soil thermal resistivity measured simply and accurately

IEEE TRANSACTIONS ON POWER APPARATUS AND SYSTEMS, VOL. PAS-89, NO. 2, FEBRUARY 1970

for the closure distance after 10 load breaks, the effect would be forthe 60-in/s line to have a slope less than one half of that shown. Inother words, this new line would not be as steep as the 30-in/s lineshown in the figure. The results would be to consistently obtain highinstantaneous voltage on closure and to produce excessive arcingtimes. Thus serious contamination of arc-quenching material canyield very poor fault-closure performance after load-break life.Materials are available which do not contaminate as readily as thatshown.

In regard to the load-break test circuit shown in Fig. 16, the authorstates that it is essentially the same as that in the proposed IEEE-NEMA standard. Fig. 19 demonstrates the difference in these twocircuits. The voltage diagrams show the half-cycle of the voltagewave in which the current is extinguished. A 70-percent power factoris assumed, and the series impedance is 10 percent of the total and ismostly reactive.

After the connector current goes to zero, the currents in theparallel R and L load impedances are equal and opposite. They decaygradually, and the voltage across this part of the circuit decays asshown. The voltage across the open connector is the difference be-tween this voltage and the source. The values of these two voltagedrops are the same for each circuit, and they are merely inter-changed with respect to ground.Assuming that the elbow is connected to the source side of the

connector, the heavy black line shows the voltage of the elbow toground. This is the most important factor since failure occurs whenthe elbow flashes over to ground. Fig. 19 clearly demonstrates whythe "standard" circuit is the more severe. At the instant of currentzero, the elbow voltage is less than 10 percent for the circuit inthe paper and 70 percent for the "standard" circuit.

In summary, the author's paper can be considered a fine contribu-tion to the understanding of this relatively new device. It must benoted however that the remarks pertain to one specific device andnot to all available load-break connectors.

Norman M. Sacks: The author wishes to express his appreciationfor the interest shown by Mr. Ruete in his paper.Mr. Ruete's comments are well taken, and they underscore points

made in the paper. As he points out, the data in Figs. 14 and 18 lendthemselves to a calculation of the velocity required for contaminatedparts as a result of load-break life. The paper shows that the termina-tor tested will give satisfactory fault-closure performance after 10load breaks using the velocity of 60 in/s. Subsequent to the publica-tion of the paper we have found that improved arcing materials candecrease the contamination that results from load switching. Thesematerials allow speeds lower than 60 in/s for successful fault closures.Mr. Ruete is correct in his conclusion that the position of the paral-

lel impedance in load-break circuits is an important element inswitching. As stated in the paper, the parallel impedance was locatedas a testing convenience before the IEEE-NEMA standard circuitbecame firm. Subsequent tests used the parallel impedance betweenthe test samples and the ground. It is agreed that this would increasethe likelihood of flashover to ground.

Manuscript received October 7, 1969.

Soil Thermal Resistivity Measured

Simply and AccuratelyJOHN STOLPE, MEMBER, IEEE

Abstract-There is a growing need for each utility to gain a

thorough knowledge of the soil thermal resistivity throughout itssystem. The volume of heavily loaded multiduct runs of power cablespromises to increase rapidly in the years ahead. We can no longerafford to plan for cable loadings based purely on an assumed soilthermal resistivity. Rather, we must compile information thataccurately describes the thermal resistivity of soils commonlyfound within specific geographic areas; then very realistic values ofsoil thermal resistivity can be used in the planning of new under-ground systems in these areas.This paper describes the design and utilization of apparatus that

has demonstrated convenience and reliability in the analysis ofthermal properties of soils and soil-like materials. The scope ofestablished test procedures is related. Test results and variouscorrelations that have been found from preliminary testing are

reviewed and illustrated.

Paper 69 C 1-PWR, recommended and approved by the InsulatedConductors Committee of the IEEE Power Group for presentationat the IEEE Underground Distribution Conference, Anaheim,Calif., May 12-16, 1969. Manuscript submitted December 2, 1968;made available for printing October 7, 1969.The author is with the Southern California Edison Company,

Los'Angeles, Calif. 90053.

INTRODUCTION

ANYONE involved in the design of underground powertransmission and distribution systems knows that in order

to accurately predetermine the ampacity of such systems, onemust know, along with other data, the thermal resistivity of thesoil surrounding the system. There are several techniquesreported in the literature [1]-[4] for determining the thermalresistivity (rho) of soils, and they have been used with varyingdegrees of accuracy, convenience, and success.A new technique, with some advantages, has been developed

to measure the thermal resistivity of soils. The apparatus iscalled a rhometer, and is basically a laboratory device. Samplesof soil are taken from the field and recompacted to any desireddensity when they are tested for rho. The soil can be tested atthe field density and over a range of moisture content; then thesoil density can be changed and the soil rho remeasured todetermine the effect of getting more compaction. This flexibilityof being able to measure rho under conditions of varying knownsoil density and varying moisture content has obvious advan-tages when making preliminary investigations in the design oflarge underground power systems. Since the rhometer can also

