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Highway Research Board Bulletin, 135, pp. 109-118, 1956
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Soil moisture movement during ice segregation
Penner, E.
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N A T I O N A L R E S E A R C H C O U N C I L
C A N A D A
SOIL MOISTURE MOVEMENT
DURING
ICE
SEGREGATION
BY
EDWARD PENNER
RESEARCH PAPER NO. 32
OF THE
DIVISION OF BUILDING RESEARCH
REPRINTED FROM HIGHWAY RESEARCH BOARD
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Soil Moisture Movement During Ice Segregation
EDWARD PENNER, Assistant Research Officer
Division of Building R e s e a r c h , National Research Council of Canada
This paper r e p o r t s the initial r e s u l t s of a continuing r e s e a r c h study of f r o s t action p r o c e s s e s in soil undertaken recently by the Division of Building R e - s e a r c h of the National Research Council of Canada.
Moisture flow to the freezing zone of s m a l l soil specimens has been m e a s - ured under controlled laboratory conditions. The experimental findings a g r e e with the now generally accepted concept of f r o s t heaving outlined by T a b e r and Beskow.
In soil s y s t e m s where a f r e e water s u r f a c e exists n e a r but below the freezing zone, the r e s u l t s indicate that m o i s t u r e flow is dependent on t h e unsaturated permeability and soil m o i s t u r e tension characteristics. Both these soil p r o p e r t i e s integrate the effect of grain s i z e s t r u c t u r e , clay c o m - position, and exchange ions among others.
C r i t i c a l desaturation beneath the f r o s t line in s o m e s o i l s appears to a c t a s a b a r r i e r to liquid m o i s t u r e transmission. Since in the heavier-textured s o i l s , rapid moisture t r a n s m i s s i o n continues to much higher tensionvalues, g r e a t e r heave r a t e s a r e observed.
.SOME s o i l s when subjected t o naturally o c c u r r i n g freezing t e m p e r a t u r e s display c e r - tain f e a t u r e s undesirable f r o m an engineering standpoint. Uniform and differential heav- ing due to i c e segregation and l o s s of skrength upon thawing a r e perhaps the m o s t notable. T h i s behavior often r e s u l t s in the deterioration of road and a i r p o r t s u r f a c e s , destruc- tive action on r a i l r o a d grades, and bad effects upon building foundations.
Much of the knowledge about f r o s t action p r o c e s s e s s t e m s f r o m the e a r l y work of T a b e r (1) and Beskow (2). More recently Haley e t a1 (3), Ruckli (4) and J u m i k i s (5, 6) have maae notable contrybutions. In his review of l i t e r s t u r e , ~ o h n c o n (7) h a s d r a w n af- tention to the m a s s of useful information in allied fields, particularly s G l s c i e n c e , available to the student of f r o s t a c t i o n .
The g e n e r a l concept of the f r o s t action phenomenon in s o i l s appears to be well estab- lished and i s available in the literature. T o a l a r g e extent f r o s t action c r i t e r i a now used in engineering have developed f r o m this g e n e r a l concept. In view of this, it appears that if any improvement in f r o s t action c r i t e r i a i s to be expected, more detailed infor- mation on the relationship between the physico-chemical p r o p e r t i e s of the s o i l and de- structive f r o s t action i s required.
A s part of a long-term study of frost action in s o i l s by the Division of Building Re- s e a r c h of the National Research Council of Canada, some initial laboratory studies a r e concerned with the nature of the moisture flow to the freezing zone. Soil m o i s t u r e po- tentials and subsequent moisture flow a r e known to result f r o m the phase change of s o i l moisture during ice segregation. In nature, t h i s p r o c e s s i s complicated by t h e hetero- geneity of the soil, the complex heat flow p a t t e r n , and the unknown s t a t u s of t h e water supply. Except in i t s s i m p l e s t f o r m , it does not lend itself to e a s y simulation in the laboratory.
The amount of moisture available f o r ice segregation may be "limited" and this is
commonly r e f e r r e d to in t h e l i t e r a t u r e a s a "closed" system. In the ''open" system water i s supplied continuously f r o m a shallow water table to the freezing zone by the mechanism of "unsaturated" liquid flow. It is with the l a t t e r condition, that i s , the open s y s t e m , under which extremely s e r i o u s heaving may occur, that this paper i s concerned.
