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Snow as Road-Building Material
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PREFA CE
Although snow r o a d s are of economic i n t e r e s t t o Canada, p a r t i c u l a r l y t o i t s P u l p a n d P a p e r I n d u s t r y , t h e r e i s as y e t v e r y l i t t l e p u b l i s h e d i n f o r m a t i o n on t h e i r c o n s t r u c t i o n and p e r f o r m a n c e which i s g e n e r a l l y a v a i l a b l e i n E n g l i s h t o e n g i n e e r s . A s one c o n t r i b u t i o n toward s o l v i n g t h i s problem t h e N a t i o n a l R e s e a r c h C o u n c i l , t h r o u g h i t s A s s o c i a t e Committee on S o i l and Snow Mechanics, i s
c o o p e r a t i n g w i t h P r o f e s s o r S e h e u l t o f t h e U n i v e r s i t y of New Brunswick on a p r o j e c t t o e v a l u a t e c u r r e n t p r a c t i c e i n t h e c o n s t r u c t i o n a n d u s e of snow r o a d a i n Canada and o t h e r c o u n t r i e s o f t h e world.
One o f t h e f i r s t s t e p s h a s b e e n t o t r a n s l a t e i n t o E n g l i s h t h e more p e r t i n e n t p u b l i c a t i o n s a v a i l - able on t h e s u b j e c t . The p r e s e n t t r a n s l a t i o n is of a p a p e r d e s c r i b i n g snow r o a d c o n s t r u c t i o n p r a c t i c e i n F i n l a n d . O t t a w a J u l y 1959 R.F. D i r e c t o r L e g g e t ,
NATIONAL RESEARCH C O U N C I L OF CANADA T e c h n i c a l T r a n s l a t i o n 822 T i t l e : Snow as r o a d - b u i l d i n g m a t e r i a l (Lumi tienrakennusmateriaalina) Author : Kalle P u t k i s t o Reference : T i e l e h t i
,
(6 ),
1957(Also r e p r i n t e d as MetsEteho Report i 3 6 of the
Department of F o r e s t r y S t u d i e s of t h e C e n t r a l A s s o c i a t i o n of t h e FinnEsh Woodworking I n d u s t r y ) T r a n s l a t o r :
E.
PeremSNOW AS ROAD-BUILDING MATERIAL
Snow h a s b e e n a r o a d - b u i l d i n g m a t e r i a l of b a s i c i m p o r t a n c e i n t h e c o u n t r i e s w i t h c o l d c l i m a t e i n t h e p a s t when permanent roads, open t o t r a f f i c t h r o u g h o u t t h e y e a r , were l a c k i n g o r were w i d e l y s c a t t e r e d . It i s known t h a t i n t h e p a s t , i n F i n l a d , f o r example, p r a c t i c a l l y a l l t r a f f i c d u r i n g t h e w i n t e r moved on snow-roads. The i m p o r t a n c e of snow r o a d s i s c o n s i d e r a b l e e v e n t o d a y a n d , as a
r u l e , i s g r e a t e r t h a n g e n e r a l l y r e c o g n i z e d . Snow r o a d s a r e a n e x c e l l e n t a d d i t i o n t o t h e permanent r o a d system a n d c o n s t i t u t e a v e r y economical s o l u t i o n of t h e problem of s e a s o n a l and o c c a s i o n a l t r a n s p o r t a t i o n . It h a s b e e n e s t i m a t e d t h a t a t p r e s e n t , i n o r d e r t o meet t h e r e q u i r e m e n t s o f o u r f o r e s t i n d u s t r y , a p p r o x i m a t e l y 70,000 t o 8 0 , 0 0 0 km. o f w i n t e r r o a d s t o be u s e d f o r h a u l a g e by h o r s e , and a p p r o x i m a t e l y 8 , 0 0 0 km. of w i n t e r r o a d s s u i t a b l e f o r motor t r a f f i c are b u i l t e a c h y e a r . The most p r i m i t i v e w i n t e r r o a d s ' a r e c r e a t e d by t h e t r a f f i c i t s e l f . A s t h e r e s u l t of t h e l o a d of t r a f f i c u n d e r c e r t a i n con- d i t i o n s snow becomes packed and h a r d e n s . The t r a n s p o r t a t i o n
p r o p e r t i e s of such r o a d s a r e , however, r a t h e r poor. W i n t e r r o a d s of b e t t e r q u a l i t y , s u c h as ploughed snow r o a d s a n d a r t i f i c i a l l y p r e p a r e d compacted snow r o a d s , r e q u i r e c o n s i d e r a b l e c a r e i n con- s t r u c t i o n and m a i n t e n a n c e . Of s u c h r o a d s t h e ploughed snow r o a d s were f o r m e r l y predominent
.
The f i n d i n g s of e x p e r i m e n t s a n dr e s e a r c h c a r r i e d o u t d u r i n g r e c e n t y e a r s i n n o r t h e r n c o u n t r i e s h a v e b r o u g h t a b o u t a w i d e r u s e of i n e x p e n s i v e compacted snow r o a d s .
The c o n s t r u c t i o n of a l l t y p e s o f w i n t e r r o a d s , e s p e c i a l l y t h a t of compacted snow r o a d s , i s b a s e d on t h e knowledge of t h e p r o p e r t i e s o f snow and on t h e u t i l i z a t i o n of t h e s e p r o p e r t i e s . In t h e f o l l o w i n g p a g e s a s h o r t r e v i e w on snow as a r o a d - b u i l d i n g m a t e r i a l w i l l b e g i v e n a g a i n s t t h e background o f p r e s e n t - d a y knowledge. P h y s i c a l P r o p e r t i e s o f Snoz Depending on p r e v a i l i n g c o n d i t i o n s , w a t e r may e x i s t e i t h e r I n
t h e l i q u i d s t a t e , i n t h e g a s e o u s s t a t e as w a t e r v a p o u r , i n t h e s o l i d s t a t e as i c e , o r a s snow. The i m p o r t a n t f a c t o r d e t e r m i n i n g t h e s t a t e of w a t e r i s t e m p e r a t u r e . With i n c r e a s i n g t e m p e r a t u r e t h e motion o f m o l e c u l e s
is
a c c e l e r a t e d a n d s t e a m , t h e g a s e o u s s t a t e , i s formed as soon as t h e c o h e s i v e f o r c e s of t h e m o l e c u l a s a r e s u r p a s s e d by t h e i r e n e r g y of motion, A s i s t y p i c a l of g a s e s , t h e d e n s i t y of s t e a m i s low a n d i t i s e a s i l y c o m p r e s s i b l e . I n t h i s s t a t e t h e v e l o c i t y of t h e w a t e r m o l e c u l e s i s s e v e r a l hundred m e t r e s p e r second. The d i f f u s i o n o f steam t h r o u g h t h e a i r i s , however, n o t s o r a p i d b e c a u s e t h e forward motion of t h e m o l e c u l e s i s slowed by collisions w i t h e a c h o t h e r a n d w i t h t h e m o l e c u l e s o f a i r . I n snow and i n o t h e r d e n s e sub- s t a n c e s t h e movement of w a t e r vapour i s s t i l l s l o w e r , When s t e a m i s e n c l o s e d by a s o l i d s u r f a c e , p r e s s u r e o r g i n a t i n g from t h e motion of m o l e c u l e s i s e x e r t e d on t h e s u r f a c e . The p r e s s u r e i s g r e a t e r f o r a g i v e n q u a n t i t y o f steam as t h e volume Is d e c r e a s e d o r t h e t e m p e r a t u r e i s i n c r e a s e d .When w a t e r i s t r a n s f o r m e d t o i c e t h e m o l e c u l e s become mutually a t t a c h e d i n such a way t h a t e a c h m o l e c u l e i s j o i n e d w i t h f o u r ad-
j a c e n t m o l e c u l e s . T h i s e x p l a i n s t h e h e x a g o n a l s t r u c t u r e of ice
( F i g . 1) and t h e f a c t t h a t t h e d e n s i t y o f m o l e c u l a r p a c k i n g i n i c e
I s l e s s t h a n t h a t o f water a n d i t s s p e c i f i c w e i g h t i s lower. The m o l e c u l e s a r e i n motion even i n i c e b u t as a r e s u l t of t h e low t e m p e r a t u r e t h e movement i s r e l a t i v e l y l i m i t e d .
