CO

NATL INST OF STAND & TECH

A11107 3“lDfifl6

NBSIR 84-2935

The Influence of Soil Type and

Gradation on the Thermal

Resistivity of Soils

Lawrence A. Salomone

Felix Y. Yokel

Herbert Wechsler

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

October 1984

tt»T OF

$</*FAU O*

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NBSIR 84-2935

National bureau

OF STANDARDS

LIBRARY

THE INFLUENCE OF SOIL TYPE AND

GRADATION ON THE THERMAL

RESISTIVITY OF SOILS

Lawrence A. Salomone

Felix Y. Yokel

Herbert Wechsler

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

October 1984

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director
\
ABSTRACT

Laboratory thermal probe tests performed on four (4) different soils were

used to study the influence of soil type and gradation on the thermal resistivity of soils. The four soils covered a wide range of gradations and

included: two sands (SP and SP-SM), a silty clay (CL), and a silt (ML).

Results are presented which indicate that as the sand content increases in a silty clay (CL), the minimum thermal resistivity and the critical moisture

content decrease for the range of compactive efforts studied. Increasing the

medium and coarse sand fraction in a granular soil significantly increases the

heat conductive properties of the soils. Also, in the stable region of each

of the major soil groups (i.e., granular and fine-grained soils) the influence

of soil type and density on the thermal resistivity of soils is negligible and

a constant value of thermal resistivity is observed. The constant value of

thermal resistivity is approximately 30 to 40°C*cm/watt and 50 to 70°C*cm/watt

for granular soils and fine-grained soils, respectively.

Keywords: compaction, compaction tests, gradation, heat flow, laboratory tests,

soil moisture, soil tests, soil type, tests, thermal conductivity,

thermal resistivity.

i i i
Table of Contents

Page

ABSTRACT iii

LIST OF FIGURES v

LIST OF TABLES vi

INTRODUCTION 1

BACKGROUND 2

APPROACH 5

DESCRIPTION OF TESTED SOILS 9

PRESENTATION AND ANALYSIS OF TEST RESULTS 9

CONCLUSIONS 26

ACKNOWLEDGEMENTS 26

REFERENCES 31

iv
List of Figures

Fi gure

Figure

Figure

Fi gure

Figure

Figure

Figure

Figure

Figure

Fi gure

Fi gure

Figure

Figure

Fi gure

Fi gure

Figure

Figure

1. Moisture content continuum showing the various states of finegrained soils and the generalized stress-strain response (modified

from Holtz and Kovacs, 1981)

2. Variation of thermal resistivity with Moisture Content for AMRL

No. 61 Silty Clay (Salomone and Kovacs, 1984^)

3. Influence of dry density on critical moisture content for AMRL

No. 61 Silty Clay (Salomone and Kovacs, 1984^)

4. Gradation and plasticity characteristics (Atterberg Limits)

of the soils studied

5. Relationship between dry density and thermal resistivity versus molding moisture content for modified energy (ASTM D 1557-78)

for AMRL No. 71 Silty Clay and Sand (CL)

6. Relationship between dry density and thermal resistivity versus molding moisture content for standard energy (ASTM D 698-78) for

AMRL No. 71 Silty Clay and Sand (CL)

7. Relationship between dry density and thermal resistivity versus molding moisture content for S6 energy (2,970 ft*lb/cu ft) for

AMRL No. 71 Silty Clay and Sand (CL)

8. Summary of compaction curves for AMRL No. 71 Silty Clay

and Sand

9. Summary of thermal resistivity versus moisture content curves for

AMRL No. 71 Silty Clay and Sand

10. Plot of: (a) minimum thermal resistivity, and (b) critical moisture

content versus compactive effort

11. Envelope of thermal behavior for AMRL No. 71 Silty Clay and

Sand (CL)

12. Envelope of thermal behavior for PEPCO thermal sand (SP)

13. Envelope of thermal behavior for Florida sand (SP-SM)

14. Comparison of envelopes of thermal behavior for PEPCO thermal

sand (SP) and Florida sand (SP-SM)

15. Envelope of thermal behavior of graul ar soils

16. Envelope of thermal behavior of fine-grained soils

17. Influence of dry density on critical moisture content

v
List of Tables

Table 1

Table 2

Table 3

Table 4

Table 5

Table 6

Textural and Other Characteristics of Soils (from Holtz and

Kovacs, 1981)

Summary of Compact ive Efforts Used During Laboratory

Testing Program

Description of Soils Used in Testing Program

USCS Definitions of Particle Size, Size Ranges, and Symbols

Compaction Characteristics

Approximate Critical Moisture for Various Soil Types

(Salomome and Kovacs, 1984^)

vi
INTRODUCTION

Heat transfer from earth-contact surfaces (such as a basement wall /floor,

slab-on grade floor, underground heat distribution system, and ground coupled

heat pump) is not well understood despite recent advances in building thermal

analysis. One difficulty in estimating the earth heat transfer is the un- certainty involved in the thermal conductivity of the soil.

