Discussion of avalanche defence works

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Discussion of avalanche defence works

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DISCUSSIGN OF dVAUCHl2 DWEiU'CE WORKS (Diskussionsbeitrag zum Lawinenverbau)

by

Edwin Bucher

t r a n s l a t e d by

D. A. S i n c l a i r

This i s the F i f t h o f the S e r i e s o f Translations Prepared f o r t h e D i v i s i o n o f Building Research.

Ottawa

6 April 194.d.

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PREFACE

This i s a r e p o r t from t h e Eidgenossische I n s t i t u t f u r Schnee- und Lawinenforschung (swiss Federal I n s t i -

t u t e f o r Snow and Avalanche ~ e s e a r c h ) Davos, Switzerland.

The t r a n s l a t i o n i s published with t h e approval of t h e authors,

T h e National Research Council through i t s Divisions

o f Mechanical Engineering and Building desearch has a c l o s e l i n k with Eidgenossische I n s t i t u t f o r Schnee- und

Lawinenforschung. Mr. G.J. Klein of t h e former Division

and t h e w r i t e r have both spent some time a t t h e I n s t i t u t e .

D r , Marcel de Quervain who i s now t h e D i r e c t o r of t h e

I n s t i t u t e spent one year a s a s p e c i a l member of t h e s t a f f

of t h e Division of Building Research during 1948 and 1949

and l a i d t h e foundation f o r t h e snow and i c e r e s e a r c h work t o which t h e Division looks forward.

The Swiss I n s t i t u t e has been a pioneer i n connection

with avalanche c o n t r o l work, This work has c l o s e relevance

t o Canada d e s p i t e t h e d i f f e r e n t c o n d i t i o n s encountered i n

t h e two countries. It i s t h e r e f o r e a pleasure t o s e e t h i s

t r a n s l a t i o n published, taken from an a r t i c l e which appeared i n "Die AlpenH f o r 'March, 1947.

Robert I?. Legget,

Director,

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Tech, T ~ m s , TT-66 DISCUSSION OF AVALAN CHE DEFENCE WORKS

I I N T R O D U C T I O N AND DEPIfTITION OF THE PROBLEM

"There i s n o t h i n g more p r a c t i c a l t h a n a good t h e o r y g v , !Phis p r o v e ~ b h a s g e n e r a l v a l f d i t y , b u t i n t h e s t r u g g l e a g a i n a t n a t u r a l f o r c e s i t a c q u i r e s a s p e c i a l a f g - n i fi c a n c e ,

P ~ c ~ $ i c u 1 a ~ I y i n t h e f i e l d of avalanche d e f e n c e , which i s t o be d i s c u s s e d h e r e , 1% i s r e v e a l e d a g a i n and a g a i n how complex t h e i n f l u e n c e s a r e which b r i n g a b o u t t h e t o t a l e f f e c t , and how df f f i c u l t i t i s , t h e r e f o r e , t o d e v l s e

e f f e c t i v e measures o f p r o t e c t i o n , u n l e s s i t f s poasibPe t o f a l l back on a g e n e r a l l y a p p l i c a b l e t h e o r y , I n r e c o g n i t i o n of t h e s e d i f f i c u l t i e s i n t e r e s t e d p e p r e s e n t a t f v e s o f b o t h t h e pure and a p p l i e d s c i encea u n f t e d i n 1931 t o form a s t u d y commission f o p t h e purpose s f c l a r i fy f n g t h e mechanical laws of a v a l a n c h e development, That was t h e c o r n e r s t o n e of t h e F e d e r a l I n s t i t u t e f o r Snow and Avalanche Research on \Veissfluhjo@h above Davos, whf ch was founded i n B94a0

What p r o g r e s s has s i n c e been made? The b a s i c

p h y s i c a l p r o p e r t i e s o f snow a r e Indeed known, I t h a s r e c e n t l y been p o s s i b l e t o gain a d e e p e r i n s i g h t i n t o the s t r e s s p r o -

c e s s e s s f t h e snow-cover and hence t o t r a c e the development of a v a l a n c h e s to mechanfcab laws o f u n i v e r s a l v a l i d i t y , F i n a l l y , t h e s e t h e o r i e s a r e now b e f n g p u t i n t o s f m p l i f f e d form, thus making them a c c e s s i b l e t o wider c i r c l e s , But when a l l this has been s a i d we must s t i l l c o n f i n e o u r s e l v e s

t o v % n d a m e n t d s D q o Before t a k i n g up t h e q u e s t i o n o f a c u r e , t h e malady must f i r s t be diagnosed, What s t a r t s an avalanche? Why does i t occur h e r e and n o t t h e r e m y t a d a y and n o t

t o m s r ~ o w ? A few f n d i c a t f ons may r e v e a l t h e d i v e k a f f f ed n a t u r e o f t h e p ~ o b l e m ,

From a chemical s t a n d p o i n t snow i s a v e r y s i m p l e s u b s t a n c e , b u t m e c h a n f c a l l y f t f s a h i g h l y complex a g g r e g a t e , When i t i s r e c a l l e d t h a t snow may have a d e n s i t y r a n g i n g

from 30 t o 600 kg, p e r m, m,, t h a t i t i s a porous c s y s t a l - l i n e a g g r e g a t e whose elements a r e cons t a n t l y changf ng In s i z e , shape, and r e l a t i v e posf t f o n , t h e n t h e scope s f t h e l a b o r a t o r y i n v e s t i g a t f o n s d e a l i n g w i t h f t wf Pl be readf l y a p p a r e n t ,