The t e r m "unsaturated" is defined in a number of ways in the literature. A s used h e r e i t does not r e f e r to the completeness with which the s o i l p o r e s a r e filled with water but r a t h e r denotes that a s t a t e of s o i l water tension exists. A t "saturation" t h e tension i s z e r o , although in the c a s e of light textured s o i l s the s o i l p o r e s may be partially filled with air. Heavy clay soils may remain completely filled with water, even when subjected to relatively high tensions, but a r e accordingly "unsaturated". To avoid any ambiguity, the d e g r e e to which the s o i l p o r e s a r e filled with a i r may be expressed in t e r m s of
void volume (8). Although "saturation" i s not used h e r e in the usual engineering s e n s e i t i s n e c e s s a r y to base the definition on conditions of soil water tension in o r d e r to ex- plain water movements by energy concepts.
METHODS AND MATERIALS Description of F r o s t Action Apparatus
The apparatus shown in F i g u r e 1 c o n s i s t s of two insulated metal tanks which a r e in- dependently mounted on portable f r a m e s . Each tank contains approximately 25 gallons of ethylene-glycol water mixture which i s pumped a t a constant r a t e t o t h e headers on the f r o s t c e l l frame. A s e r i e s of adjustable valves on the h e a d e r s p e r m i t s the condition- ing liquid to s e r v e four independent f r o s t c e l l s . The r a t e of liquid flow through the
@
FROST C E L L
@
HEADERS
@
M l
LL l VOLT RECORDER
@
CONSTANT TEMPERATURE TANK
@
CONTROL P A N E L
@
CONDENSING U N I T
@
P R E A M P L I F I E R
F i g u r e 1. F r o s t c e l l a p p a r a t u s .
headers f a r exceeds the withdrawal f o r the f r o s t cells. T h u s additional f r o s t cells m a y be mounted without greatly affecting the flow through o t h e r s in which t e s t s a r e already in p r o g r e s s . In addition, the rapid flow a c t s as a s e l f - s t i r r e r in the liquid bath. A
s e p a r a t e condensing unit mounted beneath the tank i s capable of keeping t h e liquid mix below the operating temperature. The d e s i r e d liquid t e m p e r a t u r e in the tanks i s achieved by heating the chilled liquid. The amount of reheat r e q u i r e d (a function of the ambient t e m p e r a t u r e and the number of frost c e l l s withdrawing conditioned liquid f r o m the system)
is controlled by an electronic t e m p e r a t u r e c o n t r o l l e r . The m a i n t h e r m o s t a t , consist- ing of two 500-ohm sensing e l e m e n t s , is placed in the flow path a t the outlet f r o m the tank. One 500-ohm t h e r m o s t a t i s mounted a f t e r the h e a t e r to p r e v e n t overshooting of
t h e operating t e m p e r a t u r e . Any unbalance in the electronic c o n t r o l l e r due t o a tem- p e r a t u r e change of the liquid c a u s e s the operation of a modutrol motor which is
attached to a v a r i a c for control of the e l e c t r i c a l c u r r e n t supply to the i m m e r - s i o n heater. With this a r r a n g e m e n t it has been possible to maintain the d e s i r e d tem- p e r a t u r e within ? 0.10 deg C f o r long p e r i o d s . F o r s h o r t periods of twelve hours t h e t e m p e r a t u r e s c a n easily be maintained within
+
0 . 0 5 d e g C .T h e f r o s t c e l l shown in F i g u r e 2 and d i a g r a m m a t i c a l l y in Figure 3, which con- t a i n s the s o i l s p e c i m e n , c o n s i s t s e s s e n - t i a l l y of t h r e e c o m p a r t m e n t s . T h e liquid in compartment 1 conditions the upper one-third of the s o i l specimen a n d i s the cold sectiori of t h e specimen. C o m p a r t - m e n t s 2 and 3 condition the lower two-
t e n s o m e t e r , (2) c o m p a r t m e n t 1, (3) c o m - t h i r d s of the s p e c i m e n and m a k e up the partment 2, (4) compartment 3. w a r m section. T h e s a m p l e r e s t s on a
p o r o u s c e r a m i c plate which p e r m i t s control of the water tension in the specimen. A horizontal conduit through c o m p a r t m e n t 2 p e r - m i t s thermocouple placement in t h e specimen below the f r e e z i n g zone. F o r t e m p e r a - t u r e m e a s u r e m e n t in the frozen portion, thermocouples enter compartment 1 through the s t e m of the b r a s s plunger. An e x t e n s o m e t e r i s mounted on the c e l l that m e a s u r e s the relative movement of the plunger during i c e l e n s growth. T h e f r o s t c e l l is made of lucite except f o r the b r a s s plate which holds t h e porous plate and the b r a s s i n n e r wall supporting the lower portion of t h e specimen. T h e inlets and outlets to the c o m p a r t - ment a r e s e t oblique to the p e r i p h e r y of the c e l l but p a r a l l e l to each other to facilitate even liquid circulation and good heat exchange.
