Discard Dump Failure at Duiker Mining LimitedHeritage Colliery

H.C. Hof fmann

Col l i e ry Manager

G.R. S i lk

Group Civ i l Eng inee r

S Y N O P S I S

Discard genera ted f rom Her i tage Col l i e ry ' s coa l benef ic ia t ion p lan tw a s d u m p e d , u n d e r l i m i t e d c o n t r o l c o n d i t i o n s , o n t o a g r o u n d b a s et h a t , a l t h o u g h s t r i p p e d o f t o p s o i l a n d s o m e s u b s o i l , h a d u n d e r -gone lit t le or no investigation either geologically or geohydrologically.A dump height of approximate ly 25 .0 m was achieved, fa r exceedingt h e p e r m i t t e d h e i g h t o f 5 , 0 m a n d t h e e x c e s s i v e l o a d i n g c a u s e da s l i p f a i l u r e o f t h e u n d e r l y i n g c l a y r e s u l t i n g i n e x t e n s i v e g r o u n dm o v e m e n t d o w n - s l o p e o f t h e d u m p i t s e l f . T h i s g r o u n d m o v e m e n tw a s d i r e c t l y t o w a r d s t h e R i c h a r d s B a y C o a l L i n e , s i t u a t e d i n ac u t t i n g a p p r o x i m a t e l y 1 5 0 m d o w n s l o p e o f t h e t o e o f t h e d u m p ,a n d i f a l l o w e d t o c o n t i n u e m a y h a v e r e s u l t e d i n t h e t e m p o r a r yc l o s u r e o f t h e l i n e . ( P h o t o 1 )

I N T R O D U C T I O N

In November 1995 Duiker Explora t ion (now Duiker Mining Limi ted)f i n a l i s e d n e g o t i a t i o n s f o r t h e t a k e - o v e r o f t h e p r i v a t e l y o w n e dm i n i n g c o m p a n y Ts e l e n t i s C o a l ( P t y ) L t d . P a r t o f t h e a s s e t s o fTs e l e n t i s C o a l w a s t h e i r u n d e rg r o u n d , a n t h r a c i t e m i n e , H e r i t a g eColl iery. This mine is s i tuated approximately 20 km outs ide Vryheid,N o r t h e r n K w a Z u l u N a t a l , o f f t h e N a t a l S p a / P a u l p i e t e r s b u rg r o a don the Zuinguin Mounta in . The co l l ie ry i s mining the Lower Dundasseam and i s p roduc ing approx imate ly 23 000 r.o .m. tons pe r month .

T h e a n t h r a c i t e m a r k e t s a r e s e a s o n a l . F o r 6 m o n t h s o f t h e y e a rt h e y a r e a b l e t o s c a l p o f f t h e + 4 5 m m r a w c o a l f o r e x p o r t . F o rt h e r e m a i n d e r o f t h e y e a r a l l t h e i r c o a l n e e d s t o b e b e n e f i c i a t e d .

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T h e c o a l b e n e f i c i a t i o n p l a n t i s s i t u a t e d f a i r l y c l o s e t o t h e m a i nadi t , on the s ide of the Zuinguin mounta in , and the d iscard genera tedi s t r a n s p o r t e d b y c o n v e y o r d o w n t o t h e d u m p i n g s i t e a t t h e f o o to f t h e m o u n t a i n . A t t h e q u o t e d p r o d u c t i o n r a t e , a p p r o x i m a t e l y11 0 0 0 0 t o n s o f d i s c a r d a r e p r o d u c e d a n n u a l l y. D u m p i n g o n t h es i t e i n q u e s t i o n c o m m e n c e d i n D e c e m b e r 1 9 8 9 a n d i t i s e s t i m a t e dt h a t i n e x c e s s o f 1 0 0 0 0 0 0 t o n s o f d i s c a r d w e r e t i p p e d o n t h ed u m p p r i o r t o t h e f a i l u r e .

