Terlien_HydrologicalLandslideTriggeringAshCoveredSlopesManizales

7/28/2019 Terlien_HydrologicalLandslideTriggeringAshCoveredSlopesManizales

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E L S E V I E R Geom orphology 20 (1997) 165 175G E O M O R P H O L O G Y

Hydrological landsl ide tr iggering in ash-covered slopes ofM anizales (Colombia)

Mark T.J. Terlie n *In terna t iona l Ins t i tu te jbr Aerospace Survey and Ear th Sc iences ( ITC) , Kanaalweg 3 , 2628 E B Del ft , The Ne ther lands

Received 28 Feb ruary 1996; rev ised 18 February 1997; accepted 24 February 1997

A b s t r a c t

A s t u d y o n h y d r o l o g i c a l l a n d s l i d e t r i g g e r i n g w a s c a r r i e d o u t o n a s h - c o v e r e d s l o p e s n e a r t h e c i t y o f M a n i z a l e s( C o l o m b i a ) . T h e s a t u r a t e d h y d r a u l i c c o n d u c t i v i t y o f t h e d i f f e r e n t s o i l t y p e s a n d t h e p r e s s u r e h e a d f l u c t u a t i o n s i n r e l a t i o n t or a i n f a l l w e r e d e t e r m i n e d i n o r d e r t o d e t e c t s a t u r a t e d z o n e s i n t h e s o i l p r o f i l e w h i c h f o r m p r e f e r e n t i a l z o n e s f o r t h ed e v e l o p m e n t o f s l ip s u r f a c e s o f l a n d s li d e s . T e n s i o m e t e r m e a s u r e m e n t s r e v e a l e d s a t u r a ti o n i n th e t o p s o i l a s a r e s u l t o fi n f i l t r a t i o n o f r a i n f a l l, i n t h e a s h e s a s a r e s u l t o f a p e r c h e d w a t e r t a b l e a n d a t t h e s o i l b e d r o c k c o n t a c t a s a r e s u l t o f ag r o u n d w a t e r t a b le . T h e l o c a t i o n o f th e s a t u r a te d z o n e s i n t h e s o il p r o fi l e p r o v e d t o b e i n a c c o r d a n c e w i t h t h e l o c a t i o n o f s l ips u r f a c e s o b s e r v e d i n t h e f ie l d . T h r e e l a n d s l i d e s w e r e s e l e c t e d f o r b a c k a n a l y s i s t o e s t a b l i s h t h e p r e s s u r e h e a d s t h a t c o u l dh a v e t r i g g e r e d t h e s e la n d s l id e s . T h e p r e s s u r e h e a d s o b t a i n e d b y b a c k a n a l y s i s w e r e c o m p a r e d w i t h t h e m e a s u r e d p r e s s u r eh e a d r a n g e s. T h e b a c k - c a l c u l a t e d p r e s s u r e h e a d s p r o v e d t o s u p p o r t t h e c o n c l u s i o n s o f th e h y d r o l o g i c a l s t u d y.

Keywords." landslides; rainfal l; subsurfa ce hydrolo gy; pore-w ater pressure; slope stabil i ty

1 . I n t r o d u c t i o nLands l i des i n i t i a t ed by na t u ra l p roces s es o r hu -

m a n ac t i v i t ie s fo rm one o f t he m a i n na t u ra l d i s as te r si n t rop i ca l m oun t a i nous a reas . A l t hough on l y a s m a l lpe rcen t age o f lands l i des i s ca t a s t roph ic , t he l a rgen u m b e r o f t h e m m a k e s t h e ir e c o n o m i c im p a c t ( e .g .l o s s o f f e r t i l e s o i l , d es t ruc t i on o f i n f r a s t ruc t u re )g rea t e r t han fo r o t he r , m ore s pec t acu l a r , na t u ra l haz -a rd s s uch as vo l can i c e rup t i ons and ea r t hquakes .S i nce l ands l i des a re cons i de red as p red i c t ab l e geo -l og i ca l haza rds , t he econom i c l o s s es due t o l and -s l ides can be r edu ced s i gn i f i can t l y (Hans en , 1984 ) .

* Tel .: +3 1 15 274-8847. Fax: +3 1 15 262-3961. E-mail :[email protected]

A f i r s t i m por t an t s t ep i n r educ i ng t hes e econom i cl o s s es i s t he i den t i f i ca t i on o f t he l ands l i de t r i gge r i ngm echan i s m (e . g . r a i n fa l l o r ea r t hquakes ) . Knowl edgeo n t h e t r ig g e r i n g m e c h a n i s m i s r e q u i re d f o r a n e f f e c -t i ve execu t i on o f s i t e i nves t i ga t i ons , fo r a co r rec ts e l ec t i on o f t echn i ques o f s l ope s t ab i l i t y ana l y s i s(Ander s on e t a l . , 1987 ) and fo r a p roper des i gn o fm an-m ade s l opes and s l ope s t ab i l i s a t i on works .

Hy dro l og i ca l t r i gge r i ng can be de f i ne d as t hereduc t i on o f t he s a fe t y f ac t o r o f a s l ope to un i t y a s ares u l t o f a dec reas e i n s hea r s t r eng t h due t o ani nc reas e i n po re -w a t e r p res s u re . In a reas where hy -d r o l o g i c a l t r i g g e r i n g i s v e r y c o m m o n ( e . g . h u m i dt rop i ca l m oun t a i nous r eg i ons s uch as t he C o l om bi anAndes ) , hyd ro l og i ca l s t ud i es wi l l have t o be ca r r i edou t i n o rde r t o de t e rm i ne po re -wa t e r p res s u re cond i -

0169-555X/97/$17 .00 1997 Elsev ie r Sc ience B.V. Al l r igh ts reserved .P I I S 0 1 6 9 - 5 5 5 X ( 9 7 ) 0 0 0 2 2 - 6


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166 Jl/LZZ Terlien / Geomorphology 2 0 (1997) 165 175t i o n s i n s l o p e s ( A n d e r s o n a n d K e mp , 1 9 9 1 ) a n d t oe v a l u a t e t h e h y d r o l o g i c a l p r o c e s s e s a n d me t e o r o l o g i -c a l c o n d i t i o n s t h a t ma y c a u s e f a i l u r e ( Ca r i s a n d V a nAsch , 1991) .