Progress in the determination of the thermal behavior of soils for earth contact

heat transfer problems has been hampered by the fact that information on thermal

soil properties is scattered in a variety of technical fields, and professionals

lack a common language to describe the thermal behavior of soils. Use of soil

as an insulating material or as a conductor to dissipate heat requires that

progress be made in this area and a systematic approach be developed for

selecting appropriate values of thermal resistivity.

As an aid to selecting appropriate values of soil thermal resistivity and the

calculation of heat transfer in soils, Salomone (1982) showed that the critical

moisture content can be correlated with soil limit states associated with moisture

content, which, in turn, can be established by methods (e.g., Atterberg Limits

i.e. Shrinkage Limit, Plastic Limit and Liquid Limit shown in figure 1) used

by geotechnical engineers and agronomists. The critical moisture content is the

moisture content below which the thermal resistivity increases rapidly with

further drying. Additional work by Salomone et al . (1982, 1984 a

) found that the

critical moisture content correlates with the optimum moisture content (that

will be defined later) for fine-grained soils over a wide range of densities.

However, as the density of the fine-grained soil decreases to densities typical

of unconsolidated marine deposits (less than 1600 kg/m^), the correlation between

critical moisture content and the plastic limit found by Salomone (1982) is

evident.

These correlations, optimum moisture content versus critical moisture content

and plastic limit versus critical moisture content are important because the

critical moisture content is of particular interest from a thermal instability

viewpoint as discussed in more detail later. Thermal instability occurs in

moist soils because of significant moisture movement when the soil is subjected

to thermal gradients due to the presence of a heat source (Hartley et al . , 1982).

Therefore, a large increase in the thermal resistivity of the soil occurs when

the moisture content of the soil falls below the critical moisture content

(Radhakrishna et al., 1980) (dry side of optimum). Above the critical moisture

content, the thermal resistivity is fairly constant (Boggs et al . , 1982) (wet

side of optimum).

Salomone and Kovacs (1984^) present three procedures for determining the

critical moisture content: l)the critical moisture content is the moisture

content at the knee or cusp of the thermal resistivity versus moisture

content curve, 2)the optimum moisture content and plastic limit are physical

quantities indicative of the critical moisture content in soils as explained

above and 3)the upper flex point (cusp) of the soil moisture characteristics

curve established the critical moisture content in soils.

1
This paper examines the influence of soil type and gradation on the thermal

behavior of soils. An approach for establishing the influence of these factors

is also presented.

BACKGROUND

Soil texture, plasticity, and cohesiveness form the basis for the soil

classification schemes commonly used by geotechnical engineers. Texturally,

soils are classified as either coarse-grained (sands and gravels) or fine-grained

(silts and clays) with the dividing line being whether the soil is retained

on or passes through the No. 200 (75 um) sieve. The plasticity and cohesion

of a soil are also indicators of soil type. Clays are both plastic and cohesive

while sands are nonplastic and noncohesive (cohesionless). Silts are intermediate between sands and clays. Silts are fine-grained yet nonplastic and

cohesionless. These relationships have been summarized by Holtz and Kovacs,

1981 (table 1).

The particle size distribution of sands and gravels have an important influence

on their engineering behavior. The particle size distribution (texture) of a soil is obtained by mechanical analysis. Detailed procedures for this test have

been specified by ASTM D 422-72 (Standard Method for Particle-Size Analysis of

Soils) procedures (ASTM, 1982).

The engineering properties of fine-grained soils are greatly affected by the

presence of water rather than by the texture alone. Consequently, Atterberg

Limit tests (ASTM, 1982) are used to classify fine-grained soils. The Atterberg

Limits provide a measure of the plasticity of fine-grained soils and represent

important limit states of engineering behavior (figure 1). Figure 1 shows the

Atterberg Limits (i.e.. Plastic Limit and Liquid Limit) as limiting moisture

contents that are the dividing line between the types of stress-strain behavior

shown. By knowing the natural moisture content of a soil in relation to its

Atterberg Limits, the engineering response of a soil can be estimated. Likewise,

the work of Salomone at the National Bureau of Standards (NBS) has been directed

towards developing methods to establish the limit states of thermal behavior of

soils so that the thermal response of a soil can be estimated using the same index and classification properties (e.g. particle size and Atterberg Limits) of

soils.

