Effect of Tree oots on Shallow-Seate

F o r e s t v e g e t a t i o n , e s p e c i a l l y t r e e r o o t s , h e l p s s t a b i l i z e h i l l s l o p e s by r e i n f o r c i n g s o i l s h e a r s t r e n g t h . To e v a l u a t e t h e e f f e c t of t r e e r o o t s on s l o p e s t a b i l i t y , i n f o r m a t i o n a b o u t t h e amount o f r o o t s and t h e i r s t r e n g t h s h o u l d b e known. A s i m u l a t i o n m o d e l f o r t h e r o o t d i s t r i b u t i o n o f Grvwtomer ia i a w o n i c a was proposed where t h e number of r o o t s i n each 0.5-cm d i a m e t e r c l a s s can be c a l c u l a t e d a t a r b i t r a r y d e p t h s . The p u l l o u t s t r e n g t h of r o o t s was u s e d t o a n a l y z e t h e s t a b i l i t y o f f o u r d i f f e r e n t t y p e s of f o r e s t e d s l o p e s . Root r e i n f o r c e m e n t i s i m p o r t a n t on s l o p e s where r o o t s c a n e x t e n d i n t o j o i n t s and f r a c t u r e s i n bedrock o r i n t o a w e a t h e r e d t r a n s i t i o n a l l a y e r between t h e s o i l a n d b e d r o c k . Root r e i n f o r c e m e n t o f s o i l i n c r e a s e s q u i c k l y a f t e r a f f o r e s t a t i o n f o r a b o u t t h e f i r s t 20 y e a r s , t h e n r e m a i n s &out c o n s t a n t t h e r e a f t e r . Sediment d i s a s t e r s by d e b r i s f l o w s , mud f l o w s , and l a n d s l i d e s o c c u r a l m o s t e v e r y y e a r d u r i n g t h e r a i n y J u l y t o O c t o b e r Typhoon s e a s o n i n J a p a n . I n J u l y 1982, a heavy r a i n f a l l of 488 nun i n a day, w i t h a maximum i n t e n s i t y o f 1 2 7 . 5 mm p e r h o u r , c a u s e d 4300 d e b r i s f l o w s i n N a g a s a k i p r e f e c t u r e , Kyushu I s l a n d . T h i s s t o r m d e s t r o y e d 2200 h o u s e s a n d k i l l e d 299 p e o p l e . D u r i n g J u l y 1983 , i n t e n s i v e r a i n f a l l i n i t i a t e d many d e b r i s f l o w s and 1 9 9 p e o p l e w e r e k i l l e d i n S h i m a n e p r e f e c t u r e , a l o n g J a p a n Sea on w e s t e r n Honshu I s l a n d . l p r e s e n t e d a t t h e S u b j e c t Group S1 .04 T e c h n i c a l S e s s i o n on Geomorphic Hazards i n Manaqed F o r e s t s , X I X World C o n q r e s s , I n c e r n a r l o n a l Union o f F o r e s c r y R e s e a r c h O r s a n l z a c l o n s , Aususc 5-11, 1990, Clonereal, 2 ~ e s e a r c h S c i e n t i s t , F o r e s t r y and F o r e s t P r o d u c t s R e s e a r c h I n s t i t u t e , T s u k u b a , I b a r a k i , 305 Japan; and P r i n c i p a l Research H y d r o l o g i s t , P a c i f i c S o u t h w e s t R e s e a r c h S t a t i o n , F o r e s t S e r v i c e , U n i t e d S t a t e s ~ e ~ a r t m e n t o f A g r i c u l t u r e , A r c a t a , C A . U.S.A. 95521 The c a u s e o f s o much d e s t r u c t i o n and d e a t h m i g h t b e u n p r e c e d e n t e d i n t e n s i v e r a i n f a l l . I n a d d i t i o n , e x p a n s i o n o f c i t i e s , r e s o r t s , and r o a d s o n t o h i l l s l o p e s and mountain areas h a s been a l s o thought t o be a p r i m a r y c a u s e . I n response , n a t i o n a l a n d l o c a l g o v e r n m e n t s h a v e a d o p t e d a program of a g g r e s s i v e l y c o n s t r u c t i n g many e r o s i o n c o n t r o l works a t g r e a t e x p e n s e . But , even s o , it i s i m p o s s i b l e f o r s u c h c o n s t r u c t i o n t o p r o t e c t a l l m o u n t a i n h i l l s l o p e s from d e b r i s f l o w s . An i m p o r t a n t c a u s e o f i n c r e a s e d f requency o f d e b r i s f lows i s t h e removal of f o r e s t s t o accommodate u r b a n i z a t i o n and r o a d c o n s t r u c t i o n i n mountainous a r e a s . I n monsoon a r e a s , l i k e J a p a n , where s t e e p mounta ins a r e c o v e r e d w i t h f o r e s t s , mass w a s t i n g i s t h e p r e v a i l i n g t y p e o f e r o s i o n . There i s a f r a g i l e b a l a n c e of s t a b i l i t y on s u c h s t e e p h i l l s l o p e s where t h e f o r e s t c o v e r i n t e r a c t s w i t h s o i l m o i s t u r e , s o i l s t r e n g t h , g e o l o g i c a l c o n d i t i o n , h i s t o r i c a l r a i n f a l l , and o t h e r f a c t o r s t o s t a b i l i z e t h e r e g o l i t h on a s l o p e . From a v iewpoin t o f s o i l mechanics , on many h i l l s l o p e s t h e f a c t o r of s a f e t y of a s l o p e (FS) approaches 1 . 0 d u r i n g a r a i n f a l l e v e n t t h a t o c c u r s once e v e r y s e v e r a l y e a r s . Under c o n d i t i o n s of s u c h d e l i c a t e b a l a n c e , removal of t h e t r e e s by l o g g i n g may r e s u l t i n a r e d u c t i o n i n s o i l s t r e n g t h s u f f i c i e n t t o c a u s e l a n d s l i d e s . The i n f l u e n c e o f f o r e s t s on s l o p e s t a b i l i t y h a s b e e n o n e o f t h e most i m p o r t a n t s u b j e c t s of s t u d y e s p e c i a l l y , t h e r o l e of t r e e r o o t s on r e i n f o r c i n g s o i l s h e a r s t r e n g t h . To e v a l u a t e t h e mechanical e f f e c t o f r o o t s i n s t r e n g t h e n i n g s o i l , however, t h e q u a n t i t y and d i s t r i b u t i o n o f r o o t s i n s u b s u r f a c e s o i l l a y e r s must b e q u a n t i f i e d . I n t h i s p a p e r , a s i m u l a t i o n model f o r t h e d i s t r i b u t i o n and s t a b i l i z i n g e f f e c t o f r o o t s i s i n v e s t i g a t e d u s i n g a n i n f i n i t e s l o p e s t a b i l i t y a n a l y s i s model.