During t h e f r e e z i n g p r o c e s s t h e growth o f i c e c r y s t a l s , which a r e r e l a t i v e l y p u r e , c o n c e n t r a t e s d i s s o l v e d f o r e i g n s u b s t a n c e s i n t h e m o t h e r - l i q u o r or ~ i l f r o z e n w a t e r . These s u b s t a n c e s become a s s o - c i a t e d w i t h t h e s u r f a c e s between t h e l a r g e r i c e c r y s t a l s t o g i v e t h i s r e g i o n a l o w e r m e l t i n g - p o i n t t h a n p u r e ice. When i c e i s h e a t e d , , i t s m a t e r i a l between t h e g r a i n s m e l t s f i r s t . The f r i a - b i l i t y o f i c e i n s p r i n g t i m e o r i g i n a t e s p a r t l y from t h i s phenom- enon. A good example on t h e e f f e c t of f o r e i g n s u b s t a n c e s i s t h e p r a c t i c e of mixing s a l t w i t h snow (snow i s composed of i c e c r y s t a l s ) .
A l a y e r o f s a l t s o l u t i o n i s formed a t t h e s u r f a c e o f t h e snow
Snow t r e a t e d t h i s way r e m a i n s l o o s e even when t h e t e m p e r a t u r e f a l l s below o 0 C . When two p i e c e s of i c e a r e p r e s s e d a g a i n s t e a c h o t h e r t h e y f u s e t o g e t h e r . The c o m p l e t e n e s s o f c o n j u n c t i o n depends on t h e magnitude o f s t r e s s and on t h e s t r u c t u r a l o r i e n t a t i o n of t h e samples. I n c a s e t h e s t r u c t u r a l o r i e n t a t i o n i s n o t t h e same i n b o t h p i e c e s , t h e s t r e n g t h of t h e J o i n t i s l e s s t h a n t h e s t r e n g t h of t h e i c e . Such a d h e r e n c e i s t h e r e s u l t of t h e m e l t i n g o f i c e c a u s e d by t h e p r e s s u r e , f o l l o w e d by t h e r e - f r e e z i n g of t h e m o l t e n a r e a when t h e p r e s s u r e i s t e r m i n a t e d . The r e d u c t i o n of t h e m e l t i n g - p o i n t o f ' i c e by p r e s s u r e i s i n d i c a t e d by t h e f o l l o w i n g s e r i e s o f v a l u e s ( P a u l i n g 1 9 5 2 ) . P r e s s u r e , a t m . 1 590 1090 1540 1910 2045 M e l t i n g - p o i n t , O C . 0 -5 -10 -15 -20 -22 The l o w e r t h e t e m p e r a t u r e , t h e h i g h e r i s t h e f o r c e r e q u i r e d t o c a u s e t h e i c e t o m e l t . According t o t h e s t u d i e s of Dorsey ( 1 9 4 0 ) , a t t e m p e r a t u r e s below -22OC..the p r e s s u r e d o e s n o t c a u s e m e l t i n g of i c e b u t i n s t e a d w i l l change t h e s t r u c t u r e o f i c e . The p r e s s u r e s r e q u i r e d t o c a u s e m e l t i n g o f i c e would a p p e a r t o b e t o o l a r g e f o r t h i s phenomena t o b e c o n s i d e r e d i n a p r a c t i c a l c a s e b u t s i n c e t h e a c t u a l a r e a o f c o n t a c t when two i c e s a m p l e s a r e p r e s s e d t o g e t h e r i s much smaller t h a n t h a t v i s u a l l y o b s e r v e d , t h e e f f e c t i v s p r e s s u r e s may i n d e e d b e l a r g e enough. The phenomenon d e s c r i b e d above p l a y s a n i m p o r t a n t p a r t i n t h e h a r d e n i n g p r o c e s s o f snow.
I c e c a n change d i r e c t l y t o w a t e r vapour w i t h o u t g o i n g t h r o u g h t h e l i q u i d p h a s e . T h i s p r o c e s s i s c a l l e d s u b l i m a t i o n . It i s
e x p l a i n e d by s e p a r a t i o n o f m o l e c u l e s from t h e s u r f a c e of i c e c r y s t a l s when t h e e n e r g y o f motion o f t h e m o l e c u l e s overcomes
t h e i r c o h e s i v e f o r c e s . E v a p o r a t i o n i s n a t u r a l l y f a s t e r , t h e h i g h e r t h e t e m p e r a t u r e . Vice v e r s a , w a t e r v a p o u r may change d i r e c t l y t o i c e . A c e r t a i n p o r t i o n o f vapour m o l e c u l e s which a r e above a n i c e s u r f a c e , c o l l i d e s w i t h t h e i c e c r y s t a l s a n d becomes a t t a c h e d t o them. The s p e e d of such c o n d e n s a t i o n o f v a p o u r i s r e l a t e d t o t h e number of vapour m o l e c u l e s above t h e sample of i c e . Under a par- t i c u l a r c o n d i t i o n of t e m p e r a t u r e and p r e s s u r e of t h e v n p o u r , t h e
e v a p o r a t i o n and c o n d e n s a t i o n p r o c e s s e s a r e i n e q u i l i b r i u m condi- t i o n . The v a p o u r p r e s s u r e a t t h e e q u i l i b r i u m c o n d i t i o n i s de- f i n e d a s the v a p o u r p r e s s u r e of i c e . I t d e c r e a s e s when t h e tem- p e r a t u r e d e c r e a s e s and i n c r e a s e s when t h e t e m p e r a t u r e i n c r e a s e s .
I f t h e p r e s s u r e of w a t e r v a p o u r i n t h e s u r r o u n d i n g a i r
i s
h i g h e r t h a n t h e vapour p r e s s u r e of t h e i c e , t h e i c e c r y s t a l s w i l lgrow; a n d i f i t i s l o v ~ e r t h a n t h a t of i c e , t h e i c e c r y s t a l s be- come s m a l l e r . The l o s s of i c e t a k e s p l a c e mainly a t e d g e s a n d c o r n e r s and t h e c r y s t a l s become more rounded. The p r o c e s s of c h a n g i n g from s o l i d s t a t e t o g a s e o u s s t a t e and from g a s e o u s s t a t e t o s o l i d s t a t e i s c a l l e d s u b l i m a t i o n .
The f o r m a t i o n o f snow i s b a s e d on t h e s u b l i m a t i o n o f w a t e r vapour. W i t h o u t l o o k i n g i n t o d e t a i l s of t h i s m e t c o r o l o g i c a l phenomenon, i t c a n be s a i d t h a t t h e s i z e and s h a p e o f t h e snow f l a k e s depends e s s e n t i a l l y on t h e t e m p e r a t u r e o f t h e a i r l a y e r i n which t h e y a r e formed. A t e m p e r a t u r e of -2°C. o r l o w e r i s r e -
q u i r e d b e f o r e a snow f a l l c a n s t a r t . The shape of newly-formed snow f l a k e s , formed i n d i f f e r e n t t e m p e r a t u r e c o n d i t i o n s , a r e g e n e r a l l y as f o l l o w s ( F i g . 2 ) . T e m p e r a t u r e of f o r m a t i o n , O C . Shape and s i z e
-
2....
-
7...
i c e n e e d l e s , l e n c t h 2 mm. o r less-
8 . . . . - l o . . . t a b u l a r , c l o v e n ; l e n g t h l e s s t h a n 1.5 mm. -12....-18... s t e l l a r , c l o v e n ; a p p r o x i m a t e l y 5 mm. -18... .-24,.....
, t a b u l a r , c l o v e n ; 0.5 t o 1 mm, Below -24...,
p r i s m o l d ; small. The t h i c k n e s s of snow f l a k e s i s u s u a l l y l e s s t h a n 0.1 mm. H a l l s t o n e s a r e formed when snow f l a k e s a r e exposed t o suc- c e s s i v e c o n d i t i o n s o f f r e e z i n g a n d m e l t i n g ( o . ~ . when p a s s i n gt h r o u g h r a i n c l o u d s ) , Rime i s formed when w a t e r vapour s u b l i n l a t c s on t h e s u r f a c e of a snovr d r i f t o r when small prater d r o p s i n d c v ~
f r e e z e t o u c h i n g a c o l d s u b s t r a t u m . Theso forrnes of snow, howcver, h a v e no n o t 3 b l e i m p o r t a n c e i n c o n n e c t i o n w i t h c j l n t e r r o a d s .
d i f f e r e n t c o n d i t i o n s and snow f l a k e s o f v a r i o u s s h a p e s a r e in- c l u d e d i n t h e formed snow c o v e r . I f i t i s blowing d u r i n g a snow f a l l t h e snow f l a k e s a r e ground more o r l e s s i n t o f r a g m e n t s .
F r e s h l y f a l l e n snow, i n which t h e snow f l a k e s a r e o f t e n
bound t o g e t h e r as l a r g e r f l a k e s , form normally a porous and l i g h t - w e i g h t snow c o v e r of low d e n s i t y a s i n d i c a t e d i n t h e f o l l o w i n g t a b l e ( E r i k s s o n 1 9 5 4 ) .
Type of snow D e n s i t y
F r e s h l y f a l l e n , p o r o u s snow 0.005..0.05 F r e s h l y f a l l e n , l i g h t - w e i g h t snow, packed by wlnd 0 . 0 6 . . .0.1 F r e s h l y f a l l e n , heavy snow, packed by wlnd 0 . 1 . .