I n h i n d s i g h t t h e f a i l u r e c a n b e a t t r i b u t e d t o l a c k o f u n d e r s t a n d i n gof the mechan ics o f s lope f a i lu re and the r e su l t an t l ack o f adequa teinformation on ground stability, but in the main to the non-implementationo f s t a n d a r d p r o c e d u r e s f o r t h e c o n s t r u c t i o n o f a d i s c a r d d u m p .T h e o r i g i n a l p e r m i s s i o n , a c c e p t e d b y a l l a f f e c t e d p a r t i e s , s t a t e da m a x i m u m d u m p h e i g h t o f 5 . 0 m . T h i s h e i g h t d o e s n o t a p p e a r t oh a v e b e e n b a c k e d u p b y a n y s c i e n t i f i c t h e o r y b u t w a s , r a t h e r , a na r b i t r a r y f i g u r e t h a t w a s f e l t t o b e f a i l - s a f e a n d c o u l d w i t h f u r t h e rs t u d i e s , b e i n c r e a s e d s h o u l d t h e n e e d a r i s e . ( F i g u r e 1 )

The dump des ign inco rpora t ed the cons t ruc t ion o f c l ay be rm wa l l s ,bu i l t f rom mate r ia l excava ted f rom wi th in the dump a rea , enc los ingt h e s i t e t o p r e v e n t c o n t a m i n a t e d s u r f a c e r u n - o f f f r o m t h e d u m pescap ing i n to t he nea rby wa te r cou r se s . When dumping commencedi t w a s c a r r i e d o u t b y e n d - t i p p i n g f r o m r o a d t r u c k s w i t h o u t t h ea s s i s t a n c e o f a n y e q u i p m e n t t o s p r e a d t h e d i s c a r d i n t o l a y e r s a n dthe re fo re r emained uncompac ted apar t f rom the l imi ted compac t iongained by the movement of t rucks across the dump. After a re la t ivelys h o r t p e r i o d o f t i m e t h e 5 . 0 m d u m p h e i g h t l i m i t w a s a c h i e v e da n d p a s s e d w i t h o u t a n y f u r t h e r s t u d i e s h a v i n g b e e n c a r r i e d o u tt o j u s t i f y t h e i n c r e a s e d h e i g h t . D u e t o t h e d e l a y e d p e r m i s s i o np r o c e s s f o r a n a l t e r n a t i v e d u m p i n g s i t e , t h e p l a n t c o n t i n u e d t odump the d i s ca rd on t he ex i s t i ng s i t e , r ega rd l e s s o f t he l im i t a t i onso f t h e p e r m i s s i o n .

T h e d u m p a r e a s o o n b e c a m e t o o s m a l l t o i n c r e a s e t h e h e i g h t a n yf u r t h e r a n d t h e n e x t n a t u r a l p r o g r e s s i o n w a s t o e x t e n d t h e s u r f a c ea r e a o f t h e d u m p b y e n d t i p p i n g o v e r t h e e d g e . O n c e a g a i n , i n ar e l a t i v e l y s h o r t s p a c e o f t i m e , t h e f o o t o f t h e d u m p h a d e x t e n d e dt o s u c h a d e g r e e t h a t i t b e g a n t o e n c r o a c h o n t h e c l a y b e r m w a l la round the dump , even tua l ly cove r ing i t comple t e ly. A t some po in ti t w a s n o t i c e d t h a t t h e b e r m w a l l w a s c o v e r e d a n d a s e c o n d w a l lw a s q u i c k l y c o n s t r u c t e d a h e a d o f t h e d u m p t o c o n t a i n t h e s u r f a c er u n - o f f . E v e n t u a l l y, h o w e v e r, e v e n t h i s w a l l w a s p a r t i a l l y b u r i e d

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Figure 1

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a n d m a y w e l l h a v e b e e n b u r i e d c o m p l e t e l y h a d t h e c r i t i c a l p o i n tn o t b e e n r e a c h e d a n d t h e g r o u n d f a i l u r e o c c u r r e d . T h e t i m e n o ww a s D e c e m b e r 1 9 9 5 a n d t h e f r o n t f a c e o f t h e d u m p w a s ± 2 5 , 0 mh i g h .

