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Soil Resistivity Testing

Soil Resistivity Testing

Soil Resistivity Testing

Schlumberger methods/ Wenner 4-Pole method Soil Resistivity Test. Soil resistivity testing is the process of measuring a volume of soil to determine the conductivity of the soil. The resulting soil resistivity is expressed in ohm-meter or ohm-centimeter.

Soil resistivity testing is the single most critical factor in electrical grounding design. This is true when discussing simple electrical design, to dedicated low-resistance grounding systems, or to the far more complex issues involved in Ground Potential Rise Studies (GPR). Good soil models are the basis of all grounding designs and they are developed from accurate soil resistivity testing.

Wenner Soil Resistivity Testing and Other 4-Point Tests

The Wenner 4-point Method is by far the most used test method to measure the resistivity of soil. Other methods do exist, such as the General and Schlumberger methods, however they are infrequently used for grounding design applications and vary only slightly in how the probes are spaced when compared to the Wenner Method.

Wenner 4 Point Soil Resistivity Testing

Wenner 4-Point Test

Electrical resistivity is the measurement of the specific resistance of a given material. It is expressed in ohm-meters and represents the resistance measured between two plates covering opposite sides of a 1 m cube. This soil resistivity test is commonly performed at raw land sites, during the design and planning of grounding systems specific to the tested site.

The soil resistivity test spaces four (4) probes out at equal distances to approximate the depth of the soil to be tested. Typical spacings will be 1’, 1.5’, 2’, 3’, 4.5’, 7’, 10’, etc., with each spacing increasing from the preceding one by a factor of approximately 1.5, up to a maximum spacing that is commensurate with the 1 to 3 times the maximum diagonal dimension of the grounding system being designed, resulting in a maximum distance between the outer current electrodes of 3 to 9 times the maximum diagonal dimension of the future grounding system. This is one “traverse” or set of measurements, and is typically repeated, albeit with shorter maximum spacings, several times around the location at right angles and diagonally to each other to ensure accurate readings.

The basic premise of the soil resistivity test is that probes spaced at 5’ distance across the earth, will read 5’ in depth. The same is true if you space the probes 40’ across the earth, you get a weighted average soil resistance from 0’ down to 40’ in depth, and all points in between. This raw data is usually processed with computer software to determine the actual resistivity of the soil as a function of depth.

Conducting a Wenner 4-point (or four-pin) Soil Resistivity Test

The following describes how to take one “traverse” or set of measurements. As the “4-point” indicates, the test consists of 4 pins that must be inserted into the earth. The outer two pins are called the Current probes, C1 and C2. These are the probes that inject current into the earth. The inner two probes are the Potential probes, P1 and P2. These are the probes that take the actual soil resistance measurement.

Conducting A Wenner 4 Point Soil Resistivity Test

In the following Wenner 4-Point Test Setup diagram, a probe C1 is driven into the earth at the corner of the area to be measured. Probes P1, P2, & C2 are driven at 5’, 10’ & 15’ respectively from rod C1 in a straight line to measure the soil resistivity from 0’ to 5’ in depth. C1 & C2 are the outer probes and P1 & P2 are the inner probes. At this point, a known current is applied across probes C1 & C2, while the resulting voltage is measured across P1 & P2. Ohm’s law can then be applied to calculate the measured apparent resistance.

Probes C2, P1 & P2 can then be moved out to 10’, 20’ & 30’ spacing to measure the resistance of the earth from 0’ to 10’ in depth. Continue moving the three probes (C2, P1 & P2) away from C1 at equal intervals to approximate the depth of the soil to be measured. Note that the performance of the electrode can be influenced by soil resistivities at depths that are considerably deeper than the depth of the electrode, particularly for extensive horizontal electrodes, such as water pipes, building foundations or grounding grids.

Soil Resistivity Testing: 

Soil resistivity testing is the process of measuring a volume of soil to determine the conductivity of the soil. The resulting soil resistivity is expressed in ohm-meter or ohm-centimeter.

Soil resistivity testing is the single most critical factor in electrical grounding design. This is true when discussing simple electrical design, to dedicated low-resistance grounding systems, or to the far more complex issues involved in Ground Potential Rise Studies (GPR). Good soil models are the basis of all grounding designs and they are developed from accurate soil resistivity testing.

Wenner Soil Resistivity Testing and Other 4-Point Tests

The Wenner 4-point Method is by far the most used test method to measure the resistivity of soil. Other methods do exist, such as the General and Schlumberger methods, however they are infrequently used for grounding design applications and vary only slightly in how the probes are spaced when compared to the Wenner Method.

Wenner 4 Point Soil Resistivity Testing

Wenner 4-Point Test

Electrical resistivity is the measurement of the specific resistance of a given material. It is expressed in ohm-meters and represents the resistance measured between two plates covering opposite sides of a 1 m cube. This soil resistivity test is commonly performed at raw land sites, during the design and planning of grounding systems specific to the tested site.

The soil resistivity test spaces four (4) probes out at equal distances to approximate the depth of the soil to be tested. Typical spacings will be 1’, 1.5’, 2’, 3’, 4.5’, 7’, 10’, etc., with each spacing increasing from the preceding one by a factor of approximately 1.5, up to a maximum spacing that is commensurate with the 1 to 3 times the maximum diagonal dimension of the grounding system being designed, resulting in a maximum distance between the outer current electrodes of 3 to 9 times the maximum diagonal dimension of the future grounding system. This is one “traverse” or set of measurements, and is typically repeated, albeit with shorter maximum spacings, several times around the location at right angles and diagonally to each other to ensure accurate readings.

The basic premise of the soil resistivity test is that probes spaced at 5’ distance across the earth, will read 5’ in depth. The same is true if you space the probes 40’ across the earth, you get a weighted average soil resistance from 0’ down to 40’ in depth, and all points in between. This raw data is usually processed with computer software to determine the actual resistivity of the soil as a function of depth.

Conducting a Wenner 4-point (or four-pin) Soil Resistivity Test

The following describes how to take one “traverse” or set of measurements. As the “4-point” indicates, the test consists of 4 pins that must be inserted into the earth. The outer two pins are called the Current probes, C1 and C2. These are the probes that inject current into the earth. The inner two probes are the Potential probes, P1 and P2. These are the probes that take the actual soil resistance measurement.

Conducting A Wenner 4 Point Soil Resistivity Test

In the following Wenner 4-Point Test Setup diagram, a probe C1 is driven into the earth at the corner of the area to be measured. Probes P1, P2, & C2 are driven at 5’, 10’ & 15’ respectively from rod C1 in a straight line to measure the soil resistivity from 0’ to 5’ in depth. C1 & C2 are the outer probes and P1 & P2 are the inner probes. At this point, a known current is applied across probes C1 & C2, while the resulting voltage is measured across P1 & P2. Ohm’s law can then be applied to calculate the measured apparent resistance.

Probes C2, P1 & P2 can then be moved out to 10’, 20’ & 30’ spacing to measure the resistance of the earth from 0’ to 10’ in depth. Continue moving the three probes (C2, P1 & P2) away from C1 at equal intervals to approximate the depth of the soil to be measured. Note that the performance of the electrode can be influenced by soil resistivities at depths that are considerably deeper than the depth of the electrode, particularly for extensive horizontal electrodes, such as water pipes, building foundations or grounding grids.

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