ASTM G57-20

Standard Test Method for Measurement of Soil Resistivity Using the Wenner Four-Electrode Method

SIGNIFICANCE AND USE
5.1 Measurement of soil resistivity is used for assessment and control of corrosion of buried structures. Soil resistivity is used both for the estimation of expected corrosion rates and for the design of cathodic protection systems. As an essential design parameter for cathodic protection systems, it is important to take as many measurements as necessary so as to get a sufficiently representative characterization of the soil environment to which the entire buried structure will be exposed.
SCOPE
1.1 This test method covers the equipment and procedures for the measurement of soil resistivity, both in situ and for samples removed from the ground, for use in assessment and control of corrosion of buried structures.  
1.2 The values stated in SI units are to be regarded as standard. The values given in parentheses after SI units are provided for information only and are not considered standard. Soil resistivity values are reported in ohm-centimeter.  
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 31-Oct-2020
• ICS: 93.020 - Earthworks. Excavations. Foundation construction. Underground works
• Technical Committee: G01 - Corrosion of Metals
• Drafting Committee: G01.10 - Corrosion in Soils

Relations

• Refers: ASTM G187-23 - Standard Test Method for Measurement of Soil Resistivity Using the Two-Electrode Soil Box Method
• Effective Date: 01-Nov-2023
• Refers: ASTM G187-18 - Standard Test Method for Measurement of Soil Resistivity Using the Two-Electrode Soil Box Method
• Effective Date: 01-May-2018
• Refers: ASTM E691-13 - Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
• Effective Date: 01-May-2013
• Refers: ASTM G187-12a - Standard Test Method for Measurement of Soil Resistivity Using the Two-Electrode Soil Box Method
• Effective Date: 01-Nov-2012
• Refers: ASTM G187-12 - Standard Test Method for Measurement of Soil Resistivity Using the Two-Electrode Soil Box Method
• Effective Date: 01-May-2012
• Refers: ASTM E691-11 - Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
• Effective Date: 01-Nov-2011
• Refers: ASTM E691-08 - Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
• Effective Date: 01-Oct-2008
• Refers: ASTM D1193-06 - Standard Specification for Reagent Water
• Effective Date: 01-Mar-2006
• Refers: ASTM E691-05 - Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
• Effective Date: 01-Nov-2005
• Refers: ASTM G187-05 - Standard Test Method for Measurement of Soil Resistivity Using the Two-Electrode Soil Box Method
• Effective Date: 01-Oct-2005
• Refers: ASTM E691-99 - Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
• Effective Date: 10-May-1999
• Refers: ASTM D1193-99e1 - Standard Specification for Reagent Water
• Effective Date: 10-Feb-1999
• Refers: ASTM D1193-99 - Standard Specification for Reagent Water
• Effective Date: 10-Feb-1999

Overview

ASTM G57-20 – Standard Test Method for Measurement of Soil Resistivity Using the Wenner Four-Electrode Method provides a recognized procedure for determining soil resistivity, which is a critical parameter in the assessment and control of soil corrosion for buried structures. Adopted by ASTM International, this standard is widely used in engineering and infrastructure projects, especially for the estimation of corrosion rates and the design of cathodic protection systems.

Soil resistivity measurements obtained with the Wenner four-electrode method are important for developing a representative understanding of the soil’s corrosive environment. This method enables both in situ field measurements and laboratory measurements of soil samples. The results play a vital role in the long-term durability of pipelines, tanks, cables, and other metallic structures installed underground.