The m a t e r i a l p r o p e r t i e s whf ch a r e u s u a l l y con- s t a n t I n s o l f d s i n t h i s c a s e a r e f u n c t f o n s ~f p l a c e and time,

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2

Tech, Trans, TT-66

But t h a t i s n o t a l l , The s t s a t i f f c a t i o n p a t t e r n o f t h e snow- cover f a a l s o , a s a r u l e , v e r y c o m p l i c a t e d , w h i l e v a r i a t i o n s b o t h i n t h e c h a ~ a c t e r o f t h e t e r r a i n , and i n t h e d e p t h o f t h e snow cause a c c e l e r a t f o n s and r e t a r d a t i o n s f n t h e p l a s t i c flow of t h e white cover, which, i n t h e n a t u r a l case, g e n e r a l l y r e - s u l t i n v e r y complex s t k e s s p a t t e r n s ,

The i n t ~ f n s i c weight o f the snow, l i k e t h a t sf a b r i d g e , f s borne above t h e s o l i d e a r t h b a s e by numerous s u p p o r t s , I f g r e a t e r demands a r e made upon t h e snow-cover, o r snow-b f d g e , , because o f i n c r e a s e d l o a d i n g , o s f f t h e f i m n e s s x f o f t h e s t r u c t u r a l m a t e r i a l , v e r y s l i g h t i n any c a s e , becomes damaged, then a break must o c c u r , Both p ~ o - c e s s e s a r e about e q u a l l y f r e q u e n t i n c a u s i n g breakage of t h e snow=cover: i t i s - e i t h e r a i o c a l i n c r e a s e i n s t r e s s d u e t o continuous s n o w _ f a l $ e m f c o h e s i o n i n t h e - m ~ e ~ m -

-

Thus, i n e v e r y f n v e s t f g a t i o n of s t a b i l i t y t h e concern i s w i t h t m d l f f e r e n t v a l u e s , the "degree o f s t r e s s n and t h e "ffrmneas o f t h e m a t e r i a l t ' , I f t h e s e v a r i a b l e s a r e p l a t t e d a s a f b n c t f o n o f the time, a s shown I n F i g u r e 1, t h e s a f e t y f a c t o r of each unf t may be p e r c e i v e d v e r y simply, D l -

vekgence of t h e two c u r v e s s i g n i f i e s s t a b i l f z a t f o n , convek- gence an i n c r e a s e fa? t h e d a n g e r , I f a t any p o i n t the curves of s t r e s s and f i r m n e s s i n t e r s e c t breakage w i l l be I n e v i t a b l e , Ff gure 2 i l l u s t r a t e s t h e development of breakage from s t r e s s

caused b y a n i n c r e a s e f n t h e snow d e p t h , while F i g u r e 3 shows t h e f a l l i n t h e firmness c u r v e r e s u l t i n g , f o r example, from sudden h e a t f n g ,

On t h e b a s i s o f those diagrams t h e p r o t e c t i v e measures may be f o m u l a t e d v e r y p r e c f s e l y :

E l t h e r t h e f i m e s s o f t h e m a t e r i a 1 must be i n c r e a s e d o r

%hgis

t s e s g ' must-Fe-reduZd I f t h e snbw-cgve~" ---- i

be a r % f f f ~ i h ~ ~ So r u n s the s r r n ~ l e ~ & ~ ~ theoret'f d ,

s

-

c a l ~ l a ~ ~ t h e s t u d e n t - t a t i c s , and e v e r y $ r a c t f c a l

u n d e r t a k i n g must conform t o f t ,

However, t h e snow-cover and i t s minfmum f i r m n e s s v a l u e must b e assumed a s given, There w e p r o b a b l y t e r r a i n s , such a s f f e l d s o f mountain r o s e s o r a l d e r s , on which t h e snow- cover, duo t o p a r t i a l r e t a r d a t i o n of the s e t t l e m e n t p r o c e s s 9 remains r e l a t f v e l y porous, and w h e ~ e f t can be made f i r m e r by removal of t h e s e p l a n t s , I t i s a l s o probable t i a t corn-

c o n s - o l i d a t f o n

-

~an-~artif,icial__ - -- -

- T r a n s l a t o r n o - : F e s t f g k s i t %n

the s e n s e of' " n a t i v e s t a b i i f t y N ,

or

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Tech, T ~ a n s ,

TT-66

c r e a t i o n of Dressure zones, But t h e s e measures cannot i n themselves o b t a i n t h e d e s i r e d r e s u l t , I n t h e p r a c t i c a l - c a s e ,

i t f s much_mol.eaquestion of a v m a n

-peaks, This g o a l i s a t t a f n e d e i t h e r " by a r t i f i c l a l i y i n f l u e n - c i n g t h e d i s t r i b u t i o n o f t h e masses of snow, OP e l s e by COP-

r e c t i n g t h e g i v e n c o n d i t i o n s of s t s a t f f i c a t f o n ,

I n defence works, t h e r e f o r e , two p r i n c i p a l methods a r e t o be d f f f e r e n t f a t e d :

" D r i f t defences", where an a t t e m p t is made t o i n f l u e n c e t h e d e p o s i t i n g of t h e snow, and " S t r e s s defences", where t h e s t r a t f f i c a t i o n r e l a t i o n s h i p s a r e t o b e c o r r e c t e d , Both p o s s i b f l i t f e s a r e t o b e d i s c u s s e d in t h e f o l l o w i n g s e c t i ons