297

IEEE TRANSACTIONS ON POWER APPARATUS AND SYSTEMS, FEBRUARY 1970

induce variable heat fluxes on a soil, it may become possible todevelop a correlation of heat flow through a soil and some equi-librium moisture content that can be held within the soil matrix.The foregoing idea is indeed a subject for future work.As holds true for all methods of measuring the rho of soil in the

field, the rhometer can only measure the soil rho at a point, ormany points, along a proposed underground trench. Frequencyof sampling along the route in order to locate thermal troublespots is a subject in itself and is covered in the literature [5],[6]. Once the samples have been taken, each specific soil typeencountered can be placed in the rhometer at precisely the samedensity and moisture content as in the field, and the respectivesoil rhos may be accurately determined.

It is felt that there will be no loss of accuracy by taking samplesfrom the field and remolding them to the same density andmoisture conditions as they experienced when taken from thefield. Tests have not actually been performed to establish thisbelief, but it is generally recognized that most granular soilscomposed of silt and sand do not lose their physical charac-teristics upon being remolded to the same density and moisturecontent as an undisturbed sample [7 ]. The types of soils that maycause errors when they are remolded are "sensitive" clays, but itis doubtful that even as much as 10-percent error can be intro-duced in the measurement rho of sensitive clays.

It has been stated that the correct laboratory determinationof soil rho must be made on undisturbed soil samples, andreference was made to tests on fragmented, or mechanicallycrushed, soils [8]. Undisturbed samples were taken of soil withaverage maximum particle diameters of about 10 mm andthe undisturbed soil rho was determined. Then the samplewas crushed between rollers to a maximum particle size of about2 mm, and the soil rho was again measured. These two rhovalues were then compared, and as would be expected, thevalues were quite different because, indeed, the soils were dif-ferent.The rhometer technique applies Fourier's law of heat con-

duction in a steady-state laboratory apparatus; therefore, rho isdetermined from its basic scientific definition [9]. Measuringrho of a given soil, as diagramed in Fig. 1, involves placing thesoil between two concentric cylinders and allowing heat to flowradially from the inside surface to the outside surface. Therho of the soil between concentric cylinders is found by in-tegrating Fourier's law of heat conduction in cylindrical geom-etry to get

2ir(L) (Td- TD) (1)Q ln (D/d)

where

p thermal resistivity, WC cm/WL axial length of cylinder, cmTd- TD temperature drop between d and D, °CQ total heat flowing from d to D, wattsD diameter of outer cylindrical surfaced diameter of inner cylindrical surface.

There are no simplifying assumptions in the application of(1), and the relation always holds under conditions of steady-state radial heat flow.

Since the rhometer method is an application of steady-stateheat flow, heat can be applied continuously for weeks in dura-tion without affecting the rho measurement. This permits thesoil specimen to gradually dry out, making possible comparison

Fig. 1. Radial heat flow through hollow cylinder.

of the drying rates of several soils. If it is desired to determine theeffect of compaction on the thermal resistivity of a given soil,the soil can be placed at any desired density and its rho can bemeasured as it slowly dries out.

This method also allows soil to be tested under varying heatfluxes of realistic magnitudes, giving an indication of the heatthat can be conducted by a soil without causing excessive dryingin a field application.These soil thermal properties are being acquired for "typical"

soils found in various populated regions throughout southernCalifornia. The object of such a large investigation is to be ableto establish "typical" values of rho for the soils found in thevarious regions. It is felt that the practice of assuming a stan-dard rho of 900C cm/W for the entire service territory of autility could be sacrificing a great deal of economy, and in somecases it could lead to overheated cable systems. Therefore, arather large program is being conducted in order to gain knowl-edge of typical soil conditions at the normal burial depth ofelectric power systems. It is not anticipated that every soiltype in each populated area of southern California can bedetected; however, enough samples can be taken so that theactual soil rho can be intelligently estimated instead of merelyguessed.

SOIL SAMPLING METHODS

Samples of soil are taken from various field locations andtested for moisture content and thermal resistivity in thelaboratory. Whenever possible, samples are taken from aboutfour feet below grade so that the ambient soil temperature canalso be measured. This is usually accomplished by coordinatingsoil sampling in an area with underground construction, andtaking the soil sample just after the trench is opened.The only test performed in the field is the determination of the

in-place density of the soil. Two methods can be used, both ofwhich yield good results. One that can be used in all cases isthe sand cone method (ASTM D1556-64). It involves carefullydigging a small hole and then refilling the hole with a knownvolume of specially calibrated sand. The other, the thin-walledtube method (ASTM D1587-63T), simply involves driving atube into the ground and determining the weight and volume ofthe soil sample removed. This method is much faster than thesand cone method, but it cannot be used with acceptable accuracyin very hard or gravel-type soils.