Key Topics

• Wenner Four-Electrode Method: The standard describes the arrangement of four electrodes placed in a straight line with equal spacing. A current is impressed between the outer electrodes, and the voltage drop is measured between the inner electrodes to calculate soil resistivity.
• Representativity: Multiple measurements are recommended to adequately characterize the soil environment along the full length of buried structures.
• Measurement Conditions: The test can be performed on-site (at grade) or on removed soil samples using a four-electrode soil box. Measurements must be reported in ohm-centimeter (Ω·cm).
• Equipment: Field measurements require a soil resistance meter or appropriate voltmeter/ammeter, a current source (preferably AC or a properly managed DC), four metal electrodes, and insulated copper wiring.
• Sample Preparation: For soil samples, full saturation and proper compaction are essential for accurate readings. ASTM specifies the use of distilled water for saturation and highlights the importance of temperature corrections when samples are not measured immediately.
• Statistical Analysis: The standard details repeatability and reproducibility parameters, supporting reliable soil resistivity survey planning.

Applications

• Corrosion Assessment: Determining soil resistivity helps predict the potential corrosivity of soils, as lower resistivity indicates higher corrosion risk for buried metals.
• Cathodic Protection System Design: Soil resistivity is a key design factor for cathodic protection systems shielding pipelines, tanks, and cables.
• Infrastructure Planning: Used extensively in the oil, gas, water, and utility industries to guide material selection, site assessment, and risk management associated with underground installations.
• Soil Survey and Quality Control: Environmental consultants, civil engineers, and utility companies employ standardized soil resistivity testing to identify variations in subsurface conditions and mitigate infrastructure deterioration.

Related Standards

• ASTM G187 – Test Method for Measurement of Soil Resistivity Using the Two-Electrode Soil Box Method: Alternative method for laboratory measurement on soil samples.
• ASTM D1193 – Specification for Reagent Water: Specifies water quality for sample saturation and accurate measurements.
• ASTM E691 – Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method: Guides the evaluation of precision and bias in technical standards.
• ASTM G193 – Terminology and Acronyms Relating to Corrosion: Provides definitions used throughout corrosion-related standards.

Keywords: ASTM G57-20, soil resistivity, Wenner method, four-electrode, cathodic protection, buried structures, corrosion control, pipeline protection, standardized test method, field measurement, infrastructure longevity.