The approach that is being used involves systematically testing soils in the soil

groups defined by the Unified Soil Classification System (see, for example, Holtz

and Kovacs, 1981) and establishing the relationship of thermal resistivity versus moisture content and density for these soils. The dividing line between significant

changes in the thermal resistivity versus moisture content relationship at a given density would then be an important limit state of thermal behavior.

2
Table 1. Textual and Other Character- sc Kovacs, 1981)

ics of Soils (from Holtz a

n d

Soil name: Gravel , Sands Silts Clays

Srain size: Coarse grained

Can see individual

grains by eye

F’: ne grained

Cannot see individual

grai ns

Fine grained

Cannot see

indi vi dual

grai ns C^acteri sties

Cohesionless Cohesi cnl ess Cohesi ve

Nonpl astic

Granjl ar

Nonpl asti

Granul ar Plastic

Effect of water

o

r engineering

behavi or:

Relatively unimportant

(exception: loose saturated granular materials and dynamic

loadings)

Important /ery i~portant

Effect of grain Important

size distribution

c

r engineering

behavi or:

Rel ati vely

uni -cortant

Rel ati vely

unimportant

3
Figure 1

BE

h—

CO

T3 __l

00

d>

CO

«-* go

® a>

oo

CO

Moisture content continuum showing the various states of fine-grained

soils and the generalized stress-strain response (modified from

Holtz and Kovacs, 1981)

4
Figure 2 illustrates that the critical moisture content can be an important limit

state of thermal soil behavior. By knowing the moisture content of a soil and

the critical moisture content, the thermal resistivity of a soil can be estimated.

A large increase in thermal resistivity with a small decrease in moisture content

occurs when the moisture content of the soil falls below the critical moisture

content and the term "soil thermal instability" is used for this condition

(Radhakrishna et al., 1980). Above the critical moisture content the thermal

resistivity is fairly constant and the term "soil thermal stability" is used to

describe this physical condition (Boggs et al., 1982).

APPROACH

The critical moisture content can be determined directly by using thermal probe

tests to establish the thermal resistivity versus moisture content curve for a soil at the dry density anticipated for a project or indirectly by using those

index properties of soils that have been found (Salomone, 1983) to correlate

with the critical moisture content.

The direct method was used for this study. A thermal property analyzer was used

to faciliate data acquisition and reduction from a thermal needle, the principle

of which can be found in most of the heat conduction text books such as Carslow

and Jaeger, 1959. The analyzer consists of an electric current (heat) source and

a thermocouple reader under microprocessor control. The thermal resistivity is

calculated by the microprocessor from a linear least-squares fit of the logarithms

of time and temperature data obtained from the thermal needle.

Because the critical moisture content varies with dry density as shown in figure

3, the thermal resistivity versus moisture content curve was established for the

soils used in the study at two or more densities. The densities selected were

at least 95% of the maximum density and the minimum density of the soils. For

granular soils (i.e., soils with less than 12 weight percent of soil particles

passing a No. 200 (75 ym) sieve) ASTM D 2049-69 procedures were used to determine

the maximum and minimum density. This method utilizes vibratory compaction to

obtain maximum density and pouring to obtain minimum density. For fine-grained

soils, ASTM D 698-78 and ASTM D 1557-78 procedures were used to approximate the

minimum and maximum densities. These laboratory compaction methods cover the

determination of the relationship between the moisture content and density of

soils when compacted in a mold of a given size with a 5.5 lb (2.49 kg) rammer

(ASTM D 698-78) or 10 lb (4.54 kg) rammer (ASTM D 1557-78) dropped from a height

of 12 in (305 mm) (ASTM D 698-78) or 18 in (457 mm) (ASTM D 1557-78). The number

of layers and the number of blows per layer were varied to obtain the moisturedensity relationship at the compactive efforts shown in Table 2. Test specimens

were prepared by different compaction energies to obtain the thermal resistivity

data over a wide range of density and moisture. In cases where a third density

was selected, some intermediate density was used.

By defining the thermal resistivity versus moisture content relationship at the

minimum and maximum density, the envelope of thermal behavior is established for

the soil and the influence of soil type and gradation can be studied.

