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Ager, B.
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PREFACE
Although snow roads are of economic interest in Canada, particularly to its pulp and paper industry, there is as yet very little published information on their construction and performance which is generally available in English to engineers.
As one contribution toward solving this problem the National Research Council, through its Associate Committee on Soil and Snow Mechanics, is cooperating with Professor
Seheult of the University of New Brunswick, on a project to evaluate current practice in the construction and use of snow roads in Canada and other countries of the world. One of the first steps has been to translate into English
the more pertinent publications availa.ble on the subject.
Sweden, throu* experiment and field practice has gained much valuable experience on the construction of snow roads. This translation, published with the per- mission of the author, is the second of a series of
reports to be translated which describe this Swedish ex-
perience. The Division of Building Research records its appreciation to Mr. D.A. Sinclair of the Translations Section of the National Research Council Library for preparing this translation.
Ottawa
,
June 1959
R.F.
Legget,NATIONAL RESEARCH COUNCIL OF CANADA Technical Translation 816
Title: The compacted snow road. 11. Climatic cons1deratlons
e en
snbpackade vtlgen. 11. Vaderlekens betydelse)Author:
B. H:son AgerReference: Svenska ~ko~sv&rdsf8renin~ens Tid.sl<rlft, (2): 201-227,
1955. ( SDA Meddelande no. 54)
TI33 COMPACTED
SNOW ROAD
11. THE EFFECT O F WEATHER
Foreword
I n connection with t h e p u b l i c a t i o n of Rune E r i k s s o n ' s paper
"The P m p e r t i e s of Snow1@
,
i n Svenska skogsv%rds f &eningensT i d s k r i f t No. 2 , 1954 (SDA Report No. 52*) i t was s t a t e d I n t h e
p r e f a c e t h a t a paper would follow g i v i n g an account of the p r a c t i c a l t e s t s c a r r i e d o u t t o d a t e .
I n t h e w i n t e r of 1955, however, very comprehensive s t u d i e s were c a r r i e d o u t by S D A 1 s Research S t a t i o n a t Lycksele on c l i m a t i c
f a c t o r s a c t i n g on t h e snow i n r e l a t i o n t o compacting and o t h e r
treatment. Since t h e r e s u l t s have now been processed It w a s con-
s i d e r e d a p p r o p r i a t e t o r e p o r t on t h i s i n t h e p r e s e n t paper and t h a t t h e r e p o r t should be made by Bengt H:son Ager who was appointed
C i v i l Crown F o r e s t e r with t h e SDA and who i s immediately r e s p o n s i b l e
f o r t h e s t u d i e s .
The p r a c t i c a l t e s t s c a r r i e d out t o d a t e w i l l be reviewed l a t e r .
I n t r o d u c t i o n
I n a r e c e n t number o f t h i s p e r i o d i c a l (No. 2 , 1954) Rune Eriksson d e a l s with t h e p r o p e r t i e s of t h e snow as a b u i l d i n g
m a t e r i a l and d e s c r i b e s t h e p h y s i c a l processes which t a k e p l a c e i n
t h e snow cover and i n compacted snow roads. The f a c t s and working
hypotheses which Eriksson advances a r e based on thorou& l i t e r a t u r e
s t u d i e s t o g e t h e r with snow compacting experiments c a r r i e d o u t by
*
SDA = F o r e s t r y Society and Royal Domain Administration, ResearchSDA* I n t h e p r e s e n t paper t h e importance of t h e meteorological f a c t o r s f o r t h e f r e e z i n g and hardness of the compacted snow road, o r , i n o t h e r words, t h e r e l a t i o n s between changes i n weather and t h e p h y s i c a l processes i n the compacted simw road, a r e explained.
To provide t h e r e a d e r with a b a s i s f o r the following d i s c u s s i o n s
i t was considered necessary t o r e c a p i t u l a t e b r i e f l y t h e most i m - p o r t a n t of t h e h i t h e r t o known p h y s i c a l processes i n snow ( s e e
Eriksson, B., 1954).
P h v s i c a l Processes and the Snow Cover
The t r a n s i t ion from t h e s o l i d form t o t h e gaseous form o r vice v e r s a , i . e . , i n t h e p r e s e n t case from i c e t o f r e e water vapour and
v i c e v e r s a , i s c a l l e d sublimation, An important f a c t o r i n t h i s
process i s t h e vapour p r e s s u r e , which depends i n t u r n on t h e number
of vapour molecules per u n i t volume and on t h e temperature, The
vapour p r e s s u r e decreases with d e c r e a s i n g temperature, !%en two
s u r f a c e s with d i f f e r e n t vapour p r e s s u r e s a r e contiguous, vapour molecules migrate ( d i f f u s e ) from t h e s u r f a c e with t h e h i g h e r vapour p r e s s u r e t o t h e s u r f a c e with t h e lower,
When two i c e c r y ~ s t a l s a r e pressed t o g e t h e r a t moderately low
temperature they j o i n together. The a c t u a l s u r f a c e s of c o n t a c t
between t h e c r y s t a l s a r e g e n e r a l l y only a f r a c t i o n of t h e apparent
ones, so t h a t t h e c p e c i f i c p r e s s u r e may be much higher. Since
p r e s s u r e reduces t h e melting p o i n t o f t h e i c e , t h e l a t t e r m e l t s a t t h e p o i n t s of c o n t a c t . When t h e p r e s s u r e f a l l s , tke water f r e e z e s
a g a i n and J o i n s the c r y s t a l s together. This process i s c a l l e d
r e g e l a t i o n ,
During o c c a s i o n a l periods of mild weather m e l t l m o c c u r s , followed by f r e e z i n q when t h e temperature f a l l s a g a i n below zero,
I n t h e s t a t i o n a r y snow cover t h e f i n e l y branched and b r i t t l e new snow c r y s t a l s a r e transformed i n t o rounded, o l d , l a r g e r c r y s t a l s .
The l a r g e c r y s t a l s grow a t t h e expense of t h e s m a l l e r ones. This
transformation t a k e s p l a c e through sublirna t i o n , m e l t i n g and sub-
from m e l t i n g . A t r a n s f e r of h e a t o c c u r s from t h e comparatively
warm ground upward through t h e snow cover. Vapour molecules d i f -
use from t h e ground and t h e d e e p e s t snow l a y e r s w i t h a comparatively high vapour p r e s s u r e t o t h e u s u a l l y c o o l e r upper l a y e r s with a low-
e r vapour p r e s s u r e and t h e r e t h e y condense.
I n
t h e upper l a y e r sm e l t i n g and f r e e z i n g p r o c e s s e s predominate. When t h e snow cover
i s compacted a g r e a t many bonds a r e formed between t h e p a r t i c l e s ,
t h u s g i v i n g c o n t i n u i t y t o t h e snow. The f o r m a t i o n o f bonds between t h e p a r t i c l e s i s due p a r t l y t o r e g e l a t i o n , p a r t l y t o s u b l i m a t i o n , and a l s o , when t h e t e m p e r a t u r e o c c a s i o n a l l y r i s e s above O°C., t o m e l t i n g and f r e e z i n g p r o c e s s e s . A t t e m p e r a t u r e s above O°C. t h e water of m e l t i n g with i t s s u r f a c e t e n s i o n c o n t r i b u t e s t o i n c r e a s e d
c o n t i n u i t y . Bonding w i l l be favoured by t h e mixing t o g e t h e r of
c o l d and l e s s c o l d snow and by t h e c r u s h i n g of snow p a r t i c l e s d u r i n g p r o c e s s i n g . The bonds g i v e t h e snow t h e power t o s u p p o r t t e n s i l e and s h e a r i n g f o r c e s . The f r a c t u r e l i m i t s f o r t h e s e f o r c e s a p p e a r t o i n c r e a s e w i t h d e c r e a s i n g t e m p e r a t u r e , g r e a . t e r d e n s i t y , s m a l l e r g r a i n s i z e and with time a f t e r t h e compacting of t h e snow.