DAMAGE ASSESSMENT

I n i t i a l r e p o r t s o f t h e d u m p f a i l u r e w e r e r e c e i v e d f r o m l o c a l f a r me m p l o y e e s w h o s e k r a a l w a s s i t u a t e d b e t w e e n t h e d u m p a n d t h eR i c h a r d s B a y C o a l l i n e ( s e e F i g . 2 - D u m p l a y o u t ) . T h e f a i l u r eo f t h e g r o u n d b e n e a t h t h e d u m p h a s c r e a t e d c o m p r e s s i v e f o r c e so n t h e l a y e r s o f s o f t , s u r f a c e m a t e r i a l i m m e d i a t e l y i n f r o n t o ft h e d u m p . ( P h o t o 2 ) H a v i n g v e r y l i t t l e r e s i s t a n c e t o c o m p r e s s i v ef o r c e s , a n d a l s o b e i n g s a t u r a t e d w i t h w a t e r d u e t o a v e r y w e tr a i n y s e a s o n , t h e s o f t g r o u n d b e g a n t o c o n c e r t i n a , h e a v i n g a sm u c h a s 2 , 0 m i n s o m e p l a c e s .

O n b e i n g i n f o r m e d o f t h e f a i l u r e , S e n i o r M a n a g e m e n t w e r e c a l l e dt o s i t e , a l o n g w i t h t w o g e o t e c h n i c a l c o n s u l t a n t s , t o a s s e s s t h es i t u a t i o n a n d f o r m u l a t e a p l a n o f a c t i o n . ( P h o t o 3 ) T h e f o l l o w i n gobse rva t ions were made a t t he t ime o f t he i n spec t ion o f t he dump: -

• A s e r i e s o f v e r t i c a l c r a c k s , a p p r o x i m a t e l y p a r a l l e l t o t h ec r e s t , d e v e l o p e d o n t h e s i d e s l o p e a n d o n t o p o f t h e d i s c a r dd u m p . ( P h o t o 4 )

• T h e c r e s t o f t h e d u m p m o v e d d o w n w a r d s b y a t l e a s t 2 , 0 ma t t h e c e n t r e o f t h e f a i l e d z o n e .

• S t e a m e m a n a t e d f r o m t h e c r a c k s i n d i c a t i n g r e c e n t h e a t i n gw i t h i n t h e d u m p . T h e c r a c k s i n d u c e d a c h i m n e y e f f e c t t h a tc a u s e d t h e h e a t i n g . T h e d u m p h a d n o p r e v i o u s h i s t o r y o fs p o n t a n e o u s c o m b u s t i o n e x c e p t f o r t w o o f t h r e e s u r f a c e h o ts p o t s t h a t w e r e e x t i n g u i s h e d b y c o v e r i n g .

• At the tow of the dump, the earth bund wall , for the containmentof run-off and seepage, was displaced in a hor izonta l d i rec t ionb y 1 t o 2 m . ( P h o t o 5 )

• T h e n a t u r a l m a t e r i a l d o w n s l o p e o f t h e t o e o f t h e d u m pshowed s igns o f movemen t up to a d i s t ance o f app rox ima te ly6 0 m f r o m t h e t o e o f t h e e a r t h b u n d w a l l . T h e e a r t h m o v e du p w a r d s b y u p t o 1 , 8 m i n t h i s a r e a . T h e r e w a s , h o w e v e rno ev idence o f g round movemen t w i th in t he b lue gum fo re s t ,l o c a t e d a s s h o w n i n F i g . 2 , b e t w e e n t h e r a i l w a y l i n e a n dt h e d u m p . ( P h o t o 6 )

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• T h e r e a p p e a r e d t o b e a c o n s i d e r a b l e a m o u n t o f w a t e r b e i n gh e l d u p w i t h i n t h e d u m p . M o s t o f t h i s w a t e r w a s b e l i e v e dt o h a v e a r i s e n d u e t o d i r e c t i n f i l t r a t i o n o f r a i n f a l l t h r o u g ht h e t o p s u r f a c e o f t h e d u m p . A s e e p , w i t h a f l o w r a t e o fa p p r o x i m a t e l y 1 l i t r e / s e c . , w a s o b s e r v e d n e a r t h e c e n t r eo f the f a i lu re zone . The seep was emana t ing f rom the d i sca rda t a p p r o x i m a t e l y t h e e l e v a t i o n o f t h e c r e s t o f t h e e a r t h b u n dw a l l a t t h e t o e o f t h e d u m p . ( P h o t o 7 ) . F u t h e r s e e p s w i t hl o w e r f l o w r a t e s w e r e o b s e r v e d e l s e w h e r e o n t h e d u m p a tapprox imate ly the same e leva t ion . The qua l i ty o f the seepagew a s l a t e r a n a l y s e d a s p H - 6 . 5 a n d c o n d u c t i v i t y - 2 2 0 m S /m .

