A VACUUM PERMEABILITY TEST FOR COMPACTED CLAY

by

Elmer Franklin Hart

Thesis submitted to the Graduate Faculty of the

Virginia Polytechnic Institute

APPROVED:

in partial fulfillment for the degree of

MASTER OF SCIENCE

in

Civil Engineering

Dr. Robert D. Krebs, Chairman

Dr. Richard D. Walker Dr. Henry M. Morris, Jr.

Dr. James M. Wiggert

May, 1968

Blacksburg, Virginia

LIST OF FIGURES ••

LIST OF TABLES ••

ACKNOWLEDGMENTS ••

TABLE OF CONTENTS

• • • • • • • • •• • • • • • • • • •

• • • • • • • • • • • • • • • • • •

• • • • • • • • • • • • • • • • • •

INTRODUCTION • • • • • • • • • • • • • • • • • • • •

REVIEW OF PERMEABILITY TEST METHODS •• • • • • • • • •

FACTORS INFLUENCING PERMEABILITY. • • • • • • • • • •

Page

iii

iv

V

l

3

4

Soil Composition • • • • • • • • • • • • • • • • 4 Soil Structure • • • • • • • • • • • • • • • • • 5 Degree of Saturation. • • • • • • • • • • • • • • 7 Fluid Visco~ity. • • • • • • • • • • • • • • • • 8 Entrapped .Air and Foreign Matter in Voids • • • • 8

TEST VARIABLES • • • • • • • • • • • • • • • • • • • 9

Leakage • • • • • • • • • • • • • • • • • • • • 9 Bacterial Growth. • • • • • • • • • • • • • • • • 9 Local Consolidation and Swelling. • • • • • • • • 9 Linearity of Flow Versus Hydraulic Gradient. • • 10

MATERIALS ••• • • • • • • • • • • • • • • • • • • • •

PROCEDURE AND DESCRIPTION OF TEST. • • • • •

RESULTS AND DISCUSSION. • • • • • • • • • •

• • • • • • • • • • • • • • • • CONCLUSIONS.

BIBLIOGRAPHY • • • • • • • • • • • • • • • •

VITA •• • • • • • • • • • • • • • • • • • •

ii

• • • • •

• • • • •

• • • • •

• • • • •

• • • • •

12

14

24

35

36

39

LIST OF FIGURES

Figure

1.

2.

4.

5. 6.

7.

Diagram of Pre-Formed Sample Prepared for Permeability Testing • • • • • • • • • • •

Diagram of Compacted-in-Mold Sample Prepared for Permeability Testing •••••••••

Sketch Showing Position of Permeability Test Set-.-up • • • • • • • • • ,. • • • • • • • •

• • •

• • •

• • •

Sketch Showing Pressure Reducing Device •• • • . . Typical Discharge-Time Curve. • • • • •

Flow Velocity versus Hydraulic Gradient •• • • • •

Flow Velocity versus Hydraulic Gradient. • • • • •

Page

15

17

19

21

22

25 26

8. Flow Velocity versus Hydraulic Gradient •••••• 27

9. Flow Velocity versus Hydraulic Gradient •••••• 28

iii

LIST OF TABLES

Table Page

I. Soil Properties • • • • • • • • • • • • • • • • 13

II. Comparison of Coefficients of Permeabilities for Twelve Samples. • • • • • • • • • • • • • 30

III. Effects of Permeant Temperature on Coefficient of Permeability for a Typical Sample. • • • • 32

iv

ACKNOWLEDGMENTS

The author extends thanks to Dr. Robert D. Krebs for

bis guidance, advice, and assistance both in the laboratory

and on this manuscript.

Appreciation is also extended to Mrs. R. D. Walker for

typing of the manuscript.

V

INTRODUCTION

In engineering problems involving seepage, drainage,

settlement, and stability, the hydraulic permeability of

compacted clays is of great importance. The quantity of

seepage through and under dams and the rate at which a

building settles are practical problems in which the hy-

draulic permeability of a soil can be a critical factor.

Fine-grained soils are being used increasingly to line

canals and reservoirs and to construct cores for earth dams.

Dissipation of excess pore pressure during embankment con-

struction may be critical to stability so that permissible

rate of construction depends on permeability.

Dunn (1) and Walker,et al. (2) have studied the effects

of lime stabilization on the permeabilities of compacted

fine-grained soils using a new technique. They determined

the permeability by a constant head method with high hydrau-

lic gradients induced by a vacuum. Their tests were run

using a very simple apparatus, and it was an easy method of

determining relative values of permeability.