Snow Compacting T e s t s 1954-55
During t h e p a s t w i n t e r r a t h e r e x t e n s i v e snow compacting t e s t s
were c a r r i e d o u t by t h e SDA Research S t a t i o n a t Lycksele w i t h t h e
f o l l o w i n g aims:
1. To compare v a r i o u s compacting and p r o c e s s i n g methods
such a s harrowing, r o l l i n g , d r a g g i n g , e t c ,
,
and t h e i r r e l a t i o n t o l o a d s : For t h i s purpose a b o u t 10 curved t e s t s t r i p s 1 5-
20 m.l o n g were compacted, each by a p a r t i c u l a r method,
he
impactingtook p l a c e a b o u t t h r e e times a week on a f i e l d , new curves b e i n g
compacted on each o c c a s i o n . I n t h e s e s t r i p s t h e s u r f a c e - b e a r i n g
c a p a c i t y and t e m p e r a t u r e was t h e n measured 10 and 20 cm., r e s p e c t -
i v e l y , below t h e "roadtuay" a t c e r t a i n i n t e r v a l s over a 48-hour p e r i o d . A t t h e l a s t measurement t h e road p r o f i l e was s t u d i e d and t h e d e n s i t y was measured i n t h e t o p s t r a t u m o f t h e road.
2. To g e t a n i d e a o f t h e amount of compacting and mainten-
c a p a b l e of b e a r i n g h e a v i e r wheeled v e h i c l e s : A number of 150
-
800
m.
t e s t r o a d s were compacted, each by a s p e c i a l method, andwheel t r a c t o r s h a u l i n g t r a i l e r s were d r i v e n o v e r them c o n t i n u o u s l y . Maintenance was c a r r i e d o u t as r e q u i r e d . The b e a r i n g c a p a c i t i e s o f t h e s e t e s t r o a d s were measured and t h e p r o f i l e s were s t u d i e d
c o n t i n u o u s l y d u r i n g t h e w i n t e r .
3 . To o b t a i n a d d i t i o n a l knowledge of t h e way i n which f r e e z - i n g and b e a r i n g c a p a c i t y depend on m e t e o r o l o g i c a l c o n d i t i o n s . For t h i s purpose t h e r e q u i r e d m e t e o r o l o g i c a l d a t a were r e c o r d e d , e.g. a i r t e m p e r a t u r e and r e l a t i v e humidity, cloud p e r c e n t , wind speed,
e t c . , and were c o r r e l a t e d w i t h t h e above-mentioned measurements. I n a d d i t i o n t o t h i s , s p e c i a l t e s t s were c a r r i e d o u t , o f which a more d e t a i l e d r e p o r t f o l l o w s below.
Measurements and M e a s u r i n . ~ I n s t r u m e n t s
For measurements o f t h e b e a r i n g c a p a c i t y o r hardness o f a road a P r o c t o r needle was used as i n t h e p r e v i o u s t e s t s (1950-53,
s e e Fig. 1 ) . I n p r i n c i p l e t h i s i s a s p r i n g b a l a n c e g r a d u a t e d i n kgm. f o r c e and i n d i c a t e s maximum l o a d s f o r a g i v e n s u r f a c e a r e a . The P r o c t o r n e e d l e has a number of i n t e r c h a n g e a b l e measuring d i s c s
of a r e a s r a n g i n g from 0.25 t o 3 2 sq.cm. The maximum l o a d g i v e s
t h e g r e a t e s t p r e s s u r e which t h e snow bed i s a b l e t o b e a r w i t h o u t t h e measuring d i s c b r e a k i n g through t h e s u r f a c e o f t h e bed.
However, i t h a s been found i n r e c e n t i n v e s t i g a t i o n s ( E r i k s s o n , R. 1954) t h a t t h e maximum p r e s s u r e p e r u n i t a r e a a t which break- through t a k e s p l a c e d e c r e a s e s w i t h i n c r e a s i n g l o a d a r e a , i . e . , a l a r g e r d i s c g i v e s a lower p r e s s u r e c a l c u l a t e d i n kgm. f o r c e p e r
sq.cm. t h a n a s m a l l e r d i s c on one and t h e same bed. T h i s f a c t was
conf irmed d u r i n g c a l i b l - a t i o n s o f P r o c t o r n e e d l e s l a s t w i n t e r , a l - though t h e r e s u l t s d i f f e r somewhat from t h e e a r l i e r ones. I f d i s c
3 2 ( t h e number i n d i c a t e s t h e s u r f a c e a r e a o f t h e d i s c ) i s g i v e n a
value of 1 . 0 , t h e f o l l o w i n g c o n v e r s i o n f a c t o r s a r e o b t a i n e d a f t e r
Table I Conversion f i g u r e s f o r d i f f e r e n t s i z e s of p l a t e s D I S C 0 . 2 5 0.50 1 2 4 8 16 3 2 S ~ O C ~ . Conversion 3.57 2.98 2.48 2.07 1.73 1.44 1.20 1.00 f a c t o r I n o r d e r t o e l i m i n a t e t h e i n f l u e n c e of t h e d i s c s i z e on t h e measured h a r d n e s s v a l u e s a l l measurements were reduced t o t h e
s t a n d a r d f o r d i s c 32. The
relations hi^
between t h e s i z e of t h ed i s c and t h e hardness w i l l be d i s c u s s e d f u r t h e r when t h e p r a c t i c a l
t e s t s a r e taken i n t o account. Here i t w i l l merely be p o i n t e d o u t
t h a t a l l s t a t e m e n t s below r e g a r d i n g t h e h a r d n e s s a r e t o be con- s i d e r e d r e l a t i v e v a l u e s .
A s a r u l e 2 5 measurements were taken t r a n s v e r s e l y a c r o s s t h e
e n t i r e roadway. I n measurements of t h e h a r d n e s s a t d i f f e r e n t
d e p t h s each l e v e l was exposed f o r t h e e n t i r e width of t h e road w i t h
t h e a i d of a spade. The measurement took p l a c e immediately a f t e r
exposure of t h e l e v e l i n q u e s t i o n , and h e r e a g a i n 25 measurements,
as a r u l e , were made o v e r t h e e n t i r e width of t h e roadbed.
A s an i n d i c a t i o n of t h e measuring accuracy t h e mean e r r o r i s given f o r a l l t h e h a r d n e s s va.lues r e p o r t e d .
The a i r temperature and r e l a t i v e a i r humidity were measured
with a Lambrecht thermohygrograph ( s e e Fig. 2 ) . The a p p a r a t u s
stood i n a l a t t i c e work box which s a t i s f a c t o r i l y f u l f i l l e d t h e r e q u i r e m e n t s o f a thermometer cage. The h e i g h t a t a l l t i m e s w a s
1.2 m. above t h e s u r f a c e of t h e compacted snow r o a d .
The temperature o f t h e road s u r f a c e was measured i n a r o u t i n e f a s h i o n by means of a g l a s s e a r t h thermometer graduated i n whole degrees. I n s p e c i ~ l i n v e s t i g a t i o n s t h e t e m p e r a t u r e was measured
both a t t h e road s u r f a c e and I n t h e u n d i s t u r b e d snow cover with
t h e a i d of t h e r m i s t o r s and a Norma R.W. r e s i s t a n c e gauge ( s e e Fig. 3 ) . A t h e r m i s t o r c o n s i s t s of semiconductor m a t e r i a l I n t h e
form of m e t a l o x i d e s which i s placed between two m e t a l p l a t e s .
T h i s m a t e r i a l changes i t s r e s i s t a n c e i n a r e g u l a r manner with t h e
o r d i n a r y t h e r m o m e t e r s i n d i c a t e s t h a t t h e s e g i v e v e r y a c c u r a t e measurements i n t h e snow c o v e r by p r a c t i c s . l l y e l i m i n a t i n g t h e s y s t e m a t i c e r r o r s c a u s e d , among o t h e r t h i n g o , b y a c c e s s o f c o l d o r h e a t from t h e s u r r o u n d i n g a t m o s p h e r e t o t h e m e a s u r i n g p o i n t . Py means o f r e m o t e r e a d i n g , m o r e o v e r , a t e s t o b j e c t c a n be l e f t un- d i s t u r b e d . The m e a s u r i n g e r r o r w i l l r e m a i n w i t h i n a few t e n t h s o f a d e g r e e . The wind s p e e d was measured a p p r o x i m a t e l y 2 m . above t h e p o u n d w i t h t h e a i d o f a cup anornometer ( s e e F i g . 4 ) ,
which r e g i s t e r e d wind s p e e d s a b o v e 0.5 m. p e r s e c . S i n c e t h e wind s p e e d i s r-ecorded as a mean v a l u e f o r 1 rnin., t h i s c a n o n l y be g i v e n w i t h i n 0.5 m. p e r s e c . , which i s r e g a r d e d as a d e q u a t e f o r t h e s e t e s t s .
