442-2017-ieee

IEEE Guide for Thermal Resistivity Measurements of Soils and Backﬁll Materials

IEEE Power and Energy Society

Sponsored by the Insulated Conductors Committee

IEEE 3 Park Avenue New York, NY 10016-5997 USA

IEEE Std 442™-2017 (Revision of IEEE Std 442-1981)

Authorized licensed use limited to: Escuela Superior de Ingeneria Mecanica. Downloaded Downloaded on September 26,2018 at 18:29:10 UTC from IEEE Xplore. Restrictions apply.

IEEE Std 442™-2017 (Revision of IEEE Std 442-1981)

IEEE Guide for Thermal Resistivity Measurements of Soils and Backll Materials Sponsor

Insulated Conductors Committee of the

IEEE Power and Energy Society Approved 28 September 2017

IEEE-SA Standards Board


Abstract: The measurement of thermal resistivity of soil and backll materials to include concrete, Abstract: The engineered backlls, grout, rock, sand, and any other material used to encase the cable system installed in the ground is covered in this guide. A thorough knowledge of the thermal properties of a soil or backll material enables the user to properly design, rate, and load underground cables. The method used is based on the theory that the rate of temperature rise of a line heat source is dependent upon the thermal constants of the medium in which it is placed. The designs for both laboratory and eld thermal probes are also described in this guide. Keywords: backll, IEEE 442™, soil, soil thermal properties, thermal needle, thermal probe, Keywords: thermal property analyzer, thermal resistivity

The Institute of Electrical and Electronics Engineers, Inc. 3 Park Avenue, New York, NY 10016-5997, USA Copyright © 2018 by The I nstitute of Electrical and Electronics Engineers, Inc. All rights reserved. Published Published 7 May 2018. Printed in the United States of America. IEEE is a registered trademark in the U.S. Patent & Trademark Oce, owned by The Institute of Electrical and Electronics Engineers, Incorporated. PDF:

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Participants At the time this guide was submitted to the IEEE-SASB for approval, the IEEE P442 Working Group had the following membership: Nimesh Patel, Chair Sudhakar Cherukupalli, Vice Chair Earle C. (Rusty) Bascom III William Black

Chris Grodzinski Dennis E. Johnson Rachel Mosier Lucian Munteanu

Deepak Parmar David Purnhagen Walter Zenger

The following members of the individual balloting committee voted on this guide. Balloters may have voted for approval, disapproval, or abstention. Saleman Alibhay Thomas Barnes Earle C. (Rusty) Bascom III William Black William Bloethe Kenneth Bow Kent Brown William Byrd Sudhakar Cherukupalli Michael Chirico Robert Christman Gary Donner Donald Dunn Todd Goyette Randall Groves Ajit Gwal Jeffrey Helzer

Lee Herron Lauri Hiivala Werner Hoelzl Magdi Ishac Dennis E. Johnson Jim Kulchisky Chung-Yiu Lam Michael Lauxman John Merando Daleep Mohla Rachel Mosier Jerry Murphy Arthur Neubauer Michael Newman Charles Ngethe Lorraine Padden Deepak Parmar Nimesh Patel Shashikant Patel

Branimir Petosic Christopher Petrola Thomas Proios Charles Rogers Ryandi Ryandi Daniel Sabin Bartien Sayogo Michael Smalley Jeremy Smith Jerry Smith David Tepen Peter Tirinzoni Nijam Uddin John Vergis Kenneth White Jian Yu Walter Zenger Tiebin Zhao

When the IEEE-SA Standards Board approved this guide on 28 September 2017, it had the following membership: Jean-Phillipe Faure, Chair Gary Hoffman, Vice Chair John D. Kulick, Past Chair Konstantinos Karachalios, Secretary Chuck Adams Masayuki Ariyoshi Ted Burse Stephen Dukes Doug Edwards J. Travis Grifth Michael Janezic

Thomas Koshy Joseph L. Koepnger* Kevin Lu Daleep Mohla Damir Novosel Ronald C. Petersen Annette D. Reilly

Robby Robson Dorothy Stanley Adrian Stephens Mehmet Ulema Phil Wennblom Howard Wolfman Yu Yuan

*Member Emeritus


Introduction This introduction is not part of IEEE Std 442-2017, IEEE Guide for Thermal Resistivity Measurements of Soils and Backll Materials.