There has been a need for a simple and easy test for

determining the relative permeabilities of compacted fine-

grained soils. The apparatus used in the tests by Dunn (1)

and Walke~ et al. (2) could be constructed easily with appa-

ratus commonly available in laboratories for routine soil

1

2

tests. However, knowledge is lacking on the validity of this

test and the assumption of a linear relationship between velo-

city and hydraulic gradient. In this study, the hydraulic

gradient is varied so as to study the validity of the assumed

linear relationship. Also the experimental procedure is im-

proved so as to reduce error. A falling head test was run to

compare permeability values with those found by the vacuum

test.

REVIEW OF PERMEABILITY TEST METHODS

The reliable determination of the permeability of com-

pacted clay has long been an experimental problem. Lambe (3)

devised a constant head method wherein a soil specimen was

compacted in a lucite mold, a permeant chamber was filled, a

desired gas pressure applied, and flow and time measurements

taken until a constant rate of flow was reached. Lambe's

main objections to this method are that de-aired water is

not used, since the use of gas pressure precludes it, and

small sample size leads to data scatter.

Bjerrium and Ruder (4) describe a method for measuring

the permeability of compacted clay wherein a back pressure

is used for saturation and the test is performed in a tri-

axial cell.

Mitchell, Hooper, and Campanella (5) developed a per-

meability test apparatus wherein a back pressure is applied

for saturation and the precise determination of flow both

in and out of the sample can be obtained. Samples could be

compacted directly in lucite molds. This procedure has been

found considerably easier and more rapid than the testing of

specimens in a triaxial cell described by Bjerrium and

Ruder (4).

The classical constant head and falling head tests de-

scribed by Lambe (6) in his book, Soil Testing for Engineers,

take much time and involve small flow measurements that are

difficult to arrive at accurately.

3

FACTORS INFLUENCING PERMEABILITY

The influence of the nature of the soil on permeability

is very substantial. Soil composition, soil structure, and

the degree of saturation of the soil are significant factors.

In addition, the nature of the permeant has significant ef-

fects. Some factors related to permeant are viscosity, en-

trapped air, and foreign matter.

Soil Composition

Soil composition is of little importance to the permea-

bility of silts, sands, and gravels, but with clays, it is

of major importance. Lambe (3) states that clay "composition"

includes minerals, exchangeable ions, and impurities, and

that the magnitude of permeability variation with soil com-

position ranges widely. It bas been shown by Lambe (3)

that the ratio permeability of calcium montmorillonite to

that of potassium montmorillonite at a void ratio of seven

is approximately 300 and that the permeability of kaolonite

can be 1000 times that of montmorillonite. He states that

the lower the ion exchange capacity of a soil, the lower the

effect of exchangeable ions on permeability.

It has been suggested by Lambe (3) that a composition

term with a value range for each soil mineral group could

be added to the permeability equations, but that since the

mineralogy of a soil is seldom kno1~n, the use of a compo-

sition term might be limited.

4

5

Soil Structure

Different methods of compacting or placing soil, soil

mixing, thixotropy, swell and migration of fines have signi-

ficant effects on permeability. All these factors are re-

lated to soil structure, which according to Lambe (3, 7) and

Mitchell, et al. (5), is the most important single variable

influencing permeability of compacted clay. Samples com-

pacted wet of optimum have permeability values 100 to 1000

times less than samples compacted dry of optimum, other

conditions being equal.

The method of compaction influences the permeability

of clays compacted wet of optimum. Seed and Chan (8) studied

the effects of method of compaction on structure and found

that the structure was more dispersed when induced by the

larger shear strains associated with kneading compaction as

compared with static compaction. Mitchell, et al. (5) found

that samples prepared by kneading compaction have lower

permeabilities than those prepared by static compaction.

The increased dispersion noticed by Seed and Chan (8) re-

duces the number of large flow channels and results in

smaller average pore sizes, and since permeability varies

directly with pore cross-sectional area, it decreases.