I n Ma n i z a l e s ( Co l o mb i a ) a h y d r o l o g i c a l s t u d yw a s c a r r i e d o u t t o f i n d z o n e s w i t h p o s i t iv e p o r e - w a t e rp ressu res (po ten t i a l fa i lu re zones ) in vo lcan ic so i l sw i t h s i g n i f i c a n t v e r t i c a l c h a n g e s i n h y d r a u l i c c o n -duct iv i ty . The loca t ions o f po ten t i a l fa i lu re zones asf o u n d i n t h e h y d r o l o g i c a l s t u d y w e r e v e r i f i e d i n t h ef i e ld . F i n a l l y b a c k a n a l y s i s o f f a i le d s l o p e s w e r ee x e c u t e d t o e s t a b l i s h t h e c r i t i c a l p o r e - w a t e r p r e s s u r er a n g e s t h a t ma y h a v e c a u s e d f a i l u r e .

2 . S t u d y a r e aT h e s t u d y a r e a i s lo c a t e d n o r t h e a s t o f Ma n i z a l e s

( Co l o m b i a ) a t a n a lt it u d e o f 2 1 0 0 m o n w e s t e r n f l a n k

of the Co rd i l l e ra Cen t ra l (F ig . 1 ). Th e a rea co ns i s t so f me t a - s e d i me n t a r y s c h i s t b e l o n g i n g t o t h e Q u e -b r a d a g r a n d e f o r m a t i o n o f C r e t a c e o u s a g e c o v e r e d b yp y r o c l a s t i c f a l l d e p o s it s ( v o l c a n i c a s h e s ) o f Q u a t e r -n a r y a g e ( N a r a n j o a n d R i o s , 1 9 8 9) . I n t h e w e a t h e r e ds c h i s t a s a p r o l i t e h a s d e v e l o p e d w i t h a ma x i mu mt h i c k n e s s o f 3 m . T h e t h i c k n e s s o f t h e a s h v a r i e sf rom 15 m o n the f l a t h i l l t ops to l es s than 1 m ons lopes s t ee per than 45 . The ashes a re bu i l t up o f al a r g e n u m b e r o f l a y e r s w i t h g r a i n s iz e s r a n g i n g f r o m1 c m t o l e ss th a n 7 5 / z m ( Mo o l , 1 99 2; V a n W e s t e net al . , 1993).

O n t h e b a s i s o f g e o t e c h n i c a l a n d h y d r o l o g i c a lcharac te r i s t i cs the so i l s were c l as s i f i ed in to fourg r o u p s w i t h s i m i l a r e n g i n e e r i n g p r o p e r t i e s : o r g a n i cso i l , s i l t y sand , sandy s i l t and sapro l i t e (Ter l i en ,1996, and Ta ble 1) . Fo r Cu, c ' an d q~' the ran ge o fv a l u e s i s i n d i c a t e d a s a me a n a n d s t a n d a r d d e v i a t i o nc o u l d n o t b e c a l c u l a t e d d u e t o t h e l i m i t e d n u mb e r o f

M a n i z a l e s - C o l o m b i a

LE

E

1173OOO 117400O 1~75000

Fig. 1. Location of the Manizales study area.177000 1177750


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A/L Z~ Terlien / Geomorphology 20 (1997) 165 175 167T a b l e 1S u m m a r y o f d e n s i ti e s, p o r o s i t y a n d s h e a r s t r en g t h p r o p e r t i e s o ft h e m a i n s o i l t y p e s f o u n d i n t h e s t u d y a r e a

S o i l t y p eO r g a n i c s o i l S i l t y s a n d S a n d y s i l t S a p r o l i t e

p~ (g cm 3) 1.47 1.61 1.51 1.86Of (g cm 3) 1.29 1.55 1.49 1.85Pd (g c m 3) 0 .88 1 .07 0 .87 1 .36n 0 .59 0 .54 0 .65 0 .50c u ( k P a ) 6 3 . 0 1 6 .5 3 4 . 0 3 2 . 5 3 4 . 0 4 5 . 5c ' (kPa ) 15 .6 42 .2 0 33 .6 17 .7 53 .9 17 .8 22 .7

( p e a k )9 .0 17 .8

( r e s i d u a l )qY (de g . ) 24 .8 36 .6 29 .0 39 .0 22 .4 29 .0 25 .3 30 .0

( p e a k )18.1 27.1( r e s i d u a l )

p~ = s a tu ra te d bu l k de ns i ty , p f : f i e ld bu lk de ns i ty , lOd : d ry bu lkd e n s i t y , n = p o r o s i t y , c u = u n s a t u r a t e d c o h e s i o n , c ' = e ff e c t i v ec o h e s i o n , qY = e f f e c t i v e a n g l e o f f r i ct i o n .

ana l y s ed s am p l es . In add i t i on t o t hes e fou r s o i lt ypes , l aye r s o f lap i ll i and s and w i t h a m ax i m umt h i cknes s o f 40 cm a re found wi t h i n t he a s h depos i ts .

The s i l t y s and can be c l a s s i f i ed i n t o t wo s ub -g roups on t he bas i s o f d i f f e rences i n geo t echn i ca lp roper t i e s . The f i r s t sub -g roup i s cha rac t e r i s ed by c 'va l ues l ow er t han 5 kP a w h i l e the qY va r i e s be t wee n34 and 39 . The s econd s ub -g roup i s c ha rac t e r i s ed byc ' va l ues l a rge r t han 15 kP a and qY va l ues be t we en29 and 34 .8 . The t wo s ub -g roups can a l s o be de -s c r ib e d i n te r m s o f p e r c e n ta g e o f f i n e s ( < 7 5 / , m ) :s am p l es o f t he f i r s t s ub -g roup hav e on av e rage l e s st han 40% f i nes wh i l e t hos e o f t he s econd s ub -g rouph a v e m o r e t h a n 4 0 % f i n e s .

