soil electrical resistivity

7/28/2019 Soil Electrical Resistivity

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SOIL ELECTRICAL RESISTIVITYAN ESSENTIAL PARAMETER FOR POWER PLANT DESIGN

JOHN R. DAVIEPrincipal Engineer

LLOYD W. YOUNG, JR.Chief Geotechnical Engineer

GEORGE A. STASHIKElectrical Engineer

Bechtel Power CorporationGaithersburg, Maryland

INTRODUCTIONSoil electrical resistivity is perhaps the only soil parameterthat is used by geologists and geotechnical engineers toexplore the subsurface characteristics at a power plant siteand also used by civil and electrical engineers as input totheir design. A properiy designed and executed fieldelectrical resistivity survey is one method of examining thesubsurface profile. The same program can provideinformation about the corrosion potential of the soil that canplay a major role in determining the protection needed forburied steel piping and pile foundations. At the same time,the soil conductivity interpreted from the results of theresistivity testing is an integral part of the design of theelectrical grounding system for the plant.This paper describes soil electrical resistivity, testprocedures to estimate resistivity values, and how thesevalues can best be used to obtain the parameters required forsuccessful power pknt design. Precautions andrecommendations are provided on the correct interpretationof the test results. Measures that should be taken to ensureadequate design for corrosion and grounding based onresistivity values are described and discussed.

SOIL ELECTRICAL RESISTIVITY TESTINGTrue and Apparent ResistivityElectrical resistivity, p, is the electrical resistance of a unitvolume of a material, and is measured in units of elecrncalresistance times length, typically expressed as ohm-meters.The concept is best illustrated by referring to Figure Iwhich shows the setup for an electrical soil resistivity test.Four electrodes are placed in the ground, and a measuredcurrent, I, from a battery (or, preferably, an ac source) flowsthrough the ground via the outer two electrodes. Theportion of current that flows through the ground willproduce a voltage drop, V, between the two inner

electrodes. If the soil has a constant resistivity throughout,then the resistivity that will be measured if the electrodesare art equal distance, A, apart can be shown to be:

p = 2nAvfI .................................................(l)Of course, subsurface conditions are never entirely uniform,i.e., because of the way subsurface materials are formed anddeposited, their properties vary within any specific area anddepth. Thus, in an actual field test, the computed quantityon the right hand side of Equation (1) will not be the trueresistivit y of the soil, since the resistivity of the soil willvary within the volume of soil influenced by the test. Thequantity measured is thus called the apparent resistivity.The amount the apparent and true resistivities will differwill depend on the amount of variation of the soil. It isimportant to note that there will probably be less variationof electrical resistivity within a small volume of soil thanwithin a large volume. Thus the apparent resistivitymeasured with the electrodes close together, i.e., a smallA value, will be closer to the true resist.ivity than with theelectrodes spaced far apart.Test ProceduresThe test setup with the evenly spaced electrodes shown inFigure 1 is called the Wenner electrode configuration. Thespacing A of the four electrodes depends on what resultsare desired, bearing in mind that A is typically chosen asone to two times the depth of interest. Where measurementof variations of subsurface conditions in a horizontaldirection is wanted (cakl prowling), A is held constantat the desired value and the entire spread is moved fromlocation to location. Where measurement of the variation ofresistivity with depth is wanted (called sounding), thecenter of the spread is held at a designated location, and Ais increased with successive readings. Generally the Avalues for sounding are increased logarithmically to give thebest results, e.g., spacing may beat 1, 1.75, 3, 5, 8, 12, 20,30, and 40 meters. One of the advantages of this type of testis its relatively low cost and short duration. The cost of thesounding at the A spacings noted above would be in the$200 to $500 range (including interpretation of results) andthe field portion would probably take less than 2 hours.The mild steel or martensitic stainless steel electrodes mustbe pushed or hammered into the ground until they come incontact with moist soil. Penetrations are typically around150 mm but may have to be deeper (300 mm or more) undercertain conditions. Penetration should be less for very smallA values; as a rule of thumb, electrode penetration should