By following ASTM G57-20, professionals can ensure repeatable, reliable, and internationally recognized measurements crucial for corrosion prevention and the durable design of underground assets.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: G57 − 20
Standard Test Method for
Measurement of Soil Resistivity Using the Wenner Four-
Electrode Method
This standard is issued under the fixed designation G57; the number immediately following the designation indicates the year of original
adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2.2 UNS Standards:
UNS Designation S30400 and S30403
1.1 This test method covers the equipment and procedures
UNS Designation S31600 and S31603
for the measurement of soil resistivity, both in situ and for
samples removed from the ground, for use in assessment and
3. Terminology
control of corrosion of buried structures.
3.1 Definitions:
1.2 The values stated in SI units are to be regarded as
3.1.1 four-electrode soil box, n—anon-conductivecontainer
standard. The values given in parentheses after SI units are
of known internal dimensions with four electrodes for measur-
provided for information only and are not considered standard.
ing a substance’s resistivity.
Soil resistivity values are reported in ohm-centimeter.
3.1.2 saturated soil, n—soil whose entire soil porosity is
1.3 This standard does not purport to address all of the
filled with water.
safety concerns, if any, associated with its use. It is the
3.1.3 soil resistance meter, n—an instrument capable of
responsibility of the user of this standard to establish appro-
measuring soil resistance.
priate safety, health, and environmental practices and deter-
3.1.4 soil resistivity, n—the electrical resistance between
mine the applicability of regulatory limitations prior to use.
opposite faces of a unit cube of material, typically expressed in
1.4 This international standard was developed in accor-
ohm-meter, ohm-cm, or similar units; the reciprocal of con-
dance with internationally recognized principles on standard-
ductivity
ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
3.2 The terminology used herein, if not specifically defined
mendations issued by the World Trade Organization Technical
otherwise, shall be in accordance with Terminology G193.
Barriers to Trade (TBT) Committee.
Definitions provided herein and not given in Terminology
G193 are limited only to this test method.
2. Referenced Documents
3.3 Discussion—Resistivity measurements indicate the rela-
2.1 ASTM Standards:
tive ability of a medium to carry electrical currents. When a
D1193 Specification for Reagent Water
metallic structure is immersed in a conductive medium, the
E691 Practice for Conducting an Interlaboratory Study to
ability of the medium to carry current will influence the
Determine the Precision of a Test Method
magnitude of galvanic currents and cathodic protection cur-
G187 Test Method for Measurement of Soil Resistivity
rents. The degree of electrode polarization will also affect the
Using the Two-Electrode Soil Box Method
size of such currents.
G193 Terminology and Acronyms Relating to Corrosion
4. Summary of Test Method
This test method is under the jurisdiction of ASTM Committee G01 on
4.1 The Wenner four-electrode method requires that four
Corrosion of Metals and is the direct responsibility of Subcommittee G01.10 on
metal electrodes be placed with equal separation in a straight
Corrosion in Soils.
line in the surface of the soil to a depth not exceeding 5 % of
Current edition approved Nov. 1, 2020. Published December 2020. Originally
approved in 1978. Last previous edition approved in 2012 as G57–06 (2012). DOI: the minimum separation of the electrodes. The electrode
10.1520/G0057-20.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Metals and Alloys in the Unified Numbering System (UNS), 13th Edition,
Standards volume information, refer to the standard’s Document Summary page on developed jointly by ASTM International, West Conshohocken, PA, and SAE
the ASTM website. International, Warrendale, PA, 2017.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G57−20
FIG. 1 Typical Connections for Use of Soil Box with Various Types of Instruments
separation should be selected with consideration of the soil where:
strata of interest. The resulting resistivity measurement repre-
b = outer electrode spacing, ft,
sents the average resistivity of a hemisphere of soil of a radius
a = inner electrode spacing, ft, and
equal to the electrode separation.
R = resistance, Ω.
or:
4.2 A voltage is impressed between the outer electrodes,
causing current to flow, and the voltage drop between the inner
ρ, Ω·cm 5πbR 11b⁄a (3)
~ !
electrodes is measured using a voltmeter. Alternatively, the
where:
resistance can be measured directly using a soil resistance