The l ands l ides i n t he a rea a re t he r e s u l t o f t hegeo l og i ca l and ge om orpho l og i ca l s e tt i ng o f t he a rea ,t h e w e t c l im a t e ( a p p r o x i m a t e l y 2 0 0 0 m m o f p r ec i p i-t a t i on pe r yea r ) and t he f ac t tha t t he a rea i s l oca t ed i na s e i s m i ca l l y ac ti ve r eg i on . Mos t l ands l ides ha ve s li ps u r faces e i t he r i n t he a s hes o r i n t he s ap ro l i t e ando c c u r in t h e r a in y s e a so n s ( A p r i l - M a y o r O c t o b e r -N o v e m b e r ) . T h e m a j o r i t y o f th e s h a l lo w l a n ds l id e swi t h s l i p s u r f aces be t ween 50 and 600 cm a re t r ans -l a t iona l . Mo s t l ands l ides de eper t han 600 cm a ref l ow s l i des (Ter l i en , 1996 ) .

3. Hydrological investigations3.1. Rela t ion ra i@ tl l lands l ides

The r e l a t i on be t ween l ands l i de even t s and r a i n fa l li n Man i za l e s w as s t ud i ed fo r t he yea r 1993 i n o rde rt o f i nd t he c r i ti ca l com bi na t i ons o f da i l y r a i n fa ll andan t eceden t r a i n fa l l t ha t t r i gge red l ands l i des (Lum b ,1975 ; C roz i e r , 1986 ) . The accum ul a t ed an t eceden tr a i n fa ll o f 25 days and da i l y r a in fa l l fo r days wi t hand days wi t hou t l ands l i des a re p l o t t ed i n F i g . 2 .Th i s f i gu re s hows t ha t s ha l l ow l ands l i des (w i t h am a x i m u m d e p t h o f 1 . 5 m ) a r e t ri g g e r e d b y h i g h d a i lyr a in f a ll a m o u n t s ( a p p r o x i m a t e l y 7 0 m m ) w h i l e d e e p e rl ands l i des a re t r i gge red by an accum ul a t ed an -t e c e d e n t ra i n fa l l o f 2 5 d a y s e x c e e d i n g 2 0 0 m m i ncom bi na t i on wi t h a da i l y r a i n fa l l be t ween 0 and 50m m depend i ng on t he an t eceden t r a i n fa l l . T i m e i n t e r -va l s o f 2 , 5, 10, 15 , 20 and 30 days were c ons i de re das we l l bu t a t im e i n t e rva l o f 25 days p rov ed t o g i vet he bes t d i s c r i m i na t i on be t ween days wi t h l ands l i deeven t s and days wi t hou t l ands l i de even t s , e s pec i a l l yfo r t he deeper l ands l i des . F o r t he s ha l l ow l ands l i desa ti m e i n t e rva l o f 2 days w ou l d a l r ead y be s u f f i c ien t .In o rde r t o s uppo r t t he s e l ec t i on o f a t i m e i n t e rva l o f25 days , the co r re l a t i ons be t w een acc um u l a t ed an -tece den t ra infal l o f 2 , 5 , 10 , 15 , 20 , 25 and 30 da ysand t he p res s u re heads m eas u red i n t he s o i l wereca l cu l a t ed . A l t hough t he co r re l a t i ons were i n gene ra lbe l ow 0 .5 , t he p res s u re heads dee per t han 200 c m

Daily Rainfal l [ ram]8 0 - -

Deep s l i d e s Shaflow slides

0 50 100 150 200 250 500 550A n t e c e d e n t R a i n f a l l [ t u r n ]

F i g . 2 . A n t e c e d e n t r a i n f a ll o f 2 5 d a y s a g a i n s t d a i l y r a i n f a l l f o rl a n d s l i d e a n d n o n - l a n d s l i d e e v e n t s i n 1 9 9 3 .


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168 M. ZJ . Terlien / Geomorphology 2 0 (1997) 165 175

p r o v e d t o b e b e s t c o r r e l a t e d w i t h t i me i n t e r v a l s o f 2 0to 30 days (Ter l i en , 1996) . Th i s i s in accordancew i t h t h e t i me i n t e r v a l o f 2 5 d a y s s e l e c t e d f o r t h epred ic t ion o f deeper l ands l ides . The l ine in F ig . 2 ,d r a w n b y e y e t o d i s c r i mi n a t e a s w e l l a s p o s s i b l ed a y s w i t h l a n d s l i d e s a n d d a y s w i t h o u t l a n d s l i d e s ,r e p r e s e n t s t h e p r e l i m i n a r y 5 0 % p r o b a b i l i t y t h re s h o l d .T h e t h r e s h o l d s u g g e s t s th a t i n 5 0 % o f th e c a s e s ac o mb i n a t i o n o f a n t e c e d e n t r a i n f a l l a n d d a i l y r a i n f a l lexceed ing th i s l ine wi l l t r igger a l ands l ide . The 0%a n d 1 0 0 % p r o b a b i l i t y t h r e s h o l d s c o u l d n o t b e d e t e r -m i n e d b e c a u s e o f t he l i m i te d n u m b e r o f l an d s li d ee v e n t s .

Fo r a b e t t e r u n d e r s ta n d i n g o f th e h y d r o l o g i c a lme c h a n i s ms r e s p o n s i b l e f o r s h a l l o w a n d d e e p s l o p efa i lu res , t he sa tu ra ted hydrau l i c conduct iv i ty ( K s a t )o f t h e d i f f e r e n t s o il t y p e s w a s d e t e r mi n e d . F u r t h e r-mo r e t e n s i o me t e r s w e r e i n s t a l l e d a t d i f f e r e n t d e p t h si n t h e a s h e s t o mo n i t o r p r e s s u r e h e a d f l u c t u a t i o n s i nre la t ion to ra in fa l l .

T a b l e 2K s a a n d m e a n p e r c e n t a g e o f f i n e s fo r t h e s o i l t y p e s o f th e s t u d ya r e aS o i l ty p e K sa ( c m d 1 ) N % o f f i n e s ( < 7 5 /x m )Lap i l l i 8900 9 < 12%S a n d 3 4 0 0 9 < 1 2 %O r g a n i c s o i l 3 4 . 1 1 4 b e t w e e n 1 2 a n d 5 0 %S i l ty s a n d 3 8 . 5 2 1 b e t w e e n 1 2 a n d 5 0 %San dy s i l t 7 . 0 18 > 50%Sapr ol i te 14.7 13 > 50 %N = n u m b e r o f sa m p l e s .

p r e s s u r e h e a d f l u c t u a t i o n s a t lo c a t i o n s w h e r e p e r c h e dw a t e r t a b l e s a n d g r o u n d w a t e r t a b l e s c o u l d b e p r e s e n tw e r e m o n i t o r e d b y m e a n s o f te n s i om e t e r s .