T e s t s
As mentioned a b o v e , t h e p r o f i l e s were i n v e s t i g a t e d a t t h e
c u r v e s which had been compacted f o r t h e p u r p o s e of ccmparin& d i f -
f e r e n t methods o f t r e a t m e n t . On o n e o c c a s i o n a t t h e b e g i n n i n g o f a p e r i o d o f warm a i r a h e a v y s n o w f a l l had t a k e n p l a c e o n t h e c u r v e s immedia.tely a f t e r c o m p a c t i n g . The s u r f a c e - h e a r i n g c a p a c i t y a t t h e c u r v e s 48 h o u r s a f t e r c o m p a c t i n g w a s v e r y low, I. e.
,
a b o u t 1 . 5 k c . f o r c e p e r sq.cm. On d i g g i n g o u t t h e p r o f i l e , however, i t i s found t h a t t h e roaci h a s h a r d e n e d u n d e r n e a t h . T h i s o b s e r v a t i o n l e d t o a s e r i e s o f t e s t s which a r e t o b e r e p o r t e d on i n g r e a t e r d e t a i l . S e r i e s A . On t h e 1 7 t h o f March a t 11.20 a . m . a r o a d w a s corn---
patted w i t h a c a t e r p i l l a r t r a c t o r , h a r r o w and r o l l e r and a combina- t i o n implement f o r r o l l i n g which c o r n ~ a c t e d and smoothed t h e s u r f a c e l a y e r . The p r o c e d u r e was f i r s t t o compact t h e e n t i r e w i d t h o f t h e r o a d w i t h t h e c a t e r p i l l a r t r a c t o r . T h i s was f o l l o w e d by h a r r o w i n g and f i n a l l y by one p a s s w i t h t h e above-mentioned c o m b i n a t i o n
implement. I m m e d i a t e l y a f t e r c o m p a c t i n g , a n a r b i t r a r i l y s e l e c t e d p a r t of t h e r o a d s u r f a c e was i n s u l a t e d i n t h e f o l l o w i n g way: some l a t h s were l a i d c n t h e r o a d , a s h e e t o f plywood w a s l a i d o v e r t h e l a t h s and o v e r t h e plywood a l a y e r o f snow was s p r e a d a b o u t 20 cm. t h i c k ( s e e P i g . 5 ) .
T h i s i n s u l a t i o n covered t h e e n t i r e width of a s t r e t c h of
road 2 m. long. T h e r m i s t o r s were i n t r o d u c e d both i n t h e u n i n s u l a t -
ed and t h e i n s u l a t e d roadbeds a t 0 , 5 , 1 0 , 20 and 38 c m . (bottom)
below t h e s u r f a c e . T h i s w a s done by d i g g i n g a d i t c h i n t h e road a l o n g one edge a t r i g h t a n g l e s t o t h e s u r f a c e . Holes 8 mm. i n d i a m e t e r and about 20 cm. l o n g were bored a t t h e d e s i r e d d e p t h s from t h e d i t c h i n t o t h e roadbed p a r a l l e l w i t h t h e s u r f a c e . The
t h e r m i s t o r s were i n t r o d u c e d i n t o t h e s e h o l e s , a f t e r which t h e t r e n c h was f i l l e d up a g a i n and packed. The t h e r m i s t o r s were p l a c e d i n
t h e c e n t r e of t h e i n s u l a t e d parts b e f o r e t h e road was i n s u l a t e d . The v a r i a t i o n s o f temperature and h a r d n e s s i n t h e roadbed
were followed f o r about 2 4 hours. The h a r d n e s s was measured with
t h e P r o c t o r n e e d l e a t 0 , 5, 1 0 , 15, 20 and 25 cm. below t h e road s u r f a c e . I n t h i s , a s i n t h e f o l l o w i n g s e r i e s of t e s t s , no h a r d n e s s measurements were made i n t h e lower t h i r d of t h e roadbed, f o r two
r e a s o n s . F i r s t , a t t h e bottom o f t h e roadbed l a y comparatively
c o a r s e - g r a i n e d snow ( s o - c a l l e d d e p t h h o a r ) which i s mixed n o t a t a l l
o r
o n l y s l i g h t l y w i t h t h e f i n e - g r a i n e d s u r f a c e snow, because t h e t r a c t o r and t h e implement a r e n o t c a p a b l e of working through t h e e n t i r e snow cover ( s e e Fig. 6 ) . T h i s c o a r s e - g r a i n e d snow i s very l o o s e and has much p o o r e r s t r e n g t h p r o p e r t i e s t h a n t h e f i n e - g r a i n e d snowo r
t h e s t i r r e d - u p snow w i t h v a r y i n g g r a i n s i z e s . 1Jhen t h e P r o c t o r n e e d l e s t r i k e s such a " d e p t h h o a r pocket" d u r i n g measurement of t h e d e e p e r l a y e r s , t h e t e s t v a l u e t h u s o b t a i n e d i sn o t e d , b u t i s excluded from t h e mean v a l u e c a l c u l a t i o n . Secondly,
t h e r e l a t i v e l y hard ground may p o s s i b l y i n f l u e n c e t h e h a r d n e s s measurement i n t h e d e e p e r s t r a t a of t h e r o a d , s i n c e t h e h a r d n e s s i s measured by p r e s s i n g t h e P r o c t o r n e e d l e p e r p e n d i c u l a r l y a g a i n s t t h e bed.
The a i r t e m p e r a t u r e and hwnidity were r e c o r d e d with t h e thermohygrograph.
The measured v a l u e s o b t a i n e d a r e p a r t l y assembled i n Table
I1 t o I V , and p a r t l y i l l u s t r a t e d i n Fig. 7. To make t h e diagram
c l e a r e r , o n l y a s many measured v a l u e s a r e i n c l u d e d a s a r e needed t o i l l u s t r a t e t h e r e l a t i o n s h i p s . For t h e diagram and t h e informa- t i o n p e r t a i n i n g t o s e r i e s A t h e f o l l o w i n g comments a r e c o n s i d e r e d
necessary. The temperature a t d i f f e r e n t d e p t h s of t h e road 20
minutes a f t e r compacting (11.35 a . m . ) shows an i r r e g u l a r d i s t r i b u -
t i o n because o f t h e i n t e r m i n g l i n g of w a r m and c o l d snow s t r a t a
d u r i n g t h e compacting p r o c e s s . Not u n t i l 6 o r 7 hours a f t e r com-
p a c t i n g has t h e temperature a more nnormall' d i s t r i b u t i o n , l e e . , t h e
temperature curve h a s assumed a continuous shape. T h e r e l a t i v e l y
poor c a p a c i t y of t h e snow t o conduct h e a t and c o l d i s w e l l i l l u s -
t r a t e d i n t h e diagram. While t h e temperature minimum a t t h e s u r -
f a c e drops a f t e r n i g h t f a l l , t h e minimum a t 20 cm. depth i s r e -
corded i n t h e morrling and a t t h e bottom o f t h e road proba.b)ly n o t
u n t i l midday, when t h e s u r f a c e l a y e r i s a t a maximum.
Fig. 7 shows how t h e h a r d n e s s of t h e road v a r i e s with depth
a t d i f f e r e n t times. I t i s c l e a r t h a t t h e road hardens p a r t l y from
above and p a r t l y from below. The maximum h a r d n e s s f o r l e v e l s 0 and
5 cm. w a s measured a t 7.20 a . m . A t 12.50 a . m . t h e hardness a t 0
and 5 cm. h a s dropped compared with t h e measurement a t 7.20 a . m .
A t t h e same time a c o n s i d e r a b l e r i s e i n temperature h a s t a k e n p l a c e i n t h e s u r f a c e l a y e r of t h e road. I n t h e deeper l a y e r s t h e
road has hardened s t i l l f u r t h e r between t h e s p e c i f i e d t i m e s , while
a t t h e same time t h e temperature t h e r e (below a b o u t 1 5 cm.) h a s
continued t o f a l l .
On t h e 2 6 t h of March a t 9.15 a . m . , hence about one week a f t e r
compacting, t h e h a r d n e s s of t h e u n i n s u l a t e d roadbed i s measured
a g a i n . During t h e i n t e r v e n i n g t i m e i d e a l f r e e z i n g c o n d i t i o n s had e x i s t e d with h i g h midday t e m p e r a t u r e s and c l e a r , c c l d n i g h t s . As Fig. 7b shows, t h e h a r d n e s s ha.d i n c r e a s e d r e l a t i v e l y l i t t l e corn- pared witk t h e s t r o n p i n c r e a s e o f h a r d n e s s on t h e f i r s t day.