5
MOISTURE CONTENT IN PERCENT

Figure 2. Variation of thermal resistivity with Moisture Content for AMRL

No. 61 silty clay (Salomone and Kovacs, 1984*5

)

6
DRY DENSITY IN PCF

Figure 3. Influence of dry density on critical moisture content for AMRL

No. 61 silty clay (Salomone and Kovacs, 1984b)

7
Table 2. Summary of Compactive Efforts Used During Laboratory

Testing Program

Description

Weight

of Hammer

(lbf )*

Fall

(ft)*

No. of

Layers

No. of

61 ows per

Layers

Compactive

Energy

ft*lbf/ft 3*

Modified

(ASTM D1557-78)

10 1.5 5 25 56250

S 25-5 5.5 1.0 5 25 20625

Standard

(ASTM D698-78)

5.5 1.0 3 25 12375

S 12 5.5 1.0 3 12 5940

S 6 5.5 1.0 3 6 2970

* NOTE: 1 Ibf = 4.448 N

1 ft = 0.3048 m

1 ft-lbf/ft 3 = 47.88 J/m3

8
DESCRIPTION OF TESTED SOILS

Soils were selected using the following criteria:

1. The gradation and plasticity characteri sties of the soil were different

than the other soils being studied.

2. The Unified Soil Classification symbol was different than the other soils

being studied.

3. The index properties (e.g., particle size and Atterberg Limits) of the soil were well known or easily determined.

Soils meeting these criteria included in the testing are described in table 3.

USCS definitions of particle size, size ranges, and symbols are presented in

table 4 modified from Holtz and Kovacs, 1981.

Referring to table 3, it is important to emphasize the difference between the

generic or geologic name and the soil description. The soil description and

USCS symbol is more exact for classifying soils than the generic or geologic

name. Each word in the soil description is used to indicate something of the

gradation of the soil. The major component is stated first and adjectives such

as "fine to coarse" are used to indicate particle size. Also, descriptive terms

such as the word "trace" are used to indicate the amount (or proportion) in a particular size range. Use of the generic or geologic name can often be misleading. Therefore, the soil description is recommended when classifying soils.

The gradation of the soils studied are shown on figure 4. The Atterberg Limits

of the fine-grained soils are also shown. Table 5 provides the compaction

characteristics. Using Table 5 the densities for the thermal resistivity versus moisture content curves were selected for each of the soils.

PRESENTATION AND ANALYSIS OF TEST RESULTS

The first step in determining the thermal behavior of fine-grained soils is to

correlate its trends in thermal resistivity versus moisture content with its

compaction characteristics. To illustrate the procedure, figures 5 through 7 show

the correlation for AASHTO Materials Reference Laboratory (AMRL) No. 71 silty clay

and sand (CL) for three compactive efforts: modified, standard, and S6, respectively. When the dry densities of each sample are determined and plotted versus the moisture contents, the compaction curves are obtained. Each data point on the curves represents a single compaction test for which the thermal resistivity

was determined. Each curve is unique for the AMRL reference soil and the method

of impact compaction and compactive effort used in the program. The peak point

correspondi ng to the maximum dry density is an important point and is known as the optimum moisture content. The maximum dry density varies with compactive

effort. Increasing the compactive effort increases the maximum dry density and

decreases the optimum moisture content as expected.

9
Table 3. Description of Soils Used in Testing Program

Generic

Name Letter Symbol

Pepco Thermal Sand SP

Florida Sand SP - SM

AMRL No. 71 Silty CL

Clay and Sand

Soil Description

Fine to coarse sand

trace silt

Fine to medium sand

trace silt

Silty clay, some

fine sand, trace

medium sand

Silt, trace fine

sand

Bonny Loess ML
Table 4. USCS Definitions of Particle Size, Size Ranges, and Symbols

Soil Fraction

or Component Symbol

Boulders None

Cobbles None

(1) Coarse-grained soils:

Gravel G

Coarse

Fi ne

Sand S

Coarse

Medi urn

Fine

(2)

Fine-grained soils:

Fi nes

Silt M

Clay C

(3) Organic soils: 0

(4) Peat: Pt

Size Range

Greater than 300 mm

75 mm to 300 mm

75 mm to No. 4 sieve

(4.75 mm)

75 mm to 19 mm

19 mm to No. 4 sieve (4.75mm)

No. 4 (4.75 mm) to

No. 200 (0.075 mm)

No. 4 (4.75 mm) to

No. 10 (2.0 mm)

No. 10 (2.0 mm) to

No. 40 (0.425 mm)

No. 40 (0.425 mm) to

No. 200 (0.075 mm)

Less than No. 200 sieve

(0.075 mm)

(No specific grain

size -- use Atterberg

Limits)

(No specific grain

size -- use Atterberg

Limits)

(No specific grain size)

(No specific grain size)