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Proceedings of the American Power Conferencea-c Sourceor Battery Ammeter

m.... .,. . . . ... ,. . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ,,.. .,,,.. . ...,.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Soil \ l:f:~. . Resistivity. ., 0 Flow ... . . . .. . . . . . . . . . . . . . . . . . . . . ... pfw?1963Ku.iFigure 1. Soil Electrical Resistivity Testnot be greater than 10% of A. If the surface soil is verydry, water may have to be poured around the electrode. Infrozen soil, not only is it difficult to install the electrodes,but the readings obtained in the frozen soil can bemisleading. Care must be taken to avoid positioning thesurvey line close to any conductive structures such as steelpiles, pipes or cables. Also, the cables connecting the testequipment to the electrodes must be in good condition sinceeven a minute electrical leak can seriously affect the results.If a battexy or portable ac generator is used to energize theouter electrodes, consideration should be given to the effectsof the current source on the test results. If a dc source isused, voltage and current readings should be taken in asshort a period as possible and with the polarity at the outerelectrodes in the forward and reversed directions. Thisis necessary to minimize the effects of polarization of theouter electrodes which can distort the results. If a portableac generator is used, then voltage measurements should betaken at several pin spacings prior to energizing the outerelectrodes in order to determine whether there are any strayac currents flowing through the soil which may affect thetest results.Most models of commercial resistivity measuringequipment operate at frequencies slightly higher thannormal power system frequency. This negates the effects ofdc polarization and stray ac currents. Commercial testequipment incorporates the ammeter and voltmeter into acontrol box that includes a temperature compensationdevice and a processing capability that provides readout in adirectly usable form. To obtain a resistivity reading, thereading dial on the control box is adjusted until the nullmeter in the box is balanced. Since the range of soilresistivities can be very large, a scale multiplier settingswitch is used in conjunction with the readkg dial. Formost equipment, the reading provided is the 27tV/I termfrom Equation 1. The apparent resistivity is then obtained

by multiplying the reading by A if the Werner electrodeconfiguration is used.

Interpretation of Test ResuitsThe resistivity of earth materials tends to decrease withincreasing water content and also with increasing salinity orfree ion content of the water in the pore spaces. At the highend of the resistivity scale are massive rock formations(except ores) which contain only minimal pore space. Mostclean sands and gravels also exhibit high electricalresistivity. This is almost always the case with dry sandsand gravels, but is also usually true where the porewater isclean, and free of salinity. The resistivity of rock materialsgenerally falls when they become weathered because ofincreasing moisture and ion content. Similarly, theresistivity of sands and gravels tends to decreasesignificantly when the porewater becomes dirty, i.e., whenthey contain fine grained soil particles. Most fine-grainedsoils, i.e., silts and clays, have medium or low resistivities.These can become very low in near-surface soils when porewater evaporates, leaving high concentrations of salts.Current tends to flow towards low-resistivity materials, andaway from high-resistivity materials. This is the basis forinterpreting apparent resistivity readings in the soundingprocess, i.e., where electrode spacing A is increased forsuccessive readings. Figure 2 shows a soil profile where aclay of low resistivit y p I and thickness D overlies a verythick coarse sand layer of high resistivity p2. In Figure 2(a)the A spacing is small; as a result, the sand layer hasminimal influence on the current flow and the apparentresistivity reading is close to the true resistivity reading. Asthe A spacing increases (Figure 2(b)), the current linesthat would have gone deeper are forced upwards towardsthe surface since current wants to flow away from the highresistance sand. Since a greater proportion of the currentnow flows along the surface, the voltage drop between thetwo inner electrodes will also increase. Since resistivity isproportional to voltage drop, the apparent resistivity willincrease as the A value increases.Figure 3(a) shows apparent resistivity plotted against Afor the case in Figure 2. Figure 3(b) illustrates therelationship between apparent resistivity and A for thecase where the sand overlies the clay. At A spacings thatare only a &action of D (less than about 0.5), the apparentresistivity is close to that of the upper material. Theincreasing A spacing corresponds to an increasing depthof sounding. However, this is certainly not a 1:1relationship, nor is it linear. Nevertheless, provided thereare no horizontal discontinuities, the transition curvetowards p2 will be smooth. The A spacing required to


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Proceedings of the American Power Conference

111111 111{1[

v v v 1,. ,, .,. .. . . . . . . ., ..,. . . ,: ., . . .. ... . . . . . . . .::. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . L~~ ~ ..: . . . . . . . . . . . .. . . . . :. . .....esistiVity . ....,,,. . . . . . . . . . . . . . . . . . . . . . . . . .. . .. . . . . . . . . . Clay ..,, . .. . . . .. . . . . . . . . . . . . . . . . . . . . . . . .