An important design consideration in the design of underground cable systems is to understand the thermal resistivity characteristics of the materials that surround the cable. Over the years, many utilities, consultants, and testing rms have measured soil thermal resistivity both in situ and in the laboratory on selected soil samples. Such measurements have utilized various types of equipment and measurement techniques. In many cases, these testing methods have yielded inaccurate or inconsistent measurements of soil thermal resistivity. The Insulated Conductors Committee, recognizing the need for industry guidelines for the measurement of soil thermal resistivity, has prepared this guide to provide the user information on the test equipment that should be used to perform these material resistivity measurements. In addition, the guide provides information on how to make the in-situ or laboratory resistivity measurements and interpret the data in order to provide meaningful results using this equipment. The in-situ resistivity of a soil changes from season to season, due to changes in the moisture content of the soil or due to the relocation of the water table. It is important to consider these factors when determining a soil thermal resistivity value for ampacity calculations and rating underground cables.


Contents 1. Overview..................................................................................................................................................... 9 1.1 Scope .................................................................................................................................................... 9 1.2 Purpose ................................................................................................................................................. 9 2. Normative references .................................................................................................................................. 9 3. Factors inuencing soil thermal resistivity ................................................................................................ 10 3.1 Factors inuencing measurements ..................................................................................................... 10 4. Test equipment .......................................................................................................................................... 11 4.1 Equipment required for eld measurements ....................................................................................... 11 4.2 Equipment required for laboratory measurements .............................................................................. 12 5. Test methods .............................................................................................................................................. 12 5.1 Methods for eld measurements ......................................................................................................... 12 5.2 Methods for laboratory measurements ............................................................................................... 13 6. Analysis of test results ............................................................................................................................... 14 6.1 Sample calculation ............................................................................................................................. 15 6.2 Interpretation of results....................................................................................................................... 15 Annex A (informative) Multi-sensor eld probe ............................................................................................ 22 Annex B (informative) Single-sensor laboratory and eld probes.................................................................. 23 Annex C (informative) Slide hammer assembly ............................................................................................. 25 Annex D (informative) Sample standard proctor............................................................................................ 26 Annex E (informative) Thermal dryout characteristics .................................................................................. 27 Annex F (informative) Determine critical moisture content ........................................................................... 28 Annex G (informative) Moisture migration ................................................................................................... 29 Annex H (informative) Glossary .................................................................................................................... 31 Annex I (informative) Bibliography............................................................................................................... 32


IEEE Guide for Thermal Resistivity Measurements of Soils and Backll Materials 1. Overview 1.1 Scope This guide covers the measurement of thermal resistivity of soil and backll materials to include concrete, engineered backlls, grout, rock, sand, and any other material used to encase the cable system installed in the ground. A thorough knowledge of the thermal properties of a soil or backll material enables the user to properly design, thermally rate, and load underground cables. The method is based on the theory that the rate of temperature rise of a line heat source embedded in the soil is dependent upon the thermal constants, including the thermal resistivity, of the medium in which it is placed. The designs for both laboratory and eld thermal probes are also described in this guide.

1.2 Purpose The purpose of this guide is to provide sufcient information to enable the user to select useful commercial test equipment, or to manufacture equipment that is not readily available on the market, and to make meaningful resistivity measurements with this equipment. Measurements may be made in the eld or in the laboratory on recompacted soil samples or both.