All samples are returned to the laboratory, tested for fieldmoisture content and subjected to grain size analysis (ASTMC136-63). This permits each soil to be classified as a clay, silt,or sand; and samples are stored until they can be tested for rho.

In an extensive soil investigation that is an attempt to cor-relate many variables, it is best if testing is done under thecontrolled conditions found only in a laboratory. This is why noattempts were made to measure rho in the field. Even if rhowere measured in the field, to get complete meaning from the

298

STOLPE: SOIL THERMAL RESISTIVITY MEASUREMENT

results a sample would have to be taken at the test depth inorder to know the soil moisture content and degree of soilcompaction. Thus, it is nearly as fast, and more accurate, totake one large soil sample from the field and perform the thermaltests in a laboratory.

CONSTRUCTION DETAILS

Before the methods of testing soils for thermal resistivity arediscussed, the details of construction of the rhometer will beexplained. Several materials have been chosen for use in therhometer and each has a specific physical and/or thermalpurpose. Fig. 2 shows the apparatus and the details of its con-struction; the description of each component follows.

1) Power Resistor: This serves as the heat source; it is a 2000-ohm 50-watt resistor connected to a 115-volt ac regulated powersupply. It has a temperature coefficient of resistance of onlyabout 75 ppm/°C and it is noninductive. Thus, the watts dis-sipated can be determined simply by measuring the currentflowing through the essentially constant resistance.

2) Copper Pipe: The outside diameter of the copper was chosenas large as possible in order to provide a large surface area forheat to pass into the soil, but it had to be small enough, withrespect to the concrete cylinder, so that a measurable tem-perature drop would result between the inside and outsidesurfaces of the soil sample. These opposing criteria arise becauseit is necessary that the rhometer give accurate rho valueswhen operating under a low heat flux of 10 W/ft2 at thesoil/heat source interface; for handling purposes, the overalldiameter of the soil had to be restricted to about 8 inches sothat the volume of soil being tested could be handled easily. Thematerial used for the copper tube is extra-heavy-wall copperpipe. The large thickness will equalize any temperature varia-tions, thus producing an isothermal surface at the soil inter-face. Actual measurements have shown that the copper has lessthan 0.0250 C variation in temperature over its surface.

3) Concrete Cylinder: Two requirements of the concrete pipe aresufficient strength to withstand the internal soil pressure fromcompaction, and porosity, so that moisture can be driven out ofthe soil, through the concrete, and finally to the surrounding air.This piece is mass produced as an extension neck for a waterhandhole.

4) Insulating Disks: The primary function of the disks is toinsure radial heat flow through the soil. The thermal resistanceof the foam disks, made of material with p = 30000 C - cm/W,is much greater than the thermal resistance of the soil. Becausethe heat will flow along the path of lowest thermal resistance, theheat therefore flows radially to the concrete. The disk on thebottom is reinforced with fiberglass webbing so that it can with-stand the compaction forces required to compact the soil insidethe apparatus.

5) Base: The plywood base is used to conveniently hold every-thing in position. It is light in weight, yet rigid enough to give afirm base for compacting the soil. It also provides additionalthermal insulation for the prevention of axial heat flow from thesoil.

6) Thermocouples: Copper-constantan thermocouples areused; one is placed at the concrete soil interface and the other isembedded in the copper pipe. They are electrically connected sothat the thermocouple electromotive forces (EMFs) buck eachother; thus the difference in EMF between them can be readdirectly with a good potentiometer and then converted totemperature drop.

Fig. 2. Cross section of rhometer apparatus.

These parts, or close substitutes, can easily be acquiredat low cost, and assembled without much difficulty. The com-pleted rhometer, when filled with about 15 pounds of compactedsoil, weighs less than 40 pounds and is easy to handle in thelaboratory.A special note in regard to bonding the copper pipe to the

bottom insulating disk is necessary. When soil is placed in therhometer and compacted to its maximum density, the copperpipe is very susceptible to being broken away from the fiberglasslayer over the foam. By making 1/8-inch grooves inside thecopper pipe and filling the inside with the proper epoxy cement asshown in Fig. 2, adequate strength is gained in the finished bond.For rhometer components with the dimensions used for Fig. 2,

(1) can be reduced to a simpl fied working equation:

2ir(L)(Td-TD) _ 6.28(4 X 2.54 cm)(Td- T 2)Q ln (D/d) Q ln (7.883 in/1.895 in)

= 44.77 AT/Q °C cm/W

where

AT temperature drop through soil, °CQ total heat generated in resistor, watts.

Thus, to determine rho of a given soil, one simply measures thecurrent flowing in the constant resistance, calculates the watts loss,and measures the steady-state temperature drop through the soil.Equation (2) then gives the thermal resistivity of the soil insidethe rhometer. The total heat generation is typically about 3watts, which results in temperature drops between 5 and 15°Cfor most soils.