,

DRIFT

DF,F'ENCES

-

-When " d r i f t defencesg' ape b u i l t an a t t e m p t 1s made t o a l t e r t h e configupat1 on of t h e t e r r a i n a r t i f i c i a l l y

i n such a way t h a t snow b e i n g d e p o s i t e d under the i n f l u e n c e of wind w f l l assume a favourable s t r e s s p a $ t e r n , The wfnd i s t h e r e f o r e of d e c f s f v e impostance, gpd i t is obvious t h a t t h i s type o f defence i s applfcablr= o n l y t o s l o p e s

on

which the developmant of avalanches i s due p r f n c i p a l l y t o tkle snow d e p t h d i s t r i b u t i o n a s determined by t h e a i r flow p a t t e r n ,

A 1 though t h f s probably r e p r e s e n t 8 a s tandakd c o n d i t i o n , t h e l f m i t a t f o n must nevertheLess be s t a t e d ,

Excessfvs l o c a l accumulatfons of snow can be

prevented by compelling t h e snow p a r t i c l e s t o f a l l e l s e w h e ~ s , e f t h e r i n f r o n t o f , alongside,, o r behind the p l a c e i n ques- t i o n , Observations under n a t u r a l c o n d i t f ons m e s u f f i c i e n t t o show how d e p ~ e s s i o n s , d i t c h e s and the l i k e a r e f i b l e d with snow e a r l v

In

the w i n t e r , whereas peaks a r ~ d r i dges ape aLways blown c l e a ~ ,

These i r r e g u l a r i t i e s i n t h e snow d e p o s f t a r e c l e a r l y due t o unequal wind v e l o c i t i e s , More m a t e r i a l p e r u n i t 1 s d e p o s i t e d a t p l a c e s o r low wind v e l o c i t y than a % p l a c e s of h i g h wf nd velocf t y , Thf f a c t was demonstrated

q u a n t i t a t

_JY

*ly f i r s t by iYelzenbachf

1,

and l a t e r by ~ u ~ s t e r ~ ) and Croce I n v e r y r e c e n t times v a r i o u s r e s u l t s from

n d i u m d e l t e s t s i n the wfnd t u n n e l have been made

cO"esg~Q

57, 67

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Tech, Trans, TT-66

way snow 1s deposf t e d i n the e f f e c t i v e v i c i n i t y o f o b s t a c l e s i s a v a i l a b l e ,

As t h e most i m p o r t a n t r e s u l t s f o r t h e e n g f n e e ~ t h e f o l l o w i n g w%BB s e r v e a s e s s e n t i a l p o i n t s :

When t h e snow bearing wfnd t r a v e r s e s a r e l a t i v e l y f l a t p l a t e a u a n d t h e n d e p o s i t s i t s b u r d e n on t h e l e e w a r d

s l o p e on whfch d e f e n c e s ape t o be b u i l t

-

a s o f t e n happens undek o u r c o n d i t i o n s

-

t h e approach s u r f a c e must b e p r o v i d e d

wfth s t r u c t u r e s which w f l l check t h e wind and c a u s e t h e snow p a r t i c l e s t o f a l l b e f o r e t h e y r e a c h t h e r i d g e , I n t h i s c a s e t h e o b s t a c l e s w i l l b e p l a c e d p e r p e n d i c u l a r t o t h e d i r e c t i o n of t h e wfnd, L o o s e l y woven,, g r f l l - f o m p p e s s u r e s u r f a c e s wfth a r a t i o of p e r f o r a t f o n t o m a t e r i a l c r o s s - s e e t f o n o f a p p r o x f m a t e l y

I:$

m d w i t h vepy f i n e p o r e d f s t k i b u t f o n pro- v i d e t h e b e s t braking e f f e c t , Walls of t h i s k i n d r e s u l t i n a 'wf n d w a ~ d - l e e w a r d w - d

%hey m u s t L e ~ b a e e d _-- a t T h e i r h e i g h g - A ---=---from -=-- t h e

I n a d d i t i o n t o the type of s t r u c t u r e r e p r e s e n t e d i n F i g u r e 5, which i s b e s t c o n s t r u c t e d from a wooden frame w i t h j u t e OF w i r e meshwokk, s c r e e n s o f t r e e s o r hedges and

t h e l i k e a r e t o b e c o n s i d e r e d below t h e t r e e - l i n e , I n t h f s c o n n e c t i o n t h e r e a d e r i s e f e r r e d t o t e v e r y i n f o r m a t i v e s c r e e n s t r i p s ,

8 f

i n v s s t i g a t i o n s of Nagelf ?f a n d F i n n e g c o n c e r n i n g wind Very few p r f n c f p l e s have been developed as y e t f o r t h e c o n s t r ~ u c t i s n and e r e c t i o n of "works f o r a c c e l e r a t i n g t h e windPg, The guf de w a l l s i n this c a s e s h o u l d b e smooth and compact, s h o u l d r u n i n t h e main d i r e c t i o n of t h e wfnd and, t o g e t h e r w i t h t h e t e ~ r a i n ~ s h o u l d SO c a n a l i z e t h e a i r

f l o w , t h a t t h e v e l o c i t y

w e l l

be i n c r e a s e d l o c a l % y , I n p a r - t i c u l a r c a s e a q u i t e good r e s u l t s have a l r e a d y been a t t a i n e d by t r i a l and e r r o r , e x p e ~ i n e n t 6n t h e S c h i a h s p n f o p example,

t o p r e v e n t t h e f o r m a t i o n s f d r i f t s , and a l o n g t h e r i g h t - o f - way of t h e Davos-~Parsenzn RafYway as shown i n F i g u r e 6 wheke

t h e wind i s employed t o sweep t h e r d l s ,

111 STRESS DSFEMCES -

B y d e f i n i t i o n i t i s h e r e a q u e s t f o n o f c o r r e c t i n g t h e g i v e n s _{t r a t i f}1 c a t i o n csndf t i o n s , The conff g u r a t i on o f t h e t e r r a i n i s t o be a l t e r e d i n s u c h a way t h a t no dangerous s t r e s s peaks c a n f o m , These a r e p a r t i c u l a r l y p r e v a l e n t in

t h e t e n s i l e and s h e a r zones, On t h e o t h e r hand s n o w s u b j e c t t o p r e s s u r e undergoes a p k o g r e s s f v e s o l i d i f f c a t i o n a s a

