1、Designation: G 57 06Standard Test Method forField Measurement of Soil Resistivity Using the WennerFour-Electrode Method1This standard is issued under the fixed designation G 57; the number immediately following the designation indicates the year of originaladoption or, in the case of revision, the y
2、ear of last revision. A number in parentheses indicates the year of last reapproval. A superscriptepsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers the equipment and proceduresfor the field measurement of soil resistivity, both in sit
3、u and forsamples removed from the ground, for use in the control ofcorrosion of buried structures.1.2 To convert cm (metric unit) to metre (SI unit), divide by100.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the
4、user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Terminology2.1 Definition:2.1.1 resistivitythe electrical resistance between oppositefaces of a unit cube of material; the reciprocal of conductivity
5、.Resistivity is used in preference to conductivity as an expres-sion of the electrical character of soils (and waters) since it isexpressed in whole numbers.2.1.2 Resistivity measurements indicate the relative abilityof a medium to carry electrical currents. When a metallicstructure is immersed in a
6、 conductive medium, the ability ofthe medium to carry current will influence the magnitude ofgalvanic currents and cathodic protection currents. The degreeof electrode polarization will also affect the size of suchcurrents.3. Summary of Test Method3.1 The Wenner four-electrode method requires that f
7、ourmetal electrodes be placed with equal separation in a straightline in the surface of the soil to a depth not exceeding 5 % ofthe minimum separation of the electrodes. The electrodeseparation should be selected with consideration of the soilstrata of interest. The resulting resistivity measurement
8、 repre-sents the average resistivity of a hemisphere of soil of a radiusequal to the electrode separation.3.2 A voltage is impressed between the outer electrodes,causing current to flow, and the voltage drop between the innerelectrodes is measured using a sensitive voltmeter. Alterna-tively, the res
9、istance can be measured directly. The resistivity,r, is then:r,Vcm 5 2p aR a in cm!5 191.5 aRa in ft!where:a = electrode separation, andR = resistance, V.Using dimensional analysis, the correct unit for resistivity isohm-centimetre.3.3 If the current-carrying (outside) electrodes are notspaced at th
10、e same interval as the potential-measuring (inside)electrodes, the resistivity, r is:r, Vcm 5 95.76 bR/S1 2bb 1 aDwhere:b = outer electrode spacing, ft,a = inner electrode spacing, ft, andR = resistance, V.or:r, Vcm 5pbR/S1 2bb 1 aDwhere:b = outer electrode spacing, cm,a = inner electrode spacing, c
11、m, andR = resistance, V.3.4 For soil contained in a soil box similar to the one shownin Fig. 1, the resistivity, r, is:r, Vcm 5 RA/a1This test method is under the jurisdiction of ASTM Committee G01 onCorrosion of Metals and is the direct responsibility of Subcommittee G01.10 onCorrosion in Soils.Cur
12、rent edition approved Nov. 1, 2006. Published December 2006. Originallyapproved in 1978. Last previous edition approved in 2001 as G 57 95a (2001).1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.where:R = resistance, V,A = cross sect
13、ional area of the container perpendicular tothe current flow, cm2, anda = inner electrode spacing, cm.NOTE 1The spacing between the inner electrodes should be measuredfrom the inner edges of the electrode pins, and not from the center of theelectrodes.4. Significance and Use4.1 Measurement of soil r
14、esistivity is used for the control ofcorrosion of buried structures. Soil resistivity is used both forthe estimation of expected corrosion rates and for the design ofcathodic protection systems. As an essential design parameterfor cathodic protection systems, it is important to take as manymeasureme
15、nts as necessary so as to get a sufficiently represen-tative characterization of the soil environment that the entireburied structure will experience.5. Apparatus5.1 At-Grade Measurements in situ:5.1.1 The equipment required for field resistivity measure-ments to be taken at grade consists of a curr
16、ent source, asuitable voltmeter, ammeter, or galvanometer, four metalelectrodes, and the necessary wiring to make the connectionsshown in Fig. 2.FIG. 1 Typical Connections for Use of Soil Box with Various Types of InstrumentsFIG. 2 Wiring Diagram for Typical dc Vibrator-Current SourceG570625.1.2 Cur
17、rent SourceAn ac source, usually 97 Hz, ispreferred since the use of dc will cause polarization of mostmetal electrodes, resulting in error. The current can be providedby either a cranked ac generator or a vibrator-equipped dcsource.An unaltered dc source can be used if the electrodes areabraded to