2. Normative references The following referenced documents are indispensable for the application of this document (i.e., they must be understood and used, so each referenced document is cited in text and its relationship to this document is explained). For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments or corrigenda) applies. ASTM D75, Standard Practice for Sampling Aggregates. ASTM D698, Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort (12 400 ft-lbf/ft 3(600 kN-m/m3).1 ASTM D1452, Standard Practice for Soil Exploration and Sampling by Auger Borings.

1

ASTM publications are available from the American Society for Testing and Materials (http://www.astm.org/).


IEEE Std 442-2017 IEEE Guide for Thermal Resistivity Measurements of Soils and Backll Materials

ASTM D1557, Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Modied Effort (56,000 ft-lbf/ft3 (2,700 kN-m/m3). ASTM D2049 (withdrawn), Standard Test Method for Relative Density of Cohesionless Soils. 2 ASTM D2216, Standard Test Methods for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass. ASTM D4220, Standard Practices for Preserving and Transporting Soil Samples. ASTM STP483, Sampling Soil and Rock.

3. Factors inuencing soil thermal resistivity Typical soil thermal resistivity (ρ) of various materials is listed in Table 1.

Table 1—Typical soil thermal resistivity values (ρ) (K·m/W)

Material

Quartz grains

0.11

Granite grains

0.26

Limestone grains

0.45

Sandstone grains

0.58

Mica grains

1.70

Water

1.65

Organic

1.60 Saturated — 7.00+ Dry

Air

40.00

Soil is a composite of different minerals, organics, water, and air. The thermal resistivity of a particular soil depends on the composition of the material encountered as well as the physical parameters of the soil such as moisture content and density. The conditions that most inuence the resistivity of a specic soil are the moisture content and dry density. As the moisture content or dry density or both of a soil increases, the resistivity decreases. The structural composition of the soil also affects the resistivity. The shape and size of the soil particles determines the surface contact area between particles, which affects the ability of the soil to conduct heat.

3.1 Factors inuencing measurements During the measurement of soil thermal resistivity, the following factors may adversely affect the accuracy of the test measurement: —

Migration of the soil moisture away from the probe during the test can result in higher soil thermal resistivity measurements. This migration may be signicant, and normally takes place when the input power per unit area of the probe is sufciently high to drive the moisture away from the probe.

—

Under rare circumstances, moisture migration towards the probe associated with preliminary mass transfer may lower soil thermal resistivity when initial soil moisture content is less than 5% in some soils, particularly sands.

—

Moisture migration can take place toward the end of the test resulting in increasing the apparent soil thermal resistivity.

2

ASTM D2049 has been withdrawn; however, copies can be obtained from t he IHS Markit ( http://global.ihs.com).



—

Laboratory measurements of soil thermal resistivity may be affected by the redistribution of moisture due to gravity. If gravity induced moisture redistribution takes place during the measurement, the resistivity normally goes up. The error can be signicant if the resistivity is sensitive to the change in moisture content at the dry density selected for the test.

—

Power supply stability shall be maintained throughout the test. The power dissipated in the probe shall be controlled so that variation in the magnitude of heat ux is kept within ±1%.

—

The in-situ resistivity measured using the eld probe may vary from one soil depth to the next.

—

If the surface contact area between the probe and the soil is decreased due to improper installation of the probe, the measured resistivity will be high resulting from a layer of air that is trapped between the probe and soil sample.

—

When a local nonhomogeneous material, such as a large rock, is present in the vicinity of any of the temperature sensors located in the eld probe, a misleading soil thermal resistivity will be measured. The thermal probe shall be removed and reinserted a minimum of 1.5 m away from the initial test location.

—

If multiple soil layers are present which have different soil thermal resistivities and the sensor straddles the layers the thermal resistivity of the two layers will be a composite resistivity. The eld probe should be inserted so that the temperature sensors are located at a distance of not less than 40 times the diameter of the probe away from the interface layer of the soil. The location of different soil layers can be physically determined by taking core samples at various depths.