3 .3 . T ens iom eter m eas ur em en ts3 .2 . Saturated hydraul ic conduct iv i ty

T h e K s a w a s d e t e r mi n e d i n t h e f i e l d b y t h ei n v e r s e d a u g e r h o l e me t h o d . T h i s me t h o d w a s u s e db e c a u s e n o w a t e r w a s o b s e r v e d i n t h e a u g e r h o l e s .Th e p r inc ip le o f th i s t es t is d r i l li ng a ho le to a g ive ndep th , f i l l i ng i t wi th water , and measur ing the ra t e o ff a l l o f t h e w a t e r ( K e s s l e r a n d O o s t e r b a a n , 1 9 7 4) .Mo s t t e s t s w e r e e x e c u t e d i n Ma r c h 1 9 9 2 i n t h e d r ys e a s o n . D u e t o t h e v e r y r a p i d f a l l o f th e w a t e r i n th ea u g e r h o l e t h e Ksa of sand and l ap i l l i cou ld no t beme a s u r e d w i t h t h e i n v e r s e d a u g e r h o l e me t h o d .T h e r e f o r e i t h a s b e e n e s t i ma t e d o n t h e b a s i s o fg r a i n s i z e d i s t r i b u t i o n ( L a mb e a n d W h i t ma n , 1 9 7 9 ) .

T h e K s a values fo r the d i f fe ren t so i l t ypes a res u m m a r i s e d in T a b l e 2 . T h e m e a n v a l u e s w e r e c a l c u -l a t e d a s s u mi n g a l o g n o r ma l d i s t r i b u t i o n f o r K s a .Fr o m T a b l e 2 i t c a n b e s e e n t h a t th e r e i s a s i g n i f ic a n td i f f e r e n c e i n K s a b e t w e e n t h e s o i l t y p e s a n d t h a t th eKsa i s i n v e r s e l y r e l a t e d t o t h e p e r c e n t a g e o f f i n e s( < 75 /~m).

O n t h e b a s i s o f t h e e s t a b l i s h e d K s a v a l u e s ,p e r c h e d w a t e r t a b l e s c a n b e e x p e c t e d o n c o n t a c t sw h e r e s o i l s w i t h a h i g h K s a (e.g . lapi l l i , sand, s i l tysand) over l i e so i l s wi th a lower K s a (e.g . s i l ty sando r s a n d y s i l t ) . A g r o u n d w a t e r t a b l e c a n b e e x p e c t e di n t h e s o i l a b o v e t h e s e mi - i mp e r me a b l e s c h i s t . T h e

T e n s i o me t e r s w e r e u s e d t o mo n i t o r p o s i t i v e a n dnegat ive p ressu re head f luc tua t ions in the so i l . Ten-s i o me t e r me a s u r e me n t s w e r e c o n c e n t r a t e d i n a r e p r e -s e n t a t iv e c a t c h m e n t i n t h e e a s t e r n p a r t o f t h e s t u d ya r e a ( s e e F i g . 3 ) . T h e c a t c h me n t i s c h a r a c t e r i s e d b ys t e ep s l o p e s ( 3 0 - 4 0 ) a n d a c t i v e l a n d s l id e s o f d i f f e r -e n t ty p e s . T h e l o c a t i o n o f th e t e n s i o m e t e r p l o t i ss h o w n i n F i g . 3 . T h e t e n s i o me t e r s w e r e i n s t a l l e d a td e p t h s o f 3 0 c m, 5 0 c m , 1 0 0 c m, 2 0 0 c m, 3 0 0 c m ( a tc o n t a c t s i l t y s a n d - s a n d y s i l t ) , 4 0 0 c m a n d 5 1 0 c m( c l o s e t o t h e s o i l - b e d r o c k c o n t a c t ) .

Fo r t h e i n s t a l l a ti o n o f t h e t e n s i o me t e r s a n o v e r -s i zed ho le was d r i l l ed to the requ i red dep th . Then thet e n s i o me t e r w a s l o w e r e d i n t o t h e h o l e a f t e r w h i c hthe ho le was f i l l ed up wi th s i l ty sand to a dep th o fa p p r o x i ma t e l y 5 c m a b o v e t h e c u p . T h e s i lt y s a n dw a s u s e d t o e n s u r e a g o o d h y d r a u l i c c o n t a c t b e t w e e ns o i l a n d t e n s i o me t e r a n d s u f f i c i e n t u n s a t u r a t e d h y -d r a u li c c o n d u c t i v i t y ( Ca r i s a n d V a n A s c h , 1 9 9 1) .T h e h o l e w a s s e a l e d w i t h 5 0 c m o f b e n to n i t e t op r e v e n t t h e i n f i lt r a ti o n o f w a t e r f r o m a b o v e . T h eu p p e r p a r t o f th e h o l e w a s b a c k f i l l e d w i t h t h e o r i g i -n a l s o i l ma t e r i a l. T h e t e n s i o me t e r s h a d t o b e i n s t a ll e di n a n o v e r s i z e d h o l e s i n c e a d i r e c t c o n t a c t b e t w e e ns o i l a n d t e n s i o me t e r c o u l d n o t b e o b t a i n e d w i t h t h ea v a i l a b l e e q u i p me n t . T h i s m a y l e a d t o a d i f f e r e n c e i nme a s u r e d a n d t ru e n e g a t i v e p r e s s u r e h e a d s i n t h e s o i l


5/11

AlL ZA Terlien / Geomorphology 20 (1997) 165 1752~00

169

@ / ~ / S c hi s t

j . ~ J

j . T J ~ - /

N

Fig. 3. Location of tensiometer plots and rain gauge in the study area.

w h i c h m e a n s t h a t m e a s u r e d n e g a t i v e p r e s s u r e h e a d sw i l l h a v e t o b e i n t e r p r e t e d w i t h c a r e .

