that you select equipment with electronic packages that are capable of discrimination between different types of signals when it’s essential.
High-frequency soil resistance meters typically use a pulse operating at 128 pulses per second (or other pulse rates except 60).
14. Where should you conduct the soil resistivity testing?
Soil resistivity resting should always take place as close to the proposed grounding system as possible.
If it’s possible to test in the vicinity of the site, then it’s important to do this.
However, it’s not always practical to do so.
Plus, the geology of the area can also play a role, and there can be dramatically different soil conditions only a short distance away, so keep this in mind!
15. What are common issues that can cause poor quality reading?
The top two issues that can cause poor quality readings include:
Electrical interference that causes unwanted signal noise to enter the meter
Metallic objects “short-cutting” the electrical path from probe to probe
Always maintain clearance equal to the spin spacing between the measurements traverse and any parallel buried metallic structures
Final thoughts
Electricity resistivity of the soil is its ability to oppose the passage of a current.
This can affect various fields like electrical power systems, electronics, the environment, groundwater, agriculture, etc.
When designing a grounding system or doing any construction, you must keep soil resistivity in mind.
This is where geophysical testing and geoelectric exploration come in.
If you’ve just purchased land and you need to install a grounding system, then you’ll first want to conduct soil resistivity testing.
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Disclaimer: we are not lawyers, accountants or financial advisors and the information in this article is for informational purposes only. This article is based on our own research and experience and we do our best to keep it accurate and up-to-date, but it may contain errors. Please be sure to consult a legal or financial professional before making any investment decisions.
Prior to starting Gokce Capital, Erika received a Bachelor of Architecture from the University of Southern California and a graduate degree in Urban Policy from Columbia University. She worked as both an architectural designer and engineer in New York before joining the New York City Department of Housing Preservation and Development.
Erika currently lives in the New York Metropolitan area with her spouse, daughter and cat. She is originally from Chicago and still considers herself a midwesterner at heart.
Erika also loves to read, write and travel (fun fact, she has visited all 50 states and more than 30 countries!). Her new book, Land Investing Mistakes: 11 True Stories You Need To Know Before Buying Land, is now available on Amazon.
Hi Erika,
Any requirements for the humidity conditions on the site the day of the soil resistivity test or the day of the thermal test? I am specifying to avoid rainny days and hot days, to get a worst scenario set of results. Thanks in advance. Best Regards, Marta (Electrical Engineer)
Hello Marta, I’m very sorry, but I’m afraid I cannot answer this question. You may want to consult with a geotech engineer.