I n t h e i n s u l a t e d road ( s e e Table 111) t h e temperature v a r i a -
t i o n s were small, i . e . , 2.2OC. a t t h e s u r f a c e and 0.4OC. a t t h e
20 cm. depth. A t t h e same time as t h e l a t t e r measurement, a t
12.50 a . m . , t h e i n s u l a t i o n was removed from t h e road and t h e hard-
n e s s was measured i n t h e roadbed ( s e e Fig. 7 b ) . The h a r d n e s s a t
t h e s u r f a c e i s low, h u t i n c r e a s e s s h a r p l y towards t h e bottom, The subgrade f o r t h i s s e r i e s o f t e s t s was a s l i g h t l y f r o z e n p e a t s o i l .
S e r i e s B o On t h e 2 6 t h of March a t 12.45 a.m. a road was com-
p a c t e d by t h e same method a s i n s e r i e s A . The road was i n s u l a t e d
a t t h r e e p l a c e s i n t h e manner d e s c r i b e d above, i . e . , with plywood and 20 cm. of snow. A f t e r 1 9 h o u r s t h e h a r d n e s s w a s measured i n t h e f i r s t s e c t l o n , a f t e r 4 4 h o u r s i n t h e second and a f t e r 68 h o u r s i n t h e t h i r d p i e c e of i n s u l a t e d r o a d . The r e s u l t s of t h e measure- ments a r e g i v e n i n T a b l e V and F i g , 8 . For comparison, s i m u l t a n e - c u s measurements a r e a l s o g i v e n f o r u n i n s u l a t e d p a r t s of t h e road. The i n s u l a t e d roadbeds h a r d e n s l o w l y from t h e bottom up.
S e r i e s B was c a r r i e d o u t on a b a s e o f f r o z e n moraine w i t h c a k e s of i c e formed on t h e s u r f a c e h e r e and t h e r e . Compared w i t h t h e l o c a l i t y f o r s e r i e s A t h e t e r r i t o r y where s e r i e s B was c a r r i e d
o u t was more exposed t o sun and wind, and as a r e s u l t t h e snow
c o v e r w a s r a t h e r t h i n n e r ( 6 5 cm.) and t h e snow more c o a r s e q r a i n e d .
S e r i e s L . On t h e 2 9 t h o f March a road t i a s compacted by t h e
same method as b e f o r e . A t two p l a c e s the snow w a s dug a w a y on t h e
road and a t h i n p l a s t i c c l o t h w a s s p r e a d on t h e ground. The snow was t h e n r e p l a c e d and compacted a g a i n 1 ~ 3 t h t h e a i d of a s h e e t o f
plywood on which t h e a u t h o r ( w e i g h t '70 kgm. ) jumped and stamped
f o r some time. Both t h e bottorn i n s u l a t e d s u r f a c e s and a n un-
d i s t u r b e d s e c t i o n o f t h e road were i n s u l a t e d w i t h plywood and a b o u t
20 crn. o f snow. A f t e r 2 4 h o u r s t h e h a r d n e s s was measured i n t h e p a r t o f t h e yoad which w a s o n l y s u r f a c e i n s u l a t e d as w e l l as i n t h e o t h e r two s e c t i o n s of r o a d which were i n s u l a t e d b o t h on t h e s u r -
f a c e and on t h e bottom. The s e c t i o n of t h e road i n ~ u l a t ~ e d a t b o t h t h e t o p and bottom had a h a r d n e s s v a l u e t h a t v a r i ~ d but l i t t l e w i t h t h e d e p t h , w h i l e t h e p a r t which was i n s u l a t e d o n l y on t h e
s u r f a c e showed i n c r e a s e d h a r d n e s s w i t h 3 e p t h . A f t e r a f u r t h e r
2 2 h o u r s t h e h a r d n e s s was measured i n t h e second of t h e two
s t r e t c h e s of r o a d which were i n s u l a t e d both t o p and bottom. No
a p p r e c i a b l e I n c r e a s e o f h a r d n e s s had t a k e n p l a c e ( s e e F i g . 9 ) .
S e r i e s D. T h i s s e r i e s , c a r r i e d o u t t h e 5 t h and 6 t h o f A p r i l , w a s conducted i n t h e f o l l o w i n g way: The road was compacted w i t h c a t e r p i l l a r t r a c t o r s o v e r i t s e n t i r e w i d t h ant1 was t h e n harrowed f o u r t i m e s . A t two p l a c e s on t h e r o a d t h e snow was dug away,
p l a s t i c w a s l a i d on t h e bott;om arid t h e snow t h e n thrown hack on
t h e road. Then t h e r o l l e r and t h e above d e s c r i b e d combination
implement were passed o v e r t h e e n t i r e l e n g t h of t h e road s o t h a t t h e t r a c t o r would n o t t e a r up t h e p l a s t i c a t t h e bottoms One of t h e b o t t o r n - i n s u l ~ t e d s t r e t c h e s of r o a d , t o g e t h e r w i t h a s e c t i o i ? which was u n i n s u l a t e d a t t h e bottom, was i n s u l a t e d on t h e s u r f a c e w i t h l a t h s , plywood and 30 cm. of snow. F i n a l l y , a s t r e t c h o f road w a s i n s u l a t e r l w i t h p l a s t i c on t h e s u r f a c e . The road w a s com- p a c t e d on t h e 5 t h of A p r i l a t 4 porn, A few h o u r s a f t e r w a r d s snow
began t o f a l l , and i n t h e morning t h e road was covered with a
1-ayer of snow 5 cm, t h i c k . A t 8.15 a . m . 012 t h e 6 t h of A p r i l a s u r -
f a c e lqardness of o n l y 1.53 kgn. f o r c e p e r sq.cm. under the new snow
w a s measured on t h e u n i n s u l a t e d p a r t of t h e road. The snow was
t h e n swept o f f t h e road and t h e r m i s t o r s were I n t r o d u c e d a.L v a r i o u s d e p t h s i n t o t h e u n i n s u l a t e d road, About 2 0 minutes a f t e r t h e
snow had been swept away, t h e t e m p e r a t u r e was measured i n t h e roadbed. The t e m p e r a t u r e d i s t r i b u t i o n shows t h a t t h e n e w - f a l l e n snow i n s u l a t e s t h e s u r f a c e of t h e road very e f f e c t i v e l y from t h e r e l a t i v e l y c o l d a i r above, so t h a t the h e a t corning from t h e ground l a r g e l y d e t e r m i n e s t h e t e m p e r a t u r e c o n d i t l o n s in t h e roadbed. The f o l l o w i n g day was c h a r * a c t e r i z e d by c l e a r weather w i t h a wide gap
s e p a r a t i n g maximum and minimum t e m p e r a t u r e s . On t h e morning of
t h e 7 t h A p r i l t h e h a r d n e s s was measured i n t h e s e c t i o n s of road i n s u l a t e d i n t h e v a r i o u s ways. The measured v a l u e s o b t a i n e d a r e g i v e n i n T a b l e V I I and i l l u s t r a t e d i n F i g , 10. The bottom i n s u l a t - ed s e c t i o n s ( v ) become h a r d e r towards t h e s u r f a c e , the p a r t i n s u l a t -
ed on t h e s u r f a c e w i t h plywood and snow (111) h a r d e n s towards t h e
bottom, w h i l e t h e p a r t w i t h b o t h upper and lower i n s u l a t i o n ( I v )
i s r e l a t i v e l y l ~ o s e thrcu,ghout. The d i f f e r e n c e s I n h a r d n e s s between t h e u n i n s u l a t e d p a r t s of t h e road (I) and t h o s e i n s u l a t e d w i t h
p l a s t i c (11) a r e I n s i g n i f i c a n t . Below t h e p l a s t i c c l o t h a h o a r s t r a t u m 1 t o 2 mm. t h i c k i s o b s e r v e d , and above t h e p l a s t i c c l o t h
o n l y s c a t t e r e d i c e c r y s t a l s . The h o a r underneath had probably
o r i g i n a t e d d u r i n g t h e c o l d n i g h t from t h e c o o l i n g - o f f of t h e p l a s t i c and t h e m i g r a t i o n of p a r t i c l e s of vapour from t h e w a r m
road s u r f a c e condensing on t h e p l a s t i c c l o t h ( l a y e r s n e a r t h e c o l d w a l l ) . The s c a t t e r e d i c e c r y s t a l s on t h e c l o t h showed t h a t some of
t h e water vapour i n t h e f r e e a i r had condensed on t h e p l a s t i c c l o t h .