T h e l o c a t i o n o f t h e r a i n g a u g e f o r w h i c h d a i l yr a i n f a l l me a s u r e me n t s a r e a v a i l a b l e , i s a l s o s h o w n i nF i g . 3 . D a i l y r a i n f a l l me a s u r e me n t s a r e a v a i l a b l ef ro m M arc h 1993 un t i l June 1993 . Pressu re headsw e r e u s u a l l y m e a s u r e d e v e r y t w o o r th r e e d a y sd e p e n d i n g o n t h e r a in f a ll . I n th e A p r i l a n d M a y r a i n yp e r i o d , a d a i l y r e c o r d o f p r e s s u r e h e a d r e a d i n g s f o rt h e u p p e r t w o me t r e s w a s p l a n n e d . H o w e v e r , d u e t op r o b l e ms w i t h t h e e l e c t r o n i c p r e s s u r e t r a n s d u c e r a n dt h e ma l f u n c t i o n i n g o f s e v e r a l t e n s i o me t e r s d a i l yr e a d i n g s w e r e n o t a l w a y s a c h i e v e d .3 .4 . Mean m on th ly p r es s ur e head f luc tua t ions a s a

Junc t ion o f dep thT h e me a n mo n t h l y p r e s s u r e h e a d d i s t r i b u t i o n a s a

func t ion o f dep th and the l i tho log ica l so i l p ro f i l e a reg i v e n i n F i g . 4 . T h e me a n mo n t h l y v a l u e s a r e c o n -s i d e r e d t o a n a l y s e t h e r e l a t i o n b e t w e e n v e r t i c a l p r e s-s u r e h e a d v a r i a t i o n s a n d v a r i a t i o n s i n Ksa . Fur ther -mo r e t h e y g i v e a g e n e r a l i m p r e s s i o n o f th e d i f f e r -e n c e s i n me a n p r e s s u r e h e a d s b e t w e e n t h e d r y a n d

r a i n y s e a s o n . F r o m F i g . 4 t h e f o l l o w i n g c o n c l u s i o n sc a n b e d r a w n :

( 1 ) Pe r c o l a t i n g w a t e r a c c u mu l a t e s a t a d e p t h 1 0 0cm where s i l ty sand over l i es l ap i l l i due to a reduc-t i o n i n u n s a t u r a t e d h y d r a u l i c c o n d u c t i v i t y . T h e u n -

P r es s u re h ead [ cm ]- 9 0 - 8 0 - 7 0 6 0 -5 0 - 4 0 - 3 0 2 0 - 1 0 0 1 0 2 0 3 0 L it h o l o g ica l1 . . . . ~ . _ ~ _ I I _ _ I i i J v ro n teMarch/.. June\ Ap ril Ma y j ~

~ Sap rel i te l i ' ~~ W ea t h e r ed r o ck Q G

Fig. 4. Mean monthly pressure head distribution measured withtensiometers as a function of depth and lithological soil profile.


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170 Jl//ZJi Terlien / Geomorphology 2 0 (1997) 165 175s a t u ra t ed hyd rau l i c conduc t i v i t y o f coa r s e l ap i l l i i ss m a l l e r t han t he uns a t u ra t ed hyd rau l i c conduc t i v i t yo f t he ove r l y i ng f ine r s i l t y s and due t o t he l a rgen u m b e r o f m a c r o p o r e s i n th e l a p il li w h i c h d o n o tcon t r i bu t e t o the hyd rau l i c c ondu c t i v i t y i n uns a t u -ra t ed cond i t i ons (Van den B erg , 1989 ) . Mi yazak i(1988 ) found s i m i l a r p res s u re head d i s t r i bu t i ons i ns i l t y s andy s o i l s s andwi ch i ng ve ry coa r s e l aye r s .

(2 ) P e rco l a t i ng wa t e r a l s o accum ul a t es a t a dep t ho f 3 1 0 c m w h e r e s i lt y sa n d ( w i t h a n a v e r a g e Ksa o f38 .5 cm d 1 ) ove r l i e s s andy s i l t (w i t h an ave rageKsa of 7 .0 cm d 1).

(3 ) A t t he bas e o f t he s o i l p ro f i l e wa t e r accum u-la tes in the sapro l i t e and sandy s i l t due to the pres-e n c e o f s e m i - i m p e r m e a b l e s c h is t.3 .5 . D a i ly p r es s ur e head f luc tua t ions a s a Jim c t ion o fdepth and t ime

In o rde r t o f i nd a l l zones where pos i t i ve p res s u reh e a d s h a v e b e e n m e a s u r e d a n d t o a n a l y s e t h e r e l a -

t i on be t ween p res s u re head f l uc t ua t i ons and r a i n fa l l ,t he da i l y t en s i om et e r r ead i ngs wi l l have t o be con -s idered .

F i g . 5 s hows t he i n t e rpo l a t ed da i l y p res s u re headsas func t i ons o f dep t h and t i m e i n r e l a t i on to t he d a i l yr a in f a ll . T h e n u m b e r o f m e a s u r e m e n t s f o r e a c h m o n t hi s no t exac t l y the s am e w h i ch exp l a i n s t he d i f f e rencei n leng t h be t w een Apr i l and M ay o n the t i m e ax i s . InF i g . 5 s a t u ra t ed cond i t i ons a re obs e rve d i n t he o r -gan i c s o i l at a dep t h o f 30 cm (2 m eas u rem e n t s ) , a tt he s i l t y s and - l ap i l l i con t ac t a t a dep t h o f 100 cm (1m eas u rem en t ) , a t t he s i l t y s and - s andy s i l t con t ac t a ta dep t h o f 300 cm (17 m eas u rem en t s ) , i n t he s andys il t at a dep t h o f 400 c m (4 m eas u rem en t s ) and i n t hes ap ro l i t e a t a dep t h o f 500 cm (12 m eas u rem en t s ) .

The po s i t ive p res s u re heads a t dep t h s o f 30 and100 cm were on l y obs e rved du r i ng and s ho r t l y a f t e rhea vy r a i n s to rm s . T he s a t u ra t i on i n t h i s zone i s duet o a r e d u c t i o n i n p e r c o l a ti o n ra t e w h i c h m a y b ec a u s e d b y t h e d i s a p p e a r a n c e o f m a c r o p o r e s i n t h eo rgan i c s o i l , changes i n s o i l t ex t u re o r changes i n

50e-

r,~, [

Fig. 5. Interpolated measured pressure head readings and rainfall from March 1993 until the middle of June 1993 as a function of time.


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Jl/ZZ Z Ter lien / G eomorpho logy 20 (1997 ) 165 175 171

s o i l m o i s tu re . Th e pos i t i ve p res s u re heads a t t he s i l t ys and - s andy s i l t con t ac t and a t t he s o i l bed rock con -t ac t a r e p res en t du r i ng l a rge pa r t s o f t he r a i ny s ea -son .