Another major factor to be considered while utilizing measured resistivity values is the phenomenon of moisture migration.

4. Test equipment Figure A.1 and Figure B.1 show diagrams of the system required to measure thermal resistivity in the laboratory or in the eld. The equipment for testing differs primarily in the size of the probe and the portability requirements of the devices used in the eld as shown in Annex A and Annex B.

4.1 Equipment required for eld measurements 4.1.1 Field thermal probe The eld thermal probe is fabricated from a stainless steel tubing that can be made of various diameters and lengths. The tubing contains a heater element occupying the length of the seamless stainless steel tubing. A number of temperature sensors, electrically insulated from the heater element, and the probe body are shown in Annex A. Thermal probes of long lengths (more than 1 m) may contain multiple temperature sensors, positioned at intervals of 300 mm to 450 mm from the probe tip, with suitable means at the top of the probe for making electrical connections. In order to eliminate moisture inltration in the probe and to reduce the initial thermal transient, the probe may be lled with an epoxy resin.

4.1.2 Power supply/power monitor An adjustable, regulated electric power supply should be used in the constant current mode. The unit should be capable of providing a stable and constant power ranging from 1 W up to 250 W to allow exibility to test with small lab probes and or large eld probes. The eld probes require higher power to heat the soil to the required temperatures and thus allow the determination of the soil’s thermal resistivity. If such a power source is not available in the eld, a portable generator or power inverter may be used to energize the heater in the probe. In such an application, accurate control can be exercised by using an ammeter and variable resistors to adjust the heater current. This conguration shall be able to adjust the currents between 0.1 A to 5 A and thus the heater power. Alternatively, power meter (digital or analog) may also be used to accurately measure the input power being applied to the heater in the probe.



4.1.3 Temperature monitor A multipoint portable digital instrument designed to measure temperatures with a resolution of better than 0.1 °C is preferred for eld use. Manual balance potentiometers with reference junctions have also been used successfully.

4.2 Equipment required for laboratory measurements 4.2.1 Laboratory thermal probe Laboratory measurements use a small stainless steel probe with a length to diameter ratio of 50:1. The internal parts of the probe include a heater element and a temperature sensor that are both electrically insulated from each other and from the stainless steel probe body. Detailed diagrams are depicted in Figure B.1 and Figure B.2 of Annex B.

4.2.2 Power supply/power monitor An adjustable regulated dc power supply is required with the capability of providing at least 20 W. Alternatively, power meter (digital or analog) of up to 20 W can be used in place of power supply for the power input to the thermal probe.

4.2.3 Temperature monitor A multipoint portable digital instrument designed to measure temperatures with a resolution of better than 0.1 °C is preferred for lab use. Manual balance potentiometers with reference junctions have also been used successfully.

5. Test methods 5.1 Methods for eld measurements 5.1.1 Installation of eld probe The thermal probe with multiple temperature sensors should be carefully inserted in the earth. The probe should be inserted such that the middle temperature sensor is located at the depth that the cable is to be installed. Resistivities could then be determined at the cable depth and 300 mm above and below that depth. If, due to soil conditions, insertion of the probe is difcult, a pilot hole should rst be made. Under no conditions should the thermal probe be hammered or inserted with excessive force into the soil. When a pilot hole is needed, a pilot rod of slightly smaller diameter than the thermal probe should be driven into the soil using a slide hammer and guide (Annex C), or another appropriate device. Similar means should be used to remove the pilot rod. If insertion of the thermal probe is difcult, because of unusual soil conditions, the pilot rod should be reinserted and removed again. This process can be repeated until the hole is just enlarged enough to accommodate the probe to minimize the contact resistance at the probe/soil interface. If the soil is extremely rocky, an electric drill may be used to facilitate producing a pilot hole. In any event, if the ambient temperature is raised as a result of this process, enough time should be allowed for the ambient temperature to return and stabilize at the initial condition. This may be several minutes to over an hour.