The s m a l l d i f f e r e n c e i n s u r f a c e h a r d n e s s between road s e c t i o n s I and V i n d i c a t e t h a t t h e d i g g i n g o v e r of t h e snow i n I V and V
had not a p p r e c i a b l y a l t e r e d t h e h a r d n e s s p r o p e r t i e s of t h e snow compared w i t h o t h e r p a r t s o f t h e road which had been harrowed r a t h e r I n t e n s i v e l y . The d l f f e r e n t s u r f a c e s w i l l t h e r e f o r e con- s t i t u t e l e g i t i m a t e o b j e c t s of comparison. I t may a l s o be p o l n t e d o u t t h a t f o r a l l of s e r i e s B
-
D t h e road s e c t i o n s i n s u l a t e d on t h e bottom and/or on t h e s u r f a c e were a l s o i n s u l a t e d a l o n g t h e s i d e s , s o t h a t e f f e c t s o r i g i n a t i n g from s u r r o u n d i n g a r e a s could be ignored.S e r i e s E and F. S e r i e s E and F were c a r r i e d o u t on t h e 28th t o t h e 2 9 t h March and t h e 6 t h and 7 t h A p r i l , r e s p e c t i v e l y . I n both of t h e s e t e s t s t h e temperature and h a r d n e s s v a r i a t i o n s were
s t u d i e d i n r o a d s t h a t had a l r e a d y been allowed t o harden f o r
s e v e r a l d a y s . The r e s u l t s of both E and F a r e shown I n t a b l e form ( ~ a b l e s V I I I t o X I ) , w h i l e o n l y
F
i s i l l u s t r a t e d diagramtnatically ( F i g . 11 and 1 2 ) . I n b o t h c a s e s a 24-hour p e r i o d showing wide v a r i a t i o n s of t e m p e r a t u r e was s e l e c t e d f o r t h e measurements. The measurements i n s e r i e sF
were c a r r i e d o u t on t h e 6 t hto
t h e 7 t h o f A p r i l on a road which had been compacted on t h e 3 1 s t of March andl e f t b a r e from t h e n u n t i l t h e above-mentioned s n o w f a l l on t h e n i g h t
of t h e 6 t h A p r i l . The new-fallen snow w a s swept away a t 8 a . m .
a f t e r which t h e r m i s t o r s were p l a c e d i n t h e roadbed. The rneasure-
ments were c a r r i e d o u t a t 1.50 p.m. The t e m p e r a t u r e and h a r d n e s s changes f o l l o w each o t h e r c l o s e l y , a s i s c l e a r l y shown i n F i g . 1 2 ,
where t e m p e r a t u r e and h a r d n e s s v a r i a t i o n s a t 0 , 10 and 20 crn.
d e p t h a r e i l l u s t r a t e d s e p a r a t e l y . A t 2 0 cm. t h e t e m p e r a t u r e v a r i a - t i o n s a r e indeed s m a l l and t h e s c a t t e r about t h e h a r d n e s s v a l u e s i s r e l a t i v e l y l a r g e , b u t t h e tendency i s n e v e r t h e l e ss e n t i r e l y c l e a r . The r e a s o n f o r t h e i n c r e a s e d spread i n t h e h a r d n e s s v a l u e s w i t h i n c r e a s i n g d e p t h i n t h e road i s a s s o c i a t e d w i t h t h e above-
mentioned p r e v a l e n c e o f d e p t h h o a r w i t h i n c r e a s i n g d e p t h , D i s c u s s i o n o f T e s t R e s u u
Summary of T e s t Results-
The r o a d which was s u r f a c e i n s u l a t e d w i t h plywood w a s com- p a r a t i v e l y l o o s e a t t h e s u r f a c e b u t h a r d e n e d t o w a r d s t h e bottom.
The r o a d i n s u l a t e d o n t h e s u r f a c e w i t h p l a s t i c showed t h e same h a r d n e s s as t h e c o m p l e t e l y u n i n s u l a t e d r o a d .
The r o a d i n s u l a t e d w i t h p l a s t i c on t h e bottom was c o n s i d e r a b l y l o o s e r a t t h e bottom t h a n t h e e n t i r e l y u n i n s u l a t e d r o a d ,
The r o a d which w a s s u r f a c e i n s u l a t e d w i t h plywood and snow and bottom i n s u l a t e d w i t h p l a s t i c was c o m p a r a t i v e l y l o o s e t h r o u g h - o u t .
By s i m u l t a n e o u s measurements o f h a r d n e s s and t e m p e r a t u r e i n r o a d s which had a l r e a d y h a r d e n e d , it w a s found t h a t t h e r e w a s a v e r y c l o s e r e l a t i o n s h i p between t h e changes i n h a r d n e s s and t e m - p e r a t u r e i n t h e roadbed.
D i s c u s s i o n
The t e s t r e s u l t s h a v e p r o v i d e d c e r t a i n working h y p o t h e s e s re- g a r d i n g t h e h a r d e n i n g o f t h e compacted snow r o a d , and t h e s e s h a l l now b e d i s c u s s e d ,
I f w e d i s r e g a r d t h e i n t e n s i t y o f p r o c e s s i n g d u r i n g c o m p a c t i n g and t h e p h y s i c a l phenomena i n c o n n e c t i o n w i t h m i l d w e a t h e r p e r i o d s , t h e n , a c c o r d i n g t o e a r l i e r i n v e s t i g a t i o n s , r e g e l a t i o n and sub-
l i m a t i o n are t h e p r i n c i p a l f a c t o r s which c o n t r i b u t e t o t h e f o r m a t i o n o f i c e bonds i n a compacted snow r o a d : The t e s t s i n d i c a t e t h a t
t h e s u b l i m a t i o n p r o c e s s e s h a v e t h e most e f f e c t o n t h e b o n d i n g o f t h e snow p a r t i c l e s , as E r i k s s o n a l s o assumed (see E r i k s s o n , R . , 1 9 5 4 ) . The b a l a n c e between t h e i c e and t h e w a t e r vapour I n t h e snow r o a d i s c l e a r l y a f f e c t e d by t h e f r e e atmosphere above and t h e s u b g r a d e below, I f t h e r o a d i s i n s u l a t e d from t h e I n f l u e n c e of t h e s e two f a c t o r s l i t t l e h a r d e n i n g t a k e s p l a c e (see s e r i e s C
and D ) . I n a roadbed I n s u l a t e d i n t h i s way r e l a t i v e l y uniform t e m p e r a t u r e a n d m o i s t u r e c o n d i t i o n s d e v e l o p q u i t e r a p i d l y , and t h e
s u b l i m a t i o n p r o c e s s e s a r e c o n s e q u e n t l y g r e a t l y reduced. These t e s t s show t h a t t h e s u b l i m a t i o n and r e g e l a t i o n which occur due t o
t h e mixing of w a r m and c o l d snow must b e r e l a t i v e l y unimportant
f o r t h e h a r d e n i n g p r o c e s s . I t can be o b j e c t e d h e r e t h a t t h e t e s t s took p l a c e r a t h e r l a t e i n t h e w i n t e r when t h e snow had been t r a n s -
formed and remained c o a r s e g r a i n e d . I n t h e example g l v e n above
(page 7 ) , where a s n o w f a l l followed t h e compacting and p r e v e n t e d
t h e s u r f a c e o f t h e r o a d from h a r d e n i n g , t h e compacting w a s c a r r i e d o u t on t h e 2 6 t h of J a n u a r y , i . e . , e a r l y i n t h e w i n t e r when t h e snow was comparatively f i n e g r a i n e d . The f a c t t h a t a f t e r b e i n g compacted t h e uppermost l a y e r s had hardened o n l y s l i g h t l y under t h e I n s u l a t i n g new snow, s u g g e s t s t h a t r e g e l a t i c n even i n t h e f i n e - g r a i n e d snow p l a y s a s u b o r d i n a t e p a r t i n h a r d e n l n g t h e road compared w i t h t h e s u b l i m a t i o n p r o c e s s e s . The importance of r e g e l a t i o n
probably depends i n p a r t on t h e e x t e n t of compacting, b u t a b a s i s
i s l a c k i n g f o r t h e d i s c u s s i o n of t h i s f a c t o r .