A continuous t e m p e r a t u r e r e c o r d was obtained with a s p e c i a l l y built, high precision millivolt r e c o r d e r . Copper-constantan thermocouples and i c e w a t e r r e f e r e n c e junctions w e r e used in t h e s e experiments. T h e a c c u r a c y of the t e m p e r a t u r e m e a s u r e m e n t s with the above a r r a n g e m e n t w a s much g r e a t e r than the d e g r e e of t e m p e r a t u r e c o n t r o l over the conditioning fluid. T h e f r o s t c e l l was designed to give a s h a r p t e m p e r a t u r e gradient a c r o s s the f r e e z i n g zone with a constant t e m p e r a t u r e spanning the main body of the specimen. T h i s w a s d e s i r a b l e if flow r a t e s c a u s e d only by tension gradients w e r e to be evaluated.
Soils and Specimen P r e p a r a t i o n
T h e s o i l s used in t h e s e e x p e r i m e n t s consisted of artificially blended m i x e s . Leda c l a y , a local m a r i n e deposit, w a s modified with s i l t obtained f r o m n a t u r a l d e p o s i t s a t Whitehorse and Uranium City. Leda clay and Whitehorse s i l t w e r e a i r - d r i e d , crushed, passed through a 200 s i e v e and mixed in a r b i t r a r y proportions f o r s a m p l e s 1 and 2. Samples 3, 4 and 5 w e r e m i x t u r e s of Leda c l a y (minus 200) and Uranium City s i l t (minus 325) p r e t r e a t e d in the s a m e way. Sample 6 w a s Uranium C i t y s i l t (minus 325).
The f r o s t c e l l w a s designed t o hold a c y l i n d r i c a l specimen
l5L6
inch in d i a m e t e r and 2.816 inches long. T h i s is the s i z e of a s a m p l e when molded in the M i n i a t u r e Harvard Compaction Apparatus (9). S o m e difficulty w a s encountered however, with s a t i s f a c t o r y thermocouple placement-in a premolded specimen. In p r a c t i c e the s o i l s p e c i m e n s w e r e compacted in the f r o s t c e l l in liZ-inch l a y e r s to a density of 1. 33 gm/cm3. A thermo- couple was placed between each layer. A f t e r placing the f i r s t two l a y e r s , a silicone- coated thin lucite s l e e v e , 11/4 inch inside d i a m e t e r and 2 inches long, was lowered intoTHERMOCOUPLE CONDUIT COMPARTMENT
@
THERMOCOUPLE THIN LUCITE WATER INLET I-
F i g u r e 3. S e c t i o n t h r o u g h f r o s t c e l l .the s p e c i m e n holder. T h e s o i l s p e c i m e n w a s p e r m i t t e d to f r e e z e to t h i s s l e e v e but heaving was r e s t r i c t e d only by t h e f r i c t i o n between the o u t s i d e w a l l s of the s l e e v e and the inside wa1.l of t h e s p e c i m e n holder. The f r i c t i o n plus the lea,-1 of the e x t e n s o m e t e r s t e m and b r a s s plunger amounted t o a p p r o x i m a t e l y 30 g m / c m 2 and w a s c o n s t a n t f o r a l l t e s t s . T h e r e m a i n i n g f o u r l a y e r s of s o i l w e r e p l a c e d inside t h e s l e e v e to b r i n g the s p e c i m e n height to 3 inches.
In t h e s e e x p e r i m e n t s distilled w a t e r w a s allowed to e n t e r t h e b a s e of t h e s p e c i m e n a t r o o m t e m p e r a t u r e and z e r o tension f r o m a c o n s t a n t head d e v i c e b e f o r e f r e e z i n g w a s begun. When no f u r t h e r water w a s withdrawn, i t w a s c o n s i d e r e d that the e q u i l i b r i u m condition had b e e n attained. N o r m a l l y this took about twelve h o u r s f o r t h e h e a v i e r - t e x t u r e d s o i l s .