T h e e a r t h b u n d a t t h e t o e o f t h e d i s c a r d d u m p f o r m e d a l o w -p e r m e a b i l i t y b a r r i e r t o s e e p a g e f r o m t h e d i s c a r d . ( P h o t o 8 ) i t i sl i k e l y t h a t t h e p h r e a t i c s u r f a c e w i t h i n t h e d i s c a r d d u m p h a d b u i l tu p t o a b o v e t h e l e v e l o f t h i s e a r t h b u n d w a l l . ( S w i m m i n g p o o le f f e c t ) .

REMEDIAL ACTION

I m m e d i a t e - 1 t o 7 d a y s

T h e f o l l o w i n g r e m e d i a l a c t i o n s w e r e c a r r i e d o u t w i t h i m m e d i a t ee f f e c t i n a n a t t e m p t t o p r e v e n t a n y f u r t h e r d a m a g e t o p r o p e r t yo r i n j u r y t o p e r s o n s l i v i n g i n c l o s e p r o x i m i t y t o t h e d u m p : -

• Evacuation and rehousing of all persons living in close proximityt o t h e d u m p a n d c o r d o n i n g - o f f o f t h e a r e a b e l o w t h e d u m pt o w a r n p e o p l e o f t h e d a n g e r.

• C e s s a t i o n o f d u m p i n g o p e r a t i o n s o n t h e d u m p .• N o t i f i c a t i o n o f a l l t h e i n t e r e s t e d a n d a f f e c t e d p a r t i e s o f

t h e p r o b l e m a t h a n d .( S p o o r n e t , l o c a l f a r m e r s a n d D e p a r t m e n t o f M i n e r a l s a n dE n e r g y )

• E x c a v a t i o n o f c o n t r o l l e d b r e a c h e s o f t h e b e r m w a l l t o a l l o wthe seepage wa te r wi th in the dump, which i s co l l e t ing beh indt h e b e r m w a l l , t o f l o w a w a y a n d r e d u c e t h e e l e v a t i o n o ft h e p h r e a t i c s u r f a c e w i t h i n t h e d u m p .

• M o n i t o r i n g o f a n y f u r t h e r m o v e m e n t o f t h e d u m p b y t h ei n s t a l l a t i o n o f s t a k e s / p i n s ( 2 , 0 m l o n g , b u r i e d f o r 1 , 5 0 m )at be tween 25m and 50m in terva ls a t coord ina ted and e leva ted

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p o i n t s o n t o p o f t h e d u m p , o n t h e s l o p e s o f t h e d u m p a n do n t h e h e a v e d g r o u n d b e l o w t h e d u m p . T h e s e s t a k e s w o u l dextend as far as the forest of blue gums, and would be monitoredb y s u r v e y o n a r e g u l a r b a s i s .

• R e d i s t r i b u t i o n o f t h e w e i g h t o f t h e d u m p b y d o z i n g m a t e r i a lf r o m t h e f a i l e d S o u t h - E a s t c o r n e r t o w a r d s t h e S o u t h - We s tc o r n e r. S i m i l a r l y w i t h t h e N o r t h - E a s t c o r n e r i n t o t h e N o r t h -We s t c o r n e r.

• Re -p ro f i l i ng o f t he su r f ace o f t he dump in o rde r t o m in imi set h e i n g r e s s o f p r e c i p i t a t i o n i n t h e e v e n t t h a t m o r e r a i n f a l lw a s e x p e r i e n c e d .

S h o r t Te r m - 2 t o 3 w e e k s

T h e f o l l o w i n g s h o r t - t e r m m e a s u r e s w e r e t a k e n t o r e d u c e t h e r i s ko f a n y f u r t h e r s u d d e n f a i l u r e o f t h e d u m p : -

• C o n t i n u a t i o n o f t h e r e s h a p i n g o p e r a t i o n b y b u l l d o z e r t or e l i e v e t h e w e i g h t o n t h e f r o n t f a c e o f t h e d u m p .