I n t h e f o l l o w i n g d i s c u s s i o n of t h e s u b l i m a t i o n c o n d i t i o n s i n a compacted snow road i t i s assumed t h a t t h e a i r i n t h e r o a d i s e n t i r e l y , o r almost e n t i r e l y m o i s t u r e s a t u r a t e d , 1 - e . , t h a t t h e vapour p r e s s u r e of t h e a i r i s e q u a l t o t h a t of t h e i c e a t a c e r t a i n t e m p e r a t u r e ,
The e f f e c t which i s e x e r t e d by t h e subgrade on t h e b a l a n c e between t h e i c e and t h e w a t e r vapour c o n s i s t s i n t h e f u r n i s h i n g o f
h e a t and water vapour. During t h e summer and autumn t h e ground
s t o r e s up h e a t and m o i s t u r e which i t g i v e s o f f d u r i n g t h e c o l d w i n t e r season
to
t h e snow c o v e r , o r t o t h e f r e e a i r i f t h e r e i sno snow cover. Vapour d i f f u s e s from t h e w a r m ground w i t h a h i g h
vapour p r e s s u r e t o the c o o l e r snow above w i t h a lower vapour
p r e s s u r e and condenses t h e r e on t h e snow c r y s t a l s and bonds formed
betoreen t h e c r y s t a 1 . s . The r a t e of d i f f u s i o n , which depends among o t h e r t h i n g s on t h e p o r o s i t y of t h e snow and t h e t e m p e r a t u r e
g r a d i e n t i n t h e ground*, must be r e l a t i v e l y low, a s s e r i e ~ B
*
Here t h e term !'temperature g r a d i e n t " s i g n i f i e s t h e change i n t e m p e r a t u r e p e r u n i t d i s t a n c e i n t h e v e r t i c a l d i r e c t i o n ,i n d i c a t e s , where t h e h a r d e n i n g of a s u r f a c e i n s u l a t e d road was
watched f o r t h r e e days. The t r r i n s f e r of vapour s o l e l y from t h e
lower l a y e r s t o t h e h i g h e r p a r t s o f t h e road a p p e a r s t o be of minor importance f o r t h e h a r d e n i n g of t h e r o a d , a.s t e s t s with
p l a s t i c i n s u l a t i o n of t h e bottom i n d i c a t e . The p l a s t i c i n s u l a t e d o n l y a g a i n s t m o i s t u r e and a g a i n s t h e a t which i s t r a n s p o r t e d by t h e vapour, b u t draws t h r u u & a l a r g e p a r t of t h e h e a t imparted by t h e ground t o t h e snow. T h i s q u a n t i t y of h e a t f u r n i s h e d t o t h e lower l a y e r s of t h e snow i s t h u s n o t consiclered t o be t h e cause o f any p e a t m a t e r i a l t r a n s p o r t i n t h e road.
Thus, a l t h o u g h t h e supply of m o i s t u r e from t h e subyrade does g r a d u a l l y c o n t r i b u t e t o t h e h a r d e n i n g of t h e s u r f a c e l a y e r , never- t h e l e s s t h e h a r d n e s s o f t h e s e upper l a y e r s i s principally de-
term1 ned by t h e d i r e c t i n f l u e n c e of t h e atmosphere. I f t h e road
i s t h e r m a l l y i n s u l a t e d a t t h e s u r f a c e , no h a r d e n i n g t a k e s p l a c e . I f t h e s u r f a c e of t h e road i s uninsulsteci, and t h e a i r above i s c o l d e r t h a n t h e a i r i n t h e r o a d , a wave of c o l d p e n e t r a t e s down i n t o t h e road s i m u l t a n e o u s l y w i t 1 1 i t s hardening. The e x p l a n a t i o n i s as f o l l o w s : A s t h e a i r i n t h e road c o o l s o f f i t s vapour
p r e s s u r e d e c r e a s e s . S i n c e t h i s a i r w a s p r e v i o u s l y s a t u r a . t e d w i t h h ~ ~ m i d i t y , a p r e c i p i t a t i o n of water vapour t a k e s p l a c e which con- d e n s e s on t h e bonds between tk snow g r a i n s and h a r d e n s t h e road. A wave of h e a t h a s t h e o p p o s i t e e f f e c t : A s t h e road w a r m s up, t h e
s a t u r a t i o n vapour p r e s s u r e of t h e a i r i n t h e road i n c r e a s e s .
T h i s r e s u l t s i n water vapour b e i n g evaporated from t h e snow pa:ns and t h e bonds between t h e snow g r a i n s .
Although t h e p r i n c i p a l e f f e c t of t h e f r e e atmosphere on t h e
h a r d e n i n g of t h e road c o n s i s t s i n i t s e f f e c t on t h e r o a d ' s tem- p e r a t u r e , we must n o t d i s r e g a r d t h e atmospheric e f f e c t on t h e humidity b a l a n c e i n t h e boundary l a y e r between t h e a i r and t h e
snow road s u r f a c e , i . e . , t h e b a l a n c e which d e t e r m i n e s t h e evapora- t i o n from and c o n d e n s a t i o n on t h e s u r f a c e l a y e r of t h e road. No measurements of t h i s phenomenon i n a compacted snow road have been c a r r i e d o u t . IIowever, a number of p a p e r s d e a l w i t h t h e evapora- t i o n from t h e u n d l s t u ~ * h e i i snow cover ( K b h l e r , H e 1950,
d e cuuerrvuin, M . 1951, and o t h e r s ) . A few p o i n t s i n t h i s connec- t i o n w i l l be mentioned i n t h e d i s c u s s i c n o f m e t e o r o l o g i c a l f a c t o r s . The Importance o f t k e Weather
--
!Jith t h e above p i c t u r e of t h e hardeninp, p r o c e s s t h e e f f e c t o f t h e d i f f e r e n t c l i m a t o l o g i c a l e l e m e n t s on t h e h a r d e n i n g of a compacted snow road w i l l now be d i s c u s s e d .
R a d i z t i o n . New f a l l e n snow r e f l e c t s 80
-
€382 of t h e h e a tr a d i a t i o n a r r i v i n g d u r i n g t h e d a y , w h i l e o l d , p a n u l a r snow r e - f l e c t s 4 2
-
70% ( G e i g e r , R . 1 9 5 0 ) . During t h e n i g h t h e a t r a d i a - t i o n from t h e snow c o v e r i s very s t r o n g . S i n c e a m o d e r a t e l y t h i c k snow c o v e r i s e f f e c t i v e l y i n s u l a t e d from t h e s u p p l y o f h e a t fromt h e ground, t h e upper l a y e r s of t h e snow and t h e a i r above c o o l o f f c o n s i d e r z b l y . n e c a u s e t h e c o l d a i r i s r e l a t i v e l y heavy, i t remains s t a t i ~ n a r y and a very s t a b l e s t r a t i f i c a t i o n of t h e lower a i r l a y e r s i s o b t a i n e d . I n t h i s s i t u a t i o n t h e s o - c a l l e d " t e m p e r a t u r e
i n v e r s i o n u p r e v a i l s , which means t h a t t h e v e r t i c a l t e m p e r a t u r e d i s - t r i b u t i o n i s " t u r n e d u p s i d e down" (Nyberg, A . 1939, G e i g e r , R.
1950 and o t h e r s ) . Normally, of c o u r s e , t h e t e m p e r a t u r e d e c r e a s e s w i t h i n c r e a s i n g h e i g h t . A c l o u d c o v e r g r e a t l y r e d u c e s t h e h e a t r a d i a t i o n from t h e snow c o v e r and t h u s s t r o n g l y i n h i b i t s t h i s c o o l i n g o f f a t n i h t . I n one and t h e same a 2 r mass t h e minimum t e m p e r a t u r e on a c l e a r n i g h t may be -25O, w h i l e on a c l o u d y n i g h t 2 t w i l l o n l y be - l O ° C .
An i n t e r e s t i n g example o f t h e e f f e c t o f r a d i a t i o n d u r i n g a day w i t h h i & midday t e m p e r a t u r e w a s noted on t h e 1 5 t h of March.
A r o a d was compacted a t 8 a r m . i n cloucly w e a t h e r z t -Z°C. D u r i n g i
t h e day t h e w e a t h e r c l e a r e d and t h e noon-day sun was q u i t e s t r o n g .