5.1.2 Test procedure for eld probe After insertion has been completed, the following procedure should be performed: a)

Make power and temperature monitoring connections.

b)

Conduct a verication test of the thermal probe prior to eld testing by measuring a known standard for every project and compare with the calibration certicate.



c)

Allow 10 min to 15 min for the probe to reach thermal equilibrium with the surrounding earth while monitoring the temperature at 2 min intervals. This time will depend on the difference between the air temperature and earth temperature (natural and induced by the process used for installation).

d)

Select a power level that will give at least 3 °C to 4 °C temperature rise over approximately one logarithmic cycle of time to allow for easy interpretation of the measured data. Depending on the size, heat input between 30 W/m and 50 W/m is usually applied to the eld thermal probe. The power selected depends on the resistivity of the soil and will have to be based on experience for a given type of soil. If the soil has a very low resistivity, then a high heat input is required to produce an acceptable temperature change. If the soil has a very high resistivity, then a low heat input is required to keep from overheating the probe that can also induce moisture migration. If the probe temperature reaches 50 °C at any time, the test should be terminated immediately, and a lower heat ux should be applied to the probe at the next location.

e)

Switch on the power to the probe heater.

f)

Regulate the constant current power supply.

g)

Record temperatures of each temperature sensor in turn at 30 s intervals for the rst 5 min to establish that the probe is not overheated. The temperature should not exceed 10 °C above ambient soil temperature, during the rst 5 min under normal conditions

h)

Continue recording temperatures in turn at 1 min intervals for 30 min to 40 min or until the logarithmic temperature rise becomes linear. At the end of this time, switch off the power to the probe heater.

CAUTION The power demand(s), equipment ruggedness, and measurement resolution are factors that should be considered before taking the laboratory equipment to the eld for use.

5.2 Methods for laboratory measurements 5.2.1 Sample preparation and installation of laboratory probe The laboratory thermal probe is used primarily to determine the effects of changes in density and moisture content on the resistivity of soil and backll materials. It can be used for both undisturbed tube samples as well as reconstituted or re-moulded samples. If the soil is to be tested at the maximum density, ASTM D698, ASTM D1557, or ASTM D2049 3 should be followed to determine the moisture content required at which the maximum density can be obtained. For most soils, the sample is mixed to the desired moisture content and then compacted to the desired density. Silty soils articially moistened should be allowed to equilibrate for at least 12 h in an airtight container prior to sample preparation and testing. The soil should be compacted in 25 mm thick layers so that the density of the soil in the container remains relatively uniform. The sample should be placed in a rigid cylindrical container with a minimum inside diameter of 100 mm. The height of the container would vary depending on the length of the thermal probe used. There are some sands that contain chemical deposits which form light bonds between sand particles as the sand dries. These bonds may lower the thermal resistivity of the sand due to the reduction in contact resistance between sand particles. Thus sand that is compacted at zero percent moisture could have a higher resistivity than sand that is compacted at a higher moisture content and then dried to zero percent moisture. Sand should be compacted to the condition in which it will be installed in the cable trench. Care should be taken in inserting the thermal probe into the sample. If insertion of the probe is difcult, then a probe of slightly smaller diameter may be inserted into the soil to make a pilot hole. 3

Information on references can be found in Clause 2.



Rock core or rock block should be minimum 100 mm by 100 mm taking into consideration the length of the probe. A pilot hole should be drilled to the size close to that of the diameter of the probe and the probe should be grouted-in using thermal epoxy. In cases when a probe is installed in a pre-drilled hole and grouted with epoxy, stabilization time is required for the epoxy (which may be up to 12 h) before soil thermal resistivity measurements can be made. If the native soil is to be tamped back into the trench at the same density at which it was removed, it may be desirable to make in-situ resistivity measurements along the route of the cable. If the native soil is to be placed in the trench at a density different than undisturbed soil in the same vicinity, laboratory measurements are required on soil samples recompacted to the desired density. In order to draw meaningful comparisons on selected foreign backll materials, thermal resistivity measurements should be made in the laboratory on soils that are compacted so as to provide maximum dry densities.