(a)

,V2 . . . . . . . . . . . .PI . . . .t P2,High

(a)

. .High . . . . .. . . . . .. . . . .Resistivity . . . . . .. . . . . . . . . . . . . . . . . . . . . .Coarse Sand(b) ppt-m634s2

Figure 2. Current Distribution Variation in Layered Soil SystemPa P. P.EAPEp,, High

E

pl, HighP21LOW 02, Low

P3tMedium

(b) (c)

PI

A

4P..Et p3,Medium

(d)F$MI?J63U2-3

Figure 3. Qualitative Sounding Curves for Various Soil Profiles

reach p2 is always greater than Ml = 1, and is a function ofthe difference between p, and p2. The greater thedifference, the larger the A spacing needed to achieve p2.Figure 4 (from Ref. 1) shows a theoretically correct set ofcurves for this situation, and indicates that when the p2/p 1ratio is very high (e.g., 40 or more), even at large A/D ratiosthe apparent resistivity of the pz layer is only a fraction ofthe true resistivity. Consequently, in situations where thetop few meters of soil contain a high salt residue resulting invery low resistivity values, it is difficult to obtain even anestimate of the true resistivity of the underlying clean soil.When the soil profile contains more than 2 layers, theapparent resistivity versus A plot becomescorrespondingly more complex. Figures 3(c) and 3(d) show

the relationships for a 3-layer profile. Unless eachunderlying layer is extremely thick, the actual resistivityvalue will not be reached before the influence of theunderlying layer is felt. As with the 2-layer profile, the trueresistivity of the upper layer can be closely estimated. Forthe multiple-layer profde, the shape of the apparentresistivity versus A curve can be interpreted to provide anestimate of the thicknesses of the underlying layers and aqualitative estimate of their resistivity values relative to thetop layer. The true resistivities of these lower layers aredifflcuh to assess, particularly for a series of thin layerswhere there is a large variation in resistivity between thelayers. Computer models are used to assist in theinterpretation. The task of interpretation becomes mucheasier when there are data available from an adjacent sampleborehole.


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Proceedings of the American Power Conference

10. I I 1 I I 8 1 I

5 -

3 -2 -

Pa/P,1

Y- D = Thickness of Layer 1.5 - p, = Reaiativityof Layer 1

[

.3 p.= Apparent Resistivityof Layer2

.2 p2= Resistiv ity of Layer2L---u+2 .3 .5

I I

P2=40P1

J

23 5 10 10 30 50 100ND ppt-ms+m-.l

Figure 4. Quantitative Sounding Curves for a TwoLayer System

The above discussion assumes that the subsurface strata arehorizontal. When the strata are sloping, interpretationbecomes even more difficult. Frequently, the resistivitysounding test is repeated with an orthogonal survey linethrough the center point to provide confirmation of whetherthe strata are horizontal (the two soundings give similarresistivity versus A plots) or sloping (the two plots showsome variations).

USES OF SOIL ELECTRICAL RESISTIVITY INPOWER PLANT DESIGN

Investigating the Subsurface ProfileThis use has been described briefly in the previous section.Subsurface conditions under specific structures are typicallyinvestigated by sample borings and in-situ tests such as conepenetrometer soundings. Electrical resistivity surveys canprovide confirmation of the soil layering and can be used asa rapid means for distinguishing variability of this layeringwithin a particular area of interest. The orthogonal testsnoted above provide a method of determining the slope ofthe subsurface strata. Soil profding, where the A vzdue is

kept constant and the location of the array is moved inspecified manner (e.g., tests taken at 50 m spacing), can bused as an inexpensive means of locating, for example,source of clean granular material (i.e., where the soiresistivity is significantly higher) for fdl or well water, ofor locating buried obstructions or undesirable rnateriaI,e.g., a former ash pond on an existing power plant site.Corrosiveness of the Subsurface EnvironmentThe corrosivity of a soil towards a buried metal object idependent upon a number of parameters, including soiresistivity, moisture content, dissolved salts, pH, presence obacteria, and the amount of oxygen available at the metalsurface. It is generally agreed that no one parameter can beused to accurately forecast the corrosiveness of a particularsoil. Nevertheless, electrical resistivity is commonlyutilized as an indicator of the soils corrosiveness, and as thebasis for determining whether special corrosion mitigationmeasures, such as cathodic protection, should be taken forburied steel pipes or piles in contact with the soil.