A t 2 p . ~ . t h e h a r d n e s s of t h i s r o a d , half' of which w a s exposed
t o t h e sun w h i l e t h e o t h e r h a l f was i n shadow, was measured. I n
t h e sun t h e har,:ness measurement was 2.54 kgfn. f o r c e p e r s q . cm.
and i n t h e shade 1.24 kgm. f o r c e p e r sq.cm. The g r e a t e r h a r d n e s s
i n t h e m e l t i n g , s u n l i t s e c t i o n s c a n be a s c r i b e d among o t h e r t h i n g s t o s u r f a c e t e n s i o n phenomena as w e l l as s e t t l e m e n t i n t h e s u r f a c e l a y e r s . The t e m p e r a t u r e f o r t h e 24 h o u r s imrnediat,ely b e f o r e com- p a c t i n g had been r e l a t i v e l y h i g h .
A i r t e m p e r a t u r e . The a i r t e m p e r a t u r e i s determined p a r t l y
-
by t h e a r r i v a l of a i r masses and p a r t l y by l o c a l c o n d i t i c n s such a s r a d i a t i o n phenomena, topography, v e g e t a t i o n , e t c . I n n o r t h e r n
Sweden t h e temperature i s i n f l u e n c e d by t h e w a r m , moist a i r masses
from t h e A t l a n t i c and t h e c o o l e r , r e l a t i v e l y d r y a i r masses from
R u s s i a and t h e A r c t i c . These m a t t e r s , however, w i l l h e touched
upon l a t e r . The e f f e c t of t h e topography on t h e temperature has
a c e r t a i n importance f o r t h e compacted snow road. As a r u l e , of
c o u r s e , t h e lumber t r a n s p o r t r o a d s run a l o n g d e p r e s s i o n s i n t h e t e r r a i n , where under c e r t a i n c l i m a t i c c o n d i t i o n s the h e a v i e r cold a i r c o l l e c t s . The r o a d s may t h e r e f o r e c o n s t i t u t e " c o l d a i r
r o u t e s " which a r e f a v o u r a b l e f o r t h e c o n s i s t e n c y o f t h e snow road. The a i r temperature and r b a d i a t i o n s a r e c l o s e l y r e l a t e d . T h e i r e f f e c t on t h e temperature c o n d i t i o n s i n t h e u n d i s t u r b e d snow cover and i n t h e compacted snow road may be d i s c u s s e d on t h e b a s i s o f
Fig. 13. Pig. 13 shows simultaneous temperature measurements i n
a n u n d i s t u r b e d snow cover and i n a compacted snow road d u r i n g a
twenty-four-hour period w i t h wide temperature v a r i a t i o n . The
measured v a l u e s 0 and 1 cm. below t h e s u r f a c e i n t h e u n d i s t u r b e d snow cover a r e comparatively u n c e r t a i n because of m e l t i n g p r o c e s s e s
i n t h e s u r f a c e l a y e r a t midday. The temperatures above z e r o a t a
d e p t h of 1 cm. can be a s c r i b e d t o t h e f a c t t h a t a n a i r c u s h i o n had
formed around t h e t h e r m i s t o r which w a s then h e a t e d by t h e incoming
r a d i a t i o n . I t had been demonstrated e a r l i e r , however, ( G e i g e r , R.
1950) t h a t a temperature maximum may e x i s t i n t h e u n d i s t u r b e d
snow cover a t 1 cm. d e p t h , so t h a t no g r e a t e r r o r should be pre-
s e n t i n t h e measurements c a r r i e d o u t h e r e . Under a l l c o n d i t i o n s
t h e measurements give a good comparative p i c t u r e of the tempera-
t u r e r e l a t i o n s h i p s between u n d i s t u r b e d and compacted snow. The
twenty-four-hour m p l i tu d e s a t t h e v a r i o u s d e p t h s were :
5 10 20 30 cm. below t h e surface
Undisturbed snow 15.7O 9.8O 3.4O l . O O
I n t h e porous snow cover t h e v a r i a t i o n s i n t h e s u r f a c e
l a y e r s a r e l a r g e b u t d e c r e a s e r a p i d l y towards t h e bottom. I n t h e road v a r i a t i o n s a t t h e s u r f a c e a r e much s m a l l e r , b u t a r e g r e a t e r i n t h e d e e p e r l a y e r s compared w i t h t h e u n d i s t u r b e d snow cover.
The c h i e f cause of t h i s phenomenon i s as f o l l o w s : The h e a t con-
d u c t i v i t y of t h e snow Aepenrls p r i n c i p a l l y on t h e v o l u m e t r i c weight. The comparatively porous u n d i s t u r b e d snow cover i s a very poor
h e a t conductor. The h e a t f u r n i s h e d t o t h e s u r f a c e l a y e r s of t h e l o o s e snow by r a d i a t i o n and molecular conduction t h e r e f o r e
"accumulates'' i n t h e s u r f a c e l a y e r m s , I n compacted snow, however, which h a s a h i g h e r d e n s i t y , t h e h e a t i s c c ~ u c t e d down i n t o t h e deeper l a y e r s . During t h e n i g h t , on t h e o t h e r hand, t h e s u r f s c e l a y e r s o f t h e l o o s e snow c o o l o f f t o a g r e a t e r e x t e n t t h a n t h o s e
o f t h e compacted snow. The g r e a t e r c o n d u c t i v i t y of t h e compacted
snow i n t h i s c a s e means t h a t t h e h e a t of t h e ground and t h e h e a t s t o r e d i n t h e r o a d d u r i n g t h e day a r e conducted upwards throu* t h e roadbed and a t t e n u a t e t h e c o o l i n g - o f f p r o c e s s e s . I n l o o s e
snow t h e supply of h e a t from below t a k e s p l a c e much more slowly and t h e r e f o r e h a s a l e s s damping e f f e c t on t h e c o o l i n g - o f f of t h e
s u r f a c e l a y e r s .
The d e s c r i b e d phenomena i n c r e a s e t h e r e s i s t a n c e of t h e snow r o a d t o t h e m e l t i n g e f f e c t o f t h e midday sun, a s t h e measurements
a l s o show ( F i g . 1 3 ) . On both days t e m p e r a t u r e s above z e r o had o c c u r r e d and m e l t i n g p r o c e s s e s took p l a c e i n t h e upper l a y e r s o f t h e u n d i s t u r b e d snow c o v e r , whereas t h e h i @ e s t t e m p e r a t u r e
measured i n t h e s u r f a c e l a y e r of t h e road was -1.2O. Both t h e
road and t h e snow cover were f u l l y exposed t o t h e l i g h t of t h e sun d u r i n g t h e same p e r i o d .
During t h e measurements t h e a i r t e m p e r a t u r e v a r i e d between
+2 and - l g O ( s e e Fig. 13 "Weather condition^^^). The a t t e n u a t i n g
e f f e c t of t h e cloud c o v e r on t h e cooling-off p r o c e s s i s w e l l 11-
l u s t r a t e d . During t h e p r e v i o u s n i g h t t h e r e were a few s c a t t e r e d
c l o u d s c o v e r i n g about 5% of t h e sky a t 10 p.m. and about 20% a t
t o check t h e f a l l i n g temperature which on a c l e a r n i g h t would have continued u n t i l t h e l a t e morning hours and produced c o n s i d e r a b l y lower minimum temperatures,
The r e l a t i o n between t h e a i r temperature and h a r d n e s s , and t h a t between t h e snow temperature, volumetric weight and hardness has been i n v e s t i g a t e d e a r l i e r and d i s c u s s e d by Kragelski and
Shakhov (1945) and Eriksson (1954) and o t h e r s .
Humid1t.v. The e f f e c t of the humidity i n t h e a i r on t h e con-
s i s t e n c y of t h e compacted snow road i s t o some e x t e n t unknown, and t h e r e f o r e o n l y a few p o i n t s s h a l l be d i s c u s s e d . It i s known t h a t a comparatively warm, moist flow of a i r g r e a t l y promotes t h e
m e l t i n g of t h e snow because t h e water vapour condenses on t h e sur- f a c e o f t h e snow, and t h e h e a t of condensation of t h e water c o n t r i - b u t e s t o t h e m e l t i n g process, Moreover, t h e e v a p o r a t i o n from t h e
snow cover i s a f u n c t i o n , among o t h e r t h i n g s , of t h e r e l a t i v e humidity of t h e a i r . During t h e w i n t e r , moisture i s g e n e r a l l y
e v a p o r a t i n g from t h e snow cover. Occasionally t h e e v a p o r a t i o n i s
changed t o condensation, p a r t i c u l a r l y d u r i n g c l e a r , calm n i @ t s when t h e r e l a t i v e humidity i n c r e a s e s g r e a t l y as a r e s u l t of t h e c o o l i n g of t h e a i r . F i n a l l y t h e s a t u r a t i o n p r e s s u r e of t h e vapour
i s exceeded, whereupon t h e vapour p r e c i p i t a t e s and sublimates on
t h e ground o r on cold o b j e c t s , forming hoar f r o s t . The t e s t with
t h e p l a s t i c cover on t h e s u r f a c e of t h e road i n d i c a t e s t h a t t h e sublimation phenomenon d u r i n g c l e a r , calm n i & t s h a s only a s l i g h t e f f e c t on t h e hardness p r o p e r t i e s of t h e road.