b = outer electrode spacing, cm,
meter. The resistivity, ρ, is then:
a = inner electrode spacing, cm, and
ρ, Ω·cm 5 2π aR a in cm (1)
~ !
R = resistance, Ω.
4.4 For soil contained in a soil box similar to the one shown
5191.5 aR ~a in ft!
in Fig. 1, the resistivity, ρ, is:
where:
ρ, Ω·cm 5RA/a (4)
a = electrode separation, and
R = resistance, Ω.
where:
R = resistance, Ω,
Using dimensional analysis, the correct unit for resistivity is
A = crosssectionalareaofthecontainerperpendiculartothe
ohm-centimeter.
current flow, cm , and
4.3 If the current-carrying (outside) electrodes are not
a = inner electrode spacing, cm.
spaced at the same interval as the potential-measuring (inside)
NOTE 1—The spacing between the inner electrodes should be measured
electrodes, the resistivity, ρ, is:
from the inner edges of the electrode pins, and not from the center of the
ρ, Ω·cm 5 95.76bR 11b⁄a (2) electrodes.
~ !
G57−20
FIG. 2 Wiring Diagram for Typical dc Vibrator-Current Source
5. Significance and Use different depths. The meter utilized may limit the upper range
of resistivity, which can be measured. In such cases, the
5.1 Measurement of soil resistivity is used for assessment
resistivity should be reported as greater than the meter’s upper
and control of corrosion of buried structures. Soil resistivity is
limit.
used both for the estimation of expected corrosion rates and for
6.1.4 Voltmeter—The voltmeter shall not draw appreciable
the design of cathodic protection systems. As an essential
current from the circuit to avoid influencing the results. A
design parameter for cathodic protection systems, it is impor-
galvanometer type of movement is preferred but an electronic
tant to take as many measurements as necessary so as to get a
type instrument will yield satisfactory results if the meter input
sufficiently representative characterization of the soil environ-
impedance is at least 10 megaohm.
ment to which the entire buried structure will be exposed.
6.1.5 Electrodes fabricated from mild steel or martensitic
3 1
stainless steel 0.475 cm to 0.635 cm ( ⁄16 in. to ⁄4 in.) in
6. Apparatus
diameter and 30 cm to 60 cm (1 ft to 2 ft) in length are
6.1 At-Grade Measurements in situ:
satisfactory for most field measurements. Both materials may
6.1.1 The equipment required for field resistivity measure-
require heat treatment so that they are sufficiently rigid to be
ments to be taken at grade consists of a current source, a four
inserted in dry soils, or soils containing rocks or gravel. The
terminal soil resistance meter, suitable voltmeter, ammeter, or
electrodes should be formed with a handle and a terminal for
galvanometer, four metal electrodes, and the necessary wiring
wire attachment.
to make the connections shown in Fig. 2.
6.1.6 Wiring, 18 gauge to 22 gauge insulated stranded cop-
6.1.2 Current Source—An ac source, usually 97 Hz, is
per wire. Terminals should be of good quality to ensure that
preferred since the use of dc will cause polarization of most
low-resistance contact is made at the electrodes and at the
metalelectrodes,resultinginerror.Thecurrentcanbeprovided
meter. Where regular surveys are to be made at fixed electrode
by either a cranked ac generator or a vibrator-equipped dc
spacing,ashieldedmulticonductorcablecanbefabricatedwith
source, which is internal to many soil resistance meters. An
terminals permanently located at the required intervals.
unaltered dc source can be used if the electrodes are abraded to
6.2 Soil Sample Measurement (that is, soil box):
bright metal before immersion, polarity is regularly reversed
during measurement, and measurements are averaged for each 6.2.1 The equipment required for the measurement of the
polarity. resistivity of soil samples, either in the field or in the
6.1.3 Four Terminal Soil Resistance Meter—Commercially laboratory, is identical to that needed for at-grade measure-
available, four terminal soil resistance meters are commonly ments except that the electrodes are replaced with an inert
used for measuring soil resistivity. They offer convenience, container containing four mounted electrodes (see Fig. 1).
ease of use, and repeatability. Soil resistance meters yield 6.2.2 Four-electrode soil boxes can be constructed in vari-
direct readings in ohms. Some meters allow readings to be ous sizes provided the inside dimensions are known. Design
taken at multiple depths using electrodes spaced at pre- and construction shall incorporate materials that are durable
determined intervals. These meters allow switching between and machinable. The two end plate electrodes shall be con-
electrode sets without having to rearrange electrodes and can structed of a clean, polished, corrosion-resistant metal or alloy
calculate resistivity from the entered geometry and measured (that is, UNS Designation type S30400/S30403 or UNS