.
.0.3 O l d , c o a r s e - g r a i n e d snow 0.3....0.6I c e 0.92
I n a snow d r i f t t h e s t r u c t u r e of snow u n d e r g o e s c o n t i n u o u s changes. The fundamental f a c t o r s whlch c a u s e t h e s e a l t e r a t i o n s are t h e changes i n t h e t e m p e r a t u r e o f t h e a l r a n d of t h e g r o u n d , t r a f f i c o r o t h e r t r e a t m e n t s , a n d t h e changes o c c u r r i n g as t h e r e s u l t of sublimation, m e l t i n g , f r e e z i n g , and t h e movement of w a t e r and w a t e r vapour i n s i d e t h e snow d r i f t .
The a l t e r a t i o n s a r e g r e a t e r i n t h e s u r f a c e l s y e r s of t h e snow c o v e r , b e c a u s e t h e s e a r e exposed t o more d r a s t i c c h a n g e s i n t h e t e m p e r a t u r e and m o i s t u r e c o n d i t i o n s . When e v a p o r a t i o n t a k e s p l a c e o r when snow i s m e l t i n g t h e c o r n e r s and e d g e s of t h e i c e c r y s t a l s v a n i s h f i r s t ( F i g . 3 ) . The w a t e r produced i s p a r t l y
a b s o r b e d w i t h i n t h e r e m a i n i n g l a r g e r snow f l a k e s and p a r t l y s u r r o u n d s t h e rounded c r y s t a l s and snow f l a k e s . A s a r e s u l t o f s u r f a c e t e n - s i o n t h e s u r f a c e s become more a n d more rounded and t h e s n o w f l a k e r : a r e p u l l e d c l o s e r t o e a c h o t h e r . S i m u l t a n e o u s l y t h e c o h e s i v e p r o p e r t i e s of snow ImDrove (thawy snow). The c o h e s i v e s t r e n g t h
i n c r e a s e s w i t h d e c r e a s i n g g r a i n s i z e .
When thaw i s f o l l o w e d by f r e e z i n g , c o a r s e - g r a i n e d snow i s
forrned ( F i g . 4 ) t h e p a r t i c l e s of whicl? a r e rounded. The a d h e s i o n between such g r a n u l e s i s s t r o n g e r t h a n b e f o r e because t h e w a t e r
occupying t h e space between t h e g r a n u l e s and c o v e r i n g them i s
f r o z e n and a c t s as a cement (e.g. c r u s t of snow). A s mentioned p r e v i o u s l y , m e l t i n g may t a k e p l a c e a t t e m p e r a t u r e s lower t h a n o O C .
i f f o r e i g n s u b s t a n c e s are p r e s e n t .
During t h e summer t h e ground s t o r e s h e a t which r a d i a t e s up- ward d u r i n g t h e w i n t e r . Snow f a l l e n on t h e w a r m ground m e l t s . Melting i s slowed down when t h e ground becomes c o o l e r , b u t i t may c o n t i n u e under c e r t a i n c o n d i t i o n s t h r o u g h o u t t h e w i n t e r . The h e a t c o n d u c t i v i t y of a s o f t snow c o v e r i s s o low t h a t even a t h i n l a y e r of snow p r e v e n t s f r e e z i n g of t h e s o i l . While i n s u l a t i n g t h e
s t r a t a i n t h e bottom of t h e snow c o v e r from t h e e f f e c t s of c o l d w e a t h e r , t h i s t h i n l a y e r a t t h e same t i m e i n s u l s t e s t h e upper
l a y e r s from m e l t i n g a c t i o n s of t h e ground. Temperature measure- ments c a r r i e d o u t i n t h e bottom o f t h e snow c o v e r have shown t h a t v a r i a t i o n s i n t h e t e m p e r a t u r e d u r i n g t h e w i n t e r are r e l s t i v e l y
small, and t h a t v a r i a t i o n s a r e s m a l l e r t h e g r e a t e r t h e d i s t a n c e o f t h e p o i n t o f measurement from t h e . s u r f a c e and t h e g r e a t e r t h e p o r o s i t y of snow ( F i g . 5 ) . The s u r f a c e of t h e snow c o v e r , on t h e c o n t r a r y , f o l l o w s c l o s e l y t h e f l u c t u a t i o n s of t h e a i r t e m p e r a t u r e . A t t h e very bottom of t h e snow c o v e r t h e t e m p e r a t u r e v a r i e s d u r i n g
t h e w i n t e r u s u a l l y w i t h i n t h e range of 0 ° . . . - 3 0 ~ . The t e m p e r a t u r e of t h e ground d u r i n g t h e w i n t e r h a s a s u b l i m a t i n g e f f e c t on t h e snow c r y s t a l s a t t h e bottom of t h e snow cover. The amount of w a t e r vapour s u r r o u n d i n g t h e snow c r y s t a l s i s g r e a t e r t h e d e e p e r t h e s t r a t u m i n q u e s t i o n . Water vapour t r a v e l l i n g upwards and
coming i n c o n t a c t w i t h a c o l d e r and d e n s e r l a y e r of t h e u p p e r p a r t of t h e snow cover c o n d e n s a t e s and f r e e z e s . A s a r e s u l t o f t h i s s u b l i m a t i o n d u r i n g t h e w i n t e r , some material of t h e bottom l a y e r s
i s c a r r i e d t o t h e upper l a y e r s t h u s c a u s i n g i n c r e a s e d d e n s i t y i n t h e u p p e r strata and a r e d u c t i o n i n t h e d e n s i t y of t h e lower
s t r a t a . The snow i n t h e bottom l a y e r s becomes g r i t t y and p o s s e s s e s low c o h e s i v e s t r e n g t h . T h i s phenomenon p l a y s a n important part i n u n t r e a t e d snow c o v e r s . The d e n s e r t h e snow, t h e s l o w e r and more l i m i t e d i s t h e d i f f u s i o n o f vapour and t h e accompaning movement of t h e m a t e r i a l .
The g e n e r a l c o u r s e of t h e a l t e r a t i o n s i n a snow c o v e r con- s i s t s of r o u n d i n g o f f t h e f i n e - g r a i n e d snow c r y s t a l s and f o r m a t i o n of c o a r s e - g r a i n e d p a r t i c l e s a t t h e e x p e n s e of t h e small snow
c r y s t a l s . T h i s o r i g i n a t e s from t h e f a c t t h a t t h e vapour p r e s s u r e of w a t e r i s h i g h e r t h e g r e a t e r t h e c o n v e x i t y of t h e s u r f a c e of a f i l m of w a t e r . The small snow c r y s t a l s m e l t away and e v a p o r a t e a n d t h e formed water v a p o u r s u b l i m i z e s on t h e s u r f a c e s o f t h e l a r g e r p a r t i c l e s . The i n i t i a l u n i f o r m p o r o s i t y o f a snow c o v e r i s t h u s r e d u c e d g r a d u a l l y . L a t e r t h e d e n s i t y of t h e u p p e r l a y e r s i n c r e a s e s accompanied b y d e c r e a s e s i n t h e d e n s i t y of t h e l o w e r s t r a t a . I n s p r i n g when t h e snow i s m e l t i n g , t h e d i r e c t i o n of t h e movement o f material i s r e v e r s e d . Water formed from t h e snow on t h e t o p o f t h e snow c o v e r d e s c e n d s , f i l l i n g t h e s p a c e s between snow p a r t i c l e s .
The b e a r i n g p r o p e r t i e s o f f r e s h l y f a l l e n p o r o u s snow l a c k p r a c t i c a l i m p o r t a n c e . The v e h i c l e s s i n k i n t o such snow a n d t h e
t r a c t i o n r e s i s t a n c e i s enormous. The b e a r i n g s t r e n g t h of snow which h a s h a r d e n e d n a t u r a l l y o r which h a s b e e n t r e a t e d , however, may b e c o n s i d e r a b l e .
The b e a r i n g p r o p e r t i e s o f c o a r s e - g r a i n e d Ifcorn1' snow, e v e n when i n compacted c o n d i t i o n , a r e a p p r o x i m a t e l y similar t o t h e s e o f l o o s e sand. The same a p p l i e s t o f i n e - g r a i n e d snow i n which no f r o z e n water i s p r e s e n t i n s p a c e s between t h e snow c r y s t a l s o r
i n which t h e amount of such b i n d i n g m a t e r i a l i s t o o small. When s u c h t y p e s o f snow c o v e r s are exposed t o t h e l o a d of a narrow- wheeled v e h i c l e , t h e snow p a r t i c l e s s l i p a s i d e of t h e i n t r u d i n g o b j e c t l i k e c o a r s e s a n d . The main s o u r c e o f c o h e s i v e f o r c e of s u c h snow i s t h e i n t e r n a l f r i c t i o n , t h e c o e f f i c i e n t of which i s a p p r o x i m a t e l y 0.7 when t h e t e m p e r a t u r e of t h e a t m o s p h e r e i s - 4 ' ~ . ( E r i k s s o n 1 9 5 4 ) . The f r i c t i o n , however, d o e s n o t f o l l o w t h e e x a c t l a w s which are a p p l i c a b l e t o f r i c t i o n t y p e m i n e r a l s o i l s .