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Tech0 Trans, TT-66 It i s n e c e s s a r y , t h e r e f o r e , f i r s t of a l l t o avoid t h e formation of " t e n s i l e zones" such a s occur a t every sudden convex i r r e g u l a r i t y i n t h e b a s e , OP whenever

t h e r e i s a sudden i n c r e a s e i n t h e d e p t h of the snow, A t t h e s e p o i n t s the snow-cover must be s p l i t , By t h i s meGs a s t r e s s f r e e t r a n s i t i o n from t h e upper zone of low c r e e p i n g v e l o c i t y t o the lower, faster-moving s e c t i o n s i s o b t a i n e d , P i e r c i n g s t r u c t u r e s of any t y p e of c o n s t r u c t i o n a r e s u i t a b l e f o r t h i s purpose, The o n l y a b s o l u t e requirement i s t h a t t h e e n t i r e c r o s s - s e c t i o n of t h e snow-cover s h a l l be c u t r&.$&

t h r o u e from top t o bottom, I n o t h e r words t h e h e i g h t of t h e s t r u c t u r e must be adapted t o t h e l o c a l condf t i o n s , The

p a r t of the snow-cover l y i n g above t h e s t r u c t u r e w i l l be

l a r g e l y supported by i t and w i l l t h e r e f o r e assume t h e d e s i r e d p r e s s u r e c o n d i t i o n , while d i r e c t l y below i t a s p l i t from

c r e e p i n g w i l l be formed, p e r m i t t i n g normal c r e e p i n g of t h e snow-cover, However, i f t h e s t r u c t u r e i s n o t h i g h enough, and the w h i t e cover i s t h e r e f o r e only notched from underneath s o t h a t i t s upper p a r t flows over the s t r u c t u r e , t h e n " a r t f - f i c i a l " - . e n s i l e s t r e s s e s w i l l b e produced which a r e a l l t h e more dangerous s i n c e t h e s u p p o r t i n g c r o s s - s e c t f on of t h e m a t e r i a l h a s a l r e a d y been weakened a t t h i s p o i n t , A s an

i l l u s t r a t i o n of t h i s f a c t the r e a d e r need o n l y b e remfnded of t h e d i s t r i c t s t h a t have b e e n t r a v e r s e d by e a r t h or wall

b e r r a c e s which a r e t o o low, S e v e r a l l a r g e avalanches, some of them d i s a s t r o u s , have been brought about i n r e c e n t y e a r s

t h r o u r h t h i s kind o f f a u l t y c o n s t r u c t i o n ,

Howovar, even t h e c a r e f u l avoidance of t e n s i l e zones w i l l n o t always achieve t h e d e s i r e d s t a b i l i z a t i o n , f o r t h e s t > e s s e s r u y i . p a r a l l e l t o t h e b a s e

-_L-=-

-may a l s o cause a breakage, h e i r recluction i s a l s o g a L ~ l e u by t h e method a l r e a d y mentioned, namely t h e s t r u c t u r e which goes r i g h t through the snow-cover, I f the top of t h e

s t r u c t u r s p r o j e c t s f a r enough beyond the s u r f a c e t h e r e s u l t , owing t o t h e i n f l u e n c e of t h e wind, w i l l be t o l e s s e n t h e g r a d i e n t of t h e s u r f a c e a s compared wf th t h e o r i g i n a l s l o p e , so t h a t by t h i s means alone a c e r t a i n s t a b i l f z a t i o n w i l l b e a t t a i n e d , E'urtherrnore c u t t f n g s h o r t the p r e s s u r e i n t h i s way l e a d s t o a r e d u c t i o n of t h e f n t ~ i n s i c weight of t h e snow, which causes t h e s h e a r i n g s t r e s s e s , I n many c a s e s , t h e r e -

f o r e , tho h e i g h t of t h e s t r u c t u r e s and t h e d i s t a n c e s between s u c c e s s i v e ones must be determined n o t o n l y by t h e p o s i t i o n of the t e n s i l e zones but a l s o by the s h e a r i n g s t r e s s e s , I t i s obvious, of c o u r s e , t h a t t h e l o c a l c h a r a c t e r of t h e snow f s of d e c i s i v e importance a l o n g w i t h the c o n f f g u r a t f on of t h e t e r r a i n ,

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I V EFFECTIVENESS O F DIFFERENT STRUCTURAL

UNITS

OF S

In t h e f o r e g o i n g e x p l a n a t i o n s the n e c e s s i t y o f s p l i t t i n g o r d i v f d i n g up t h e snow-cover h a s been mentfoned s e v e r a l times, The a p p r o p r i a t e s t m c t u r a l p r o v i s f ons a r e now t o b e d i s c u s s e d , Some o f t h e i d e a s r e g a r d i n g t h i s a r e

s t f l l q u i t e mnf'used, b u t s f n c e i t I s t h e s t m c t u r a l a s p e c t which i s of s p e c i a l i n t e r e s t t o t h e e n g i n e e r some f b r t h e r t r e a t m e n t o f i t seems t o b e i n d i c a t e d a t t h i s p o i n t ,