Gradation Symbols

Well-graded, W

Poorly-graded P

Description Term

Trace

Little

Some

And

Liquid Limit Symbols

High LL, H Low LL, L

Range of Proportion

1 - 10%

10 - 20%

20 - 35%

35 - 50%

11
Table 5. Compaction Characteristics

ASTM D2049-69 AstM D698-78

GENERIC

NAME

MINIMUM

DENSITY

kg/m3

(PCF)

MAXIMUM

DENSITY

kg/m3

(PCF)

OPTIMUM

MOISTURE

CONTENT

%

MAXIMUM

DENSITY

kg/m3

(PCF)

PEPCO THERMAL SAND

(SP)

1578

(98.5)

1861

(116.1)

N.A. N.A.

FLORIDA SAND

(SP-SM)

1282

(80.0)

1590

(99.2)

12.5 1843

(115.0)

AMRL NO. 71 SILTY

CLAY AND SAND

(CL)

N.A. N.A. 14.7 1828

(114.1)

BONNY LOESS

(ML)

N.A. N.A. 16.5 1667

(104.0)

12
igure 4. Gradation and plasticity characteristics (Atterberg Limits

of the soils studied

13

2087

AlPlACCt

«CS

roWM

ho

<X4>.

>!*

INI,

WHICH

it

ONBOLCTC

(

mANSLVCMNT

)
THERMAL

RESISTIVITY

IN

°C*cm/watt

UNIT

WEIGHT

IN

PCF

MODIFIED ENERGY

MOISTURE-DENSITY RELATIONSHIP

(Compaction Curve)

coE

o> 2

>

CO

z

111 o

>•

cco

60 r

50 -

40 -

30 -

20 1 1 1 1 10 20 30 40

MOISTURE CONTENT IN % BY WEIGHT

Figure 5. Relationship between dry density and thermal resistivity versus molding moisture content for modified energy (ASTM D 1557-78)

for AMRL No. 71 Silty Clay and Sand (CL)

14
THERMAL

RESISTIVITY

IN

°C-cm/watt

UNIT

WEIGHT

IN

PCF

STANDARD ENERGY

MOISTURE-DENSITY RELATIONSHIP

(Compaction Curve)

coE

U)

>

COz

LUo

>-

ccQ

higure 6. Relationship between dry density and thermal resistivity versus molding moisture content for standard energy (ASTM 0 698-78)

for AMRL No. 71 Silty Clay and Sand (CL)

15
THERMAL

RESISTIVITY

IN

°C-cm/watt

UNIT

WEIGHT

IN

PCF

S6 ENERGY

MOISTURE-DENSITY RELATIONSHIP

(Compaction Curve)

70 r

50

40

30 l l l l 10 20 30 40

MOISTURE CONTENT IN % BY WEIGHT

Figure 7. Relationship between dry density and thermal resistivity versus molding moisture content for S6 energy (2,970 ft*lb/cu ft)

for ANIRL No. 71 Silty Clay and Sand (CL)

16

DRY

DENSITY

IN

Mg/m
A summary of the compaction curves and the thermal resistivity versus moisture content curves for the five compactive efforts are shown on figures 8 and

9, respectively. The minimum thermal resistivity for each compactive effort

(see figures 5 through 7) generally occurs at the point of optimum moisture

content (and maximum dry density). Therefore, the moisture content corresponding

to the minimum value of thermal resistivity (or the critical moisture content)

is approximately equal to the optimum moisture content. When a plot of minimum

thermal resistivity versus compactive effort is made, the importance of compactive

effort (or density) in achieving the minimum thermal resistivity during placement

of this material is seen (figure 10(a)). Using the fact that the optimum moisture

content is equal to the critical moisture content, the relationship between

critical moisture content and compactive efforts is shown iri figure 10(b). The

relationship is determined from the line of optimums, which is the line drawn

through the peak points of the compaction curves at the five compactive efforts.

Comparing the curve for AMRL No. 61 silty clay (CL) from Salomone and Kovacs,

1 984 a with the curve for AMRL No. 71 silty clay and sand (CL) on figure 10, one can see the influence of increasing the amount of sand in a clay soil on the

relationships shown. AMRL No. 61 has 10 percent by weight sand while AMRL No.

71 has 35 percent by weight sand. As the sand content increases, the minimum

thermal resistivity and the critical moisture content decrease for the range of

compactive efforts studied.