The h a r d e n e d snow c o v e r , i n which t h e snow c r y s t a l s a r e j o i n - e d t o g e t h e r by f r o z e n w a t e r i n t h e v o i d s , a c t s u n d e r a l o a d i n somewhat s i r n i l ~ r way a s a c o h e s i v e t y p e s o i l , e.g. a c l a y . Snow h a s a p p r e c i a b l e c o m p r e s s i v e , t e n s i l e , and t o r s i o n a l s t r e n i ~ t h s .
The t e n s i l e s t r e n g t h o f snow, f o r example, may exceed 1 k5m./cned
(Bucher 1 9 4 8 )
.
Hardening of SnowA s shown a b o v e , t h e c o n d i t i o n s n e c e s s a r y f o r h a r d e n i n g of a
snow c o v e r may b e d i s s i m i l a r depending on t h e t y p e of snow and on t h e t e m p e r a t u r e .
F r e s h snow f a l l e n i n e a r l y w i n t e r i s , a s a r u l e , e a s i l y h a r d e n e d . I t c o n s i s t s of f i n e - g r a i n e d snow c r y s t a l s o r c a n b e e a s i l y c o n v e r t e d t o such f i n e n e s s by p r o c e s s i n g . The s m a l l e r t h e c r y s t a l s , t h e g r e a t e r a r e t h e a r e a s of c o n t a c t between t h e
snow c r y s t a l s which a r e j o i n e d b y f r e e z i n g . Poor h a r d e n i n g of c o a r s e - g r a i n e d l l c o r n t l snow of t h e l a t e w i n t e r i s a l s o t h e r e s u l t of t h i s phenomenon. I n o r d e r t o a c h i e v e d e s i r a b l e h a r d n e s s i n s u c h snow t h e p a r t i c l e s have t o b e f i r s t p u l v e r i z e d .
The f i n e n e s s of snow, however, i s n o t s u f f i c i e n t f o r harden- i n g of t h e snow c o v e r . The d e n s i t y of snow h a s t o be h i g h as w e l l s o t h a t t h e c o n t a c t a r e a s between t h e snow f l a k e s and c r y s t a l s a r e as g r e a t as p o s s i b l e . Not c o n s i d e r i n g e x c e p t i o n a l c a s e s , s u c h
d e n s i t y of snow c a n n o t be o b t a i n e d w i t h o u t c o m p a c t i v e t r e a t m e n t . I n o r d e r t o c r e a t e a d e q u a t e b e a r i n g p r o p e r t i e s , t h e s p e c i f i c w e i g h t of snow h a s t o b e 0.3..
.
-0.4 o r more ( s e e V u o r i s t o - M a l l e n b e r g 1937 ).The t h i r d r e q u i r e m e n t f o r h a r d e n i n g i s t h e p r e s e n c e of w a t e r o r water vapour between t h e snow c r y s t a l s and p a r t i c l e s . I f t h e r e i s no water o r w a t e r vapour t h e p a r t i c l e s w i l l n o t b e a b l e t o J o i n t o g e t h e r by s u b l i m a t i o n a n d f r e e z i n g . A s mentioned p r e v i o u s l y , t h e amount of w a t e r vapour between t h e snow p a r t i c l e s i n c r e a s e s toward t h e b o t t o m l a y e r of snow c o v e r . The snow l a y e r which h a s b e e n exposed by p l o u g h i n g , h a r d e n s r a t h e r e a s i l y . Such " b i n d i n g m a t e r i a l n , on t h e o t h e r h a n d , may b e l a c k i n g i n t h e u p p e r l a y e r s of t h e snow c o v e r . I n t h e e v e n t of low r e l a t i v e h u m i d i t y of t h e a i r , when accompanied by t e m p e r a t u r e s c o n s i s t e n t l y w i t h i n t h e
r a n g e of D O . . . . - ~ O C . , t h e low r e l a t i v e h u m i d i t y becomes t h e c r i t -
i c a l f a c t o r i n h a r d e n i n g of snow. I n t h e above-mentioned condi- t i o n t h e d i f f e r e n c e s between t h e t e m p e r a t u r e of day and of n i g h t
are s o small t h a t no a p p r e c i a b l e cementing of t h e g r a i n s by s u b l i m a t i o n can t a k e p l a c e . A s a r e s u l t t h e snow remains d r y . A t lower a i r t e m p e r a t u r e s t h e c o n d e n s a t i o n and f r e e z i n g of t h e w a t e r v a p o u r , which o r i g i n a t e s from t h e ground a t a h i g h e r
tem-
p e r a t u r e , i s a c c e l e r a t e d and t h e d a i l y f l u c t u a t i o n s i n t h e tem- p e r a t u r e a r e l a r g e s o t h a t h a r d e n i n g of t h e snow by s u b l i m a t i o n can take p l a c e .
- B e s i d e s t h e above-mentioned f a c t o r s , however, c o l d w e a t h e r
i s r e q u i r e d t o f r e e z e t h e w a t e r i n t h e snow. The lower t h e t e m - p e r a t u r e , t h e g r e a t e r t h e h a r d n e s s of snow o b t a i n e d . I f t h e t e m p e r a t u r e of a i r i s n o t s u f f i c i e n t l y low f o r f r e e z i n g t h e
"binding water" i n t h e snow,
it
i s a p p a r e n t l y f e a s i b l e u n d e r c e r - t a i n c o n d i t i o n s t o u t i l i z e t h e below z e r o t e m p e r a t u r e s of t h e snow p a r t i c l e s . The r e s u l t s of some e x p e r i m e n t s d u r i n g which s o l i d i c e was formed from thawy snow a t t h e a i r t e m p e r a t u r e of 6OC. i n d i c a t e such p o s s i b i l i t i e s .Ager (1955) c a r r i e d o u t e x p e r i m e n t s on h a r d e n i n g of compacted snow d u r i n g which t h e s u r f a c e of t h e snow c o v e r was s e p a r a t e d from t h e s u r r o u n d i n g atmosphere by s h e e t s of plywood o v e r which a n
a p p r o x i m a t e l y 20 cm. t h i c k l a y e r of l o o s e snow was p l a c e d . The s u r f a c e l a y e r s of t h e road remained l o o s e , b u t h a r d e n i n g had t a k e n p l a c e i n t h e bottom l a y e r s . When s h e e t s of p l a s t i c f i l m , which d i d n o t have temperature- i n s u l a t i n g p r o p e r t i e s b u t p r e v e n t e d
movement of water vapour o n l y , were used f o r s e p a r a t i o n , t h e sur- f a c e l a y e r s of t h e snow c o v e r hardened i n t h e same way as i n t h e c a s e of u n l n s u l a t e d snow. When t h e bottom l a y e r s were s e p a r a t e d from t h e ground by s h e e t s of p l a s t i c f i l m , t h e s u r f a c e l a y e r s hardened i n much t h e same way as t h o s e of u n l n s u l a t e d snow, b u t t h e bottom l a y e r s remained n o t i c e a b l y l o o s e r . S e p a r a t i o n of b o t h t h e s u r f a c e and t h e bottom, t h e f i r s t by s h e e t s of plywood and a l a y e r of snow, t h e second by s h e e t s of f i l m , p r e v e n t e d h a r d e n i n g of t h e snow a l m o s t completely.
These e x p e r i m e n t s which were a c t u a l l y c a r r i e d o u t u n d e r l a t e w i n t e r c o n d i t i o n s i n d i c a t e d t h a t a newly-packed snow road h a r d e n s
c o n d i t i o n s , b u t 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 s depends
l a r g e l y on t h e low t e m p e r a t u r e s of t h e a i r and on t h e p o s s i b i l i - t i e s of p e n e t r a t i o n of low t e m p e r a t u r e s deep i n t o t h e snow.
I t h a s b e e n e s t a b l i s h e d t h a t f r e s h l y f a l l e n snow r e f l e c t s a p p r o x i m a t e l y 8 0
-
88% of t h e r a d i a t i o n as compared w i t h o n l y 4 2-
70% r e f l e c t i o n of o l d " c o r n t t snow ( G e i g e r 1 9 5 0 ) . Thus r a d i a - t i o n i s more e f f e c t i v e i n m e l t i n g c o a r s e - g r a i n e d snow t h a n i n m e l t i n g f r e s h , f i n e - g r a i n e d snow. During t h e n i g h t , e s p e c i a l l y when t h e sky i s c l o u d l e s s , h e a t r a d i a t i o n t a k e s p l a c e from t h e body of t h e r o a d t o atmosphere. T h i s k i n d of h e a t t r a n s f e r , whichi s t y p i c a l f o r l a t e w i n t e r c o n d i t i o n s , h a s a h a r d e n i n g e f f e c t on t h e u p p e r l a y e r s of t h e w i n t e r r o a d .