A f i r s t and immediately obvious measure i n view of t h e t h e o r e t i c a l r e q u i r e m e n t c o n s i s t s i n t h e f a m i l i a r

b r e a k i n g up of t h e s l o p e , o r formation of s t e p s , s i m i l a r t o t h e method used i n damming t o r r e n t s , To make t h i s method of t r e a t m e n t e f f e c t f v e the i n d i v i d u a l s t e p s must be approximately v e r t i c a l and must b e a t l e a s t a s h i g h a s t h e maximum d e p t h o f snow which o c c u r s i n t h e d i s t r i c t concerned,

However, .every t e r r a s s i n g work h a s t h e disadvan- t a g e t h a t i n t h e course o f normal c r e e p f n g o f the snow-cover, c o r n i c e s a r e formed, a s shown i n F i g u r e 9 A , which b r e a k o f f and which may cause avalanches to develop, There a r e two p o s s i b l e mehsures which may be t a k e n t o a v o i d t h i s : t h e t e r r a c e may b e p e r m i t t e d t o r u n o u t l e v e l , o r i t may be provided with a p r e s s u r e s u p p o r t , Both methods a f f e c t t h e

s t r e s s e s : t h e n a t u r a l c r e e p i n g p r o c e s s i s r e t a r d e d , and

the

o r i g i n a l l y n e u t r a a -O

Considerable advantages a r e t h u s r e a l i z e d , s i n c e t h e g r a d i e n t of t h e c o r r e c t e d s l o p e may now b e chosen s t e e p e r thm-when simple s h e a r s t r e s s i s involved, w h i l e a t t h e same t h e e q u a l s e c u r i t y i s maintained,

C o n s i d e r a t i o n s o f e conomy r a i s e t h e f u r t h e r q u e s t i o n whether t h e formation o f a p r e s s u r e zone due t o t h e l e v e l l i n g o f f of t h e t e r r a i n a t t h e lower end o f t h e c o r r e c t e d s l o p e , o r t o a s u p p o r t i n g w a l l a t t h a t p o i n t , does n o t i n any case r e n d e r t h e r e d u c t i o n of t h e s l o p e g r a d i e n t s u p e r f l u o u s , I n answering t h i s q u e s t f o n i t must be c o n s i d e r e d t h a t s u p p o r t s o n l y make t h e s t r e s s p a t t e r n more c o h e r e n t b u t cannot a f f e c t l o o s e t y p e s o f snow, f o r t h e s imple r e a s o n t h a t porous snow l y i n g on t h e s u r f a c e i s n o t s u b j e c t t o t h e i n f l u e n c e of p r e s -

s u r e . As a n example, t h e f a c t need o n l y b e r e c a l l e d t h a t even the most massive s t r u c t u r e cannot p r e v e n t t h e develop-

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Tech, Trans, TT-66 I n most c a s e s , howevep, i n s t e a d of employing a s t e p formation of the s l o p e i t w i l l be c o n s i d e r a b l y cheaper

in such a way t h a t t t e r than t h a t o f i t s of t h e wind t h i s i s r e l a t i v e l y easy t o achieve,, It i s o n l y n e c e s s a r y t o have t h e peak of

t h e s t r u c t u r e ~ r o j e c t f a r enough from the s l o p e , The v e r y f i r s t snowfalls w i l l then f i l l f n t h e a n g l e between the p r e s - s u r e s u r f a c e and t h e t e r r a i n , so t h a t t h e s u r f a c e of t h e snow undergoes a p r o g r e s s 1 ve r e d u c t i o n of i t s maximum g p a d i e n t ,

C e r t a i n s p e c i f i c a t i o n s f o r t h e dimensions of t h e c o n s t r u e t i o n a r e t h e r e b y gaf ned, However t h e i r t h e o r e t i c a l development would t a k e u s beyond t h e scope of t h e p r e s e n t d i s c u s s i o n , Of much more irnmed e i n t e r e s t a t t h i s p o i n t i s t h e problem, p r e s s u r e s u ~ f a c e ,

r a f s e d by Oechslin

,

of t h e angle of I n c l i n a t i o n o f t h e As i s e a s i l y seen from t h e f o r e g o f n g t h e h o r f q z o n t a l t e r r a c e and t h e v e r t f c a l wall have t h e same e f f e c t

m z t

t-o the f o r m a t f o n f p ~ s s u r e zones, I n b o t h e a s e s t h e c r e e p i n g motion i n t h e d i r e c t i o n of t h e l i n e of g r a d i e n t i s e n t i r e l y a r r e s t e d , I t i s t h e r e f o r e n e c e s s a r y t o t r y t o compare the advantages and d i s a d v a n t a g e s o f t h e two systems, and t o t h i s end t h e h o r f z o n t a l e a r t h o r s t o n e - work t e r r a c e , a s i n F i g u r e 7B, w i l l f i r s t b e examined, I n

t h i s c a s e t h e s t a t i c p r e s s u r e has b e e n t ~ a n s f e r r e d t o t h e t e r r a c e w i t h the h e l p of f r i c t i o n f o r c e s , However, i f t h e h e a t of t h e ground now t a k e s e f f e c t on t h e u n d e r s i d e of t h e snow-coves i t w i l l g i v e r i s e t o g r a t flowing movements and c o r n i c e s w i l l be formed, Fabereleg s o l v e d s i m i l a r problems of p r e v e n t i n g t h e formation of snow c o r n i c e s on r o o f s by having c u r r e n t s of a i r pass b e n e a t h f l a t r o o f s , The aame

e f f e c t i s o b t a i n e d i n avalanche d e f e n c e works i f , i n s t e a d of s o l i d t e r r a c e s open b r i d g e s a r e b u i l t , as shown i n F i g u r e 76, The c o l d a i r then i n f l u e n c e s t h e snow-cover f ~ o m below and r e d u c e s c r e e p i n g t o a mfnimum, Open c o n s t r u @ t f o n a have t h e f u r t h e r advantage t h a t t h e y p r e s e n t l e s s r e s i s t a n c e t o t h e wind and t h e r e f o r e become s a t u r a t e d l e s s q u i e k l g , Hence, i n moat c a s e s they a r e p r e f e r a b l e t o t h e s o l i d t e r r a c e s ,