• O n - g o i n g m o n i t o r i n g b y s u r v e y o f t h e c o o r d i n a t e d s t a k e s ,t h e r e s u l t s o f w h i c h w e r e p a s s e d o n t o G o r d o n M c P h a i la n d A s s o c i a t e s ( C o n s u l t i n g G e o h y d r o l o g i s t s ) o n a d a i l yb a s i s w h o , w i t h t h e a i d o f c o m p u t e r m o d e l l i n g , u s e d t h einformat ion to determine more c lear ly the extent of the fa i lurez o n e .

• Preparation of a topographical plan of the dump and surroundinga r e a s h o w i n g a l l r e l e v a n t i n f o r m a t i o n . T h i s i n c l u d e d t h ed u m p p o s i t i o n , c r e s t a n d t o e o f t h e e a r t h b u n d w a l l , e x t e n to f t h e f a i l u r e z o n e i n t h e d u m p a n d o n t h e n a t u r a l g r o u n d ,n e a r e s t r i v e r c o u r s e s , f o r e s t o f b l u e g u m s , R . B . r a i l w a yl i n e a n d a l l t h e m o n i t o r i n g s t a k e p o s i t i o n s .

• Three su rveyed sec t ions were to be p repa red fo r t he s t ab i l i t ya n a l y s i s . T h e f i r s t o f t h e s e t o b e a t r i g h t a n g l e s t o , a n dt h r o u g h t h e c e n t r e o f , t h e f a c e o f t h e f a i l u r e z o n e , t h e o t h e rt w o t o b e p a r a l l e l t o t h e f i r s t a n d 4 0 m e i t h e r s i d e i t .

M e d i u m Te r m - o n e t o t w o m o n t h s

Once suf f ic ien t in format ion had been ga thered regard ing the fa i lu rez o n e , a d e t a i l e d s t a b i l i t y a n a l y s i s w a s c a r r i e d o u t o n t h e d u m p ,t h e r e s u l t s o f w h i c h w e r e u s e d i n t h e d e s i g n o f s u i t a b l e r e m e d i a lm e a s u r e s t o p r e v e n t f u r t h e r f a i l u r e . I n t h e m e a n t i m e , p r e l i m i n a r y

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d e s i g n s o f a b u t t r e s s a n d u n d e r - d r a i n a g e s y s t e m i n f r o n t o f t h ed u m p w e r e p r e p a r e d s o t h a t t h e i r c o n s t r u c t i o n c o u l d c o m m e n c e .T h e f i n a l s h a p e a n d h e i g h t o f t h e b u t t r e s s w a s t o b e d e t e r m i n e df r o m t h e d e t a i l e d s t a b i l i t y a n a l y s i s .

L o n g Te r m - L i f e o f m i n e

F i n a l d e s i g n s f o r t h e b u t t r e s s a n d u n d e r - d r a i n a g e s y s t e m s w e r em a d e a v a i l a b l e a n d i m p l e m e n t e d . T h e d e s i g n w a s r e q u i r e d t o t a k ein to cons ide ra t i on t he r equ i r emen t s fo r t he con t a inmen t o f su r f acer u n - o f f w a t e r f r o m t h e d u m p i n a 1 i n 5 0 y e a r f l o o d s i t u a t i o na n d t h e c o n t r o l o f s e e p a g e w a t e r e m a n a t i n g f r o m w i t h i n t h e d u m p .

O n g o i n g m o n i t o r i n g o f t h e d u m p b y s u r v e y w a s t o b e u n d e r t a k e no n a r e g u l a r b a s i s t o e n s u r e t h a t n o f u r t h e r m o v e m e n t w a s t a k i n gp l a c e .

DETAILS OF CONSTRUCTION PROCEDURES ADOPTEDTO PERMANENTLY STABILISE THE FAILED DUMP AND

UNDERLYING GROUND

G o e h y d r o l o g i c a l c o n s u l t a n t s w e r e c o m m i s s i o n e d t o c a r r y o u t ade ta i led s tab i l i ty ana lys i s on the fa i l ed dump and assoc ia ted groundf a i l u r e z o n e t o f a c i l i t a t e t h e d e s i g n o f s u i t a b l e r e m e d i a l m e a s u r e st o p r e v e n t f u r t h e r f a i l u r e a n d p r o v i d e a l o n g - t e r m s o l u t i o n t ot h e p r o b l e m .