The d i r e c t e f f e c t of t h e r e l a t i v e h u m i d i t y of a i r on t h e h a r d e n i n g of snow i s p r o b a b l y c o r n p a r i t i v e l y small. I t , however, i n f l u e n c e s t h e o c c u r r e n c e of p r e c i p i t a t i o n s . I n a d d i t i o n ,
e x c e p t i o n a l l y low r e l a t i v e h u m i d i t y may c a u s e s o i n t e n s e a sub- l i m a t i o n o f snow, t h a t t h e u p p e r l a y e r s of t h e r o a d s o f t e n , o r t h e h a r d e n i n g i s r e s t r a i n e d . L i k e w i s e , t h e d i r e c t e f f e c t of wind,
i f i t s d r i f t i n g a c t i o n i s n o t c o n s i d e r e d , i s r a t h e r similar t o t h a t o f r e l a t i v e h u m i d i t y . I n o t h e r words, i n e x c e p t i o n a l c a s e s wind may a c c e l e r a t e s u b l i m a t i o n o f snow t o s u c h an e x t e n t t h a t t h e u p p e r l a y e r s of snow become s o f t e r .
Snow compacted d u r i n g a snow f a l l o r i m m e d i a t e l y b e f o r e d o e s n o t h a r d e n , s i n c e f r e s h , l o o s e snow c o l l e c t e d on t h e packed snow l a y e r a c t s as i n s u l a t i o n . A l a y e r of s o f t snow o n l y 5 cm. d e e p ,
may have t h i s i n s u l a t i n g e f f e c t . I f t h e r o a d i s h a r d e n e d , f r e s h snow c o v e r i n g t h e compacted l a y e r s , p r e v e n t s o r slows down s o f t e n - i n g o f d e n s e snow d u r i n g a thaw. P r o v i d e d t h e ground and t h e r o a d b e d were w a r m b e f o r e t h e f a l l of t h e i n s u l a t i n g snow l a y e r , t h e r o a d s o f t e n s s i n c e it becomes i n s u l a t e d from t h e c o l d atmos- p h e r e . Packed snow r o a d s may t h u s s o f t e n from below.
Hardening which s t a r t s from t h e b o t t o m h a s b e e n found t o b e f a s t e r on p e a t s o i l t h a n on m i n e r a l s o i l . In f o r e s t a r e a s , d u e t o l o c a l c l i m a t e , t h e h s r d e n i n g i s p r o b a b l y somewhat s l o w e r t h a n i n o p e n a r e a s , b u t t h e p r e s e r v a t i o n o f h a r d n e s s i s b e t t e r i n f o r e s t
a r e a s . Up t o a c e r t a i n l i m i t , t h e h a r d n e s s v a l u e s of t h e snow g e n e r a l l y improve w i t h t i m e , e s p e c i a l l y i f t h e r o a d s a r e k e p t i n c o n t i n u o u s use. S i n c e u n d e r o u r c o n d i t i o n s t h e w e a t h e r d u r i n g t h e w i n t e r changes c o n s i d e r a b l y , t h e c o n t r u c t i o n o f snow r o a d s h a s t o be u n d e r t a k e n w h i l e t h e c o n d i t i o n s n e c e s s a r y f o r h a r d e n i n g a r e f a v o u r a b l e and t h e p r e s e r v a t i o n of t h e h a r d n e s s i s a s s u r e d . Com- p a c t i n g of a r o a d bed s h o u l d n o t b e c a r r i e d o u t d u r i n g a p e r i o d of heavy snow f a l l s , s i n c e f r e s h snow w i l l h i n d e r t h e h a r d e n i n g of t h e road. On t h e o t h e r hand, f a v o u r a b l e w e a t h e r c o n d i t i o n s f o r such u n d e r t a k i n g s e x i s t immediately f o l l o w i n g a weakening o r a c e s s a t i o n of t h e p e r i o d s of low p r e s s u r e . High p r e s s u r e
c o n d i t i o n s w i t h accornpaning c o o l a i r c u r r e n t s and l a c k of p r e c i p i t a t i o n a r e d e s i r a b l e . It i s a d v i s a b l e t o s t a r t t h e p r o c e s s of compacting d u r i n g t h e e a r l y w i n t e r , even i f t h e r o a d s are n o t b e i n g u s e d u n t i l a l a t e r d a t e . A t h i n l a y e r of snow h a r d e n s more
r a p i d l y t h a n a t h i c k o n e , and f u r t h e r m o r e , t h e s i z e of snow
p a r t i c l e s i s , as p o i n t e d o u t p r e v i o u s l y , more a d v a n t a g e o u s i n t h e e a r l y w i n t e r as r e g a r d s t o h a r d e n i n g .
A f t e r n o o n i s c o n s i d e r e d t h e most f a v o u r a b l e t i m e of t h e day f o r compacting snow. The snow h a s t h e n a r e l a t i v e l y h i g h tempera- t u r e and a c c o r d i n g l y t h e r e i s p l e n t y of water v a p o u r and w a t e r between t h e snow p a r t i c l e s . F u r t h e r m o r e , t h e t e n ~ p e r a t u r e f a l l s
d u r i n g t h e n i g h t s o t h a t f r e e z i n g w i l l o c c u r .
The Reauirements o f D i f f e r e n t Types of T r a f f i c on a Snow Road U t i l i z a t i o n o f snow as a r o a d - b u i l d i n g material i s b a s e d on
i t s p r o p e r t i e s t o r e s i s t changes i n i t s volume and form. An i n c r e a s e i n t h e d e n s i t y o f snow i n c r e a s e s i t s c a p a c i t y t o r e s i s t compression and t h e f r i c t i o n between t h e p a r t i c l e s , t o g e t h e r w i t h t h e e x i s t i n g a d h e s i o n o f t h e p a r t i c l e s as a r e s u l t of f r e e z i n g , i n c r e a s e t h e c a p a c i t y o f snow t o r e s i s t d e f o r m a t i o n .
A s p o i n t e d o u t p r e v i o u s l y , v i r g i n snow may o n l y i n e x c e p t i o n -
s u p p o r t t r a f f i c . Such s t r e n g t h p r o p e r t i e s , however, c a n b e pro- duced t h r o u g h c e r t a i n methods of snow t r e a t m e n t . B e f o r e we review t h e methods u s e d i n t h e p r a c t i c e f o r c o n s t r u c t i o n of snow r o a d s , t h e r e q u i r e m e n t s of d i f f e r e n t t y p e s o f r o a d t r a f f i c have t o b e d i s c u s s e d .