For samples compacted dry of optimum, permeability may

increase or decrease with increasing water content. Mitchell,

et al. (5) stated that this behavior would appear to represent

6

the results of a complex interaction between soil type, com-

pactive effort, small changes in structure that develop with

increasing water content dry of optimum, and effects such as

non-uniform saturation and the migration of fines that may

develop during permeation. Lambe (3) found that samples

compacted dry of optimum picked up moisture, swelled, and

became more nearly saturated upon permeation; samples com-

pacted near optimum showed little change; samples wetter than

optimum did not behave consistently. Mitchell, et al. (5)

have also shown that variations in permeability of two to

three orders of magnitude may develop within relatively nar-

row ranges of compaction water content and density, especially

when compacted wet of optimum.

Dunn (1) discusses a point of permutation which is some

molding water content at which the permeability changes sig-

nificantly. He found that permeability of clay molded at a

water content less than the point of permutation is greater

than the permeability of a soil molded at a water content

greater than this point. This point of permutation is not

necessarily related to the optimum water content and may

vary by several per cent moisture on either side.

Sample mixing influences permeability values for clays.

The fines in a soil plug voids among the larger particles if

they are well distributed, thus decreasing the permeability.

Mechanical mixing breaks down the soil aggregates and

distributes the fines.

7

Mitchell and Younger (9) and Olsen (10) have reported

evidence of particle migrations in clays. Tests by Mitchell

and Younger (9) have sho'W!l the effect of particle migration

to be very sensitive to initial compacted density,and water

content, with the most significant changes developing in

soils at the lowest water contents and densities. Lambe (3)

suggests that as permeation occurs, particles tend to move

to positions of greater stability to seepage forces and that

this particle shifting always results in lower permeability.

Mitchell, et al. (5) found that thixotropic hardening

led to an increase in permeability. They suggest that this

is probably due to a change to a more flocoulent structure

on aging. Thixotropy is defined as the ability to gain

strength with time of rest after compaction while at con-

stant water content and density. In most experimental work,

the water content is changed soon after compaction, so that

thixotropic effects are difficult to measure.

Degree of Saturation

The study by Mitchell, et al. (5) showed an increase in

permeability with an increase in degree of saturation. This

increase in permeability amounts to a factor of as much as

four or five over ranges in saturation from 85 to 98 per cent.

It could be of importance when working with partially

saturated clays.

8

Fluid Viscosity

Values of permeability are customarily expressed at a

standard temperature, 20°C. A temperature correction is ap-

plied to the test permeability so as to have some standard

for comparing different permeabilities. Terzhagi and Peck (11)

state that in clays, temperature seems to have a greater in-

fluence on viscosity than it has in coarser soils and that

the average viscosity of pore water of clay appears to in-

crease with decreasing pore space.

Entrapped Air and Foreign Matter in Voids

Taylor (12) states that if the volume of entrapped gas

remains constant, decreases in permeability can possibly oc-

cur during the test because of migration of gas bubbles to

critical points in the pore channels. Also, he suggests

that if tap water is used, even though it be deaerated, it

may contain enough solid matter of microscopic size to plug

up pore passages and cause a decrease of permeability with

time.

TEST VARIABLES

In the determination of permeability of compacted clays

many experimental problems have been found. Some of these

are leakage, bacterial growth, local consolidation and swell-

ing of sample, and lack of linearity in flow velocity versus

hydraulic gradient.

Leakage

Mitchell, et al. (5) have found that undetected leakage

may easily account for an apparent permeability of 1 x 10- 8

cm/sec. They state that at low permeabilities, the slightest

leakage past a valve, or the slightest flaw in sealing the

sample in the permeameter can completely invalidate results.

Bacterial Growth

Gupta and Swartzendruber (13) have found that the growth

of bacteria within samples influences flow behavior. To

avoid this problem, some investigators attempted treating

the permeant with formaldahyde and other bacterial agents.

However, it is believed that the organic ions of those

agents may influence permeability in clays, since the

nature of the exchange ion influences the permeability of

soil.

Local Consolidation and Swelling

Even when a sample is confined during a test, local

consolidation and swelling may occur. Mitchell and

9

10

Younger (9) state that since the application of a hydraulic

gradient results in different pore pressures, and therefore

changed effective stresses, at different points along the

length of the sample, non-uniform void ratios may develop

within the sample. When swelling occurs in clays, there is

an increase in void ratio. Taylor (12) indicates that the

logarithm of the coefficient of permeability varies directly

with void ratio. Thus, if swelling occurs, there should be

an increase in the coefficient of permeability.