The l a rges t p res s u re head f l uc t ua t i ons were ob -s e rved i n t he upper m e t re o f t he s o i l p ro f i l e and a rec l o s e l y r e l a t ed t o r a i n fa l l and evapo t r ans p i r a t i on . A tg rea t e r dep t h s t he p res s u re head r i s e s s l owl yt h roug hou t March , A pr i l and Ma y as a r e su l t o f t heaccum ul a t i on o f the pe rc o l a t ed r a i n fa l l . A t the en d o fMay and i n J une t he p res s u re head dec reas es i n t heen t i r e s o i l p ro f i l e due t o cons ecu t i ve days wi t hou tra infal l .

4 . I mpl ica t ions on hydro log ica l t r igger ing cond i -t ions

The pos i t i ve p res s u re heads i n t he o rgan i c s o i land a t s i l t y s and - l ap i l l i con t ac t can t r i gge r s ha l l owlands l ides (so i l s l ips ) on s teep s lopes ( > 35 ) in so i l sw i t h a c o h e s i o n l o w e r t h a n a p p r o x i m a t e l y 5 k P a(Ter l i en , 1996 ) . S i nce o rgan i c s o i l has a r e l a t i ve l yh i gh cohes i on ( s ee Tab l e 1 ) , pos i t i ve p res s u re headsa re no t cap ab l e o f t r i gge r i ng l ands l ides w i t h s l ips u r faces i n t he o rgan i c s o i l and t he re fo re t he s l i ps u r face o f s o i l s l i p s i s u s ua l l y found be l ow t heo rgan i c s o i l i n the s i l ty s and . The dep t h o f t hepo t en t i a l s l i p s u r f ace depends on t he geo t echn i ca lp roper t i e s o f t he s i l t y s and and t he s l ope ang l e .F a i l u re w i ll on l y take p l ace w hen t he s o i l beco m e ss a t u ra t ed f rom t he t e r r a i n s u r f ace t o t he dep t h o f thepotent ia l s l ip surface (Ter l i en , 1996) . This sa turat iondep t h i s a func t i on o f t he s o i l p ro f i l e , the i n i ti a lve r t i ca l s o i l m o i s t u re d i s t r i bu ti on , t he r a i n fa l l am ou n tand ra infal l in tens i ty .

T h e p e r c h e d w a t e r t a b l e m a y t r i g g e r l a n d s l i d e swi t h s l i p s u r f aces a t t he s i l t y s and - s andy s i l t con t ac to n s lo p e s s t e e p e r t h a n a p p r o x i m a t e l y 3 0 T h eg roundwat e r t ab l e i n t he s ap ro l i t e and s andy s i l t i scapab l e o f t r igge r i ng l ands l i des w i t h s li p s u r f aces i nt he s ap ro l i te o n m oder a t e l y s t eep ( c~ > 25 ) a s we l las on gent le s lopes (c~ < 25 ) . Lands l ides t r iggeredby pe rched wa t e r t ab l es a s we l l a s l ands l i des t r i g -g e r e d b y g r o u n d w a t e r w i l l o n l y o c c u r a f t e r s e v e r a lwee ks o f s i gn i f i can t r a in fa l l .

5. Back analys is5.1 . Procedu re

In t he f i e l d one l ands l i de a s s um ed t o be t r i gge redby s a t u ra t ion o f t he t ops o i l , one a s s um ed t o bet r ig g e r e d b y a p e r c h e d w a t e r t ab l e a n d o n e a s s u m e dt o b e t r i g g e r e d b y g r o u n d w a t e r w e r e s e l e c t e d f o r aback ana l y s i s in o rde r t o eva l ua t e whe t he r t hes el a n d s l i d e s c o u l d b e e x p l a i n e d b y t h e m e c h a n i s m sdes c r i bed above . S i nce t he geom orpho l og i ca l s e t t i ngo f the s e l ec t ed l ands li des i n t e rm s o f s l ope ang l e ands l ope m orpho l ogy d i f f e r s s i gn i f i can t l y f rom t he geo -m orp ho l og i ca l s e t t i ng o f t he m on i t o red s l ope , a d i -r e c t c o m p a r i s o n o f o b s e r v e d p r e s s u r e h e a d s a n dback -ca l cu l a t ed p res s u re heads r e s u l t i ng i n s l opefa i l u re i s no t j u s t i f i ed . Ho we ver w hen t he r e s u l ts o ft he back ana l y s i s a r e no t con f l i c t i ng wi t h t he p re l i m -i n a r y c o n c l u s i o n s o f t h e h y d r o l o g i c a l s u r v e y w i t hres pec t t o s l i p s u r f ace l oca t i ons and p res s u re heads ,i t i s a fu r t he r i nd i ca t i on t ha t t he p ropo s ed hy d ro l og i -ca l t r i gge r i ng m echan i s m s a re co r rec t .

To e nab l e a back ana l y s i s o f f a i l ed s l opes w i t h theob j ec t i ve o f f i nd i ng c r i t i ca l p res s u re heads , t he fo l -l owi ng i n fo rm at i on i s r equ i r ed : (1 ) t he p res en t t o -pog r aphy o f t he l ands l i de ( f rom f i e l d s u rveys ) ; (2 )t he t opog raphy and t he s o i l p ro f i l e be fo re f a i l u re( f rom f i e l d s u rveys and o l d t opog raph i c m aps ) ; (3 )t he l oca t i on o f the s l ip s u r f ace ( f rom d r i ll i ng) ; (4 )s hea r s t r eng t h p roper t i e s and dens i t i e s o f s o i l t ypesi nvo l ve d ( l abo ra t o ry t e st s ).The s t ab i li t y ca lcu l a t i ons were p e r fo rm ed wi t h t he5ZIDE p r o g r a m ( D e V o s , 1 9 8 8) . B e c a u s e t h e r e c o n -s t ruc t ed s l i p s u r f aces d i d no t have a c i r cu l a r fo rm ,J a n b u ' s s i m p l i f i e d m e t h o d w a s u s e d t o c a l c u l a t es a fe t y f ac t o r s .