18、bright metal before immersion, polarity is regularlyreversed during measurement, and measurements are averagedfor each polarity.5.1.3 VoltmeterThe voltmeter shall not draw appreciablecurrent from the circuit to avoid polarization effects. A galva-nometer type of movement is preferred but an electron
19、ic typeinstrument will yield satisfactory results if the meter inputimpedance is at least 10 megaohm.5.1.4 Electrodes fabricated from mild steel or martensiticstainless steel 0.475 to 0.635 cm (316 to14 in.) in diameter and30 to 60 cm (1 to 2 ft) in length are satisfactory for most fieldmeasurements
20、. Both materials may require heat treatment sothat they are sufficiently rigid to be inserted in dry or gravelsoils. The electrodes should be formed with a handle and aterminal for wire attachment.5.1.5 Wiring, 18 to 22-gage insulated stranded copper wire.Terminals should be of good quality to ensur
21、e that low-resistance contact is made at the electrodes and at the meter.Where regular surveys are to be made at fixed electrodespacing, a shielded multiconductor cable can be fabricated withterminals permanently located at the required intervals.5.2 Soil Sample Measurement:5.2.1 The equipment requi
22、red for the measurement of theresistivity of soil samples, either in the field or in thelaboratory, is identical to that needed for at-grade measure-ments except that the electrodes are replaced with an inertcontainer containing four permanently mounted electrodes (seeFig. 1).5.2.2 If the current-ca
23、rrying (outside) electrodes are notspaced at the same interval as the potential-measuring (inside)electrodes, the resistivity, r, is:r,Vcm 5 95.76 bR/S1 2bb 1 aDwhere:b = outer electrode spacing, ft,a = inner electrode spacing, ft, andR = resistance, V.or:r,Vcm 5pbR/S1 2bb 1 aDwhere:b = outer electr
24、ode spacing, cma = inner electrode spacing, cm, andR = resistance, V.5.2.3 The dimensions of the box can be established so thatresistivity is read directly from the voltmeter without furthercalculation. The box should be readily cleanable to avoidcontamination by previous samples.6. Standardization6
25、.1 Periodically check the accuracy of resistance metersusing a commercial resistance decade box. Meter error shouldnot exceed 5 % over the range of the instrument. If errorexceeds this limit, prepare a calibration curve and correct allmeasurements accordingly. A soil box can be calibrated usingsolut
26、ions of known resistivity. Solutions of sodium chlorideand distilled water with resistivities of 1000, 5000, and 10 000Vcm are recommended for this purpose. These solutionsshould be prepared under laboratory conditions using a com-mercial conductivity meter, itself calibrated to standard solu-tions
27、at 20C (68F).27. Field Procedures7.1 At-Grade Measurements:7.1.1 Select the alignment of the measurement to includeuniform topography over the limits of the electrode span. Donot include large nonconductive bodies such as frozen soil,boulders, concrete foundations, and so forth, which are notreprese
28、ntative of the soil of interest, in the electrode span.Conductive structures such as pipes and cables should not bewithin12 a of the electrode span unless they are at right anglesto the span.7.1.2 Select electrode spacings with regard to the structureof interest. Since most pipelines are installed a
29、t depths of from1.5 to 4.5 m (5 to 15 ft), electrode spacings of 1.5, 3.0, and 4.5m (5, 10, and 15 ft) are commonly used. The a spacing shouldequal the maximum depth of interest. To facilitate fieldcalculation of resistivities, spacings of 1.58, 3.16, and 4.75 m(5.2, 10.4, and 15.6 ft), which result
30、 in multiplication factors of1000, 2000, and 3000, can be used when a d-c vibrator-galvanometer instrument is used.7.1.3 Impress a voltage across the outer electrodes. Measurethe voltage drop across the inner electrodes and record both thecurrent and voltage drop if a separate ammeter and voltmetera
31、re used. Where a resistivity meter is used, read the resistancedirectly and record.7.1.4 Make a record of electrode spacing, resistance oramperes and volts, date, time, air temperature, topography,drainage, and indications of contamination to facilitate subse-quent interpretation.7.2 Soil Sample Mea
32、surement:7.2.1 Soil samples should be representative of the area ofinterest where the stratum of interest contains a variety of soiltypes. It is desirable to sample each type separately. It will alsobe necessary to prepare a mixed sample. The sample should bereasonably large and thoroughly mixed so
33、that it will berepresentative. The soil should be well-compacted in layers inthe soil box, with air spaces eliminated as far as practicable.Fill the box flush to the top and take measurements aspreviously detailed (7.1.3). The meter used may limit the upperrange of resistivity, which can be measured
34、. In such cases, theresistivity should be recorded as 10 000 Vcm, and so forth.7.2.2 The measured resistivity will be dependent on thedegree of compaction, moisture content, constituent solubility,and temperature. The effect of variations in compaction andmoisture content can be reduced by fully sat