S o i l R e p l a c e m e n t Tr e n c h e s(To be read in con junc t ion wi th d rawing 112 /001 /2RO) in Append ix1 ) .

The area in which the soi l replacement t renches were to be excavatedw a s f i r s t l y c l e a r e d o f a l l u s e a b l e t o p s o i l , a n d s o m e s u b s o i l , t h i sb e i n g s t o c k p i l e d f o r l a t e r u s e i n t h e r e h a b i l i t a t i o n o f t h e d u m p .( P h o t o 9 )

A t o t a l o f 6 0 t r e n c h e s , 5 0 , 0 m l o n g b y 1 . 0 m w i d e , w e r e d u grad ia l l y f rom the f ace o f t he f a i l ed dump . (Pho to 10 ) . The t r enchesw e r e d u g f r o m 4 . 0 m t o 6 . 0 m d e e p , e n s u r i n g e a c h t i m e t h a t t h ee x c a v a t i o n h a d g o n e t h r o u g h t h e f a i l u r e z o n e i n t o t h e u n d i s t u r b e dg r o u n d b e l o w, w i t h a t r e n c h f l o o r g r a d i e n t o f a p p r o x i m a t e l y 1 i n

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2 5 t o p r o m o t e d r a i n a g e o f g r o u n d w a t e r a w a y f r o m t h e d u m p a r e a .T h e e x c a v a t i o n s e q u e n c e s t a r t e d o n t h e s o u t h e r n - m o s t t r e n c h e sa n d c o n t i n u e d n o r t h w a r d s . T h e i n i t i a l t r e n c h e s w e r e p o s i t i o n e douts ide the fa i lure zone and, being undis turbed, were se l f suppor t ingeven a t 6 . 0 m deep . (Pho to 11 & 12 ) Th i s a l l owed t he en t i r e l eng tho f t r e n c h t o b e e x c a v a t e d b e f o r e a n y b a c k f i l l m a t e r i a l w a s p l a c e d ,s i m p l i f y i n g t h e w h o l e o p e r a t i o n . H o w e v e r , w h e r e t h e g r o u n d h a db e e n b a d l y a f f e c t e d b y t h e f a i l u r e , d i f f i c u l t y w a s e x p e r i e n c e d i nm a i n t a i n i n g s u p p o r t o f t h e t r e n c h s i d e s f o r a n y l e n g t h o f t i m ea n d s h o r t s e c t i o n s o f t r e n c h h a d t o b e e x c a v a t e d a n d b a c k f i l l e di m m e d i a t e l y t o r e d u c e t h e r i s k o f c o l l a p s e . ( P h o t o 1 3 ) . E v e n s o ,short s t re tches of the worst affected t renches did col lapse, sometimesa s w i d e a s 5 , 0 m a t t h e s u r f a c e , ( P h o t o 1 4 ) , i n d i c a t i n g t h e d e g r e eo f g r o u n d m o v e m e n t e x p e r i e n c e d i n t h o s e a r e a s . ( P h o t o 1 5 )

B u l k b a c k f i l l i n g o f t h e t r e n c h e s w i t h s e l e c t e d p l a n t d i s c a r d w a sb r o u g h t t o w i t h i n 1 , 0 m o f t h e s u r f a c e , w i t h n o c o m p a c t i o n . ( P h o t o1 6 ) T h e u s e o f d i s c a r d a s t h e b a c k f i l l m a t e r i a l c h o s e n w a s r e a d i l ya v a i l a b l e o n s i t e a n d w a s o f a n a c c e p t a b l e s i z e g r a d i n g a n d s h e a rs t r e n g t h . T h e q u e s t i o n o f a d d i t i o n a l p o l l u t i o n p o t e n t i a l w a s f o u n dt o b e i n a p p l i c a b l e a s t h e s i t e w a s a d i s c a r d d u m p .