5.2.2 Test procedure for laboratory probe An input power between 10 W/m and 30 W/m is usually applied to the thermal probe. The heat input selection depends on the resistivity of the soil. If a soil with a higher moisture content has been compacted to a high density, a high heat input is needed to produce an acceptable temperature change over the interval of the test. If a soil with lower moisture content has been compacted in the container to a very low dry density, the resistivity will be high and a low heat input is required. Temperature data is recorded at 15 s intervals for 10 min. If at any time, the probe temperature reaches 25 °C above ambient soil temperature, the test should be terminated.

6. Analysis of test results The analytical model used to calculate thermal resistivity was derived assuming a line heat source of innite length dissipating heat in an innite medium. Under these conditions, the following Equation (1) is valid: ρ =

4π

(T − T ) 2

1

(1)

 t  q ln  2   t  1

where ρ

is the thermal resistivity (K·m/W)

T 1 is the temperature measured at some arbitrary elapsed time (K) T 2 is the temperature measured at another arbitrary elapsed time (K) q

is the heat dissipated per unit length (W/m)

t 1

is the elapsed time at which temperature T 1 is recorded (min)

t 2

is the elapsed time at which temperature T 2 is recorded (min)

Initial transients exist due to the nite diameter of the probe. Boundary effects are possible due to the nite medium of the soil sample. A convenient way of determining when the initial transients are over and when the nite boundary begins to effect measurements is to plot temperatures versus the log of time for the duration of the test. The data points located on the linear section of the curve can be used to compute the resistivity of the soil. If the temperatures plotted at the beginning of the test deviate from the straight line, the initial transients have not yet settled out. If the temperatures deviate from the straight line at the end of the test, the nite boundary or moisture migration is causing the sample to be thermally unstable, affecting the test. In either case, these data should not be used in resistivity computations.



To simplify the resistivity calculations, extend the straight-line section of the curve to intersect at least one cycle on the semi-log graph. By recording the temperature change over one logarithmic cycle, the resistivity computation reduces to Equation (2): ρ

=

4π 2.303q

(2)

∆T

6.1 Sample calculation Data, including times and temperatures, should be tabulated during the test on an appropriate data table. Subsequently, the temperatures versus log time should be plotted for each temperature sensor until a straight line can be tted. A sample calculation follows for a test performed with a laboratory probe. Data from a typical test are shown in Figure 1. The data have been plotted in Figure 2. A similar procedure is followed when calculating the in-situ resistivity of a soil using the eld probe. It should be noted that since the time span required to make a eld resistivity measurement is longer than when using a laboratory-scale probe, the time elapsed shown on the x-axis should be increased to at least 30 min. The coefcient of determination is based on the R 2 least t.

6.2 Interpretation of results To judge the reliability of the thermal resistivity data gathered in the eld or laboratory, one shall make comparisons to existing data gathered in previous tests for similar types of soil and backll materials. Figure 2 shows some characteristic thermal resistivity versus moisture content curves for soils including sands, clays, and silts. Figure 3 through Figure 10 show various graphs that depict the variability in the probe readings and the reasons for the observed trends are described alongside each respective graph. If test results vary from these trends, then reinvestigation of results should be made. For Figure 3 through Figure 10, the thermal resistivity can usually be determined from the straight line portion of the curve at long time (i.e., after the initial lag time), even though it may not have the ideal shape. In some cases, the test curve may appear reasonable but the calculated thermal resistivity would be in error. Figure 3 through Figure 10 show the following trends: —

The trend shown in Figure 3 suggests that the probe has a large heat capacity that results in longer initial transient time.

—

The trend shown in Figure 4 suggests that the initial temperature rise is high due to the air gap at soil/ probe interface.