The s t r e s s c r e a t e d by t h e t r a f f i c o r i g i n a t e s p a r t l y from t h e l o a d on t h e s u r f a c e of t h e r o a d ( s u r f a c e l o a d ) and p a r t l y from t h e m e c h a n i c a l wear. The stresses c a u s e d by d i f f e r e n t t y p e s of t r s f f i c a r e d i f f e r e n t ; s l e d g e t r a f f i c d i f f e r s from t h e t r s f f i c of wheelsd v e h i c l e s and from t h a t of c r s w l e r t y p e of v e h i c l e s . F u r t h e r m o r e i f t h e c a r g o i s n o t c a r r i e d by t h e t r a c t i o n v e h i c l e b u t by a s e p a r a t e v e h i c l e , t h e stresses t o which t h e r o a d i s sub-
j e c t a r e d i f f e r e n t f o r b o t h v e h i c l e s . The r e q u i r e m e n t s of a r o a d a r e n o t c o n c e r n e d w i t h
i t s
b e a r i n g p r o p e r t i e s and i t s r e s i s t a n c e t o wear o n l y , b u t a l s o w i t h i t s r o l l i n g a n d s l i d i n g f r i c t i o n , The above-mentioned f a c t o r s , however, a r e c l o s e l y i n t e r r e l a t e d .The i n f l u e n c e of a l o a d on t h e r o a d s u r f a c e d i m i n i s h e s r a p i d - l y w i t h i n c r e a s e of d i s t a n c e from t h e s u r f a c e . S i m u l t a n e o u s l y w i t h t h e d e c r e a s e of t h e p r e s s u r e toward t h e b o t t o m of t h e r o a d b e d , t h e s t r e s s s p r e a d s l a t e r a l l y . From t h i s i t f o l l o w s t h a t t h e s m a l l e r t h e l o a d area t h e g r e a t e r i s t h e f o r c e , c a l c u l a t e d p o r u n i t a r e a , r e q u i r e d t o b r e a k t h e r o a d s u r f a c e o r , i n o t h e r words, t h e r o a d s u r f a c e w i l l r e s i s t h i g h e r l o a d s p e r u n i t a r e a , t h e s m a l l e r t h e t o t a l a r e a u n d e r u n i f o r m l o a d i n g ( F i g . 6 ) . S u r f a c e h a r d n e s s , however, i s n o t t h e o n l y r e q u i r e m e n t f o r r e s i s t a n c e a g a i n s t s u b s t a n t i a l m e c h a n i c a l wear on t h e road. Wheeled v e h i c l e t r a f f i c w i l l e a s i l y b r e a k t h r o u g h a road i f o n l y t h e s u r f a c e of t h e road i s hardened. Furtkiermore, t h e s u r f a c e stress of t h e t r a f f i c sf wheeled v e h i c l e s i s g e n e r a l l y h i g h e r t h a n t h a t of c r a w l e r t y p e v e h i c l e s a n d s l e i g h s . W i n t e r r o a d s i n t e n d e d f o r t h e t r a f f i c o f wheeled v e h i c l e s h a v e t o be ploughed s o t h a t t h e t h i c k n e s s o f t h e r e m a i n i n g snow l a y e r i s small enough t o a l l o w t h e snow t o h a r d e n t h r a u g h o u t . According t o p r e s e n t
knowledge, a compacted snow r o a d w i l l s t a n d t h e t r a f f i c of wheeled v e h i c l e s o n l y u n d e r t h e f o l l o w i n g c o n d i t i o n s :
1. snow t h a t h a s f a l l e n on t h e road h a s b e e n compacted l a y e r by l a y e r s t a r t i n g from t h e b o t t o m of t h e ground s u r f a c e s o t h a t it i s h a r d e n e d t h r o u g h o u t t h e r o a d b e d ;
2. t h e amount of snow F a l l e n u p t o t h e t i m e of o p e n i n g t h e
t r a f f i c has b e e n l i m i t e d ( n o t more t h a n 30....40 cm. depending on t h e t y p e of snow) a n d snow f a l l d u r i n g t h e p e r i o d of t r a f f i c i s low;
3. t h e snow compaction was c a r r i s d o u t d u r i n g a thaw when t h e snow w a s s o wet t h a t a f t e r f r e e z i n g t h e r o a d shows t h e a p p e a r a n c e of a w a t e r - s p r a y e d w i n t e r r o a d ; 4. t h e r o a d i s t h o r o u g h l y w a t e r - s p r s y e d , o r 5. t h e r o a d , b e f o r e o p e n i n g t o wheeled v e h i c l e s , h a s b e e n u s e d f o r a s u f f i c i e n f ; p e r i o d o f t i m e by c r a w l e r t r a c t o r s a n d h a l f - t r a c k e d v e h i c l e s . The b a s i c a i m o f t h e s t u d i e s of w i n t e r r o a d s i s a t p r e s e n t t o f i n d a s i m p l e and i n e x p e n s i v e method o f snow p r o c e s s i n g which w i l l e n a b l e compaction of d e e p snow c o v e r s s o t h a t it w i l l s t a n d t r a f f i c of wheeled v e h i c l e s .
If t h e c a r g o i s c a r r i e d by s l e i g h , drawn e i t h e r S y h o r s e , c r a w l e r t r a c t o r o r h a l f - t r a c k e d v e h i c l e , g e n e r a l l y o n l y t h e h a r d - n e s s of t h e s u r f a c e of t h e r o a d i s i m p o r t a n t .
B e s i d e s ploughed r o a d s and compacted snow r o a d s , a l s o " h a l f - compactedn snow r o a d s are u s e d . Such r o a d b e d s are p r e p a r e d b y compacting snow, b u t t h e y a r e k e p t i n r e p a i r by p l o u g h i n g . T h i s t y p e of r o a d i s s u i t a b l e , depending on t h e t h i c k n e s s of t h e com- p a c t e d snow l a y e r , f o r t h e t y p e s of t r a f f i c which f u r t h e r compact t h e snow, o r f o r t h e t r a f f i c of wheeled v e h i c l e s .
The problems c o n c e r n e d w i t h t h e magnitude of f r i c t i o n on snow r o a d s w i l l n o t b e d i s c u s s e d here.
C o n s t r u c t i o n and Maintenance of Snow Boads
The term n c o n s t r u c t i o n o f w i n t e r r o a d s " i n c l u d e s many o t h e r o p e r a t i o n s b e s i d e s c o n s t r u c t i o n o f snow r o a d s . E x p l a n a t i o n s of s u c h o p e r a t i o n s , however, do n o t b e l o n g w i t h i n t h e bounds of t h i s p a p e r .
T h e r e a r e two r e a s o n s f o r u s i n g w i n t e r r o a d s : t h e y c a n be b u i l t r a p i d l y and t h e y a r e i n e x p e n s i v e . However, t h e y a r e i n e x p e n s i v e o n l y u n d e r t h e c o n d i t i o n t h a t t h e p r o c e d u r e s f o r con- s t r u c t i o n makes t h e maximum u s e of p r e v a i l i n g w e a t h e r c o n d i t i o n s t o improve t h e h a r d n e s s o f t h e snow. I n o t h e r words t h e c o n s t r u c t i o n p r o c e d u r e s h o u l d :
a )
b r e a k up* snow p a r t i c l e s , i f t h e y a r e n o t s u f f i c i e n t l y small ( t h u s i n c r e a s e of c o n t a c t between p a r t i c l e s ) ;b )
mix
t h e snow o f t h e b o t t o m l a y e r s which h a s a h i g h e r t e m p e r a t u r e , w i t h snow from u p p e r l a y e r s which h a s l o n e r t e m p e r a t u r e s ( r e s u l t s-
s t r o n g e r b o n d i n g between p a r t i c l e s ),
a n d c ) compact t h e snow l a y e r s ( t h i s i n c r e a s e s t h e c o n t a c t b e t w e e n snow p a r t i c l e s , i n t e r n a l f r i c t i o n of snow a n d h e a t c o n d u c t i v i t y . The a i m of t h e m a i n t e n a n c e o p e r a t i o n s , on t h e o t h e r h a n d , i s p r i m a r i l y t o p r e s e r v e t h e b e a r i n g p r o p e r t i e s of t h e r o a d by c o u n t e r a c t i n g t h e o c c u r r e n c e of s o f t e n i n g o f snow due t o n a t u r a l c o n d i t i o n s . These measures h a v e t o p r e v e n t a ) growth of t h e s i z e of t h e snow p a r t i c l e s , b ) change o f s h a p e of t h e p a r t i c l e s t o a s p h e r e w i t h accompaning smoothening o f t h e s u r f a c e s , c ) m e l t i n g o f t h e b i n d i n g i c e between t h e p a r t i c l e s , and d ) movement o f t h ematerial i n form of w a t e r vapour from t h e b o t t o m l a y e r s t o u p p e r l a y e r s .
The s u r f a c e of t h e r o a d , n a t u r a l l y , h a s t o b e k e p t l e v e l d u r i n g u s e .
Nowadays snow i s p r o c e s s e d w i t h c r a w l e r t r a c t o r s , h a l f - t r a c k e d wheel t r a c t o r s , snow t r a c t o r s , r a c k r o l l e r s , r o t a r y til-
l e r s , snow m i x e r s , snow p a c k i n g d i s c s , v i b r a t i n g p l a t e s , d r a g s a n d , i n case of ploughed r o a d s , a l s o by snow p l o u g h s and snow b l o w e r s .
T r a c k - t y p e t r a c t o r ( c r a w l e r t r a c t o r ) mixes and compacts t h e snow u n d e r t h e t r a c k s by p a s s i n g o v e r i t r e p e a t e d l y . The s u r f a c e i s l e v e l l e d l a t e r w i t h a d r a g t o p r o d u c e a r e l a t i v e l y good r o a d .
If t h e snow c o v e r i s s o t h i c k t h a t t h e i n f l u e n c e of t h e t r a c k s d o e s n o t r e a c h t h e g r o u n d , t h e bottom l a y e r s r e m a i n s o f t . Compacting i s more e f f e c t i v e , t h e g r e a t e r t h e l o a d i n g s u r f a c e of t h e t r a c k s . If t h e snow l a y e r i s t h i c k o n l y a h a r d s u r f a c e c r u s t i s formed. H a l f - t r a c k e d wheeled t r a c t o r ( F i g . 7 ) g e n e r a l l y p r o d u c e s similar r e s u l t s a s t h e p r e v i o u s l y d e s c r i b e d v e h i c l e , b u t i t s com- p a c t i n g e f f e c t i n d e e p snow i s s t i l l more l i m i t e d t o t h e s u r f a c e l a y e r s , The same i s t r u e f o r wheeled t r a c t o r s e q u i p p e d w i t h t r a c k s of b o m b a r d i e r t y p e .