Soil F r e e z i n g P r o c e d u r e
Upon completion of the s a t u r a t i n g p r o c e s s , t h e conditioning fluid w a s c i r c u l a t e d through the two c o m p a r t m e n t s t h a t condition t h e l o w e r t w o - t h i r d s of the s p e c i m e n until an equilibrium t e m p e r a t u r e w a s r e a c h e d . F o r t h e s e e x p e r i m e n t s an a r b i t r a r y t e m p e r - a t u r e of a p p r o x i m a t e l y
+
1 . 3 d e g C w a s used which took s o m e two h o u r s to attain. C i r -F i g u r e 4. Mechanical a n a l y s i s o f s o i l s . SAMPLE NO. I
-
SAMPLE NO. 2 SAMPLE N 0 . 3 SAMPLE NO. 4 SAMPLE NO. 5 G R A I N S I Z E (MMJculation of t h e cold liquid a t a t e m p e r a t u r e of -6 d e g C was then begun through c o m p a r t - m e n t 1, conditioning the top inch of t h e 3-inch s p e c i m e n . T h e t e m p e r a t u r e of the con- ditioning liquids w a s held constant (within the l i m i t s of t h e t e m p e r a t u r e c o n t r o l s y s t e m ) throughout t h e e x p e r i m e n t . Supercooling was a l w a y s evident but p a r t i c u l a r l y i n the h e a v i e r - t e x t u r e d s o i l s . In the f i r s t few e x p e r i m e n t s c r y s t a l l i z a t i o n was p e r m i t t e d to begin n a t u r a l l y but b e t t e r reproducibility w a s a t t a i n e d by inducing c r y s t a l l i z a t i o n a t a fixed t e m p e r a t u r e . A s s t a t e d b e f o r e , a continuous t e m p e r a t u r e r e c o r d m a d e i t possible to follow t h e p r o g r e s s of t h e f r e e z i n g zone in t h e s p e c i m e n . T h e amount of m o i s t u r e flow into the s a m p l e and extent of heaving w e r e m e a s u r e d a t f r e q u e n t i n t e r v a l s . In all
the e x p e r i m e n t s r e p o r t e d h e r e a f r e e w a t e r s u r f a c e was maintained l e v e l with the b a s e of t h e s p e c i m e n . T h e s m a l l hydraulic head l o s s c a u s e d by t h e p o r o u s p l a t e a t t h e b a s e of the s p e c i m e n i n t h e s e e x p e r i m e n t s was c o n s i d e r e d to b e negligible.
Unsaturated P e r m e a b i l i t y and M o i s t u r e - C o n t e n t / ~ e n s i o n D e t e r m i n a t i o n s
T h e g e n e r a l c h a r a c t e r i s t i c s of the r e l a t i o n s h i p between m o i s t u r e content a n d m o i s t u r e suction o r tension f o r s o i l s and t h e i r significance a r e well known. I t h a s b e e n shown in
COARSE F I N E U N D
2 4 6 2 4 6 8 T I M E ( H R )
F i g u r e 5. R a t e o f m o i s t u r e f l o w i n d u c e d by i c e s e g r e g a t i o n a s a f u n c t i o n o f t i m e .
the past that t h e r e is h y s t e r e s i s between the wetting and drying conditions, the sample being d r i e r at a given suction when that suction i s approached f r o m the wet condition than when i t i s approached f r o m the d r y condition.
The f r e e z i n g p r o c e s s , c a r r i e d out a c - cording to the method d e s c r i b e d above, i s essentially a drying p r o c e s s . T h e r e - f o r e i t was n e c e s s a r y to determine only t h e drying moisture-content/tension r e - lationship. T h i s was done b y the usual porous plate method. A s o i l i s f i r s t p e r - mitted t o wet against z e r o w a t e r tension on a porous c e r a m i c plate. The plate is
then conditioned to various tensions using a i r p r e s s u r e . Moisture contents w e r e determined b y oven drying a t 1 1 0 deg F a f t e r equilibrium conditions had b e e n attained a t the d e s i r e d moisture tensions.
A modified method of the technique de- s c r i b e d by R i c h a r d s (10) w a s used to d e - t e r m i n e the u n ~ a t u r a t e d p e r m e a b i l i t ~ of the soil. B r i e f l y the p r o c e d u r e i s to c r e a t e known moisture-tension gradients
in a s m a l l s o i l specimen and to m e a s u r e the flow r a t e s under steady-state condi- tions. D a r c y ' s law, v = ki, which h a s been shown to apply also to unsaturated flow, p e r m i t s the calculation of s a t u r a t e d p e r m e - ability coefficients kuf. T h e s e a r e usually presented graphically a s a function of either tension o r m o i s t u r e content.