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Proceedings of the American Power ConferenceTable 1- Soil Corrosiveness Criteria for Buried Steel

Soil Property Property value range for corrosiveness indicated belowLittle Mildly Moderately Corrosive Very

Corrosive Corrosive Corrosive CorrosiveResistivity, Ohm-m >100(1.2) 20-100(1) 10-20() 5-IO(I) .@)

50-100(2) 20-50(2) 7-20(2) 30(3)pH >5.0 and


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Proceedings of the American Power Conferencecurrent probes (three times A in the Wennerconfiguration) determines the volume of soil being tested.Some advocates support using a traverse equal to twice thediagonal length of the site. For large sites this becomesimpractical. A maximum traverse of 30 to 100 m isgenerally considered acceptable. As with the soundingsurvey described earlier, measurements are started at closeprobe spacings (shallow depths) and increasedincrementally until the selected traverse length is reached.Preferably, two sets of measurements are taken at eachlocation, using orthogonal traverses. NormalIy, the two setsof results will be similar. Small differences may indicatesloping s&ata. Large differences could irdcate possiblenoise problems from nearby power lines, buried metallicstructures, or soil anomalies. Soil anomalies can usually bechecked from soil boring logs.After any extraneous data have been discarded, the soilresistivity information collected at the various locations andprobe spacings are analyzed using a computer program.The output is generally in the form of a multi-layer soilmodel that provides an estimate of soil resistivity andthickness for each soil layer. The soil model providesimportant data for designing the ground grid. For example,normal practice may be to bury the ground grid conductor at0.5 m depth; however, the soil model may indicate that thesoil at 1 m depth is more conductive. A cost comparisonwould then be made to determine whether the savings inconductor material outweigh the additional costs for thedeeper installation.By studying the soil resistivity model, decisions can bemade about using ground rods. These copper rods are morecostly than a conventional ground grid, and are typicallyused when the near-surface soils have very high resistivity.The authors have experienced this situation with coarsegranular soils (gravels and cobbles), as well as calcareoussands and lateritic soils. In all cases, the ground water tablewas below the high resistivity layer, and the grounding rodshad to extend into the underlying saturated stratum. Thisinvolved installing the rods to as deep as 20 to 30 m.Indirectly, the telephone company is also concerned withthe soil resistivity at the power plant site. During a fault onthe high voltage system, a portion of the fault current flows

from the ground grid through the earth and back to thesource. The resistance of the ground grid to earth, which isa function of the soil resistivity, in conjunction with thecurrent flowing from the ground grid results in a groundpotential rise (GPR). If the GPR is high from the telephonecompanys viewpoint, they will install equipment to protecttheir lines from being damaged. GPR hiconcern with the use of fiber optic cablesmetallic path to remote locations.

CONCLUS1ONS

become less of asince there is no

The field soil resistivity test where soil resistivities forsuccessive strata are obtained by performing tests atincreasing electrode spacing provides data that can be used .(1) as an inexpensive means of interpreting the subsurfaceprofile, (2) for determining the corrosion potential of thesoil towxds buried piping, and (3) as input for designingthe plant ground grid. The test provides apparent soilresistivity values. True soil resistivity can be interpretedfrom apparent resistivity; this interpretation becomes moredit%cuh with more complex soil profiles. Near-surface soilswith high resistivity generally provide a suitably non-corrosive environment for buried piping, but increase thecost of the ground grid. Low resistance near-surface soilsgenerally result in an economical ground grid design, butalso result in a need for corrosion protection of the buriedpiping.

REFERENCES1. Bison Instruments Incorporated. Instruction

Manual for Earth Resistivitv Meters, 1973.2. American Petroleum Institute. Cathodic Protection

of Abovemound Petroleum Storage Tanks, APIRecommended Practice 651, Washington, D.C.,1991.

3. STS Consultants, Inc. Reinforced Soil Structures.Vol. 1. Desire and Construction Guidelines,FHWA Report No. FHWA-RD-89-043, McLean,VA, 1990.

soil electrical resistivity

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