After that the backfi l l was placed in 300m thick layers and compacted,b y m e a n s o f a h a n d o p e r a t e d v i b r a t i n g r o l l e r , u p t o t h e s u r f a c el e v e l . ( P h o t o 1 7 & 1 8 )

T h e d i s c a r d - f i l l e d s o i l r e p l a c e m e n t t r e n c h e s ( S RT ) , a s p r e v i o u s l ym e n t i o n e d , a r e a l s o i n t e n d e d t o a c t a s a d r a i n a g e m e d i u m f o rthe su r round ing c l ays . I n t h i s r ega rd , e ach o f t he SRT ' s was l i nkedb y m e a n s o f a r i n g t r e n c h , a l s o b a c k f i l l e d w i t h c o a r s e d i s c a r d ,a n d 6 r a d i a l t r e n c h e x t e n s i o n s , ( P h o t o 1 9 ) l e a d i n g t o t h e s u r f a c eso lu t i on t r ench , t o a l l ow f r ee d r a inage o f t he g round wa te r. (Pho to2 0 & 2 1 )

S u b - S o i l D r a i n s N o . 1 a n d N o . 2

In o rde r to p reven t the deve lopment o f an e l eva ted phrea t i c su r facew i t h i n e i t h e r t h e d u m p o r b u t t r e s s a d e q u a t e s u b - s o i l d r a i n a g eh a d t o b e e s t a b l i s h e d . Tw o d i f f e r e n t s y s t e m s w e r e i n s t a l l e d , t h ef i r s t e n s u r e d t h a t t h e g r o u n d b e n e a t h t h e c o m p a c t e d b u t t r e s s w a sa d e q u a t e l y d r a i n e d a n d t h e s e c o n d h a n d l e d a n y b u i l d u p o f w a t e rw i t h i n t h e d u m p i t s e l f .

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T h e b u t t r e s s d r a i n c o n s i s t e d o f a r i n g t r e n c h r u n n i n g p a r a l l e lt o , a n d a p p r o x i m a t e l y 2 , 0 m f r o m , t h e f o o t o f t h e e x i s t i n g c l a yberm a t the base o f the dump. The t r ench and an a rea approx imate ly3 ,5m wide to the foo t o f the c l ay be rm wal l were l ined wi th KaymatU 3 4 g e o f a b r i c ( P h o t o 2 2 ) i n o r d e r t o p r e v e n t a n a c c u m u l a t i o no f f i n e s i n t o t h e d r a i n a g e s y s t e m t h a t w o u l d e v e n t u a l l y c h o k ei t . 1 5 0 m m d i a m e t e r , o p e n j o i n t e d , c l a y v i t r e o u s p i p e s w e r e l a i do n a b e d o f 1 9 m m s t o n e i n t h e t r e n c h a n d t h e n b a c k f l l e d w i t h1 9 m m s t o n e t o g r o u n d l e v e l ( P h o t o 2 3 ) Tw o 0 . 5 m t h i c k l a y e r s o f1 9 m m a n d 6 m m s t o n e w e r e t h e n p l a c e d o v e r t h e t r e n c h a n d t ot h e f o o t o f t h e c l a y b e r m , o n t o p o f t h e g e o f a b r i c , i n o r d e r t op r o v i d e a p e r m a n e n t d r a i n a g e p a t h f o r p r e c i p i t a t i o n f a l l i n g o n t oa n d p e n e t r a t i n g t h e c o m p a c t e d b u t t r e s s t h a t w i l l e v e n t u a l l y c o v e rt h e a r e a . T h r e e o u t l e t d r a i n s w e r e l a i d f r o m t h e r i n g d r a i n t o t h eso lu t ion t r ench , be low the evapora t ion paddocks , wh ich even tua l lyl e d i n t o a n e x i s t i n g s u r f a c e w a t e r s t o r a g e d a m .

I n a n a t t e m p t t o p r o v i d e a d r a i n a g e p a t h f o r t r a p p e d w a t e r, a s i xi n c h s t e e l p i p e w a s p u s h e d i n e x c e s s o f 2 0 m i n t o t h e f a c e o f t h ed u m p . T h i s w a s d o n e a t ± 1 m a b o v e g r o u n d l e v e l t o p e n e t r a t e t h ec l a y b e r m w a l l i n o r d e r t o d r a i n w a t e r t h a t w a s t r a p p e d b e h i n dt h i s b e r m w a l l .

