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)
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IEEE Std 442™-2017
(Revision of IEEE Std 442-1981)
IEEE Guide for Thermal
Resistivity Measurements of
Soils and Backfill Materials
Sponsor
Insulated Conductors Committee
of the
IEEE Power and Energy Society
Approved 28 September 2017
IEEE-SA Standards Board
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Abstract: The measurement of thermal resistivity of soil and backfill materials to include concrete,
engineered backfills, 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 backfill 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 field thermal probes are also described in this guide.
Keywords: backfill, IEEE 442™, soil, soil thermal properties, thermal needle, thermal probe,
thermal property analyzer, thermal resistivity
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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 Griffith
Michael Janezic
Thomas Koshy
Joseph L. Koepfinger*
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
6
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Introduction
This introduction is not part of IEEE Std 442-2017, IEEE Guide for Thermal Resistivity Measurements of Soils and
Backfill 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 firms 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.
7
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Contents
1. Overview����������������������������������������������������������������������������������������������������������������������������������������������������� 9
1.1 Scope���������������������������������������������������������������������������������������������������������������������������������������������������� 9
1.2 Purpose������������������������������������������������������������������������������������������������������������������������������������������������� 9
2. Normative references���������������������������������������������������������������������������������������������������������������������������������� 9
3. Factors influencing soil thermal resistivity������������������������������������������������������������������������������������������������ 10
3.1 Factors influencing measurements����������������������������������������������������������������������������������������������������� 10
4. Test equipment������������������������������������������������������������������������������������������������������������������������������������������ 11
4.1 Equipment required for field measurements��������������������������������������������������������������������������������������� 11
4.2 Equipment required for laboratory measurements������������������������������������������������������������������������������ 12
5. Test methods���������������������������������������������������������������������������������������������������������������������������������������������� 12
5.1 Methods for field 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 field probe�������������������������������������������������������������������������������������������� 22
Annex B (informative) Single-sensor laboratory and field 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
8
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IEEE Guide for Thermal
Resistivity Measurements of
Soils and Backfill Materials
1. Overview
1.1 Scope
This guide covers the measurement of thermal resistivity of soil and backfill materials to include concrete,
engineered backfills, 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 backfill 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 field
thermal probes are also described in this guide.
1.2 Purpose
The purpose of this guide is to provide sufficient 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 field 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/ft3(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/​).
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IEEE Std 442-2017
IEEE Guide for Thermal Resistivity Measurements of Soils and Backfill Materials
ASTM D1557, Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Modified
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 influencing soil thermal resistivity
Typical soil thermal resistivity (ρ) of various materials is listed in Table 1.
Table 1—Typical soil thermal resistivity values
Material
(ρ) (K·m/W)
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 influence the resistivity of a specific 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 influencing measurements
During the measurement of soil thermal resistivity, the following factors may adversely affect the accuracy of
the test measurement:
2
—
Migration of the soil moisture away from the probe during the test can result in higher soil thermal
resistivity measurements. This migration may be significant, and normally takes place when the input
power per unit area of the probe is sufficiently 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.
ASTM D2049 has been withdrawn; however, copies can be obtained from the IHS Markit (http://​global​.ihs​.com).
10
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IEEE Std 442-2017
IEEE Guide for Thermal Resistivity Measurements of Soils and Backfill Materials
—
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 significant 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 flux is kept within ±1%.
—
The in-situ resistivity measured using the field 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 field 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 field 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 field. The equipment for testing differs primarily in the size of the probe and the portability
requirements of the devices used in the field as shown in Annex A and Annex B.
4.1 Equipment required for field measurements
4.1.1 Field thermal probe
The field 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 infiltration in the probe and to reduce the initial
thermal transient, the probe may be filled 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 flexibility to test with
small lab probes and or large field probes. The field 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 field, 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 configuration 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.
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IEEE Std 442-2017
IEEE Guide for Thermal Resistivity Measurements of Soils and Backfill Materials
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 field 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 field measurements
5.1.1 Installation of field 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 difficult, a pilot hole should first 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 difficult, 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 field probe
After insertion has been completed, the following procedure should be performed:
a)
Make power and temperature monitoring connections.
b)
Conduct a verification test of the thermal probe prior to field testing by measuring a known standard
for every project and compare with the calibration certificate.
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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 field 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 flux 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 first 5 min to establish
that the probe is not overheated. The temperature should not exceed 10 °C above ambient soil
temperature, during the first 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 field 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 backfill 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 D20493 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 artificially 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 difficult, 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.
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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 backfill 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 infinite
length dissipating heat in an infinite medium. Under these conditions, the following Equation (1) is valid:
ρ=
4π (T2 − T1 )
t 
q ln  2 
 t1 
(1)
where
ρ
T1
T2
q
t1
t2
is the thermal resistivity (K·m/W)
is the temperature measured at some arbitrary elapsed time (K)
is the temperature measured at another arbitrary elapsed time (K)
is the heat dissipated per unit length (W/m)
is the elapsed time at which temperature T1 is recorded (min)
is the elapsed time at which temperature T2 is recorded (min)
Initial transients exist due to the finite diameter of the probe. Boundary effects are possible due to the finite
medium of the soil sample. A convenient way of determining when the initial transients are over and when the
finite 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 finite
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.
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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
∆T
2.303q
(2)
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 fitted. 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 field probe. It should
be noted that since the time span required to make a field 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
coefficient of determination is based on the R2 least fit.
6.2 Interpretation of results
To judge the reliability of the thermal resistivity data gathered in the field or laboratory, one shall make
comparisons to existing data gathered in previous tests for similar types of soil and backfill 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.
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Figure 1—Temperature versus log of time
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Figure 2—Thermal property characteristics of soils
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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)
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Figure 5—Interpretation of thermal probe test curves: Sample boundary effect
Figure 6—Interpretation of thermal probe test curves: Probe axial heat loss
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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
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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
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Annex A
(informative)
Multi-sensor field probe
Figure A.1 provides typical construction of a multi-sensor field probe used for thermal resistivity measurement.
Figure A.1—Field probe
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Annex B
(informative)
Single-sensor laboratory and field probes
Figure B.1 and Figure B.2 provide typical construction of single-sensor laboratory and field probes used for
thermal resistivity measurements.
Figure B.1—Probe for laboratory applications
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Figure B.2—Small probe for field use
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Annex C
(informative)
Slide hammer assembly
Typical apparatus for installation of a field probe is shown in Figure C.1.
Figure C.1—Slide hammer assembly
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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
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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
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Annex F
(informative)
Determine critical moisture content
The critical moisture content is defined 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
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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—Effect of moisture migration on thermal resistivity
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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 infinite 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 flux 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 field to obtain a temperature and thermal resistivity profile 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
filled 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 backfill is installed, the dry density seldom changes.
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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 Backfill 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 Backfill, 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.
4
EPRI publications are available from the Electric Power Research Institute (http://​www​.epri​.com).
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