In the assembly and actual use of the rhometer, it is mostimportant to be certain that none of the heat generated by thepower resistor is lost except by passing through the soil. Thus,any leakage of heat throuah small air Dassages or out excessively

299


large metallic conductors must be prevented. Heat loss throughpaths other than the soil would cause error in the measuredvalue of rho; a rho value less than actual would result becausethe measured AT would be less than if all the heat flowed throughthe soil. Therefore, the soil would appear to conduct heat betterthan its actual capability, because some of the heat would notpass through the soil.

LABORATORY PROCEDUTRES

It is very desirable to be able to conduct tests under lab-oratory conditions and to have them represent actual fieldconditions. The rhometer was designed to do this in the measure-ment of thermal resistivity of soil. Following is a description ofthe various aspects in the use of the rhometer apparatus.

All the soil that has been tested in the rhometer thus far hasbeen sieved through a 1/4-inch mesh screen. This eliminatesany gravel-size soil particles that might cause a nonuniform soildensity and thus cause heat to flow unevenly in the soil mediumunder test. Larger size particles (up to 3 inches) can be testedin the rhometer, but an accurate knowledge of the true densityof the medium under test would be very obscure.As stated previously, a complete graph of rho versus moisture

content can be developed for any soil. These curves are generatedby knowing the weight of the rhometer apparatus and the weightof the dry soil inside the rhometer. Any additional weight isdue to moisture in the soil and this can be expressed as a per-centage of the weight of the dry soil in the rhometer. The mois-ture content of the soil after some moisture has been drivenaway is computed by knowing the initial soil moisture contentas the soil is placed in the rhometer; then any weight loss of theoverall apparatus then can be readily converted to a new soilmoisture content for the soil medium under test.

It is noted that the new soil moisture content is an averagevalue through the soil rather than a perfectly uniform moisturecontent. This is because a soil moisture gradient exists betweenthe heat source and the concrete shell caused by greater moisturemigration at the heat source than at the concrete shell. This is arather trivial yet permanent inaccuracy that cannot be over-come as long as heat is forced to flow in cylindrical geometry.

Probably the most valuable capability of the rhometer is itcan test the hypothesis that soil moisture migration is dependenton a soil heat flux rather than on absolute soil temperaturealone. There is general discussion today about a "critical tem-perature" of soil being between 55-65°C, above which a soil islikely to become thermally unstable and produce thermal run-away. However, from the heat flux approach we look at the heatper unit area that passes through a soil medium.The heat flux approach is analogous to the temperature

gradient approach [10], in that for a given soil the heat fluxthrough a soil is directly proportional to the temperature gradientin the direction of heat flow, q/A - aT/8x. The constant ofproportionality is the soil thermal resistivity, yielding

q =1I d9T. (3)A pOx

Therefore, if the temperature gradient is known anywhere inthe heat flow path, the heat flux is also known from (3), andconversely. Unless temperature measurements are made in anactual underground installation, the temperature gradientat any point in the heat path is difficult to determine. The heatflux entering the soil is easy to determine from the heat generatedwithin the duct bank and the peripheral area of the duct bankstructure.

F-

E

I

I-

U)

L/)

100

o~~~~~~~~~~1PCF)l

ooe,-_ z, < -1 £PCF)

150 c

loo 24 6 8 lo "CF)

SOIL MOISTURE CONTENT - PERCENT

Fig. 3. Comparison of thermal resistivities of five different soils atoptimum density and 100 lb/ft' dry density.

The important result that could be used in field applicationsis knowledge of the allowable heat flux that could be passedthrough a given soil without creating excessive drying. As yetthere are not enough test results to place quantitative values onallowable heat fluxes for various- soils, but the rhometer shouldbe able to ultimately measure the value.Removing the soil from the apparatus without damaging the

rhometer is somewhat tedious. In most cases, the soil is totallydry and very hard when it is ready to be removed from theapparatus. The best technique devised so far is to use a wooddrill, about one inch in diameter, and make enough holes toallow the soil to be broken out of the apparatus in large chunks.This technique works well for all soils; however, soaking withwater may be effective for sandy soils because they readilyabsorb water and thus become workable.

300

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25HR.45MIN. 7HR.15MIN10

0

zU)4tn

I-zw

wa.

HYDROMETER ANALY'TIME READINGS

I. 60 MIN. 19MIN.

a O 0 0 0 O 0 00-.0 0 OO*.002 .005 .009

_MN._ IMIN. 200I-4MIN. ---I MIN. 200VV

0 0

.01gI03 q o o

.037 .074

CLAY IPLASTIC) TO SI LT (NON -PLASTIC) L

U.S. STANDARD SERIES100 50 30 16

.149 .P.97

I- -t - ----t--

.590

SIEVE ANALYSIS

8 4 3/E

2.38DIAMETER OF PARTICLE IN MILLIMETERS

SAND _

4.76

5/4 1- 1/2

9

19.1

38.,inI4 ..2.38.1

1 FINE MEDIUM I COARSE FINE I COARSE

Fig. 4. Particle gradation curves for soil samples in Fig. 3.