—

The trend shown in Figure 5 suggests the test duration was too long or the sample diameter was too small.

—

The trend shown in Figure 6 suggests the test duration was too long or the sample length was too short.

—

The trend shown in Figure 7 suggests moisture movement in liquid or vapor form in moist soil and air convection in dry soil.

—

The trend shown in Figure 8 suggests the sample is either warming or cooling with the ambient.

—

The trend shown in Figure 9 suggests that probe temperature is not in equilibrium with the soil or the sample is warming or cooling.

—

The trend shown in Figure 10 suggests temperature reader requires calibration or has malfunctioned.



Figure 1—Temperature versus log of time



Figure 2—Thermal property characteristics of soils



Figure 3—Interpretation of thermal probe test curves: Large probe heat capacity

Figure 4—Interpretation of thermal probe test curves: Poor probe to soil contact (extends initial lag time)



Figure 5—Interpretation of thermal probe test curves: Sample boundary eect

Figure 6—Interpretation of thermal probe test curves: Probe axial heat loss



Figure 7—Interpretation of thermal probe test curves: Moisture or air convection in soil

Figure 8—Interpretation of thermal probe test curves: Probe temperature not at equilibrium with soil



Figure 9—Interpretation of thermal probe test curves: Ambient temperature drift or probe power drift

Figure 10—Interpretation of thermal probe test curves: Unstable temperature reader



Annex A (informative)

Multi-sensor eld probe Figure A.1 provides typical construction of a multi-sensor eld probe used for thermal resistivity measurement.

Figure A.1—Field probe



Annex B (informative)

Single-sensor laboratory and eld probes Figure B.1 and Figure B.2 provide typical construction of single-sensor laboratory and eld probes used for thermal resistivity measurements.

Figure B.1—Probe for laboratory applications



Figure B.2—Small probe for eld use



Annex C (informative)

Slide hammer assembly Typical apparatus for installation of a eld probe is shown in Figure C.1.

Figure C.1—Slide hammer assembly



Annex D (informative)

Sample standard proctor Figure D.1 shows typical standard Proctor values for various soil types.

Figure D.1—Typical standard proctor values for various soil types



Annex E (informative)

Thermal dryout characteristics Figure E.1 shows trends of thermal resistivity values as a function of moisture content for various soil types.

Figure E.1—Thermal resistivity versus moisture content for typical soils



Annex F (informative)

Determine critical moisture content The critical moisture content is dened as the intersection of the tangents to the moist and dry line segments of the thermal resistivity versus moisture content curves, shown in Figure F.1.

Figure F.1—Determination of a soil’s critical moisture content 28 Copyright © 2018 IEEE. All rights reserved. Authorized licensed use limited to: Escuela Superior de Ingeneria Mecanica. Downloaded on September 26,2018 at 18:29:10 UTC from IEEE Xplore. Restrictions apply.


Annex G (informative)

Moisture migration Figure G.1 shows the effect of moisture migration on thermal resistivity of materials.

NOTE—The test data and graphs are for illustration purposes only.

Figure G.1—Eect of moisture migration on thermal resistivity



Annex H (informative)