The back ana l y s i s s t a r t s w i t h t he ca l cu l a t i on o ft h e m i n i m u m s a f e t y f a c t o r u s i n g u n s a t u r a te d s h e a rs t r eng t h pa ram e t e r s . S ubs equ en t l y t he m i n i m ums afe t y f ac t o r i s ca l cu l a t ed u s i ng s a t u ra t ed s hea rs t r eng t h pa ram et e r s a s s um i ng a ze ro p res s u re heado n t h e s li p s u r f a ce . W h e n t h e m i n i m u m s a f e t y f a c t o rs ti ll exc eeds un i t y , eve n whe n t he l owes t c ' an d q~'va l ues a re u s ed , t he pos i t i ve p res s u re heads r educ i ngt he s a fe t y f ac t o r t o un i t y a re ca l cu l a t ed .5.2. A landslide tr igge red by saturation o f the topsoil

O n t h e 1 3 th o f M a y 1 99 3 a 6 9 m m r a i n st o r m w i t ha du ra t i on o f l e ss t han t wo hou r s t r i gge red a s ha l l ow


8/11


9/11

iV[ Z $ Terlien Geom orphology 20 (1997) 165 175 173L e g e n d~...un~ S i l t y s a n d

S an dy s i l t . :. :: iC o l l u v i u m . /

~ W e a t h e r e d r o c k Q G .. ... ... .. / ~ i i i ~ 1 5Or ig i na l top ogr aph y .. .. .. ... .. .. . / / " 3 ~

I.... - " . ~ < ' ; ' . - " - - . - - ~ - , - . .. .. .. .... ~ R < - - ..... , .,- .........

0 1 0 2 0 2 2D i s t a n c e [ m ]

Fig. 7.table.

g .

Cross-section of a landslide triggered by a perched water

observed in the slide scarp 80 cm above the slipsurface, a pressure head of + 90 cm seems to be arealistic possibility. From the tensiometer measure-ments it can be seen that such a pressure head willonly develop after several weeks with significantrainfall.5 .4 . A l a n d s l i d e t r i g g e r e d b y g r o u n d w a t e r

A groundwater-triggered landslide with a slip sur-face in the saprolite at a depth of 1200 cm (Fig. 8)was selected for a back analysis. In the field it couldnot be established whether this was a first-timefailure in the saprolite which would imply that peakstrength characteristics have to be used, or a remobi-lization of a prior failure, requiring residual strengthcharacteristics. Therefore safety factors were calcu-

Table 4Results of back analysis for different cases for a landslide trig-gered by a perched water table (landslide B)Case Pressurehead Silty sand Safety factor1 Negative cu = 16.5 kPa 3.50q) = 39.02 Zero c' = 0 kPa 1.29qY = 34.03 + 90 cm on slip surface c' = 0 kPa 1.00qY = 34.0

LegendSilty sand and Sandy s il t

~ - ~ Col luv iumWeathered ock. . Or iginal opogr

i0 25 50 70Distance m]

2

Fig. 8. Cross-sectionof a landslide riggered by groundwater.

lated using the peak as well as the residual shearstrength properties of the saprolite.

The minimum safety factor in unsaturated condi-tion s is 2.28 (c a = 45.5 kPa, q~ = 25.3). Wh en thesaturated shear strength properties are used and thepressure head is assumed to be zero on the slipsurface, the minimum safety factor is 1.69 in thecase o f peak shear strengt h values (C'p = 22.7 kPa,% = 25.3 ) and 1.22 in the case of residual shearstrength values ( C ' = 17.8 kPa , q~[ = 18.1). Thesesafety factors show that a groundwater pressure isrequired to trigger the landslide regardless whichpeak or residual strength values are used.

Two groundwater levels were considered to eval-uate the effect of groundwater on the stability of theslope. The first groundwater level is based on thepresence of gley features observed in augerholes onadjacent slopes which suggests a me an pressure head

S a f e t y f a c t o r1 8 T iL _ : !1 6 4 . . . " ~- 2" ~ ., .< . . . . . . i . . . . . . . . . . . . . . . . . . ./

i . . .. " - 4 . I S t r e n g t h s a p r o l i t e! ! ~ - ~ . ~ ~ P e a k

~,2 ,. . .. . .. . .. . .. . . .. .. .. .. ~ :... k'~a~s~duo 2

0 1 0 0 2 0 0 5 0 0 4 0 0 5 0 0M e a n p r e s s u r e h e a d [ c m ]

F i g . 9 . S a f e t y f a c t o r a s a f u n c t i o n o f m e a n p r e s s u r e h e a d f o r al ands lide t r iggered by groundwater.


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174 A/LEA Terlien / Geomorphology 20 (1997) 165 175on the s l ip s u r face + 460 cm . A s eco nd hypo t he t i ca lg roundwat e r l eve l wh i ch r e s u l t s i n a m ean p res s u rehead o f + 320 cm on t he s li p s u r face is i nc l uded i nt he ana l y s is t o eva l ua t e t he e f f ec t o f a l ow er g round -wa t e r l eve l on t he s a fe t y f ac t o r .

T h e s a f e t y f a c t o r a s a f u n c t i o n o f m e a n p r e s s u r ehead and s hea r s t r eng t h p roper t i e s i s s hown i n F i g . 9 .W hen l ow es t pea k c ' and qF va l ues a re u s ed , thefac t o r o f s a fe t y i s 1 . 29 . I t becom es l e s s t han un i t ywhen t he g roundwat e r t ab l e i s a t t he t e r r a i n s u r face .F i e l d o b s e r v a t i o n s a n d t e n s i o m e t e r m e a s u r e m e n t shave s hown t ha t s uch a g roundwat e r t ab l e i s no tr ea l is t ic . W he n t he low es t r e s i dua l c ' and qF va l uesa re u s ed , the s a fe t y f ac t o r i s 0 .93 a s s um i ng a m ea np r e s s u r e h e a d o f + 4 6 0 c m o n t h e s l i p s u r f a c e ( s e eF i g . 9 ) . Due t o de fo rm at i ons i n t he s l ope t ha t m ayh a v e b e e n c a u s e d b y p r e v i o u s e x t r e m e l y w e t p e r i o d so r ea r t hquakes , t he s ap ro l i t e m a y have l o s t (a pa r t o f )i ts peak s t r eng t h wh i ch m akes t he u s e o f r e s i dua ls hea r s t r eng t h pa ram et e r s accep t ab l e .

