1、Designation: G57 06 (Reapproved 2012)Standard Test Method forField Measurement of Soil Resistivity Using the WennerFour-Electrode Method1This standard is issued under the fixed designation G57; the number immediately following the designation indicates the year of originaladoption or, in the case of
2、 revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscriptepsilon () 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 resistivi
3、ty, both in situ 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 therespons
4、ibility of the 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 Definitions:2.1.1 resistivitythe electrical resistance between oppositefaces of a unit cube of material; the reciprocal
5、 of conductivity.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.2.1.1.1 DiscussionResistivity measurements indicate therelative ability of a medium to carry electrical currents. Whena metalli
6、c structure is immersed in a conductive medium, theability of the medium to carry current will influence themagnitude of galvanic currents and cathodic protection cur-rents. The degree of electrode polarization will also affect thesize of such currents.3. Summary of Test Method3.1 The Wenner four-el
7、ectrode method requires that fourmetal 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 res
8、ulting resistivity measurement 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 volt
9、meter. Alterna-tively, the resistance can be measured directly. The resistivity,r, is then:r, Vcm 5 2p aR a in cm!5 191.5 aR a in ft! (1)where:a = electrode separation, andR = resistance, V.Using dimensional analysis, the correct unit for resistivity isohm-centimetre.3.3 If the current-carrying (out
10、side) 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 aD(2)where:b = outer electrode spacing, ft,a = inner electrode spacing, ft, andR = resistance, V.or:r, Vcm 5pbR/S1 2bb 1 aD(3)where:b = outer electrode
11、 spacing, cm,a = inner electrode spacing, cm, 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/a (4)1This test method is under the jurisdiction of ASTM Committee G01 onCorrosion of Metals and is the direct responsibility
12、of Subcommittee G01.10 onCorrosion in Soils.Current edition approved May 1, 2012. Published June 2012. Originallyapproved in 1978. Last previous edition approved in 2006 as G5706. DOI:10.1520/G0057-06R12.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2
13、959, United States.where:R = resistance, V,A = cross sectional 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 theel
14、ectrodes.4. Significance and Use4.1 Measurement of soil resistivity 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 pro
15、tection systems, it is important to take as manymeasurements 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 resistiv
16、ity measure-ments to be taken at grade consists of a current source, asuitable voltmeter, ammeter, or galvanometer, four metalelectrodes, and the necessary wiring to make the connectionsshown in Fig. 2.5.1.2 Current SourceAn ac source, usually 97 Hz, ispreferred since the use of dc will cause polari
17、zation 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 bright metal before immersion, polarity is regularlyreversed during measurement, and meas
18、urements 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 electronic typeinstrument will yield satisfactory results if the meter inputimpedance is at least
19、 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. Both materials may require heat treatment sothat they are sufficiently rigid to be inse
20、rted 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 ensure that low-resistance contact is made at the electrodes and at the meter.Where regular su
21、rveys 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 required for the measurement of theresistivity of soil samples, either in the field or in thel
22、aboratory, 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-carrying (outside) electrodes are notspaced at the same interval as the potential-measuring
23、 (inside)electrodes, the resistivity, r, is:FIG. 1 Typical Connections for Use of Soil Box with Various Types of InstrumentsG57 06 (2012)2r, Vcm 5 95.76 bR/S1 2bb 1 aD(5)where:b = outer electrode spacing, ft,a = inner electrode spacing, ft, andR = resistance, V.or:r, Vcm 5pbR/S1 2bb 1 aD(6)where:b =
24、 outer electrode 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. Sta
25、ndardization6.1 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 calibrat
26、ed usingsolutions 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 standar
27、d solu-tions 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 a
28、re notrepresentative 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 ar
29、e installed at 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),
30、 which result 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 a
31、nd voltmeterare 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 So
32、il Sample Measurement: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 thoroug
33、hly mixed so 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 upper2Handbook of Chemistry and Phy
34、sics, 41st ed., The Chemical Rubber Co., p.2606.FIG. 2 Wiring Diagram for Typical dc Vibrator-Current SourceG57 06 (2012)3range of resistivity, which can be measured. In such cases, theresistivity should be recorded as 10 000 Vcm, and so forth.7.2.2 The measured resistivity will be dependent on thed
35、egree of compaction, moisture content, constituent solubility,and temperature. The effect of variations in compaction andmoisture content can be reduced by fully saturating the samplebefore placing it in the box. This can be done by preparing astiff slurry of the sample, adding only sufficient water
36、 toproduce a slight amount of surface water, which should beallowed to evaporate 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, l
37、ocal tapwater can be used without introducing serious error. Some soilsabsorb moisture 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
38、usefully com-pared with “as-received” resistivity measurements. Surpluswater should 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 su
39、bsequent measurement,correct the resistivity if the measurement temperature issubstantially different from the ground temperature. Correctionto 15.5C (60F) is recommended if the sample temperatureexceeds 21C (70F).R15.55 RT S24.5 1 T40D(7)where:T = soil temperature, C, andRT= resistivity at T C.A no
40、mograph for this correction is shown in Fig. 3.38. Planning and Interpretation8.1 Planning: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 soi
41、l condition. The lattermethod permits precise mathematical treatment, such as cumu-lative probability analysis. This test method permits the deter-mination of the probability of the presence of a soil with a3National Institute of Standards and Technology Circular No. 579, p. 157.FIG. 3 Nomogram or C
42、onversion 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)G57 06 (2012)4resistivity equal to or greater than a particular value.4Whererandom resistivities are measured over a plant site, these canbest be
43、displayed on a plot plan or similar layout. In either case,use pedological surveys in the planning and interpretation ofany extensive survey. Measurements could be made in eachsoil classification under a variety of drainage conditions tosimplify survey planning.8.1.2 If resistivity information is re
44、quired 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. Risk and
45、 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 with r
46、easonable 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. Sharp ch
47、anges 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 corros
48、ion 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 contamination t
49、ends 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
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