I n t h e course of s t u d i e s of the above-mentioned curved t e s t
r o a d s , on one o r two o c c a s i o n s t h e r e l a t i v e h u m i d i t i e s v a r i e d
while t h e temperature remained approximately c o n s t a n t . These
v a r i a t i o n s had no s i g n i f i c a n t e f f e c t on t h e f r e e z i n g of t h e mad.
However, it i s probable t h a t t h e very low value of t h e r e l a -
t i v e humidity i s accompanied by such s t r o n g e v a p o r a t i o n t h a t t h e s u r f a c e l a y e r of t h e road d i s i n t e g r a t e s , e s p e c i a l l y when a t t h e
same time a h i g h temperature p r e v a i l s and a s t r o n g wind blows
t e s t r o a d s , foll-owed by t h e i r b e i n g d r i v e n o v e r , took p l a c e on a day with a mldday sun, low r e l a t i v e humidity and s t r o n g wind.
Wind. An example of t h e e f f e c t of t h e wind h a s a l r e a d y been
--
mentioned. I n c r e a s i n g wind i s g e n e r a l l y accompanied by i n c r e a s i n g
e v a p o r a t i o n . I n t h e boundary l a y e r between t h e snow and t h e a i r t h e r e i s a c o n s t a n t exchange of vapour, and d u r l n g calm weather
a c e r t a i n e q u i l i b r i u m p r e v a i l s i n t h i s p r o c e s s . The wind u ~ s e t s t h i s e q u i l i b r l u m by c a r r y i n g away w a t e r vapour into tfie a i r , s o t h a t t h e snow g i v e s up more vapour i n a n e f f o r t t o r e s t o r e t h e
e q u i l i b r i u m .
However, t h e wind i s r a r e l y a b l e t o e x e r t as powerful a n
e f f e c t on t h e compacted snow road as on an exposed h o r i z o n t a l snow
cover. The roadbed, a t l e a s t immediately a f t e r compacting, i s
c o n s i d e r a b l y lower t h a n t h e s u r r o u n d i n g , u n d i s t u r b e d snow and i s t h e r e f o r e r a t k e r w e l l s h e l t e r e d a g a i n s t t h e wind. On p a r t s of t h e road which a r e very exposed t o t h e wind i t d r i f t s a g a i n , and t h e road g r a d u a l l y a t t a i n s t h e h e i g h t o f t h e s u r r o u n d i n g snow and t h e p r o t e c t i o n i s removed.
P r e c i p i t a t i o n . P r e c i p i t a t i o n i n t h e form o f snow immediately a f t e r packing I n s u l a t e s t h e road t o some e x t e n t from t h e e f f e c t s of t h e atmosphere on t h e f r e e z i n g o f t h e rood. The e x t e n t o f t h e
i n s u l a t i o n depends, of c o u r s e , on t h e t h i c k n e s s of t h e l n s u l a t i n g snow c o v e r and on t h e c o n s i s t e n c y of t h e snow. Even 5 cm. t h i c k - n e s s of snow cover i n c e r t a i n c a s e s c a n have a s t r o n g l y I n s u l a t i n g e f f e c t and r e t a r d t h e h a r d e n i n g of t h e r o a d . Except i n t h e c a s e of extremely low a i r t e m p e r a t u r e s , t h e h a r d e n i n g of t h e r o a d t h e n t a k e s p l a c e c h i e f l y from below. The r a t e a t which t h e road
hardens a p p e a r s t o depend t o a l a r g e e x t e n t on t h e t e m p e r a t u r e and m o i s t u r e p r o p e r t i e s of t h e subgrade. S l n c e a snow road i s g e n e r a l -
l y c l e a r e d o v e r many d i f f e r e n t k i n d s of p o u n d , i t i s n e c e s s a r y t o conform t o " t h e w o r s t " and probably t o w a i t f o r s e v e r a l d a y s
b e f o r e t h e road w i l l he c a p a b l e o f s u p p o r t i n g l o a d s o v e r i t s e n t i r e l e n g t h .
Importance of' t h e s u b r r a d e . A s w a s e v i d e n t from t e s t s e r i e s A
-
D , t h e r o a d s which w e r e t e m p e r a t u r e and m o i s t u r e i n s u l a t e d a tt h e s u r f a c e hardened c o n s i d e r a b l y more r a p i d l y (from t h e bottom) o v e r p e a t bog t h a n on morraine. Some of t h i s d i f f e r e n c e i s due probably t o t h e d i f f e r e n c e i n g r a i n s i z e . The snow c o v e r o v e r a
morraine subgrade i n t h i s c a s e was more exposed t o t h e sun and
wind, and was t h e r e f o r e more c o a r s e grained. However, most of t h e
d i f f e r e n c e i n h a r d n e s s can b e a t t r i b u t e d t o d i f f e r e n c e s i n t h e subgrade. I t i s a l s o h i g l l l y p r o b a b l y t h a t t h e t e m p e r a t u r e and m o i s t u r e p r o p e r t i e s of d i f f e r e n t k i n d s of ground produce d i f f e r - e n c e s i n t h e d e l i v e r y o f m o l s t u r e t o t h e snow c o v e r and t h u s
a f f e c t t h e h a r d n e s s o f t h e r o a d t o d i f f e r e n t d e g r e e s , Swampy s o i l s , which c o n t a i n a g r e a t d e a l of water and a r e c o m p a r a t i v e l y w a r m i n w i n t e r , w i l l g i v e up more m o i s t u r e t o t h e snow c o v e r t h a n , f o r example, r e l a t i v e l y d r y and c o l d sandy s o i l s . To ,g i n t o t h i s problem i n g r e a t e r d e t a i l would l e a d beyond t h e scope of t h e pre-
s e n t paper. Moreover t h e t e s t m a t e r i a l i s t o o s p a r s e t o p e r m i t any c e r t a i n c o n c l u s i o n s t o be drawn c o n c e r n i n g t h i s p o i n t , b u t i t h a s g i v e n a n i n d i c a t i o n t h a t t h e subgrade p r o b a b l y h a s a c e r t a i n
importance a s f a r as t h e c o n s i s t e n c y of t h e r o a d i s concerned. Among o t h e r f a c t o r s which a r e o f importance i n c o n n e c t i o n w i t h t h e l o c a l c l i m a t e o f t h e compacted snow r o a d , t h e e x t e n t of f o r e s t p r o w v may a l s o be mentioned. A s t a n d of t r e e s g r e a t l y r e d u c e s t h e r a n g e of t h e weather f o r c e s . The t e m p e r a t u r e v a r i a - t i o n s a r e s m a l l e r , t h e r a d i a t i o n i s absorbed by t h e t r e e t o p s , t h e wind i s moderated, e t c . A l l t h i s p r o b a b l y l e a d s t o a r e l a t i v e l y f a v o u r a b l e m i c r o c l i m a t e f o r t h e c o n s i s t e n c y of t h e r o a d i n a s t a n d o f t r e e s , compared w i t h one o v e r a n open f i e l d . The snow a l s o remains l o n g e r i n a woods t h a n o n a f i e l d .
The E f f e c t o f D i f f e r e n t Kinas of Weather
The v a r i o u s m e t e o r o l o g i c a l e l e m e n t s d i s c u s s e d above must of c o u r s e he i n c l u d e d i n t h e broad c o n c e p t of " w e a t h e r " , which i s , o f
c o u r s e a complicated and fascinating phenomenon.
The a n a l y s i s of t h e v a r i o u s k i n d s o f w e a t h e r and t h e i r e f f e c t on a compacted snow r o a d would f i l l a number of t h i c k volumes i f one were t o g i v e a complete p i c t u r e . Here we s h a l l o n l y d i s c u s s