LABORATORY RESULTS

Thus far, over 50 different soil samples have been obtainedfrom the southern California area and 15 have been tested in therhometer. The soils tested range in texture from clays to coarsesands, and each soil shows an improved rho with increased densitysand moisture content. Thesegeneral results are quite predictable.

Fig. 3 shows the thermal resistivity of various soils as a func-tion of soil moisture content and soil dry density. Fig. 4 shows thegradation curves for each of these soils, indicating a wide rangeof soil textures and grading. The soil sample A is a fine siltysand; B is a skip-graded silty sand; C and D are well-gradedsands containing small quantities of silt; E is a well-graded sandwith essentially no silt particles.Each soil has been thermally tested near its optimum dry

density and over a range of moisture content; each has alsobeen tested at dry density of about 100 lb/ft3 over a range ofmoisture. The optimum density is commonly termed 100-per-cent compaction, and it is practically the most dense packingthat can be obtained on a given mixture of soil particles.Comparing the soils in Fig. 3 shows that reducing the optimum

density of a given soil by about 20 percent can cause an increasein the soil rho as much as 100 percent. In some cases the inplacedensity of the native soil is 20 percent less than the optimumdensity, and it may be prudent to compact the native soil in thearea of the duct bank to take advantage of a better soil rho.Another benefit from increased soil density can be seen in

that the slope of the rho versus moisture content curve is reducedwhen the soil density is increased. This provides a more stablesoil rho because a smaller change in rho results from a givenchange in moisture content. All the soil samples tested thus far

show this characteristic. Note that the preliminary test dataobtained with the rhometer show no sudden increase in rho ata "critical" moisture content. Rather, smooth curves are pro-duced with gradually increasing slope for decreasing moisturecontent.

It is seen that samples C and D have almost identical grada-tion characteristics but very different values of rho. Even moreinteresting is the fact that the soil with the lowest density hasthe lowest rho. Explanations for this kind of soil thermal be-havior will be sought from further, more extensive testing.The soils in Fig. 4 have been tested under three different heat

flow rates. The heat fluxes at the entrance to the soil were about10, 20, and 30 W/ft2, with corresponding heat fluxes atthe exit from the soil of 2.4, 4.8, and 7.2 W/ft2. A record ofthe average soil moisture content with time was kept to deter-mine whether the moisture in the soil would approach a constantvalue for, say, 10 W/ft2, and then drop to a new value whenthe heat flux was increased to 20 W/ft2, etc. Thus far, ithas not been found that such behavior definitely exists. Fig. 5is a drying curve for the soil sample B at almost optimumdensity. A heat flux of 10 W/ft2 was applied for 60 days, thenit was increased to 20 W/ft2 for 45 days, and then to 30 W/ft2 for 40 more days. It is seen that there is a tendency for thesoil moisture content to level out at the end of each period,followed by slightly increased rate of moisture loss after eachincreased heat flux. More testing will be required in order toestablish the validity of this hypothesis.An interesting observation has been made in connection with

soil moisture migration after an increase in heat flux is imposedon the soil. Some soils actually show a slightly improved rhofor about one week after a suddenly increased heat flux is passed

-

301

a.0

-1


Qa1aL)Q0.

CaL)c0U0L)

0

I

CL I-O 40 80 120

Days of DryingFig. 5. Drying curve for sample B at optimum density with heat

fluxes of 10, 20, and 30 W/ft' for durations of 60, 45, and 40days, respectively.

through the soil. It seems that some of the heat is carried out ofthe soil by the moving moisture, in addition to the heat that isconducted through the stationary soil particles. This effectchanges the soil rho by only about 3 percent, and it only lasts forthe short time during which the moisture is rapidly being redis-tributed in the soil.The rhometer has also been used to measure the thermal

resistivity of a pea-gravel concrete mix used in duct banks,and the measured rho values differ greatly from the recom-mended values commonly used in underground cable ampacitycalculations. After the concrete had cured for two weeks insidethe rhometer, the rho was measured to be 330 C -cm/W. Toget the rho of the concrete without moisture, the sample wasplaced in an oven at 800 C for three weeks to dry out the concrete.After cooling, the rho of the dry concrete was measured as650 C cm/W. These results show the excellent thermalresistivity of concrete in comparison to soil, and it invites thequestion of whether concrete should be used as an importedbackfill if special soil with good thermal resistivity is not readilyavailable.The above results are easily and accurately obtained if two

important precautions are observed. First, the rhometer must beallowed to run for about ten hours before making measurements;this is sufficient time for attainment of steady-state conditions.Second, to achieve the greatest accuracy in measuring the actualtemperature between the inside and outside surfaces of the soil,the air around the rhometer should be maintained at a reasonablyconstant temperature. This prevents the outer thermocouplefrom fluctuating in temperature, and thus the temperaturegradient between the heat source and the outer thermocoupleremains constant. It should be noted that the outer concreteshell serves to damp out small temperature swings of shortduration, thus minimizing the effect of air temperature fluctua-tions. To further reduce the effect of fluctuating air temperaturein the laboratory, it has been found very convenient to maketemperature measurements early in the morning when thelaboratory is a uniform temperature after having been idleduring the night.