The snow mixing and compacting e f f e c t of snow t r a c t o r s ( F i g , 8 ) i s , l i k e t h a t of o t h e r similar l i g h t - w e i g h t t r a c k - t y p e t r a c t o r s , l i m i t e d t o t h e s u r f a c e l a y e r of t h e r o a d , b e c a u s e of t h e small p r e s s u r e of t h e t r a c k s . ' h e y produce r o a d s which h a v e a d e q u a t e b e a r i n g p r o p e r t i e s f o r t h e i r own u s e , b u t g e n e r a l l y d o n o t b e a r h e a v i e r t r a f f i c .
The b e a r i n g c a p a c i t y o f snow i s . c o n s i d e r a b l y improved i f a r a c k r o l l e r o r some o t h e r snow compacting mechanism i s a t t a c h e d t o t h e motor v e h i c l e . I f t h e r o l l e r i s of c l o s e d t y p e , it h a s a compacting e f f e c t o n l y , which i n c r e a s e s w i t h w e i g h t . I n o u r
c o u n t r y a s p e c i a l r o l l e r o f open s t r u c t u r e h a s b e e n developed f o r b u i l d i n g snow r o a d s , l e e . , t h e r a c k r o l l e r ( r a i l r o l l e r ) ( F i g . 10).
It h a s b o t h c r u s h i n g a n d compacting e f f e c t on t h e snow. F u r t h e r - more, it mixes t h e u p p e r l a y e r of snow t o some e x t e n t , t h u s making p o s s i b l e t h e p e n e t r a t i o n o f c o o l e r t e m p e r a t u r e s d e e p e r i n t o t h e snow c o v e r . The r a c k r o l l e r i s more e f f e c t i v e i n d e e p snow t h a n t h e c l o s e d r o l l e r , The u s e of r a c k r o l l e r s h a s s p r e a d a l s o t o o t h e r n o r t h e r n c o u n t r i e s . I n Sweden t h e r a c k r o l l e r s a r e power d r i v e n . The power i s t r a n s n ~ i t t e d from t h e t r a c t o r t o t h e a x i s of t h e r o l l e r . The c i r c u m f e r e n t i a l v e l o c i t y of t h e r o l l e r i s h i g h e r t h a n t h a t of t h o t r a c t o r w h e e l s , a n d as a r e s u l t t h e d e p t h o f I t s mixing and c r u s h i n g e f f e c t e x t e n d s a s d e e p as 70 cm. T h i s w a s e s t a b l i s h e d d u r i n g e x p e r i m e n t s c a r r i e d o u t by M e t s a t e h o i n t h e w i n t e r of 1 9 5 7 , u s i n g a r e d d y e s p r a y e d i n f r o n t of t h e r o l l e r . A b e t t e r h a r d e n i n g e f f e c t on t h e snow c a n b e a c h i e v e d
w i t h a power-driven r o l l e r t h a n by any of t h e equipment mentioned above. E f f e c t i v e h a r d e n i n g , however, d o e s n o t e x t e n d t o t h e v e r y b o t t o m of t h e r o a d , s i n c e i t s compacting e f f e c t d o e s n o t r e a c h t h e bottom. T h i s r o l l e r a l s o h e l p s t h e advancement of t h e draw- i n g v e h i c l e i n t h i c k snow.
A r o t a r y t i l l e r , i n which t h e k n i v e s h a v e b e e n r e p l a c e d w i t h
"snow whippingn a t t a c h m e n t s ( F i g . l l ) , h a s b e e n t e s t e d i n Sweden.
It g r i n d s and mixes snow more e f f e c t i v e l y t h a n any of t h e e q u i p - ment d e s c r i b e d above. The snow becomes a t t h e same t i m e somewhat compacted. I f t h e r o a d bed i s t r e a t e d a f t e r t h e u s e of a t i l l e r by a snow p a c k i n g d i s c a n d t h e n l e v e l l e d , t h e compaction e x t e n d s t o t h e b o t t o m of t h e snow c o v e r . The 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 s i s , however, even i n t h i s c a s e , g r e a t e r t h a n it i s i n t h e b o t t o m l a y e r s . T h e t t P u l v i m i x e r t t i s a d e v i c e d e v e l o p e d by t h e U . S . Army f o r t h e p u r p o s e of b u i l d i n g a i r f i e l d s from snow. A c c o r d i n g t o i n f o r - m a t i o n o b t a i n e d from t h e l i t e r a t u r e , t h i s d e v i c e mixes t h e snow and h e a t s it w i t h 8 b u r n e r s , t h u s c a u s i n g b i n d i n g of t h e snow p a r t i c l e s t o e a c h o t h e r t h r o u g h f r e e z i n g . F u r t h e r m o r e ,
it
h a s a compacting e f f e c t on t h e snow. The u s e of t h i s d e v i c e , on which more d e t a i l e d i n f o r m a t i o n i s l a c k i n g , would p r o b a b l y b e t o oe x p e n s i v e u n d e r o u r c o n d i t i o n s .
The snow p a c k i n g d i s c d e v e l o p e d i n t h e SDA Experiment S t a t i o n i n Sweden, i s a s i m p l e d e v i c e made of metal p l a t e ( F i g . 1 2 ) . I t s p e r f o r m a n c e , however, h a s b e e n found t o b e similar t o t h a t of a c l o s e d r o l l e r . I n o t h e r w o r d s , it i n c r e a s e s c o n s i d e r a b l y t h e s u r f a c e h a r d n e s s of t h e snow c o v e r . The h e a v i e r t h e e q u i p m e n t , t h e more e f f e c t i v e i t i s . E x p e r i m e n t s w i t h t h i s d e v i c e h a v e b e e n f a v o u r a b l e , e s p e c i a l l y when t r e a t i n g wet snow.
The v i b r a t i n g p l a t e ( F i g . 13) w a s t e s t e d by M e t s a t e h o
i n
t h e w i n t e r of 1957. It w a s found t h a t t h i s d e v i c e compacted t h e snow c o v e r down t o t h e b o t t o m s o t h a t t h e h a r d n e s s v a l u e s o f t h e snowwere more f a v o u r a b l e w i t h d e p t h t h a n w i t h o t h e r methods. I n c o l d w e a t h e r i t l e f t a l o o s e s u r f a c e l a y e r , which u n d e r t h e p r e v a i l i n s
c o n d i t i o n s , however, hardened r e a d i l y by t h e a c t i o n of t r a f f i c and l e v e l l i n g . During a thaw t h e s u r f a c e l a y e r s a l s o h a r d e n e d . By u s i n g o n l y a v i b r a t i n g p l a t e , drawn by a h o r s e , a snow r o a d
c a n b e produced i n wet snow o r i n t h e f i n e - g r a i n e d snow of e a r l y w i n t e r , i n d e p e n d e n t of t h e t h i c k n e s s of t h e snow, which w i l l b e a r b o t h h o r s e a n d c r a w l e r t r a c t o r t r a f f i c . The p l a t e h a s , a c r u s h i n g e f f e c t on t h e snow i n a d d i t i o n t o compacting. Dragging i s a n e c e s s a r y measure f o r l e v e l l i n g a l l w i n t e r r o a d s d u r i n g t h e i r c o n s t r u c t i o n . F o r t h i s p u r p o s e d r a g s of s p e c i f i c d e s i g n have b e e n b u i l t ( F i g . 1 4 ) , which i n a d d i t i o n t o l e v e l l i n g h a v e mixing a n d compacting e f f e c t s on t h e s u r f a c e l a y e r s of t h e snow r o a d .
Snow ploug& e x p o s e t h o s e l a y e r s of t h e snow c o v e r which due t o t h e i r h i g h c o n t e n t of b i n d i n g water f r e e z e i n c o l d w e a t h e r f i r m l y and r a p i d l y . S i n c e a ploughed w i n t e r r o a d r e q u i r e s a
c o n s i d e r a b l y g r e a t e r smoothness of t h e u n d e r l y i n g g r o u n d - l e v e l o f t h e r o a d t h a n a compacted snow r o a d , and s i n c e b o t h i n i t i a l p l o u g h i n g and maintenance p l o u g h i n g s a r e c o s t l y , such r o a d s
are
more e x p e n s i v e t h a n compacted snow r o a d s . The u s e o f snow ploughs i n t h e c o n s t r u c t i o n of w i n t e r r o a d s w i l l n o t b e d i s c u s s e d i n
d e t a i l h e r e .
The e f f e c t of a snow b l o w e r i s s i m i l a r t o ploughing. Snow b l o w e r s which have b e e n a v a i l a b l e on t h e m a r k e t up t o now have b e e n t o o e x p e n s i v e and t h e i r c a p a c i t y t o o low s o t h a t t h e y have n o t been c o n s i d e r e d f o r u s e u n d e r o u r c o n d i t i o n s .
