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What is Soil Resistivity?

Soil resistivity is the measure of how much the soil can resist the flow of electrical current. Low soil resistivity means that the soil can easily conduct electrical current and ensures a safe passage of electric charge into the earth.  High soil resistivity means that the soil is less conductive and makes it more difficult for the electric charge to dissipate into the earth. 

Low soil resistivity is more desirable since it makes it easier to avoid the risk of electrical shock and short circuiting in the event of an electrical charge.  Soil resistivity also influences the corrosiveness of the soil.  Unfortunately, the lower the soil resistivity means the soil will have a higher corrosiveness and higher soil resistivity means it will be less corrosive.   

Factors That Influence Soil Resistivity?

Moisture – Soil with a higher moisture content typically has a lower resistivity which would enhance the soil conductivity.   

Soil Type – Soils with high organic content have a tendency to hold more moisture which leads to it being a better conductor.  Sandy soils don’t retain moisture as long and rocky soils or volcanic ash have nearly no moisture at all, which make them the worst type of conductive soils.  

Temperature – Colder climates are the main concern, specifically in areas that have below freezing temperatures on a regular basis.  Soil conductivity decreases sharply once the soil water goes below the freezing point.  

Depth – Soils typically have varying compositions the deeper you go which ties into the three factors above.  The moisture content, soil type, and temperature are all susceptible to change as you go deeper into the soil layers. 

Chemical Content, Mineral Content, And Contaminantes – Depending on which or how much of any of these three are present, they will likely affect how conductive the soil is due to their specific conduciveness.   

Factors That Influence Soil Corrosiveness:

Soil Resistivity – Low soil resistivity relates to higher corrosiveness and high soil resistivity relates to lower corrosiveness.

Moisture – Higher moisture content lowers the soil resistivity and in turn increases the soil corrosiveness and vice versa.

Temperature – Lower temperatures increase the soil resistivity and in turn decrease the soil corrosiveness and vice versa.

Soil Acidity (pH Level) – The more acidic a soil is, the more susceptible metal buried in that soil is to corroding and pitting. 

The Importance Of Testing The Soil Resistivity 

Two major reasons in determining your soil resistivity are due to its impact on grounding system designs and corrosion of underground metals.  Both are important in avoiding potential equipment or structure damage that could result in a loss of services and/or costly repairs.  In the specific event of corrosion on a steel guy anchor shaft, you could be looking at a catastrophic tower failure and collapse which would have many severe consequences.  

The Purpose Of Testing The Soil Resistivity

Soil resistivity testing is used to determine how much the soil resists or conducts electric currents and how corrosive the soil is to underground metals. Soil resistivity is a critical factor in designing and analyzing grounding systems. The primary objective of a grounding system is to intentionally connect non-current carrying conductive materials, such a coaxial cables, conduits, junction boxes, and electrical enclosures to the earth ground.

This allows for the low resistance path of electrical flow from the equipment down to the ground to protect against sudden high voltage discharges that could damage the equipment. Soil resistivity also has a direct correlation with the soil’s corrosiveness. Determining how corrosive the soil is will influence which protective measures will need to be addressed to ensure the integrity of any underground metals or pipelines remains undamaged.

Soil Resistivity Testing Methods

The two most common methods for testing soil resistivity are the Wenner Method and Schlumberger Method.  The Wenner Method is the most common since the resistivity is easily calculated in the field and the sensitivity of testing instruments is not as critical as compared to other methods.  The Schlumberger Method is not as labor intensive but requires more sensitive testing instruments and the field setups can be more difficult and harder to coordinate with the field crew.   

Wenner Method

The Wenner alpha four-pin method is performed by placing four pins, or electrodes, at equal distances apart and introducing a controlled current through the outermost electrodes, and then recording the voltage between the innermost electrodes. These electrodes are usually copper rods or pipes and are driven into the soil using a hammer. The figure below shows a visualization of this method and its field setup.

 

I = Injected current (Amp)
V = Measured voltage (V)
a = Equidistance between the electrodes (m)
b = Depth of electrodes (m)
V/I = Resistance (Ω)

When the depth of the electrodes (b) is significantly less than the distance between the electrodes (a), the soil resistivity can be calculated using the equation below. This would be typical for this testing method.

ρE = Measured apparent soil resistivity (Ωm)

 

The spacing of the electrodes determines the depths of soil that can be measured for resistivity. The closer the spacing provides shallower results of soil resistivity and longer spacing provides deeper results. Shorter electrode spacing is typically used to determine which grounding system is most appropriate. Longer electrode spacing is typically used where potential soil variations occur with depth.

Schlumberger Method

In the Schlumberger method, the distances between electrodes are not equal. The introduction of current, type and quantity of electrodes used is the same as the Wenner method.

 

I = Injected current (Amp)
V = Measured voltage (V)
a = Distance between voltage electrodes (m)
b = Depth of electrodes (m)
c = Distance between voltage and current electrodes (m)
V/I = Resistance (Ω)

When the depth of the electrodes (b) is significantly less than the distance between voltage electrodes (a) and less than the distance between voltage and current electrodes (c), the soil resistivity can be calculated using the equation below. This would be typical for this testing method.

ρE = Measured apparent soil resistivity (Ωm)

 

 

As with the Wenner method, the spacing of the electrodes determines the depth of the soil that can be measured for resistance.

Challenges Of Field Testing And Measurements

Both methods of testing have similar challenges for obtaining accurate measurements in the field. The continuity of the electrode/probe material, buried metallic systems not native to area, and insufficient power and/or sensitivity of the measuring devices can all play a role in the accuracy of the recorded soil resistivity.

The time of year or season can affect the measurements recorded as well. As discussed above, the temperature and moisture of the soil play an important role in determining the soil resistivity and these can vary greatly depending on the time of year and specific location of test site where the field test is performed.

Competent and experienced technicians and engineers can recognize these issues and modify the testing procedure accordingly, use alternative techniques, as well as interpret the recorded measurements to determine their accuracy based on typical local soil conditions.