The p r e v i ous argument a p p l i e s e q u a l l y t o v e r t i c a l w a l l s , and the whole problem of t h e c h o i c e o f c o n s t r u c t i o n i s f i n a l l y reduced t o t h a t of t h e a n g l e of e f f e c t i v e p r e s s u r e s u r f a c e . Is t h f s t o be h o r i z o n t a l , a t a c e ~ t a i n angle &I t h e base, o r v e r t f c a l ?

The s t m c t u r e s may a l s o be c l a s s 1 f l e d a c c o r d i n g t o the angle and type of eons t r u c t f o n o f the p r e s s u r e s u r -

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f a c e , I n the i n t e r e s t of various conceptions a l r e a d y long employed i n p r a c t i c e i t i s perhaps p e r m i s s i b l e t o i n t r o d u c e c e r t a i n d e f i n i t i o n s h e r e , and f o r t h i s purpose t o u s e as c r i t e r i a , the angle t h e p r e s s u r e s u r f a c e makes w i t h t h e

h o r i z o n t a l , and t h e type of c o n s t r u c t i o n , I n accordance with t h e s e c t o r s r e p r e s e n t e d i n Figure 7 D open c o n s t r u c t i o n s w i l l

be designated a s b r i d g e s , rakes o r f e n c e s , and s o l i d - - construe- _---

t i o n s a s t e r r a c e s , s l a b s o r w a l l s ,

A s a l r e a d y mentioned, the open c o n s t r u c t i o n s a r e p r e f e r r e d and we w i l l now r e t u r n t o our q u e s t i o n of t h e angle of g r a d i e n t of t h e p r e s s u r e s u r f a c e , I n t h i s connection i t need h a r d l y be s a i d t h a t o t h e r c o n s i d e r a t i o n s such a s the p o s s i b i l i t i e s f o r foundations, t h e n a t u r e of the m a t e r i a l ,

and t h e supporting s t r u c t u r e a r e n o t of subordinate importance, b u t f i r s t an attempt w i l l be made t o solve the problem on a predominately snow- t e c h n i c a l b a s i s ,

A lower l i m i t t o t h e group of angles i n q u e s t i o n i s given by the attempt t o hold back

-

a l l snow p a r t i c l e s , With f l a t b r i d g e s t h e r e i s d o u b t l e s s a danger t h a t l o o s e snow avalanches w i l l overshoot them, whereas r a k e s o r f e n c e s w i l l h a l t

the l o o s e snow, For t h i s reason t h e tendency i s t o favour t h e v e r t i c a l s u p p o r t o

A s i s f'urther shown by Figure 7 D the p o s i t i o n of t h e p r e s s u r e g r i l l - s u r f a c e , as mentioned, pequfres a mfnf- mum expenditure o f m a t e r i a l , To be s u r e t h e r e a r support must be made longer w i t h i n c r e a s i n g s t e e p n e s s of t h e p r e s s u r e s u r - face b u t i n c o n s i d e r a t i o n of the d i s t a n c e between props t h i s c o a t component i s o f l e s s importance t h a n an i n c r e a s e i n t h e t o t a l m o u n t of g r i l l work, The determination of the e x a c t p r e s s u r e s u r f a c e angle w i t h i n t h i s range i s a m a t t e r f o r t h e c o n t r a c t i n g engineer, s i n c e m a t e r i a l - t e c h n i c a l and ffnanef ak f a c t o r s p l a y the c h i e f p a r t ,

Prom

snow-technical con-

s i d e r a t i o n s , however, we come t o t h e conclusf on t h a t

-

-l o p e may be magbe-

no

Along with t h e s e c o n s t m c t i o n s a s e r i e s of a d d i t i o n a l s t r u c t u r a l measures need t o be taken i n each defence d i s t r i c t , These can only be touched upon h e r e , S h o r t s t a k e s , f o r examplg a r e an e x c e l l e n t means of i n c r e a s i n g the f r i c t i o n r e s i s t a n c e s be tween the snow-cover and t h e ground, I n o t h e r i n s t a n c e s d r i f t cleavages w i l l come i n f o r s p e c f a l a t t e n t i o n , and c e r t a i n s p e c i a l c o n s t r u c t i o n s such as b r i d g e s and t h e l i k e w i l l have t o b e c a r r f e d o u t ,