The second step in determining the thermal behavior of a fine-grained soil is to use the data from plots similar to those presented on figures 5 through 7 to

establish the thermal resistivity versus moisture content relationship for the

range of densities anticipated for the project. The lowest density can be the

minimum density anticipated for the soil and the maximum density can be the

maximum density defined by ASTM D 698-78 procedures. The curves of thermal

resistivity versus moisture content at the minimum and maximum density establish

the envelope of thermal behavior for the soil as illustrated in figure 11 for

the AMRL No. 71 silty clay and sand (CL).

The envelope of thermal behavior for a granular soil is more easily determined

than for a fine-grained soil. The minimum and maximum densities are determined

using ASTM D 2049 procedures (or if there is more than 12 weight % of soil

particles passing a No. 200 (75 urn) sieve, ASTM D 698-78 procedures are used to

determine the maximum density). Samples are then prepared at the minimum and

maximum density for a full range of moisture contents and thermal probe tests

are performed using the following two methods. Method 1 involves stage drying

a sample at the desired dry density. Method 2 involves measuring the thermal

resistivity of reconstituted soil samples at different moisture contents and

at the desired dry density. The results of these thermal probe tests for the

PEPC0 thermal sand (SP) and the Florida sand (SP-SM) are shown in figures 12 and

13, respectively.

Comparing the envelopes of thermal behavior for the PEPC0 thermal sand (SP) and

the Florida sand (SP-SM) in figure 14, one can see the advantages of increasing

17
UNIT

WEIGHT

IN

PCF

Figure 8. Summary of compaction curves for AMRL No. 71 silty

clay and sand

18

DRY

DENSITY

IN

Mg/m
THERMAL

RESISTIVITY

IN

°C*cm/watt

Figure 9

MOISTURE CONTENT IN % BY WEIGHT

Summary of thermal resistivity versus moisture content curves

for AMRL No. 71 silty clay and sand

19
CRITICAL

MOISTURE

(

PERCENT )

MINIMUM

THERMAL

RESISTIVITY

(

°C

•

cm/watt

)

COMPACTIVE EFFORT ( * 10

4 ft • lbf/ft3

]

COMPACTIVE EFFORT |x 104 ft - lbf/ft3

)

Figure 10. Plot of: (a) minimum thermal resistivity, and (b) critical

moisture content versus compactive effort

20
CO

»bm/uio.Oo ni AiiAiisisaa nvwaaHi

Figure 11. Envelope of thermal behavior for AMRL No. 71 silty clay and

sand (CL)

21

MOISTURE

CONTENT

IN

%

BY

WEIGHT
360

320

280

240

200

160

120

80

40

0

Fig

t

r—i—i—i—i—i—i—i—i—i—

PEPCO THERMAL SAND (SP)

Dry density = 1580 kg/m3 (99 PCF)

1688 kg/m3 (105 PCF)

1792 kg/m 3 (112 PCF)

±_ L l l I l I J L I 2 4 6 8 10 12 14

MOISTURE CONTENT IN % BY WEIGHT

16

ire 12. Envelope of thermal behavior for PEPCO thermal sand (SP)

22
480

440

400

360

320

280

240

200

160

120

80

40

0

F i gu

2 4 6 8 10 12 14 16 18

MOISTURE CONTENT IN % BY WEIGHT

13. Envelope of thermal behavior for Florida sand (SP-SM)

23
280

260

240

220

200

180

160

140

120

100

80

60

40

20

0

Fig

PEPCO fine to coarse

4 sand, trace silt

Florida fine sand, trace silt

l J L J L 1

3 4 5 6 7 8 9 10 11 12 13 14 15

Moisture Content in % by Weight

14. Comparison of envelopes of thermal behavior for PEPCO thermal

sand (SP) and Florida sand (SP-SM)

24
the medium and coarse sand fraction in a granular soil. The envelope of thermal

behavior for the PEPCO thermal sand (SP) is narrow and the upper bound is

significantly lower than that for the Florida sand (SP-SM). Considering the

range of densities possible (i.e. the minimum and maximum densities for these

soils) the PEPCO thermal sand should be a better soil to conduct heat.

By systematically testing soils for their thermal behavior the influence of

soil type and gradation was seen. To use these data to predict the thermal

behavior of other granular and fine-grained soils, figures 15 through 17 were

prepared. Figure 15 provides the envelope of thermal behavior for the granular

soils shown. Note that in the stable region the influences of soil type and

density are negligible and a constant value of thermal resistivity of approximately 30 to 40°C-cm/watt is obtained. The shapes of the curves for the various

soils seem to be determined by the stable region value (approximately 40°C-cm/

watt), the thermal resistivity in the dry state and the critical moisture content.