Linearity of Flow Versus Hydraulic Gradient

Dunn (1) suggests that there is little hope that the

permeability coefficient for compacted clays is indeed re-

lated to flow velocity and hydraulic gradient, as is assumed

for the purposes of his computations. Mitchell and Younger

(9) give a considerable amount of evidence to indicate de-

viations from Darcy's Law in many fine-grained soils when

they are subjected to low hydraulic gradients. However,

they suggest that some of this evidence may be questionable

because of undetected experimental error. Two possible

causes of this non-linearity suggested by Mitchell and

Younger (9) are abnormal water properties and migration of

particles.

Experiments by Miller and Low (14) were performed to

determine whether or not a threshold gradient for flow

actually exists in clays. They concluded that a threshold

11

gradient for flow could exist and that the water in those

clays could be classed as solid when subjected to gradients

less than the threshold value. However, in studies on

saturated kaolinite and saturated, compacted silty clay,

Mitchell and Younger (9) found no evidence of a threshold

gradient. They suggest that experimental difficulties may

be the cause of an apparent threshold gradient.

MATERIALS

The soil used in this study is a reddish-brown silty

clay commonly known as Cecil soil. It is derived from Colum-

bia granite under well-drained conditions in the gently

sloping Virginia Piedmont Upland. Kaolinite is the major

clay mineral with soil vermiculite and illite present in

small amounts. Some of the soil properties are summarized

in Table I.

The Cecil soil was chosen for study because it was

readily available and has a high clay content. The soil was

obtained for previous research work at V.P.I., and after

being air-dried, the larger chunks were ground up with the

use of a Los Angeles Abrasion Machine. It was then passed

through a No. 10 sieve (2 mm diameter) and placed in covered

metal containers in a dry environment.

12

13

Table I. Soil Properties (2, 15)

Properties

% Clay <2.tl

% Silt

1~ Sand

Liquid Limit

Plastic Limit

Plasticity Index

Max. Dry Density* pcf

Opt. Moisture*%

AASHO Classification

uses Classification

53

21

26

62

44

18

90.5

29.5

A-7-5 MH

*Equivalent to Standard AASHO

PROCEDURE AND DESCRIPTION OF TEST

The first part of the investigation was the determi-

nation of the permeability of 12 compacted clay samples using

the method described by Dunn (1), He compacted the clay

using the kneading compactor, quartered the clay extracted

from the mold along its longitudinal axis, and carved each

quarter on a soil lathe until the finished samples approxi-

mated the size of a Harvard miniature sample. Each sample

was then wrapped in Saran Wrap and dipped in Protex Coat for

a seal. When preparing the samples for permeability determi-

nation, the samples were removed from their coatings and each

end was broken off. The length and diameter were measured,

and the sample was seated in the sand and sealed with asphalt

in a glass tube. (See Fig. 1). The results using this pro-

cedure were erratic because of the author's inability to

create a good bond between the asphalt and the clay sample.

It was then decided to compact the clay in the glass tube

for the remaining part of this investigation. Twelve samples

were prepared by the procedure described below.

A calculated amount of distilled water was thoroughly

mixed with the soil to achieve a moisture content near opti-

mum. The mixture of soil and water was worked by hand until

no cluster of clay particles exceeded 1/8 inch in diameter.

A water content sample was taken to serve as a check before

14

Plastic Tube

Rubber Stoppers

Plastic Tube

15

,, I .··.. . ,. .

'~-~--~:-:/>-·:: -~i-~.:.) ·. : ...

·,. · .. · .. ·· ..

Water

Glass Cylinder

Asphalt Seal

Soil Sample

Wax Seal

Sand

Filter Paper

--------::•""'- Vacuum

Figure 1. Diagram of Pre--Formed Sample Preparerl for Permeability Testing.

16

compacting the soil. The soil was then allowed to cure over-

night in a sealed container.

The soil was compacted inside the permeameter as shown

in Figure 2. Problems involving an imperfect seal between

the permeameter and the soil sample thus were eliminated.

Eliminating seepage along the seal between the permeameter

and the sample is normally difficult under the high vacuums

used in this test.

The permeameter used in this experiment was a glass

cylinder with a diameter and length of approximately l½

inches and 5 inches respectively. The actual inside diameter

of the glass cylinder was obtained by taking the average of

several readings made with a micrometer. This diameter was

used to obtain the area used in permeability calculations.

The average length of the sample was taken after the comple-

tion of the test in order to calculate the hydraulic gradient

for determining permeability.

