Three purposes of soil resistivity testing
Soil resistivity is a measure of how much the soil resists the flow of electricity. The resulting soil resistivity is expressed in ohm-meter or ohm-centimeter.
Soil resistivity was conducted mainly for the following purposes:
- Sub-survey geophysical surveys: use to identify ore locations, depth of bedrock and other geological phenomena
- Level of Corrosion: soil resistivity has a high impact on the degree of corrosion in underground pipelines. A lower soil resistively means increase of corrosion activity. The soil corrosiveness is classified based on soil electrical resistivity by the British Standard BS-1377
- Designing a grounding system: It is advisable to locate the area of lowest soil resistivity in order to achieve the most economical grounding installation
Conditions influencing soil resistivity
Several factors, such as soil composition, temperature, moisture content, and salts level determine soil resistivity.
- Soil composition: soil with a high organic contents are usually good conductors in comparison with sandy soils which drain fasters because of higher moisture levels and have a higher electrolyte level
- Moisture contents: Moisture content has the largest influence on soil resistivity. Moisture retention can be influenced by local climate conditions and electrolytic mechanisms such as mineral content, soil ionization, consistent grain size, even distribution and packing density
- Temperature: High temperatures and drought, or temperatures below freezing and deep frosts, can cause high resistivity in soils that have much lower values in the preceding months
|Fills – Ashes, cinders, brine, wastes||2,370||590||7,000|
|Clay, shale, gumbo, loam||4,060||340||16,300|
|Clay, shale gumbo, loam with varying proportions of sand and gravel||15,800||1,020||135,000|
|Gravel, sand, stones with little clay or loam||94,000||59,000||458,000|
Source: U.S. Bureau of Standards Technical Report 108
In Western Australia, soil type was typically dry and lacking of moisture contents resulting to a high soil resistivity. However, there are products such as Ground Enhancement Material (GEM) which dramatically reduces earth resistance and impedance measurements. It is the ideal material to use in areas of poor conductivity, such as rocky ground, mountain tops and sandy soil.
Conducting a Wenner 4-point soil resistivity test
There are two test methods, the Wenner and the Schlumberger. The Wenner method is more popular and easier to use for testing soil resistivity for a grounding electrode system. Below are the steps to conduct the test:
- Four test stakes are positioned in a straight line an equal distance apart and are hammered into the ground to be surveyed to a depth of not more than 1/20 the distance between the adjacent stakes
- An earth resistance tester is connected to these four stakes as shown in Figure 1
- The DC test option on the tester is then selected and performed, and the resistance figure “R” recorded
- The soil resistivity level “r” (in ohms/cm) is then calculated using the formula: r = 2 ρaR where:
R = the resistance figure (in ohms)
a = the separation of the test stakes, in meters
A = distance between the electrodes in centimeters
B = electrode depth in centimeters
If A > 20 B, the formula becomes:
Ρ = (with A in cm)
Ρ = 191.5 AR (with A in feet)
Ρ = Soil resistivity (ohm-cm)
This value is average resistivity of the ground at a depth equivalent to the distance “A” between two electrodes
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