The trend in figure 16 for the fine-grained soils appear to be similar except

the stable region value is approximately 50 to 70°C-cm/watt. Note the stable

region value should be even greater than 70°C-cm/watt for highly organic silts

and clays and peaty soils if the trends continue for these traditionally more

thermally resistive soils.

Figure 17 provides the relationship between critical moisture content and density

for the soils studied. The relationships for the fine-grained soils (AMRL No. 61

silty clay, AMRL No. 71 silty clay and sand, and Bonny Loess) were determined

using the peak points of the compaction curves and the fact that the critical

moisture content is approximately equal to the optimum moisture content. The

relationships are determined from the line of optimums, which is the line drawn

through the peak points of the compaction curves.

For the granular soils (PEPCO thermal sand and Florida sand) the task of determining the critical moisture content is more difficult. No standard method

exists for establishing the critical moisture content. Traditionally, the moisture

content at the knee (or cusp) of the thermal resistivity versus moisture content

curve was selected as the critical moisture content. The knee or cusp is evident

for the Florida sand at densities of 1747 kg/m^ (109 PC F ) and 1506 kg/nr (94

PCF) (see figure 13), and these data were used to establish the relationship

between critical moisture content and density for Florida sand shown in figure

17. On the other hand, it is not possible to determine the critical moisture

content for the PEPCO thermal sand using the knee-of-the-curve approach. As seen in figure 12, the knee of the thermal resistivity versus moisture content

curve at the three densities shown is not evident. Consequently, the critical

moisture contents for the PEPCO thermal sand at densities of 1580 kg/m^ (99 PCF),

1688 kg/rrP (105 PCF) and 1792 kg/m^ (112 PCF) were defined as the moisture

content at which the thermal resistivity was equal to the stable region value

(approximately 40°C*cm/watt). It is also important to emphasize that if this

definition of critical moisture content was used for the Florida sand, the line

for Florida sand would move close to the line for PEPCO thermal sand. Additional

data is therefore required to esablish the appropriate definition of critical

moisture content for granular soils and to confirm the trends observed. In the

meantime, this figure and table 6 can aid in the estimation of critical moisture

contents, i.e., the dividing line between "stability and instability."

25
If these trends are confirmed as more data becomes available, it would appear

that the thermal resistivity versus moisture content relationship for a given

dry density can be approximated by knowing:

a) stable region value of thermal resistivity,

b) critical moisture content, and

c) thermal resistivity in the dry state.

CONCLUSIONS

Based on this study, the following conclusions are warranted:

1. The fact that the critical moisture content for fine-grained soils is

approximately equal to the optimum moisture content is confirmed.

2. As the sand content increases in a silty clay (CL), the minimum thermal

resistivity and the critical moisture content decrease for the range of

compactive efforts studied.

3. Increasing the medium and coarse sand fraction in a granular soil

significantly increases its heat conductive properties as seen in

figure 14.

4. In the stable region of each of the major soil groups (i.e. granular

and fine-grained soils) the influence of soil type and density on the

thermal resistivity of soils are negligible and a constant value of thermal

resistivity is observed. The constant value of thermal resistivity is

approximately 30 to 40°C-cm/watt and 50 to 70°C-cm/watt for granular soils

and fine-grained soils, respectively.

5. If the trends observed during this study are confirmed as more data becomes

available, it would appear that the thermal resistivity versus moisture

content relationship for a soil at a given density can be approximated by:

stable region value of thermal resistivity, critical moisture content and

the thermal resistivity in the dry state.

ACKNOWLEDGMENT

J. Lichtenberg of Potomac Electric Power Company (PEPCO) and J. Pielert and 0.

W. McIntosh of the AASHTO Materials Reference Laboratory (AMRL) provided the

required quantities of PEPCO thermal sand and AMRL No. 71 silty clay and sand,

respectively, used in the testing program. J. Farrar of the Bureau of Reclamation

supplied the Bonny Loess material. The authors wish to thank these individuals

for their cooperation.

26
Table 6. Approximate Critical Moisture for Various Soil Types

(Salomone and Kovacs, 1984^)

Soil Description

(1)

Approximate Standard Maximum

Dry Unit Weight (ASTM D698-78)

Approximate

critical moisture

content*, as

a percentage

(4)

Pounds per

cubic feet

(2)

Megagrams

per cubic meter

(3)

Granul ar 120 to 135 1.92 to 2.16 <12

Si 1 ts 110 to 120 1.76 to 1.92 12 to 16

Clays 100 to 110 1.60 to 1.76 16 to 22

Organic silts and

expansive clays <100 <1.60 >22

* Critical moisture content is defined for a dry density that is

100% of standard maximum density (ASTM D 698-78)

27
THERMAL

RESISTIVITY

IN

°Ccm/

watt

Figure 15. Envelope of thermal behavior of granular soils

28
Figure 16. Envelope of thermal behavior of fine-grained soils

29
T T

O

lN30d3d Nl

1N31NOO 3dniSIOIAI IVOIlldO

Figure 17. Influence of dry density on critical moisture content

30

DRY

DENSITY

IN

PCF
REFERENCES

American Society for Testing and Materials, Annual Book of ASTM Standards, Soil

and Rock, Building Stones, Part 19 , 1982, Philadelphia, PA, 650 pp.