Glossary boundary effect: During laboratory measurement of thermal resistivity, the diameter of the heat-front generated by the thermal probe is a function of time and thermal characteristics of the material. If the sample diameter is not appropriate (too small) or if the test duration is too long, the heat-front will reach the boundary of the sample and adversely affect the test result. The transient thermal probe test assumes an innite boundary of homogenous material. critical moisture content: The increase in thermal resistivity as the moisture content decreases is only ‘marginal and almost proportional’ from a higher moisture content level (near saturation or optimum). At a certain (lower) moisture content; referred to as ‘critical moisture’ the increase in thermal resistivity becomes disproportional. As a rule of thumb approximation, this value is the intersection of the two tangents as in Figure F.1. Below this moisture level, the thermal resistivity increases multi-fold. Alternatively, CIGRE (Electra 145) proposes taking a value that intercepts the thermal resistivity versus moisture content curve 10% higher than the asymptotic thermal resistivities. This will produce critical moisture contents that are higher than those values determined by the two tangent methods. moisture migration: During thermal testing of a moist or wet soil, the moisture may start to move away from the probe/soil interface as a result of the thermal gradient generated by the thermal probe. In partially saturated soil, the moisture may move in vapor form, under vapor pressure. The extent of moisture migration is a function of the heat output and heat ux of the probe, its duration, and the thermal characteristics of the soil. multi-sensor probe: A thermal probe may contain more than one temperature sensor. This type of probe is used mainly in the eld to obtain a temperature and thermal resistivity prole in a single test-run. The location of each sensor must satisfy the length-to-diameter ratio. probe: A thin-wall stainless steel tubing containing a heater element and a temperature sensor. The annulus is lled with a thermal epoxy that insulates the components from each other and the probe body. With a lengthto-diameter ratio of ~40, it simulates ‘line heat source’. thermal dry-out curve: Depicts the change in thermal resistivity as a function of moisture content at a constant dry density. Once soil or backll is installed, the dry density seldom changes.



Annex I (informative)

Bibliography Bibliographical references are resources that provide additional or helpful material but do not need to be understood or used to implement this standard. Reference to these resources is made for informational use only. [B1] EPRI Project 7861, Soil Thermal Resistivity and Thermal Stability Measuring Instrument, EPRI EL2128, November 1981.4 [B2] EPRI Project 7883, Thermal Stability of Soils Adjacent to Underground Transmission Power Cables, EPRI EL-2595, September 1982. [B3] EPRI Project 7883, Thermal Stability of Soils Adjacent to Underground Transmission Power Cables Phase II, January 1985. [B4] EPRI Project 7883-1, Thermal Stability of Soils Adjacent to Underground Transmission Power Cables, EPRI EL-5090, March 1987. [B5] EPRI Report 3002003493, Soil and Special Backll Thermal Resistivity Considerations for Underground Transmission Cables, 5 December 2014. [B6] Hartley, J. G. and W. Z. Black, “Transient Simultaneous Heat and Mass Transfer in an Unsaturated Porous Media,” ASME Journal of Heat Transfer, vol. 103, no. 2, pp. 376–382, 1981, http://dx.doi.org/10.1115/ 1.3244469. [B7] Hartley, J. G., W. Z. Black, M. A. Martin, and R. A. Bush, “Thermal Limits on Soil Stability,” Underground Cable Thermal Backll, Eds: Boggs, Chu, Radhakrishna, and Steinmans. Toronto: Pergamon Press, 1982, http://dx.doi.org/10.1016/B978-0-08-025387-9.50014-2. [B8] Hooper, F. C. and F. R. Lepper, “Transient Heat Flow Apparatus for the Determination of Thermal Conductivities,” Transactions of the American Society of Heating and Ventilating Engineers, paper 1395, pp. 309–324, presented at the Semi-Annual Meeting, Ontario Canada, June 1950. [B9] Mantel, C. L., Engineering Materials Handbook, 1st Ed., McGraw-Hill, 1958. [B10] Martin, M. A., W. Z. Black, R. A. Bush, and J. G. Hartley, “Practical Application of Thermal Stability Measurements for Use on Underground Power Cables,” IEEE Transactions on Power Apparatus and Systems, vol. PAS-100, no. 9, pp. 4236–4249, September 1981, http://dx.doi.org/10.1109/TPAS.1981.316975. [B11] Mason, V. V. and M. Kurtz, “Rapid Measurement of the Thermal Resistivity of Soil,” Transactions of the American Institute of Electrical Engineers, August, pp. 570–577, 1952.

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EPRI publications are available from the Electric Power Research Institute (http://www.epri.com).


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