A pre-compaction water content sample was taken, since

there was no convenient way of obtaining a sample after com-

pacting the soil in the permeameter. A single hole rubber

stopper was used to seal one end of the glass tube with a

piece of filter paper placed over the stopper opening. Wet

sand was firmly compacted in two layers to a height equal to

approximately one-third the length of the :,tube. The permea-

meter was now ready for compaction to take place.

Plastic Tube

Rubber Stoppers

Plastic Tube

17

.. : . . ~-. '-::"~. -... •, _._._ •• 11-<1----. .. . ... .

. . ... •-" .. :. \· .: ·. ' .... ·. ·: .,. .. , ,,. . . .. ... · .... . .

.- ...

. ' . .,_ ........

Water

Filter Paper Glass Cylinder

Sand

Soil Sample

Sand

Filter Paper

'--------->- Vacuum

Figure 2. Diagram of Compacted-In-Mold Sample Prepared for Permeability Testing.

18

Forty grams of the prepared soil were weighed out by

adding soil to the glass permeameter until the total weight

bad been increased by 40 grams. The sample was then compacted

with a Harvard miniature compactor using 50 applications and

an 18 pound spring. Another 40 gram layer of soil was then

compacted using the same compactive effort. It was attempted

to distribute the points of application of tb.e compactor uni-

formly over the area. The average water content of the soil

prior to compaction was 32.6 per cent.

The compactive effort and compaction moisture content

were a result of observing the procedure that seemed to give

the best results. Difficulty in making accurate density

measurements precluded direct comparison with standard com-

paction procedure.

Wet sand was firmly compacted over the soil in two

layers until the glass cylinder was almost filled. Then a

rubber stopper with a filter covering the opening was used

to seal the open end of the permeameter, and the sample was

ready for testing.

The apparatus for the test (see Fig. 3) was set up be-

fore the samples were prepared, so as to reduce any loss in

water content. Ringstands, fly clamps, and arm clamps are

used to hold the permeameters, burettes, and pipettes in

place. One-eighth inch plastic tubing, plastic tees, and

rubber tubing serve the purpose of transmitting the flow of

.19

C

Burette · -Manometer

Permeameter

Ring Stand

""'l--::=======i~cj,~,~».:=~::c::::=~'..:::c:====71 To other

Permeameters

Vacuum Reservoir

,:;::::.==__,_ To Aspirator at Sink and to Pressure-Reducing Device

Figure 3. Sketch Showing :P,osi tion of Permeability Test Set-Up.

20

water and carrying the vacuum pressure. A Fisher Air Ejec-

tor aspirator was used to pull a vacuum of about 25 inches

of mercury below atmospheric pressure. rrhe aspirator was

connected to a tap water faucet and a lead carried the pres-

sure on to a five gallon jug, which was used as a reservoir.

From the reservoir, the plastic tubing carried the vacuum to

the sample and to an open end manometer which measured the

vacuum. A set of reducing nozzles (See Fig. 4) along with

one-half inch H-clamps was used to reduce the vacuum and

hold the pressure fairly constant at intervals down to a

vacuum of 2J inches of mercury. Thus, the hydraulic gradi-

ent could easily be changed any time during the test.

The full available vacuum (about 25 inches of mercury)

was gradually (in steps) applied to the samples so as to

vacuum-saturate them. Water was allowed to pass through

the samples from the burettes. The elevation head, the

vacuum pressure head, and tho volume of water passing

through the sample over periods of 24 hours were recorded.

The flow rate was calculated and plotted for each time. A

typical discharge-time curve is shown in Figure 5. When

the flow rate became constant, readings were taken and a

value for permeability was calculated using Darcy's Law.

To study the relationship between velocity and hy-

draulic gradient, the vacuum was dropped in increments to

obtain five different values of hydraulic gradient. The

Rubber Tubing

21

Plastic Tees

To Water Fauce r.---:::,- _ __._ _ ___._ ____ --,~

Aspirator

To Vacuum Reservoir

Figure 4. Sketch Showing Pressure-Reducing Device.

M ..c.::

s 0

C1) bD H t'tj .Cl 0 r/.l

•r-1 Cl

22

4

3 0

2

1

Q.,___ ___ ,__ __ __. ___ -.J. ___________ _

50 100 Elapsed Time - hours

-150

Figure 5. Typical Discharge-Time Curve.