In dealing with the overall accuracy of the rhometer techniqueof measuring soil thermal resistivity, even under very simplelaboratory conditions it is possible to get 95-percent accuracy.Under the most favorable conditions, 99-percent accuracy isabout the best that can be achieved. In either case, the greatestsingle source of error is the measurement of temperature dropbetween the copper and concrete cylinders.

CONCLUSIONSA new method has been developed to accurately measure the

thermal resistivity of soils at any possible density and moisturecontent; this same basic information can be obtained with othertest devices, referenced earlier. The transient methods are at anadvantage in that they are faster than the steady-state methods,but the steady-state methods have the advantage of beingable to create long-term drying in the soil being tested. Possibly,the most valuable capability of the rhometer is that it caininduce a variable, realistic amount of heat into a soil samplewhile continuously measuring its rho. With some more work, itshould be possible to get a quantitative correlation betweenheat flow in a soil and the extent of moisture migration out of it.This correlation might be established for several soils and allwork done in the laboratory under controlled conditions, therebyeliminating the high cost of many full-scale controlled tests inthe field.The rhometer method of measuring soil thermal resistivity can

make possible the simple acquisition of the essential thermalinformation needed to design underground power systems.The device has been used in an extensive soil investigation alongmajor proposed underground power right of way. It will also beused in a research project to provide more information aboutheat conduction in soils than is presently available in publishedmaterial.

REFERENCES[1] V. V. Mason and M. Kurtz, "Rapid measurement of the ther-

mal resistivity of soil," AIEE Trans. (Power Apparatus andSystems), vol. 71, pp. 570-577, August 1952.

[2] W. L. Shannon and W. A. Wells, "Tests for thermal diffiusivityof granular materials," Proc. ASTM, vol. 47, p. 1044, 1947.

[3] M. S. Kersten, "The thermal conductivity of soils," Proc.Highway Res. Board, vol. 28, p. 390, 1948.

[4] J. H. Neher, "The temperature rise of buried cables and pipes,"AIEE Trans., vol. 68, pp. 9-21, 1949.

[5] L. H. Fink, "Evaluation of soil thermal characteristics,"IEEE Trans. Power Apparatus and Systems, vol. PAS-84,pp. 807-814, September 1965.

[6] R. J. Wiseman and R. W. Burrell, "Soil thermal characteristicsin relation to underground power cables, pt. I: soil types:identification and physical properties," AIEE Trans. (PowerApparatus and Systems), vol. 79, pp. 792-795, December 1960.

[7] B. K. Hough, Basic Soils Engineering. New York: Ronald,1957.

t8] W. D. Smith, "The thermal conductivity of dry soil," SoilSciences, vol. 53, p. 435, June 1942.

[9] J. P. Holman, Heat Transfer. New York: McGraw-Hill,1963.

[10] J. I. Adams and A. F. Baljet, "The thermal behavior of cablebackfill materials," IEEE Trans. Power Apparatus and Sys-tems, vol. PAS-87, pp. 1149-1161, April 1968.

Discussion

A. F. Baljet (The Hydro-Electric Power Commission of Ontario,Toronto, Ont., Canada): The steady-state method of measuringsoil thermal resistivity described by the author should yield accurateresults without the need for specialized equipment. On the otherhand, the cost of taking soil samples and placing these in the testcylinders could be substantially higher than the cost of makingin-place measurements with the transient needle.

Manuscript received June 2, 1969.

302


280DRY DENSITY

260 MATERIAL PCF

240 tSTONE SCREENINGS 124.9NATURAL SAND 97.6

220 UNIFORM BEDDING 103.3H ~~~~~SAND103200

LiJ 180a.

o 160

to 140

a 1200

100 - UNIFORM BACKFILL SAND

a: NATURAL SANDN880

60

20LIMESTONE SCREENINGS

00 2 4 6 8 10 12 14 16 10 20 22 24

MOISTURE CONTENT (PERCENT OF DRY WEIGHT)

Fig. 6. Thermal resistivities of three typical soils at densities shown.

The reported results seem to indicate that, even for poorly gradedsoils such as sample E, the thermal resistivity for a given degree ofcompaction varies relatively little with moisture content.