current and voltages. Other models have the capability of S31600/S31603 stainless steel) that will not form a heavy
providing one or two dimensional analysis of soil resistivity at oxide film or otherwise add significant resistance. The body of
G57−20
the box shall be constructed of a material that is non- multiplication factors of 1000, 2000, and 3000, can be used
conductive and able to maintain its desired dimensions (poly- when a d-c vibrator-galvanometer instrument is used.
carbonate plastics). The box shall be readily cleanable to avoid
9.1.3 Impress a voltage to create a current across the outer
contamination by previous samples. Some soil box designs
electrodes. Measure the voltage drop across the inner elec-
incorporate removable pins (that is, electrodes) to facilitate
trodes and record both the current and voltage drop if a
cleaning. Other soil box designs incorporate two electrodes in
separate ammeter and voltmeter are used. Where a resistivity
lieu of four electrodes (see Test Method G187).
meter is used, read the resistance directly and record.
6.2.3 If the current-carrying (outside) electrodes are not
NOTE 2—Some newer models of soil resistivity meters allow resistivity
spaced at the same interval as the potential-measuring (inside)
measurements to be collected in such a way as to automatically analyze
electrodes, the resistivity, ρ, is determined using Eq 2, where
soil resistivity at different depths. A number of pins are driven at equal
the spacing is given in feet, and Eq 3, where the electrode
spacing and the instrument switches between different combinations of
spacing is given in centimeters. four pins. The information can then be processed using software to give
either one or two dimensional profiles
6.2.4 The dimensions of the box can be established so that
resistivity is read directly from the voltmeter without further
9.1.4 Make a record of electrode spacing, resistance or
calculation. The box should be readily cleanable to avoid
amperes and volts, date, time, ambient temperature,
contamination by previous samples.
topography, drainage, and indications of contamination to
facilitate subsequent interpretation.
7. Reagents and Materials
9.2 Soil Sample Measurement (that is, soil box measure-
7.1 Distilled or deionized water (Type IV grade as refer-
ments
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: G57 − 06 (Reapproved 2012) G57 − 20
Standard Test Method for
Field Measurement of Soil Resistivity Using the Wenner
Four-Electrode Method
This standard is issued under the fixed designation G57; the number immediately following the designation indicates the year of original
adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method covers the equipment and procedures for the field measurement of soil resistivity, both in situ and for samples
removed from the ground, for use in the assessment and control of corrosion of buried structures.
1.2 To convert cm (metric unit) to metre (SI unit), divide by 100.The values stated in SI units are to be regarded as standard. The
values given in parentheses after SI units are provided for information only and are not considered standard. Soil resistivity values
are reported in ohm-centimeter.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D1193 Specification for Reagent Water
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
G187 Test Method for Measurement of Soil Resistivity Using the Two-Electrode Soil Box Method
G193 Terminology and Acronyms Relating to Corrosion
2.2 UNS Standards:
UNS Designation S30400 and S30403
UNS Designation S31600 and S31603
3. Terminology
3.1 Definitions:
This test method is under the jurisdiction of ASTM Committee G01 on Corrosion of Metals and is the direct responsibility of Subcommittee G01.10 on Corrosion in
Soils.
Current edition approved May 1, 2012Nov. 1, 2020. Published June 2012December 2020. Originally approved in 1978. Last previous edition approved in 20062012 as
G57–06. –06 (2012). DOI: 10.1520/G0057-06R12. 10.1520/G0057-20.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Metals and Alloys in the Unified Numbering System (UNS), 13th Edition, developed jointly by ASTM International, West Conshohocken, PA, and SAE International,
Warrendale, PA, 2017.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G57 − 20
3.1.1 four-electrode soil box, n—a non-conductive container of known internal dimensions with four electrodes for measuring a
substance’s resistivity.
3.1.2 saturated soil, n—soil whose entire soil porosity is filled with water.
3.1.3 soil resistance meter, n—an instrument capable of measuring soil resistance.
3.1.4 resistivity—soil resistivity, n—the electrical resistance between opposite faces of a unit cube of material; the reciprocal of