The i d e a l method of c o n s t r u c t i o n of snow r o a d s , a c c o r d i n g t o p r e s e n t - d a y e x p e r i e n c e s i s as f o l l o w s . The snow i s t r e a t e d w i t h a snow m i x e r which mixes t h e snow down t o t h e bottom of t h e r o a d , and s i m u l t a n e o u s l y g r i n d s t h e snow p a r t i c l e s as f i n e a s
p o s s i b l e . T h i s i s f o l l o w e d by d r a g g i n g i n o r d e r t o l e v e l t h e r o a d , and by compaction w i t h a v i b r a t i n g p l a t e . The r e s u l t w i l l
p r o b a b l y b e a snow r o a d compacted t o t h e ground l e v e l . The b e a r - i n g c a p a c i t y and r e s i s t a n c e t o t h e s o f t e n i n g e f f e c t of t h e w e a t h e r of s u c h r o a d s w i l l b e b e t t e r t h a n t h o s e o f r o a d s b u i l t a c c o r d i n g t o any of t h e e a r l i e r methods.
I n normal c o n d i t i o n s t h e road h a s t o b e b l l i l t i n one o r two o p e r a t i o n s i n o r d e r t o keep down t h e c o s t s . Furthermore, t h e speed of compacting h a s t o a v e r a g e 4 t o 8 km. p e r hour. F o r snow of a n p r o x i m a t e l y 40 t o 50 cm. deep t h e p r o c e s s i n g equipment i s p u l l e d e f f e c t i v e l y by h a l f - t r a c k e d wheeled t r a c t o r s and wheeled t r a c t o r s equipped with t r a c k s of bombaraier t y p e . A f a s t t r a c t o r
w i t h s u f f i c i e n t l y low o p e r a t i n g c o s t s which can o p e r a t e i n snow of any depth i s s t i l l l a c k i n g . Whether t h e s o l u t i o n w i l l b e a j e e p equipped w i t h f u l l - t r a c k e d c h a i n s ( F i g . 1 5 ) , t h e new Swedish "Barnsen" c r a w l e r t r a c t o r , o r something e l s e , i s an open q u e s t i o n .
During c e r t a i n experiments (Ager 1956) i n snow 90 t o 95 cm. deep t h e f o l l o w i n g range of v a l u e s f o r drawbar p u l l were r e c o r d e d f o r t h e d i f f e r e n t t y p e s of t r a c t o r s .
F i a t CP and C l e t r a c IIG 42 w i t h A l f a t r a c k s 400- 600 kgm.
F i a t OM w i t h A l f a t r a c k s 800-1000 kgm.
Fordson Major w i t h f u l l t r a c k s 800-1000 kgm.
V e s s l a , o r i g i n a l 600- 700 kgm.
Nove snow t r a c t o r approx.800 kgm.
The p r a c t i c a l maintenance measures f o r compacted snow r o a d s a r e t r a f f i c , d r a g g i n g , ploughing and s p r a y i n g w i t h w a t e r . The
compacting o p e r a t i o n s g i v e t o a road i n i t i a l b e a r i n g s t r e n g t h , e n s u r i n g t h e i n i t i a l t r a f f i ~ a b i l i t y ~ The f i n a l h a r d e n i n g of a road i s brought about by t r a f f i c ( F i g . 1 7 ) , which i s a l s o t h e
most i m p o r t a n t maintenance measure. It h a s a c o n t i n u a l compacting e f f e c t on t h e snow. F u r t h e r m o r e , t h e loaded s l e i g h r u n n e r s melt t h e s u r f a c e snow b e n e a t h them, which i n c o l d w e a t h e r r e f r e e z e s immediately. The h a r d e n i n g e f f e c t of t h e t r a f f i c e x t e n d s t o some d i s t a n c e below t h e s u r f a c e , b u t I n t h e c a s e of a t h i c k snow l a y e r n e v e r down t o t h e bottom. On t h e o t h e r hand, t h e t r a c k s , h a l f -
t r a c k s and a n t i - s k i d d e v i c e s of t r a c t o r s g r i n d t h e snow on t h e s u r f a c e of t h e road t o f i n e p a r t i c l e s . During a thaw s l u s h i s formed from t h i s l o o s e m a t e r i a l . It i s i m p o r t a n t t h a t t h e s l e i g h r u n n e r s f o l l o w t h e t r a c k s of t h e t r a c t o r i n o r d e r t o repack t h e loosened snow. It i s n o t a d v i s a b l e t o u s e snow c h a i n s on t h e t r a c t o r s equipped w i t h s p i k e s and t e e t h . Tho t r a f f i c h a s t o b e
of a road improving t y p e .
I n or$.er t o keep t h e road s u r f a c e l e v e l and t o e l i m i n a t e t h e harmful e f f e c t of wear, dragging h a s t o be c a r r i e d out on a l l t y p e s of w i n t e r roads.
The i n c r e a s e of p o r o s i t y of t h e snow i n t h e bottom l a y e r s of a compacted snow road h a s been found t o b e r e l a t i v e l y s m a l l . The movement of m a t e r i a l from t h e bottom t o t h e upper l a y e r s i s
t h u s r a t h e r l i m i t e d and w i t h o u t importance, s i n c e compacted snow h i n d e r s e f f e c t i v e l y t h e upward d i f f u s i o n of w a t e r vapour, I f snow h a s been mixed b e f o r e compacting no n o t i c e a b l e f o r m a t i o n of porous snow and Itcorn snowtt t a k e s p l a c e ,
Ploughing as a maintenance o p e r a t i o n can be s u c c e s s f u l l y employed f o r compacted snow r o a d s t o remove t h e l o o s e l a y e r of snow produced by t h e t r a f f i c on t h e road. The o r i g i n a l c o n d i t i o n s of t h e road can be r e s t o r e d much f a s t e r a f t e r a snow f a l l by
ploughing t h a n by dragging o r through t r a f f i c . Embankments
formed on t h e s i d e s of t h e road b y t h e plough may be troublesome because of t h e snow d r i f t i n g from t h e embankments t o t h e road.
S p r a y i n g of w a t e r on snow roads i s employed o n l y , a c c o r d i n g t o t h e present-day c o n c e p t i o n , a s a maintenance measure o r i n
c o n d i t i o n s where t h e l a c k of w a t e r , which i s t h e j o i n i n g s u b s t a n c e between t h e snow p a r t i c l e s , i s t h e a f f e c t i v e f a c t o r In t h e hard-
e n i n g o r maintenance of h a r d n e s s of an e s t a b l i s h e d road.
During t h e l a s t f o r t y y e a r s i t h a s been custornary t o c o n s i d e r snow mainly a s an o b s t a c l e t o t r a f f i c ( F i g , 2 1 ) , s i n c e t h e know- l e d ~ e of t h e p r o p e r t i e s of snow ss a r o a d - b u i l d i n g m a t e r i a l h a s been i n a d e q u a t e .
For t h e c o s t of t h e a n n u a l i n t e r e s t of t h e investment f o r a sirnple road which i s t r a f f i c a b l e t h r o u g h o u t t h e y e a r a w i n t e r road w i t h good t r a f f i c - b e a r i n s p r o p e r t i e s can be b u i l t . I n t h i s way a r e a s which a r e n o t connected b y perenaneat roads can be
i n c l u d e d a t l e a s t one p a r t of t h e y e a r i n t h e g e n e r a l road system. For t h e s e r e s s m s i t i s worthwhile t o pay s z r i a u s a t t s n t i 3 n t o t h e development of b u i l d i n g t e c h n l q u c s f o r w i n t s r ~ D T J Z ,
L i t e r a t u r e
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H e l s i n k i , 1952. P u t k i s t o , K a l l e . S u i l d i n g a n d m a i n t e n a n c e o f w i n t e r r o a d s f o r motor t r a f f i c . M e t s a t e h o I n s t r u c t i o n Book. H e l s i n k i , 1952. P u t k i s t o , K a l l e . F o r e s t r o a d f o r m o t o r t r a f f i c o r w i n t e r r o a d f o r motor t r a f f i c . M e t s 5 t e h o R e p o r t No. 8 3 . H e l s i n k i , 1952. P u t k i s t o , K a l l e . Temporary r o a d s i n w i n t e r . P r o c e e d i n g s o f t h e
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F i g . 1
The hexagonal s t r u c t u r e of i c e , a c c o r d i n g t o PAULING. On t h e l e f t t h e m o l e c u l e s a r e drawn t o a p p r o x i m a t e l y
t h e n a t u r a l s c a l e , on t h e r i g h t t o a s m a l l e r s c a l e
F i g . 2
T y p i c a l snow c r y s t a l s t h a t o r i g i n a t e a t d i f f e r e n t t e m p e r a t u r e s , a c c o r 3 i n g t o WALL. The u n s p e c i f i e d s c a l e shown below