It has been our experience that these soils exhibit thermal resis-tivities above 2500 C-cm/W at moisture contents of 0.5 percentor less in a lightly compacted state, and above 1500 C-cm/W inthe well-compacted state. This means in practice that with direct-buried cables the danger of thermal runaway can be much moreserious than suggested by the author's published data.We have on several occasions measured such high thermal re-

sistivity values on backfill samples taken from older cable instal-lations where dryout of the backfill was discovered during excava-tion. A typical comparison of backfills at in-place densities in aparticular cable installation is shown in Fig. 6.

It should be interesting in this context to refer to the study ofDr. Radhakrishna [11] who concluded on a theoretical basis thatmoisture migration is initiated when the moisture content of the soildrops below a critical value. This results in an unbalance of outwardvapor flow and return liquid flow. This phenomenon is well knownand has often been reported in the literature. Not generally rec-ognized, however, is the fact that the critical moisture contentcan vary over a very wide range, depending on the composition andthe degree of compaction of the soil. Further work showed that forrelatively uniformly graded sand, such unstable conditions canfrequently occur during periods of dry weather.Once an unstable condition is attained, the rate of moisture

migration is primarily governed by the heat flux from the cable andis still significant at low heat fluxes. This was substantiated by ourdiscovery of soil dry-out, to less than 0.5-percent moisture content,on a cable circuit loaded well below rated ampacity. In this case thecable sheath temperature was approximately 30°C dispelling theassumption that certain (relatively high) minimum sheath tem-peratures were prerequisite to moisture migration.We therefore tend to accept the hypothesis presented in [11]

and would be reluctant to assume that below a certain heat flux nomoisture migration will take place and that no excessively highthermal resistivities can be expected, as implied by the author in thepaper.

REFERENCES[11] H. S. Radhakrishna, "Heat flow and moisture migration in

cable backfill," Ontario Hydro Res. Q., vol. 20, no. 2, pp. 10-20,1968.

Herman Halperin (Consulting Engineer, Menlo Park, Calif.):This paper presents valuable data and an interesting laboratorytool. Such information is very helpful in designing cable systems.Various utilities and others have been obtaining thermal data in thelaboratory and in cable system installations for some 50 years.There are many kinds of parameters that have a bearing in deter-

mining the allowable loading of underground cable systems duringnormal or emergency operation. During the past 25 or 30 years, themaximum allowable temperatures for normal operation have beenincreased considerably for both paper-insulated and synthetic-insulated cable; and the practice of establishing extra-high tem-peratures for emergency operation was started.Recognition of these facts and developments led AEIC Cable

Engineering Section, in consultation with cable factory engineers, toprepare in 1959-1960 the "Guide for Application of AEIC MaximumInsulation Temperatures at the Conductor for Impregnated-Paper-Insulated Cables." (A second edition was issued in April 1968.) Thepoints in the Guide really apply to all kinds of cable insulations.As indicated in the Guide, utilities here and abroad have found

large variations in the thermal characteristics of given cable systeminstallations from year to year, and very large variations betweenlocations in temperature rises for a given amount of heating. As aresult, field surveys of temperatures are considered necessary,particularly for locations where moderate to high temperaturerises are apt to occur, in order to obtain optimum economic utiliza-tion of the facilities without having serious operating troubles.

Referring to Fig. 4, it is of interest that the increase in resistivityas the moisture content was decreased from a few percent to zero isappreciably less sharp than others have found. What were themaximum temperatures at the heat source when the soils were soheated that the moisture content was zero?

It has been demonstrated previously in various laboratory testsand by many measurements that the thermal characteristics ofsoils change little within a band of moderate temperatures, and thenstart to increase more sharply with still higher temperatures. Forobvious reasons, there is a desire to avoid the sad experiences of linefailures and shortened cable life that have resulted from excessiveincreases in soil temperatures. After considering this and otherpoints, the net practical result has been the employment of so-called critical temperatures, which may vary from 35 to 50 or 60°C(or even higher for very special cases) for normal operation, de-pending on the circumstances.


L. T. Frey, III (General Electric Company, Hickory, N. C.): It isindeed gratifying to see the author and his company pursuing thepath of gathering and evaluating information on the thermal char-acteristics of the soil in the area they serve. Only through suchprograms as this can we gain the knowledge necessary to intel-ligently design and apply underground distribution systems.

In utilizing the rhometer, I believe these precautions should beobserved:

1) Since the output voltage of the copper-constantan thermo-couple is nonlinear, the difference in voltage read between buckingthermocouples is not necessarily the difference in temperature.When the temperature difference being measured is small, theerror is small, but as the temperature difference increases, the errorincreases. If the individual voltages are read, converted to tem-perature, and then subtracted, the probability of error will bereduced.

2) Highly accurate instrumentation and careful voltage (tem-perature) readings should be taken, since a difference in tempera-ture rise of as little as 0.5 degree can result in a difference of 20 inrho, as calculated from (2).