35、urating the samplebefore placing it in the box. This can be done by preparing a2Handbook of Chemistry and Physics, 41st ed., The Chemical Rubber Co., p.2606.G57063stiff slurry of the sample, adding only sufficient water toproduce a slight amount of surface water, which should beallowed to evaporate
36、before the slurry is remixed and placed inthe box. Where available, use ground water from the sampleexcavation for saturation. Otherwise, use distilled water. If thesoil resistivity is expected to be below 10 000 Vcm, local tapwater can be used without introducing serious error. Some soilsabsorb moi
37、sture slowly and contain constituents that dissolveslowly, and the resistivity may not stabilize for as much as 24h after saturation. The saturated measurement will provide anapproaching minimum resistivity, and can be usefully com-pared with “as-received” resistivity measurements. Surpluswater shou
38、ld not be poured off as this will remove solubleconstituents.7.2.3 Temperature correction will not be required if mea-surement is made in-the-ditch or immediately after the sampleis taken. If samples are retained for subsequent measurement,correct the resistivity if the measurement temperature issub
39、stantially different from the ground temperature. Correctionto 15.5C (60F) is recommended if the sample temperatureexceeds 21C (70F).R15.55 RT S24.5 1 T40Dwhere:T = soil temperature, C, andRT= resistivity at T C.A nomograph for this correction is shown in Fig. 3.38. Planning and Interpretation8.1 Pl
40、anning:8.1.1 Surveys may be conducted at regular or randomintervals. The former method is suited to graphical presentationand plotting resistivity versus distance, and will identifygradients and abrupt changes in soil condition. The lattermethod permits precise mathematical treatment, such as cumu-l
41、ative probability analysis. This test method permits the deter-mination of the probability of the presence of a soil with aresistivity equal to or greater than a particular value.4Whererandom resistivities are measured over a plant site, these canbest be displayed on a plot plan or similar layout. I
42、n either case,use pedological surveys in the planning and interpretation ofany extensive survey. Measurements could be made in each3National Institute of Standards and Technology Circular No. 579, p. 157.4Scott, G. N., “Corrosion,” National Association of Corrosion Engineers,Vol14, No. 8, August 195
43、8.FIG. 3 Nomogram or Conversion Chart for Reducing Soil Paste Resistance in ohms at a Particular Temperature as Measured in theBureau of Soils Cup, to Resistance at 15.6C (60F)G57064soil classification under a variety of drainage conditions tosimplify survey planning.8.1.2 If resistivity information
44、 is required to assess therequirement for corrosion control measures, it is recommendedthat the tests be made on a true random basis. Since the numberof soil sections that could be inspected is essentially unlimited,infinite population characteristics can be used to simplifystatistical treatment. Ri
45、sk and error must be arbitrarily selectedto allow determination of the number of measurements. A riskof 5 % of an error greater than 100 Vcm should be suitable formost situations. The error limit should be about 10 % of theanticipated mean resistivity. Where mean or median valuescannot be estimated
46、with reasonable accuracy, sequentialsampling techniques can be employed.8.2 InterpretationInterpretation of the results of resistiv-ity surveys will largely depend on the experience of the personsconcerned. The mean and median resistivity values will indi-cate the general corrosivity of the soil. Sh
47、arp changes inresistivity with distance and appreciable variations in moisturecontent and drainage are indicative of local severe conditions.Cumulative probability plots will indicate the homogeneity ofthe soil over the area or route and will indicate the probabilityof severe, moderate, and minimal
48、corrosion of the variousconstruction materials. Available pedological data should beused to facilitate interpretation.9. General9.1 It should be recognized that subsurface conditions canvary greatly in a short distance, particularly where other buriedstructures have been installed. Surface contamina
49、tion tends toconcentrate in existing ditches with surface run-off, apprecia-bly lowering the resistivity below the natural level. Since apipeline ditch cannot be included in the span of at-grademeasurements, soil box samples should be obtained where theopportunity exists. To evaluate contamination effects when anew route is being evaluated, soil samples can be obtained atcrossings of existing pipelines, cables, etc, or by intentionalsampling using soil augers.9.2 Other field resistivity measurement techniques andequipment are available. These commonly use