EXPERIMENTAL RESULTS , 3 2 1 o 1 2 3 4 3 2 1 o 1 2 ~ 4 ~ 2 1 0 1 2 ~
AND DISCUSSION DEGREES c
The hydrometer analyses f o r the s o i l s /I / I I l l 1 1
W
a r e shown in F i g u r e 4. T h e g r a i n - s i z e 6 J 5
. SAMPLE 1 SAMPLE I-! S k Y P L E 1-2
distribution c u r v e s a r e f a i r l y s i m i l a r f o r s a m p l e s 1 and 2 and f o r s a m p l e s 4 and 5. These c u r v e s i l l u s t r a t e the reproducibility of s a m p l e p r e t r e a t m e n t such a s crushing, sieving, and mixing.
F i g u r e 5 gives the r a t e of moisture movement into the s a m p l e v e r s u s time
in hours induced by i c e segregation f o r -1-J-2-I o I 2 J - 1 - 5 - 2 - 1 o , 2 J - 4 - 3 - 2 - 1 O 1 2 3
s a m p l e s 1 t o 5. No heaving occurred in DEGREES c
6. The curves have the same F i g u r e 6.. T e m p e r a t u r e d i s t r i b u t i o n i n t h e
general f o r m although the s t a r t i n g t i m e s s o i l 1 5 ,,,inUtes p r i o r t o a n d 1 5 m i n u t e s
a f t e r t h e maximum h e a v e r a t e .
' k U = v
( s l
",,
1
v = linear velocity of flow in cm/hr. L = length- of sample in cm.
S1 and SZ = the respective tensions at the two f a c e s of the specimen e x p r e s s e d in c m of water
vary. This i s largely due t o the lack of control over the amount of supercooling
and the initiation of crystallization. The reduction in r a t e of heave with t i m e i s caused by many factors, some of them not fully understood. One of these is the r e - duction in heat flow a s the z e r o isotherm penetrates the s o i l specimen. Temperature distributions in the soil specimens that existed 15 minutes prior to and 1 5 minutes following the maximum heave r a t e a r e shown in Figure 6. These graphs also show the penetration (if any) of the 0 deg C iso- t h e r m during this time.
A comparison of the grain-size distri-
bution curves with the maximum r a t e of - -
heave shows that the heavy textured soils F i g u r e 7 . Sample 2 , showing i c e s e g r e g a -
heaved a t the greatest rate. In heavy soils t i o n b e g i n n i n g w i t h n o e v i d e n c e o f s o i l
such a s sample 5 the limiting f a c t o r appears S e p a r a t i o n and p r o g r e s s i n g t o a l m o s t c o n A t i n u o u s i c e . ( T o t a l h e a v e f o r 12 h o u r
to be the r a t e of heat removal. Further
p e r i e d w a s 1,2 i n . ) t o p o f sample,
evidence was obtained on this point when ( 2 ) p o i n t o f maximum r a t e o f h e a v e , a n d sample 5-2 was treated similarly in all r e - ( 3 ) f i n a l f r o s t l i n e .
s p e c t s to sample 5-1 with the exception of
the temperature on the cold side. This was increased from -6 deg to -3 deg C on the sample 5-2 which reduced the maximum heaving r a t e fromO.l to 0.75cm/hr (Figure 5).
Examination of the frozen samples showed that for all s a m p l e s no d i s c r e t e ice lensing was evident a t the position of the z e r o isotherm during the maximum heaving rate. As the f r o s t line penetrated the sample, the separation of soil particles increased, gradually blending into solid ice. At the stage where a c l e a r ice lens was growing, the ratedid not become completely constant but progressed a t a decreasing r a t e . The general nature of the distribution of ice i s shown in Figure 7.
200 400 600 200 400 GOO 200 400 GOO T E N S I O N
-
C M WATERF i g u r e 8. R e l a t i o n s h i p between m o i s t u r e t e n s i o n and m o i s t u r e con- t e n t f o r a l l s a m p l e s .