3) Careful control of test environmental conditions is required toassure that steady-state thermal equilibrium has been achieved.

In conclusion, if these precautions are followed, the test resultswill be meaningful.


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John Stolpe: The discussions are appreciated and are a valuableaddition to the paper. The comments of both Mr. Baljet and Mr.Halperin in regard to the straightness of the curves in Fig. 4 are welltaken. Unfortunately, the soils at 100 PCF dry density were thefirst soils tested in the device, and they were not tested long enoughto achieve complete dryness. If the soils' had been fully dried, themore or less typical abrupt rise in soil rho would have been ob-served.The same soils, at optimum density, were tested to complete

dryness under a heat flux of 30 W/ft2, which produced a tem-perature drop through the soil of about 10°C. At the completion ofthe tests, the moisture content of about 100 grams of soil midwaythrough the sample was checked to be between 0.2 and 0.4 percent.This is the hygroscopic moisture content of the soil, which is thelowest moisture content of practical interest in the field. Thesehygroscopic levels were entered as zero in Fig. 4, and thus theconfusion. Nevertheless, they are the maximum rho values thatcould be expected in practice for these highly compacted specificsoils.To clear up any beliefs that high soil rho values have not been

measured with the rhometer, a typical southern California beachsand was tested at the field dry density and at the hygroscopicmoisture content and it produced a rho of 2950 C-cm/W. Evenwhen fully compacted, the rho of the dry sample was 240. Soilssuch as this, and the upper two in Fig. 6 have very high porosities andare obviously poor soils for thermal applications.

In regard to the equilibrium moisture content that may be es-tablished in a given soil, this is definitely a function of the "ambient"soil moisture content, which varies with the seasons of the year.Thus, the equilibrium must be some fraction of the "ambient"content, which would decrease with increasing heat flux.Mr. Frey has emphasized some very important details that must

be observed when making thermocouple measurements. Especiallyat low heat fluxes, the accuracy of the rhometer is mostly dependenton the measured temperature drop through the soil sample. Byconnecting thermocouples to buck each other, the difference involtage is obtained. If the temperature of the cooler thermocouple,which is very close to room temperature, is approximately known,the temperature difference between bucking thermocouples is simplyobtained from a graph of a standard temperature-EMF table.This technique introduces only 1-percent error even if the tem-perature of the cool thermocouple is as much as 5°C from themeasured temperature of the air around the rhometer.The final point made by Mr. Halperin of establishing a critical

soil temperature as the threshold of thermal runaway and theresulting cable failure is good. Unfortunately, these limits areusually established by trial and error. However, looking a stepfarther into the amount of heat that is the cause of the resultingtemperatures, heat flux passing into the soil may offer some addi-tional convenient design information.

Manuscript received July 17, 1969.

Experimental Installation of 69-ky Synthetic-InsulatedUnderground Cables and Components

R. CARTER BLANKENBURG, FELLOW, IEEE, WOLFGANG W. STEINKE, ASSOCIATE MEMBER, IEEE,AND JOHN STOLPE, MEMBER, IEEE

Abstract-This paper describes the test installation, its operation,and the intermediate results of a continuous test program of syn-thetic-insulated 69-kV underground cables and joints, and termina-tions.The purpose of the test program is to gain knowledge in respect to

installation, operation and maintenance, and particularly perfor-mance and aging of different solid-insulated power cables at normaland emergency loads.

Various test data, including periodic corona and insulation powerfactor measurements, are given and discussed. Methods are shownto determine conductor temperature and the thermal resistivity ofsoil adjacent to a buried duct bank using results of the field tests.The solutions to a variety of minor problems conceming design andinstallation practices are presented.

Paper 69 C 1-PWR, recommended and approved by the InsulatedConductors Committee of the IEEE Power Group for presentationat the IEEE Underground Distribution Conference, Anaheim,Calif., May 12-16, 1969. Manuscript submitted December 9, 1968;made available for printing October 7, 1969.The authors are with the Southern California Edison Company,

Los Angeles, Calif. 90053.

INTRODUCTION

EXTENSIVE landscape beautification programs initiatedthe development of low-cost underground distribution

systems. The use of synthetic insulations on distribution-classcables has already contributed substantially to a system costreduction.More than five years of operating experience in this company

with over 1000 miles of polyethylene-insulated distribution cableexhibited excellent performance. Constant improvement ofcable insulations and designs as the result of joint efforts bymanufacturers and users perfected the cable to a point that madeits operation at higher voltages feasible.

In 1965 the Southern California Edison Company decided toevaluate 69-kV synthetic-insulated cables, joints, and termina-tions for application and use on the Edison system.A test program was developed in cooperation with several

manufacturers. A suitable test location in one of the substationswas found and several different cable systems, together withtest equipment, were installed. It is the purpose of this paper to

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soil thermal resistivity measured simply and accurately

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