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9

Tech, Trans, TT-66 As w e s e e , each defence p r o j e c t r e q u i r e s s p e q f a l s t u d y if i t i s t o b e c a r r i e d o u t c o r r e c t l y from t h e t e c h n i c a l s t a n d p o i n t , and e s p e c i a l l y i f i t i s t o be completed a t a

minimum c o s t , This f a c t l e a d s t o the f o l l o w i n g conclusfons,

CONCLUDING FIEMARKS

The e n l i g h t e n e d e n g i n e e r i s a s o r u l e r e c e p t i v e t o a l l t h e o r e t i c a l information, y e t on the o t h e r hand h i s o b l i g a t i o n t o a u t h o r i t i e s , t h e procurement o f the n e c e s s a r y means f o r c a r r y i n g o u t a p r o j e c t , and, n o t l e a s t , h i s d e s i r e

t o complete a c u r r e n t pfece of work w i t h i n a

mfnfxnum

time, o f t e n r e s u l t i n h i s f a i l i n g t o a r r i v e a t

a

p o s i t i v e l y e x a c t c l a r i f i c a t i o n of a l l a s p e c t s o f t h e problem, On _{t h e b a s i s}

of experiences vhich may o r i g i n a t e from d i s t r i c t s wf t h en- t i r e l y d i f f e r e n t c l i m a t i c c o n d i t i o n s , o r from the unconscfoue tendency t o p r e f e r a system which has been employed anywhere e l s e w i t h good r e a u l t a , pro j e e t s a r e then completed without any e x a c t accounting being made b e f o r e h n d . o f the aim, purpose and l o c a l c o n d i t i o n s , It must be admitted a t once

t h a t i n many c a s e s s o l u t i o n s have been a r r i v e d a t i n t h f s way which were t h e o r e t i c a l l y i n c o n t e s t a b l e , Indeed, one can o n l y admire the remarkable s k i l l with which our f o r e f a t h e r s , w i t h no b e t t e r knowledge than they had o f t h e m a t e r i a l pro- p e r t i e s o f t h e snow, b u i l t defence works which m e q u i t e simply

t o be d e s c r i b e d a s exemplary, On t h e o t h e r hand every e n g i n e e r knows only t o o w e l l how o f t e n t h f s procedure can l e a d t o o u t - and-out f a i l u r e s ,

I t i s p l a i n , of c o u r s e , and s u r e l y u n d e r s t a n d a b l e t o everyone, t h a t t h e p r o f e s s i o n a l f o r e s t e r cannot m a s t e r o v e r n i g h t a s p e c i a l i z e d branch of technology, i n which i t h a s taken a r a t h e r l a r g e t e c h n i c a l l y t r a i n e d r e s e a ~ c h s t a f f , provided w i t h a l l p o s s i b l e equipment, me a s u r i n g appasatua and i n s t a l l a t i ons, more than ten y e a r s j u s t t o c l a r i f y the most important p r i n c i p l e s , With the i n t r o d u c t i o n of young

s t u d e n t s of f o r e s t r y e n g i n e e r i n g t o t h e s c i e n c e of avalanches

i t

f s to be hoped t h a t t h e s e p r o f e s s i o n a l people w i 11 l a t e r approach the problem w i t h more modern methods ; b u t f o r t h e p r e s e n t t h i s t h e o r e t i c a l knowledge i s handicapped by t h e l a c k of p r a c t f c a l experience

In t h e meantime o n l y the c l o s e co-operation of t h e o r e t i c f a n s and p r a c t f t f o n e r s can l e a d t o t h e a t t a i n m e n t of t h e g o a l , I t i s up t o t h e f o r e s t e r s t o s t u d y the l o c a l condf t f ons and t h u s t o work o u t the u n d e r l y i n g d e t a i l s f o r the b u i l d i n g o f t h e defence works, The r e s p o n s i b i l i t y o f the I n s t i t u t e , on the o t h e r hand, i s t o work o u t the f a c t s

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f u r t h e r and t o f'urnish these t o the engineer In c o r r e c t e d form, To t h i s end we d e s i r e

-

t o u s e t h e words of Oechslfn

-

"to get the f o r e s t e r onto a t h e o r e t i c a l t r a c k w , We do n o t

want t o leave him s i t t i n g t h e r e , however, but t o l e a d him d i r e c t l y t o our common goal, f o r in avalanche defence work t h e r e is nothing b e t t e r than a good theory,

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-

11

Tech, T P ~ s , TT-66

1 ) Welzenbach,

W,

2 U n t e ~ s u c h u n g e n u e b e r d i e S t r a t f g r a p h f e d e r Schneeablagerung und d i e Mechanik d e ~ Schneebewegungen nebs t S c h l u s s f o l g e r u n g e n auf d l e Methodfk d e r Vekbauung

( I n v e s t i g a t f ons i n t o t h e S t r a t i g r a p h y of t h e Snow Deposf t and t h e Mechanics of Snow Movements, Together w i t h Con- c l u s f ons foxn t h e Methods of Buf l d i n g Defences ), Wf ssen- s c h a f t l i c h e V e r o e f f e n t l f c l ~ u n g e n N r , 9 des Deutschen und

O e s t e r r , Alpenverefns, Innsbruck, 1930,

2 ) E u g s t e r , E , : Schneestudien fm Oberwallis und f h r e Anwendungen auf den Lawfnenverbau (Snow S t u d i e s I n t h e Upper Valafs and t h e i r A p p l i c a t i o n t o Avalanche Defence Work), B e f t r a e g e z u ~ Geologfe d e r Schweiz, Geotechnfsche S e r f e (Contrfbut ions t o Geology from t h e Swf s s Geotechnf c a l S e r i e s ) - - ~ y d r o l o g i e , No, 2 , Berne, 1938,

3) Croce, Karln U n v e r o e f f e n t l f c h t e M f t t e f l u n g (Unpublished

r e p o r t ) ,

4 ) Ffnney, E,A,: Snow c o n t r o l on t h e h i ~ h w a y s , B u l l e t i n

No, 37, M i chfgan Engineering Experimental S t a t f o n , E a s t Lansing, MfchOD May, $934,