6 . D i s c u s s i o n

The ana l y s i s o f t he r e l a t i on be t w een l ands l i deeven t s and r a i n fa l l s how n i n F ig . 2 r eve a l ed t ha ts ha l l ow l ands l i des a re t r i gge red by h i gh da i l y r a i n fa l l(_+ 7 0 m m / d a y ) w h i l e d e e p e r l a n d s li d e s a r e t ri g -g e r e d b y a n a c c u m u l a t e d a n t e c e d e n t r a i n f a l l o f a tl eas t 200 m m i n 25 days i n com bi na t i on wi t h a da i l yr a i n f a l l a m o u n t w h i c h r e d u c e s w h e n t h e a n t e c e d e n tra i n fa l l i nc reas es . The p re l i m i na ry conc l u s i ons o ft h i s ana l y s is a re s upp o r t ed by t he hyd ro l og i ca l s tudy .In t he uppe r m e t re o f the s o i l p ro f i l e p res s u re headf l uc t ua t i ons a re p r i nc i pa l l y con t ro l l ed by r a i n fa l l andt he re fo re one s i ng l e ex t r em e ra i n fa l l even t can l eadt o s a t u ra t ion o f the t ops o i l eve n i f t h i s s o i l is r e l a -t i ve l y d ry a t t he ons e t o f t he r a i n s t o rm . At g rea t e rdep t h s t he r i s e i n p res s u re head i s caus ed by a s l owa c c u m u l a t i o n o f p e r c o l a t i o n ra i n a t t h e s i lt y s a n d -s andy s i l t con t ac t and a t t he s o i l -bed rock con t ac tdue t o s i gn i f ican t r educ t i ons i n Ks a . In t hes e zo ness a t u ra t i on can on l y occu r a f t e r s eve ra l weeks wi t hs i gn i f i can t am oun t s o f r a i n fa l l.

The s l ip s u r faces o f f a i l ed s l opes were o f t enl oca t ed i n t he zones wi t h pos i t i ve p res s u re headsi den t i f i ed by t he h yd ro l og i ca l s u rvey . F o r t h ree f a i l eds l opes t he c r i t i ca l p res s u re heads r educ i ng t he s a fe t y

fac t o r t o un i t y we re ca l cu l a t ed by ba ck ana l y s is .T h e y p r o v e d t o b e s i g n i f i c a n t l y h i g h e r t h a n t h ep res s u re heads m eas u re d i n t he d i f f e ren t po t en t i a lf a i l u re zone s even whe n t he l owes t c ' and q~' va l uesw e r e u s e d . T h e s e d i f f e r e n c e s c a n b e e x p l a i n e d a sfo l l ows :

(1 ) The hyd ro l og i ca l s t udy was no t ca r r i ed ou t i nm o n t h s w i t h e x t r e m e l y h ig h r a i n f al l a m o u n t s a n dt h e r e f o r e th e o b s e r v e d p r e s su r e h e a d s a r e p r o b a b l yn o t th e m a x i m u m p r e s s u re h e a d s t h a t c o u l d o c c u r inthe so i l .

(2 ) D i f f e rences i n back -ca l cu l a t ed p res s u re headsa n d o b s e r v e d p r e ss u r e h e a d s m a y a l s o b e c a u s e d b yd i f f e rences i n t he geom orpho l og i ca l s e t t i ng ( s l opes t eepnes s , con f i gu ra t i on and d i s t ance t o s t r eam ) be -t ween t he i n s t rum en t ed s l ope and t he f a i l ed s l opess e l ec t ed fo r back ana l y s i s . S t eep and convex s l opesa re u s ua l l y d r i e r t han gen tt le and conc ave s l opes .

A l t h o u g h t h e h y d r o l o g i c a l t r ig g e r i n g m e c h a n i s m sp r o p o s e d o n t h e b a s is o f t h e h y d r o l o g i c a l s u r v e ycanno t be va l i da t ed wi t h t he back ana l y s i s becaus eo f th e r e a s o n s m e n t i o n e d a b o v e , t h e r e s u lt s o f b a c kana l y s i s s uppo r t t he con c l u s i ons o f t he hyd ro l og i ca ls u rvey wi t h r e s pec t t o t he l oca t i on o f s l i p s u r facesand t he f ac t t ha t pos i t i ve p res s u re heads a re r equ i redfo r f a i l u re .

7 . C o n c l u s i o n

On t he bas i s o f t he r e s u l ts p res en t ed i n t h i s pape ri t can be c onc l ud ed t ha t an ana l y s i s o f r a i n fa ll andl ands l i de even t s i n com bi na t i on wi t h a hyd ro l og i ca ls t udy o f l ands l i de -p rone s l opes a re i nd i s pens ab l e fo ra co r rec t under s t and i ng o f t he hyd ro l og i ca l tr i gge r -i ng o f l ands li des . Ten s i om e t e r s a re r equ i red t o de t ec ts a t u ra t ed zones wh i ch u s ua l l y fo rm po t en t i a l f a i l u rezones i n s o i l p ro f i l e s w i t h s i gn i f i can t ve r t ica l change si n Ks a . The m eas u re d p res s u re head f l uc t ua t i ons canbe r e l a t ed t o r a i n fa l l wh i ch , i n com bi na t i on wi t h t heana l y s i s o f r a i n fa ll and l ands li des , g i ves an i nd i ca -t i on o f t he c r i t ica l m e t eo ro l og i ca l cond i t i ons andrequ i red r a i n fa ll am oun t s . B ack a na l y s i s o f f a il eds l opes a re r equ i red t o s ee whe t he r s l i p s u r faces a rerea l l y l oca t ed i n t he po t en t i a l f a i l u re zones i de n t i f i edon t he bas i s o f t he hyd ro l og i ca l s t udy and wh e t he rt he p res s u re heads r equ i red t o caus e f a i l u re a re r ea l -


11/11

~ Z~ Terlien / Geomorph ology 2 0 (1997) 165 175 17 5i s ti c i n c o mp a r i s o n t o th e p r e s s u r e h e a d s me a s u r e d i nthe f i e ld .

Acknow l e dg emen t sJ . D . N i e u w e n h u i s , T h . W . J . V a n A s c h a n d N .Re n g e r s a r e t h a n k e d f o r r e v i e w i n g t h e d r a f t . M.

M o s s e l m a n a n d D . A l k e m a a r e t h a n k e d f o r t h e i rass i s t ance in the f i e ld .

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