2';

flow rate usually became constant after about five days, so

the pressure was changed about every seven days in this ex-

periment. The permeability, flow velocity, and hydraulic

gradient were calculated for each pressure level, and the

relationship between them was plotted on arithmetic graph

paper. The straight-line fit was attained by applying the

method of least squares.

RESULTS AND DISCUSSION

The vacuum permeability test devised by Dunn (1) and

altered somewhat by the author has been investigated and

found to give adequate values of coefficient of permeabili-

ty for most experimental work on compacted clays. The as-

smnption of a linear relationship between flow velocity and

hydraulic gradient used in calculating the coefficient of

permeability is found to exist, as shown in Figures 6 - 9.

The main reason for the different slopes (coefficients

of permeability) given by the curves in Figures 6 - 9 is

believed to be due to differences in compactive effort, and

thus compaction density, from one sample to another. Opera-

tor inexperience together with the nature of the Harvard

miniature compactor are likely to lead to differences in

compactive effort in this study, even though the number of

blows per layer was held constant. There was no attempt to

check the compacted density of the test specimens, since

this might interfere with the permeability test procedure.

Walker, et al. (2) and Dunn (1) ran their tests by ap-

plying the full available vacuum (approximately 25 inches of

mercury) from a water faucet aspirator and calculating the

coefficient of permeability after assuming a linear relation-

ship between flow velocity and hydraulic gradient. The co-

efficient of permeability calculated from readings at this

full available vacuum is consistently the truest value as

24

\.0 0 ,-{

0 (l) rn

'-..... s 0

>, .p ·r-i 0 0

,-{ (l)

::> I ;:

0 ,-{

20

15

10

5

0

8

/ /

Sample 1

Sample 3

Sample 6

/y· //

25

,,.,.

0 // ,, /

100 200

Hydraulic Gradient

Figure 6. Flow Velocity Versus Hydraulic Gradient.

300

26

20 0 Sample 2

Sample 11 I..O

0 8 Sample 7 ,-j

C) 15 (l) r/).

'---..,. s 0

>, .µ 10 •rl 0 0

..--! (l)

>-0

..--! 5 i:-x..

0 100 200 300

Hydraulic Gradient

Figure 7. Flov, Velocity Versus Hydraulic Gradient.

20

I.O 0 ,--l

0 15

Q) 00

"---s 0

>, .p 10 •r-i 0 0

,--l Q)

P-;::: 0

,-l 5 i:...

27

0 Sample 10 Sample 9

8 Sample 5

· 100 200 300 Hydraulic Gradient

Figure 8. Flow Velocity Versus Hydraulic Gradient.

28

40 0 Sample 4

D Sample 8 I.D

0 8 Sample 12 ,-;

0 30 / (I) [/J

'---... s 0

>, +:> ·r-i 20 0 0 ,-; (I)

::>

0 ,-;

10

/

0 ~//

100 200 300

Hydraulic Gradient

Figure 9. Flow Velocity Versus Hydraulic Gradient.

29

compared with those calculated for lower hydraulic gradients

(See Table II),

The available vacuum from the water faucet aspirator bas

been used to vacuum-saturate aggregate. Thus, it is reason-

able to expect that it effectively saturates compacted clay

as well. Moisture contents of several samples were taken be-

fore and after the test. After assuming reasonable values

for dry density and specific gravity of solids, the change

in degree of saturation during the test was found to be an

increase of from 8 to 14 per cent. Since clays compacted

near optimum moisture content are about 90 per cent saturated,

the samples in this test are believed to become saturated.

Because of the nearly steady flow that exists after three or

four days, it is believed that saturation occurs after this

small lapse in time.

The test is acceptable because such factors that may

present experimental problems such as leakage, capillary

effects, entrapped air and swell do not seem to produce ap-

preciable error. A characteristic of leakage of permeant

during vacuum permeability testing is that leaks become pro-

gressively worse. Thus, flow is not observed to be steady,

but flow rate increases with time. Such increases were not

detected during the final part of this investigation in

which the sample was compacted in the permeameter. However,

these increases were detected when using the pre-formed

30

Table II. Comparison of Coefficients of Permeabilities for Twelve Samples

Sample Vac. kl F.H.k.J Vac. k Vac.k No. cmLsec x 108

H. Vac. k28 cmLsec x 10 cmLsec x 10 8 H.Vac k F.H.k

1 6.09 7.13 12.80 0.85 0.48 2 7.06 7.30 9.05 0.97 0.78 3 6.27 6.41 0.98 4 15.86 14.80 12.90 1.07 1.23 5 2.74 2.98 4.65 0.92 0.59 6 3.75 3.92 5.50 0.94 o.6s 7 3.88 4.06 5.50 0.96 0.71 8 10.11 10.50 9.15 0.96 1.11 9 3.51 3.77 0.93