The question a r i s e s why no heaving o c c u r r e d in s a m p l e 6. Figure 8, which gives the moisture-content/tension c u r v e s f o r a l l s o i l s , shows that the m o i s t u r e content of s a m p l e 6 was approximately 30 percent when freezing was initiated. The unsaturated p e r m e a b i l i t y f o r this s o i l , shown in F i g u r e 9 , d e m o n s t r a t e s i t s ability to p a s s water a t
low tensions. In fact a comparison of the unsaturated permeability coefficients a t low tensions with the flow r a t e s in the f r e e z i n g e x p e r i m e n t s shows that a n i n v e r s e c o r r e l a - tion e x i s t s f o r the s o i l s studied. T h e s e e x p e r i m e n t s s u g g e s t that the crystallization of the s o i l m o i s t u r e i s able t o c r e a t e a sufficient tension to c a u s e a high d e g r e e of d e s a t -
TENSION - C M WATER
F i g u r e 9. U n s a t u r a t e d p e r m e a b i l i t y c o e f f i c i e n t s a s a f u n c t i o n o f t e n s i o n .
uration immediately beneath the freezing zone. F o r the l i g h t e r textured s o i l s (such a s s a m p l e s 3 and 6) this m e a n s a g r e a t reduction in m o i s t u r e content ( F i g u r e 8), t h e r e f o r e , a low unsaturated permeability, and consequently a s m a l l heaving r a t e a s in sample 3 .
o r no heaving a s in s a m p l e 6.
Other s t u d i e s support the belief that c r i t i c a l desaturation beneath the f r o s t line m a y a c t a s a b a r r i e r to m o i s t u r e movement in l i g h t e r textured s o i l s . R e s e a r c h workers in s o i l s c i e n c e have shown t h e i n v e r s e relationship of u n s a t u r a t e d p e r m e a b i l i t y coefficients
and s o i l moisture tension (10). In moisture flow studies of the Division of Building R e s e a r c h , the s o i l m o i s t u r e a t one face of a s o i l specimen was held at z e r o tension(Sz) and the r a t e of flow was m e a s u r e d with increasing tension up to 1000 c m w a t e r on the opposite f a c e (S,). Figure 10 shows that f o r a predominantly s i l t y s o i l t h e r e i s an in- c r e a s e of flow with tension which r i s e s to a maximum and then rapidly r e d u c e s to a low value. F o r c l a y s the maximum flow r a t e i s lower but the-ability to t r a n s m i t w a t e r ex- tends to higher tension values. T h i s a p p e a r s t o be the explanation for the higher r a t e of heave observed in these experiments f o r clayey s o i l s a s compared to c o a r s e r grained soils. It a l s o explains how a no-heaving condition may o c c u r even though high initial moisture contents exist.
S , - T E N S I O N CM WATER
F i g u r e 1 0 . F l o w r a t e a s a f u n c t i o n o f S1 w i t h S q h e l d a t z e r o .
CONCLUSION
T h i s paper i s not a r e p o r t on a completed study but r a t h e r d e s c r i b e s the g e n e r a l ap- proach and gives the initial experimental r e s u l t s .
T h e freezing experiments described w e r e designed to m e a s u r e moisture flow r a t e s to the freezing zone for a number of soils under s i m i l a r but a r b i t r a r y conditions. The r e s u l t s can b e s t be understood in t e r m s of unsaturated permeability c h a r a c t e r i s t i c s and
moisture-content/tension relationships.
/ -T---.-,
T h e use of t h e s e concepts h a s the advantage of ~ n t e g 'ng a l l the f a c t o r s , s u c h a s g r a i n s i z e , s t r u c t u r e , clay composition, and exchange ion which together r e s u l t in a heaving r a t e peculiar to a given soil.
T
-
,i
ACKNOWLEDGMENTS
T h e author gratefully acknowledges the a s s i s t a n c e of K. N. Burn and K. R. Solvason in designing the apparatus and to C. B. Crawford and M a r g a r e t G e r a r d f o r t h e i r valuable c r i t i c i s m of the manuscript. S i n c e r e thanks a r e due to the D i r e c t o r , R. F. Legget and the Assistant D i r e c t o r , N. B. Hutcheon, of the Division of Building R e s e a r c h , National
R e s e a r c h Council of Canada, f o r t h e i r constant encouragement and continuing i n t e r e s t i n f r o s t action r e s e a r c h . T h i s p a p e r i s a contribution f r o m t h e Division of Building Re- s e a r c h ; National R e s e a r c h Council of Canada and is published with the Approval of the D i r e c t o r .
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