Boggs, S. A., Chu, F. Y., and Radhakrishna, H. S., "Measurements for Underground

Thermal Transmission Design," Underground Cable Thermal Backfill , S. A. Boggs

et al., eds, Pergamon Press, Inc., Toronto, Canada, 1982, pp. 134-146.

Carslow, H. S. and Jaeger, J. C., Conduction of Heat in Solids , 2nd ed., Oxford

University Press, London, England, 1959.

Hartley, J. G., Black, W. Z., Bush, R. A., and Martin, M. A., "Measurements,

Correlations and Limitations of Soil Thermal Stability," Underground Cable

Thermal Backfill , S. A. Boggs et al., eds, Pergamon Press, Inc., Toronto,

Canada, 1982, pp. 121-133.

Holtz, R. D. and Kovacs, W. D. , An Introduction to Geotechnical Engineering , Prentice-Hall, Inc., Englewood Cliffs, NJ, 1981, 732 pp.

Radhakrishna, H. S. , Chu, F. Y., and Boggs, S. A., "Thermal Instability and Its

Prediction in Cable Backfill Soils," IEEE Transactions on Power Apparatus and

Systems, Vol . PAS-99, No. 3, 1980, pp. 856-857.

Salomone, L. A., "Procedures Used to Predict the Thermal Behavior of Soils,"

Proceedings of the International Conference on Earth Sheltered Buildings,

Sydney, Australia, 1983, pp. 49-53.

Salomone, L. A., "Improving Geotechnical Investigations for Underground Transmission

Lines," Underground Cable Thermal Backfill , S. A. Boggs et al., eds, Pergamon

Press, Inc., Toronto, Canada, 1982, pp. 57-71.

Salomone, L. A. and Kovacs, W. D., "Thermal Resistivity of Soils," Journal of

Geotechnical Engineeing , ASCE, Vol. 110, No. 3, March 1984 b

, pp. 375-389.

Salomone, L. A., Kovacs, W. D., and Kusuda, T., "Thermal Performance of FineGrained Soils," Journal of Geotechnical Engineering, ASCE, Vol. 110, No. 3,

March 1984^, pp. 359-374.

Salomone, L. A., Kovacs, W. D., Wechsler, H., "Thermal Behavior of Fine-Grained

Soils," Building Science Series 149, National Bureau of Standards,

Washington, DC, 1982.

31
NBS-1MA (REV. 2-60

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

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1. PUBLICATION OR

REPORT NO.

NBSIR 84-2935

2. Performing Organ. Report No. 3. Publication Date

4. TITLE AND SUBTITLE

THE INFLUENCE OF SOIL TYPE AND GRADATION ON THE THERMAL RESISTIVITY OF SOILS

5. AUTHOR(S)

Lawrence A. Salomone, Felix Y. Yokel and Herbert Wechsler

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8 . Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

10.

SUPPLEMENTARY NOTES

] Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bl iography or literature survey, mention it here)

Laboratory thermal probe tests performed on four (4) different soils were used to

study the influence of soil type and gradation on the thermal resistivity of soils.

The four soils covered a wide range of gradations and included: two sands (SP and

SP-SM) , a silty clay (CL) , and a silt (ML)

Results are presented which indicate that as the sand content increases in a silty

clay (CL) , the minimum thermal resistivity and the critical moisture content

decrease for the range of ccmpactive efforts studied. Increasing the medium and

coarse sand fraction in a granular soil significantly increases the heat conductive

properties of the soils. Also, in the stable region of each of the major soil

groups (i.e. granular and fine-grained soils) , the influence of soil type and

density on the thermal resistivity of soils is negligible and a constant value

of thermal resistivity is observed. The constant value of thermal resistivity

is approximately 30 to 40°C*cm/watt and 50 to 70°C*cm/watt for granular soils

and fine-grained soils, respectively.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) compaction; compaction tests; gradation; heat flow; laboratory tests; soil

moisture; soil tests; soil type; tests; thermal conductivity; thermal resistivity

13.

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