10 4.97 5.23 6.98 0.95 0.71 11 6.14 6.62 8.92 0.93 0.69 12 7.00 7.12 8.88 0.98 0.79

Ave;. t>.42 ii.l>5 8.43 0.90 0.77 1Denotes vacuum permeability as determined by slope of

flow velocity-hydraulic gradient curve. 2Denotes vacuum permeability as determined by obtaining

flow rate at about 25 inches of mercury below atmospheric pressure and assuming a linear relationship of flow velocity versus hydraulic gradient.

3nenotes permeability as determined by falling head test.

31

samples and an asphalt bond. It is believed that this was

due to the author's inability to create a good bond with

asphalt when preparing the samples for testing.

Anticipated difficulties of capillary effects and per-

meability reduction by entrapped air do not introduce appre-

ciable error as shown by the linearity of flow velocity-

hydraulic gradient curves and the general agreement of test

results with those not using a vacuum.

Swelling was not believed to cause an increase in per-

meability during this test, since the sample was confined,

and since there was no trend established of an increase in

permeability with time as might be expected. Kaolinite

clays are known to have a low swell potential, and kaolinite

is the major mineral in the Cecil soil used in this study.

Dunn's (1) method of using the asphalt bond with a pre-

formed sample would allow for more lateral swell than the

method used in this study since the asphalt would allow for

more expansion than glass.

Sand was compacted on top of the sample for the test.

This compacted sand could act as a filter for foreign matter,

and by compacting the sand on both sides of the specimen the

flow direction could be reversed.

The data would have been more consistent if a correction

for fluid viscosity had been applied. In Table III, the data

for coefficient of permeability is shown to be more

32

Table III. Effects of Permeant Temperature on Coefficient of Permeability for a Typical Sample.

Temperature Permeability Permeability T at T°C, kT at 20°0, k20°C

in "C in cm/sec x 108 in cm/sec x 108

22.5 4.06 3.83

21.0 3.68 3.60

23.2 4.02 3.72

23.2 3.92 3.63

24.0 4.78 4.35

33

consistent for a single sample after a correction is applied

for the change in viscosity of water due to temperature. The

difference in extreme values for the uncorrected coefficient

of permeability is 1.09, and the difference in extreme values

for corrected coefficient of permeability is 0.75. This cor-

rection was not applied to the data compiled for Figures

6 - 9. This inexpensive test has proven to be a very rapid

test. Within four or five days after compacting the clay in

the permeameter, a value of coefficient of permeability can

be obtained that is adequate for most experimental work on

compacted clays.

Since hydraulic gradients of almost 300 were used in

this test, and hydraulic gradients in the field seldom ex-

ceed unity, the laboratory test results should be applied

to field conditions with caution. There is need for a per-

meability test of clay actually compacted in the field so

a comparison can be made with laboratory results.

A falling head test was run for comparison purposes.

The results of the falling head test showed the same trend

of values among different samples as the vacuum test. How-

ever, the average coefficient of permeability obtained using

the vacuum test was about eight-tenths the average coeffi-

cient obtained using the falling head test. Experimental

errors are more pronounced at lower hydraulic gradients in

34

all tests on compacted clays. It was difficult to hold the

vacuum constant at lower gradients, and the measurement of

bead may have caused some error while applying these gradi-

ents. It is also believed that smaller diameter standpipes

should have been used when running the falling head test to

reduce error. Comparisons of vacuum test results and fall-

ing head test results are shown in Table II.

CONCLUSIONS

1. This rapid, inexpensive test gives adequate values of

coefficient of permeability for most experimental work

on compacted clay.

2. There exists a linear relationship between flow velocity

and hydraulic gradient for the soil studied and the test

employed.

3. The coefficient of permeability obtained with maximum

vacuum application (approximately 25 inches of mercury)

is consistently truer than that found for percolation

under reduced vacuums.

4. Anticipated experimental errors such as capillary ef-

fects, entrapped air and foreign matter, and undetected

leakage do not introduce appreciable error as indicated

by the linearity of the flow velocity-hydraulic gradient

curves, the constant nature of flow rate and the general

agreement of vacuum test results with those obtained

from the non-vacuum falling head test.

