Soil Resistivity Test or Soil Resistivity Measurement

Needs of Soil Resistivity Test

The soil resistivity test becomes the most prior activity for designing an underground earthing system. It becomes essential to construct a proper earthing system during the installation of an electrical system. Otherwise,  any unwanted surge current generated in the system does not get a smooth way to flow to the earth without a proper conductive path to the earth. Actually, a surge current tries to flow to the earth through any available low impedance path. Without a proper earthing path, the current may flow through the human body or other healthy equipment of the system. So it causes major electrical shock or damages to the system equipment.

So a proper earthing system of installation is necessary to avoid this type of incidents. Hence, we need to know the actual resistivity of the soil to know the designing parameters of the earthing system.

Resistivity of Soil

The resistivity of soil depends mainly on the electrolysis action of the soil. The electrolytes present in the soil due to the presence of bore water and dissolved salts in the bore water. So the resistivity of the soil increases with an increase in the bore water content. Again up to 1% of the concentration of the dissolved salts the resistivity remains low and constant. But beyond 1% of the salt concentration, the resistivity of the soil increases.

Factors Affecting the Soil Resistivity

  1. Dry soil shows higher resistivity than the wed one.
  2. More mineral causes less resistivity.
  3. The resistivity of the soil is high at low temperature.
  4. Different types of soil show different resistivities.


Theory of Soil Resistivity Test

Two electrodes embedded in the ground close the current circuit. The soil resistivity measuring instrument injects a constant current to the soil. This injected current gets a path through the soil to complete the circuit. So, a soil resistivity tester must have a constant current source. The current always flows through the shortest possible path through the soil. Certainly, this path is the straight line connecting two current electrodes.

Now we have to measure the voltage drop of a specified distance along the same straight line. We do this with the help of two probes embedded in the ground along the same straight line and in the same depth as that of the current electrodes.

Then we get the corresponding resistance from the value of the injected current and the measured voltage drop. Finally, we can calculate the resistivity of the soil from this resistance value.

Variation of Soil Resistivity

The soil resistivity widely depends on the type of terrain. The silt on a riverbank may have a resistivity of 1.5 Ω – m. On the other hand, dry sandy soil may have the resistivity higher than 10,000 Ω – m. Again, for the moisture content above 20% in the soil, the conductivity of the soil does not increase much rapidly with increasing moisture content. But below 20% of the moisture content, the rate of increase of conductivity with increasing moisture is very rapid. The variation of resistivity with a variation of temperature is negligible below the freezing point.

Again, the presence of underground metal pipes, concrete pipes, metal or concrete tanks, large slabs, cable ducts, rail tracks, etc, may affect the resistivity of soils.

The Resistivity of Different Soils

  • Seawater 0.1 to 10 Ω – m
  • Clay 8 to 70 Ω – m
  • Ground spring water 10 to 150 Ω – m
  • Clay and sand mixture 4 to 300 Ω – m
  • Shale, Slates, Sandstone 10 to 100 Ω – m
  • Peat, loam, mud 5 to 250 Ω – m
  • Lake, brook water 100 to 400 Ω – m
  • Sand 200 to 3000 Ω – m
  • Moraine gravel 40 to 10,000 Ω – m
  • Ridge gravel 3,000 to 30,000 Ω – m
  • Solid granite 10,000 to 50,000 Ω – m
  • Ice 10,000 to 1,00,000 Ω – m

Method of Soil Resistivity Test

We have to place the resistivity measuring instrument at the center of the measuring location.

soil resistivity test

Then we embed two spikes (electrodes) to the right and left of the measuring instrument. The distance of the electrodes from the measuring instrument is equal for both left and right spikes. This distance is maintained as per the requirement of the test. These electrodes are to be embedded 5 to 25 cm into the ground depending on the condition of the soil.

Then we have to embed the other two spikes along the same straight line. The distance of these spikes also depends on the requirement of the test.

We connect the inner spikes with potential terminals of the instrument.

Then we connect the outer spikes (electrodes) with the current terminals of the measuring instrument.

We call the spikes connected with the potential terminals as potential probes.

On the other hand, we call the electrodes connected with the current terminals as current probes.

Wenner Method

It the distance between the potential probes is ‘a’, the distance between the current probes is 3a.

The probes must be embedded in a straight line. This is because, the current flowing from one current probe to the other current probe through the soil, follows the shortest possible path. This is the straight-line connecting these two current probes.

The potential probes measure the voltage drop. Hence we must embed them on the same straight line.

The instrument has a current source. This current source injects a constant current into the soil. Then the instrument measures the voltage drop in the soil between the potential probes.

Now, by dividing the measured voltage and the constant injected current the instrument shows the resistance on the instrument screen.

Calculation of Soil Resistivity

As per the Wenner method of soil resistivity test, we can calculate the resistivity of the soil with the formula given below,

soil resistivity

Here r is the measured soil resistance as shown on the screen of the instrument during measurement. And ‘a’ is the distance between the probes.

The same measurement is taken for at least 4 times with four different values of a, on the same straight line.

Then with the same values of ‘a’, the measurement is being repeated along the imaginary straight line perpendicular to the previous straight line.

Now, we get a reliable value of the actual test result by taking an average of these eight test results.

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