5 ) Becker, A,: Der n a t u e r l i c h e Schneeschutz an V e ~ k e h ~ s w e g e n ( N a t u r a l P r o t e c t f o n Agafns t Snow on ~f ghways), Die

Bautechnfk, B e ~ l f n , 22 September, 1944,

6 ) Kohler, H, : Df e Vekhue tung von Schneeverwehungen (The P r e v e n t i o n o f Snow ~ r f f t s ) ~

7 ) Naegelf,

W,x

Untersuchungen u e b e r d i e Wfndverhaeltnfsse

fm

Berefch von W f n d s c h u t z s t ~ e f f e n ( I n v e s t i g a t i o n s on Wind Conditions

in

t h e v i c i n i t y of S h e l t e r e d A r e a s ) , M i t t e f l u n - gen d e r Schwef

z,

A n s t a l t h e r d a s f o r s t l f che Versuchswesen, Vol*

XXIIIO

8 ) Finney, E , A , ; Snow C o n t r o l by Tree P l a n t i n g , Michigan

Engineering Experimental S t a t i o n , E a s t Lansing, Mf c h i gan, B u l l e t f n No, 75, J u l y , 1937,

9 ) Gabarel,

R,

: Davoser Bauten (Bufldfng, Cons t r u e t i o n i n

~ a v o s ) , ~ Schwefz, Bauzeftung, Vol,

160,

No, 8, 20 AugOs 1932,

1 0 ) Oechslfn, M,: Efnfge Errahrungen und Gedanken b e t k e f f e n d

Lawfnenvekbau (Some Experiences and Thoughts on Avalanche Defence Work), Schwef z , Zef t s c h r , f o Forstwesenc 9 7 t h Year, Decembe~, 1946, No, 12,

(15)

FIG.1 STABILITY DIAGRAM

THE STRESS CURVE

FIG.3 DROP IN FIRMNESS CURVE

(16)

DIRECTION OF WIND

EFENCE WORK AREA

(LEEWARD SIDE )

DISTANCE A AT LEAST TEN TIMES HEIGHT OF FENCE

FIG. 4

ERECTION OF FENCES FOR RETENTION OF SNOW

FENCE POSTS

WOOD OR WIRE PERCENTAGE OF

FIG. 5

(17)

Swl t c h i n g p"i11t on t h e u p p e r sec tj on of t h e

Davoa-:'na1attnn Hrl _.ilway_showing _{g u i d e}_{w a l l s}_{f o r t h e}

(18)

FIG. 7a,b,c.

T T - 6 6

SCHEMATIC

REPRESENTATION OF DIFFERENT TYPES OF DEFENCE WORKS

SLOPE GRADIENT Advantages : S t a b i l i z a t i o n by r e d u c t i o n of s l o p e g r a d i e n t , s i m i l a r t o t o r r e n t damming. D i s a d v a n t a g e s : C o s t l y e a r t h t r a n s p o r t .

~ a a o n i - ~ has t o beaE weight o f f i l l i n g a n d s u r c h a r g e . C o r n i c e a n d d a n g e r of i t s b r e a k i n g o f f

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a v a l a n c h e development. Loose snow a v a l a n c h e s i r e n o t a r r e s t e d . . A, B r e a k i n g up of s l o p e g r a d i e n t by f o r m a t i o n of s t e p s . Advanteges: S t a b i l i z a t i o n by f o r m a t i o n of HORIZONTAL - 9 --- p r e s s u r e zones. R e d u c t i o n of c o r n i c e s . D i s a d v a n t a g e s : The e n t i r e p r e s s u r e f o r c e i s a b s o r b e d b y - f r i c t i o n . When h e a t i n g t a k e s p l a c e from below t h e r e a r e f l o w p r o c e s s e s and p r e s s u r e d i s p l a c e m e n t s . Loose snow a v a l a n c h e s a r e n o t a r r e s t e d . B. H o r i z o n t a l t e r r a c e s . CF ORIGINAL Advantages: S t a b i l i z a t i o n by f o r m a t i o n of p r e s s u r e zones. Simple d e s i g n . I n c r e a s e of f r i c t i o n by u n d e r - n e a t h v e n t i l a t i o n . D i s a d v a n t a ~ e s : Loose snow a v a l a n c h e s a r e n o t a r r e s t e d . C . B r i d g e s .

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2 2 \ D . G r a d i e n t of p r e s s u r e s u r f a c e s . &%%%%$no by f o r m a t i o n of p r e s s u r e zones. Large f i l l i n g s p a c e . R e t a i n s l o o s e snow e v e l a n c h e s . D i s a d v a n t ~ g e s : I t i s d i f f i c u l t s t r u c t u r a l l y t o ~ n c h 0 r t h e f e n c e s from above.

GLE OF ORIGINAL There i s no s u p p o r t i n g e f f e c t f o r

LOPE GRADIENT the f o r c e s due t o t h e i n t r i n s i c w e i g h t p r e p e n d i c u l a r t o t h e f o u n d e t i o n .

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V e r t i c a l Fences. 6%'8%E%on by f o r m a t i o n of p r e s s l r e zones. Large f i l l i n g s p a c e . R e t a i n s l o o s e snow a v a l a n c h e s . Good s u p p o r t i n g e f f e c t f o r t h e f o r c e s due t o i n t r i n s i c w e i g h t p e r p e n d i c u l a r t o t h e f o u n d d t b n . Simple c o n s t r u c t i o n due t o p o s s i b i l i t y of s u p p o r t from below. ANGLE OF ORIGINAL SLOPE GRADIENT F. Rake C o n s t r u c t i o n s .