35

BIBLIOGRAPHY

1. Dunn, H. c., "The Effect of Lime Stabilization on the

Permeabilities of Two Virginia Clays," Master of

Science Thesis, Virginia Polytechnic Institute, De-

partment of Civil Engineering, 1966.

2. Walker, R. D., Krebs, R. D. and Esmer, E., "Effect of

Freezing and Thawing on the Strength, Permeability,

and Pore Characteristics of Lime Stabilized Soils,"

Highway Research Record No. 198.

3. Lambe, T. W., "The Permeability of Compacted Fine-

Grained Soils," Special Technical Publication No.

163, ASTM, 1954, PP• 56-67.

4. Bjerrium, L. and Buder, J., "Measurement of the Permea-

bility of Compacted Clays," Proceedings, Fourth Inter-

national Conference on Soil Mechanics and Foundations

Engineering, London, Vol. I, 1957, pp. 6-10.

5. Mitchell, J. K., Hooper, D.R., and Campanella, R. G.,

"Permeability of Compacted Clay," Journal of Soil

Mechanics and Foundations Division, ASCE, Vol. 91,

No. SM4, July, 1965, pp. 41-65.

6. Lambe, T. w., Soil Testing for Engineers, John Wiley

and Sons, Inc., New York, 1967, pp. 52-62.

36

37

7. Lambe, T. w., "The Engineering Behavior of Compacted

Clays," Journal of Soil Mechanics and Foundations Di-

vision, ASCE, Vol. 84, No. SM2, Proc. Paper 1655,

May 1958.

8. Seed, H.B. and Chan, c. K., "Structure and Strength

Characteristics of Compacted Clays," Journal of Soil

Mechanics and Foundations Division, ASCE, Vol. 85,

No. SM5, Proc. Paper 2216, October 1959.

9. Mitchell, J. K. and Younger, J. s., "Abnormalities in

Hydraulic Flow Through Fine-Grained Soils, 11 Symposium

on Permeability and Capillarity, 69th Annual Meeting

of American Society for Testing and Materials, Atlan-

tic City, New Jersey, July 1966.

10. Olsen, H. W., "Deviations :from Darcy's Law in Saturated

Clays," Proc. Soil Science Society of America, 1965,

pp. 135-140.

11. Terzaghi, K. and Peck, R. P., Soil Mechanics in Engi-

neering Practice, John Wiley and Sons, Inc., New York,

1967, pp. 46-47.

12. Taylor, D. w., Soil Mechanics, John Wiley and Sons, Inc.,

New York, 1965, pp. 119-122.

38

13.- Gupta, R. P. and Swartzendruber, D., "Flow-Associated

Reduction in the Hydraulic Conductivity of Quartz Sand,"

Proc. Soil Science Society of America, 1962, pp. 6-10.

14. Miller, R. J. and Low, P. F., "Threshold Gradient for

Water Flow in Clay Systems," Proc. Soil Science Society

of America, Vol. 27, No. 6, Nov.-Dec. 1963, pp. 605-609.

15. Certain Properties of Selected Southeastern Soils and

Mineralogical Procedures for Their Study, Southern

Cooperative Series Bulletin 61, p. 23, 1959.

The vita has been removed from the scanned document

A VACUUM PERMEABILITY TEST FOR COMPACTED CLAY

by

Elmer Franklin Hart

Abstract

A vacuum permeability test utilizing high hydraulic

gradients has been devised for compacted clay of low permea-

bility. The test induces easily measurable flow rates in

virtually impervious soils by placing a vacuum at the drain-

age end of the sample and an elevation head at the inflow

end. The apparatus used in the test could easily be con-

structed with materials commonly available in laboratories

for routine soil tests. The sample can either be compacted

in the permeameter (a glass cylinder) or can be seated in

sand and sealed with an asphalt bond.

The anticipated difficulties, capillary effects, un-

detected leakage, and permeability reduction by entrapped

air, do not introduce appreciable error as shown by the

linearity of flow velocity-hydraulic gradient curves and the

general agreement of test results with those obtained from

a falling bead, non-vacuum test.

Relative coefficients of permeability can be obtained

within a few days after the start of the test. It is con-

cluded that this rapid, inexpensive method gives adequate

values of coefficient of permeability for most experimental

work on compacted clays.