conductivity. Resistivity is used in preference to conductivity as an expression of the electrical character of soils (and waters) since
it is expressed in whole numbers.material, typically expressed in ohm-meter, ohm-cm, or similar units; the reciprocal of
conductivity
2.1.1.1 Discussion—
Resistivity measurements indicate the relative ability of a medium to carry electrical currents. When a metallic structure is
immersed in a conductive medium, the ability of the medium to carry current will influence the magnitude of galvanic currents
and cathodic protection currents. The degree of electrode polarization will also affect the size of such currents.
3.2 The terminology used herein, if not specifically defined otherwise, shall be in accordance with Terminology G193. Definitions
provided herein and not given in Terminology G193 are limited only to this test method.
3.3 Discussion—Resistivity measurements indicate the relative ability of a medium to carry electrical currents. When a metallic
structure is immersed in a conductive medium, the ability of the medium to carry current will influence the magnitude of galvanic
currents and cathodic protection currents. The degree of electrode polarization will also affect the size of such currents.
4. Summary of Test Method
4.1 The Wenner four-electrode method requires that four metal electrodes be placed with equal separation in a straight line in the
surface of the soil to a depth not exceeding 5 % of the minimum separation of the electrodes. The electrode separation should be
selected with consideration of the soil strata of interest. The resulting resistivity measurement represents the average resistivity of
a hemisphere of soil of a radius equal to the electrode separation.
4.2 A voltage is impressed between the outer electrodes, causing current to flow, and the voltage drop between the inner electrodes
is measured using a sensitive voltmeter. Alternatively, the resistance can be measured directly. directly using a soil resistance meter.
The resistivity, ρ, is then:
ρ, Ω·cm 5 2π aR a in cm (1)
~ !
5191.5 aR a in ft
~ !
where:
where:
a = electrode separation, and
R = resistance, Ω.
Using dimensional analysis, the correct unit for resistivity is ohm-centimetre.ohm-centimeter.
4.3 If the current-carrying (outside) electrodes are not spaced at the same interval as the potential-measuring (inside) electrodes,
the resistivity, ρ, is:
b
ρ, Ω·cm 5 95.76b R/ 12 (2)
S D
b1a
ρ, Ω·cm 5 95.76 bR 11b⁄a (2)
~ !
where:
where:
b = outer electrode spacing, ft,
a = inner electrode spacing, ft, and
G57 − 20
FIG. 1 Typical Connections for Use of Soil Box with Various Types of Instruments
R = resistance, Ω.
or:
b
ρ, Ω·cm 5 πb R/ 12 (3)
S D
b1a
ρ, Ω·cm 5 πbR~11b⁄a ! (3)
where:
where:
b = outer electrode spacing, cm,
a = inner electrode spacing, cm, and
R = resistance, Ω.
4.4 For soil contained in a soil box similar to the one shown in Fig. 1, the resistivity, ρ, is:
ρ, Ω·cm 5 R A/a (4)
where:
where:
R = resistance, Ω,
A = cross sectional area of the container perpendicular to the current flow, cm , and
a = inner electrode spacing, cm.
NOTE 1—The spacing between the inner electrodes should be measured from the inner edges of the electrode pins, and not from the center of the
electrodes.
G57 − 20
FIG. 2 Wiring Diagram for Typical dc Vibrator-Current Source
5. Significance and Use
5.1 Measurement of soil resistivity is used for the assessment and control of corrosion of buried structures. Soil resistivity is used
both for the estimation of expected corrosion rates and for the design of cathodic protection systems. As an essential design
parameter for cathodic protection systems, it is important to take as many measurements as necessary so as to get a sufficiently
representative characterization of the soil environment that to which the entire buried structure will experience.be exposed.
6. Apparatus
6.1 At-Grade Measurements in situ:
6.1.1 The equipment required for field resistivity measurements to be taken at grade consists of a current source, a four terminal
soil resistance meter, suitable voltmeter, ammeter, or galvanometer, four metal electrodes, and the necessary wiring to make the
connections shown in Fig. 2.
6.1.2 Current Source—An ac source, usually 97 Hz, is preferred since the use of dc will cause polarization of most metal
electrodes, resulting in error. The current can be provided by either a cranked ac generator or a vibrator-equipped dc source. source,
which is internal to many soil resistance meters. An unaltered dc source can be used if the electrodes are abraded to bright metal
before immersion, polarity is regularly reversed during measurement, and measurements are averaged for each polarity.
6.1.3 Four Terminal Soil Resistance Meter—Commercially available, four terminal soil resistance meters are commonly used for
measuring soil resistivity. They offer convenience, ease of use, and repeatability. Soil resistance meters yield direct readings in
ohms. Some meters allow readings to be taken at multiple depths using electrodes spaced at pre-determined intervals. These meters
allow switching between electrode sets without having to rearrange electrodes and can calculate resistivity from the entered
geometry and measured current and voltages. Other models have the capability of providing one or two dimensional analysis of
soil resistivity at different depths. The meter utilized may limit the upper range of resistivity, which can be measured. In such cases,
the resistivity should be reported as greater than the meter’s upper limit.
6.1.4 Voltmeter—The voltmeter shall not draw appreciable current from the circuit to avoid polarization effects. influencing the
results. A galvanometer type of movement is preferred but an electronic type instrument will yield satisfactory results if the meter
input impedance is at least 10 megaohm.
3 1
6.1.5 Electrodes fabricated from mild steel or martensitic stainless steel 0.4750.475 cm to 0.635 cm ( ⁄16 in. to ⁄4 in.) in diameter
and 3030 cm to 60 cm (1(1 ft to 2 ft) in length are satisfactory for most field measurements. Both materials may require heat
treatment so that they are sufficiently rigid to be inserted in dry or gravel soils. soils, or soils containing rocks or gravel. The
electrodes should be formed with a handle and a terminal for wire attachment.
G57 − 20
6.1.6 Wiring, 1818 gauge to 22-gage22 gauge insulated stranded copper wire. Terminals should be of good quality to ensure that
low-resistance contact is made at the electrodes and at the meter. Where regular surveys are to be made at fixed electrode spacing,
a shielded multiconductor cable can be fabricated with terminals permanently located at the required intervals.
6.2 Soil Sample Measurement:Measurement (that is, soil box):
6.2.1 The equipment required for the measurement of the resistivity of soil samples, either in the field or in the laboratory, is
identical to that needed for at-grade measurements except that the electrodes are replaced with an inert container containing four
permanently mounted electrodes (see Fig. 1).
6.2.2 Four-electrode soil boxes can be constructed in various sizes provided the inside dimensions are known. Design and
construction shall incorporate materials that are durable and machinable. The two end plate electrodes shall be constructed of a
clean, polished, corrosion-resistant metal or alloy (that is, UNS Designation type S30400/S30403 or UNS S31600/S31603 stainless
steel) that will not form a heavy oxide film or otherwise add significant resistance. The body of the box shall be constructed of
a material that is non-conductive and able to maintain its desired dimensions (polycarbonate plastics). The box shall be readily
cleanable to avoid contamination by previous samples. Some soil box designs incorporate removable pins (that is, electrodes) to
facilitate cleaning. Other soil box designs incorporate two electrodes in lieu of four electrodes (see Test Method G187).
6.2.3 If the current-carrying (outside) electrodes are not spaced at the same interval as the potential-measuring (inside) electrodes,
the resistivity, ρ, is:is
b
ρ, Ω·cm 5 95.76 b R/ 12 (5)
S D
b1a
determined
where:
b = outer electrode spacing, ft,
a = inner electrode spacing, ft, and
R = resistance, Ω.
using Eq 2, where the spacing is given in feet, and Eq 3, where the electrode spacing is given in centimeters.
or:
b
ρ, Ω·cm 5 πb R/ 12 (6)
S D
b1a
where:
b = outer electrode spacing, cm
a = inner electrode spacing, cm, and
R = resistance, Ω.
6.2.4 The dimensions of the box can be established so that resistivity is read directly from the voltmeter without further
calculation. The box should be readily cleanable to avoid contamination by previous samples.
7. Reagents and Materials
7.1 Distilled or deionized water (Type IV grade as referenced in Specification D1193) to saturate samples.
7.2 Commercially available solutions in the range of 1000 ohm-cm, 5000 ohm-cm, and 10 000 ohm-cm are recommended for the
purpose of calibration.
8. Standardization
8.1 Periodically check the accuracy of resistance meters using a commercial resistance decade box. manufacturer’s recommen-
dations. Meter error should not exceed 5 % over the range of the instrument. If error exceeds this limit, prepare a calibration curve
and correct all measurements accordingly. A soil box can be calibrated using solutions of known resistivity. Solutions of sodium
chloride and distilled water with resistivities of 1000, 5000, and 10 000 Ω·cm 1000 Ω·cm, 5000 Ω·cm, and 10 000 Ω·cm are
G57 − 20
recomm
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