Two sub- so i l d ra ins were in s t a l l ed a t t he base o f and pe rpend icu la rt o t h e f a i l e d f a c e o f t h e d u m p t o p r o v i d e a d r a i n a g e p a t h f o r a n yw a t e r b u i l d i n g u p i n s i d e t h e d i s c a r d d u m p i t s e l f . Tw o c u t s w e r eexcava ted fo r app rox ima te ly 30m in to the f ace o f t he dump, (Pho to2 4 ) d o w n t o o r i g i n a l g r o u n d l e v e l . T h e s i t e s c h o s e n f o r t h e s ee x c a v a t i o n s w e r e t h e e x i s t i n g " s e e p s " e m a n a t i n g f r o m t h e b a s eof the dump and therefore already showed signs of providing drainagep a t h s f o r w a t e r i n s i d e t h e d u m p . T h e c o n s t r u c t i o n o f t h e d r a i n sw e r e s i m i l a r t o t h o s e p r e v i o u s l y d e s c r i b e d , ( P h o t o 2 5 ) w i t h o p e njointed clay vi t reous pipes la id on a coarse granular bed and enclosedi n a g e o f a b r i c b e f o r e b e i n g b a c k f i l l e d . ( P h o t o 2 6 ) T h e s e d r a i n sa g a i n l e d i n t o t h e s o l u t i o n t r e n c h a n d e v e n t u a l l y t o t h e s t o r a g ed a m . ( p h o t o 2 7 ) ( I t s h o u l d b e n o t e d h e r e t h a t b o t h o f t h e s e d r a i n ss t a r t e d d r a i n i n g t h e d u m p i m m e d i a t e l y. )

C o m p a c t e d B u t t r e s s

On complet ion of the sub-soi l drainage construct ion of the compactedbu t t r e s s cou ld commence . (Pho to 28 ) The ma jo r i ty o f t he ma te r i a l sf o r t h e b u t t r e s s w a s t a k e n f r o m t h e f a i l e d d u m p i t s e l f . T h i s h a d92

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t w o d i s t i n c t a d v a n t a g e s , f i r s t l y i t k e p t h a u l a g e c o s t s d o w n t o am i n i m u m a n d s e c o n d l y i t e f f e c t i v e l y r e d u c e d t h e s i z e o f t h e d u m pwi th a resu l t an t r educ t ion in the load ing be ing p laced on the fa i lu rep l a n e o f t h e u n d e r l y i n g s o i l . T h e o n l y m a t e r i a l t h a t w a s i m p o r t e dw a s a r e l a t i v e l y s m a l l v o l u m e o f s e l e c t e d , c o a r s e d i s c a r d , t a k e nd i r e c t l y f r o m t h e c o a l b e n e f i c i a t i o n p l a n t , f o r u s e i m m e d i a t e l ya b o v e t h e s u b - s o i l d r a i n s i n o r d e r t o p r o m o t e t h e d r a i n a g e p a t ht o t h e s e d r a i n s .

T h e b u t t r e s s w a s l a i d d o w n i n l a y e r s , ( P h o t o 2 9 ) m a x i m u m 2 0 0 m mt h i c k , a n d t h e n c o m p a c t e d b y m e a n s o f a v i b r a t i n g r o l l e r t o 9 5 %M o d A A S H TO d e n s i t y. I n t h e i n i t i a l s t a g e s c o m p a c t i o n t e s t s w e r ecar r ied out on t r ia l a reas in order to es tab l i sh a s tandard compact ionp r o c e d u r e t h a t w o u l d a c h i e v e t h e r e q u i r e d d e n s i t y. O n c e t h i s h a dbeen established this procedure was followed religiously and compactionsampl ing was r educed to spo t checks on each l aye r a s a mon i to r ingp r o c e d u r e o n l y. ( P h o t o 3 0 )

T h e c o n t r a c t c a l l e d f o r t h e b u t t r e s s t o b e r a i s e d i n 5 , 0 m s t e p sto a maximum height of 9 .0m, but the but t ress design made al lowancefor th i s he igh t to be inc reased shou ld the need a r i se . A managementd e c i s i o n w a s t a k e n , h o w e v e r , n o t t o p l a c e a n y f u r t h e r d i s c a r do n t h i s d u m p s i t e b u t t o c o m p l e t e l y r e h a b i l i t a t e t h e d u m p a n de s t a b l i s h a n e w d u m p i n g a r e a o n a n a l t e r n a t i v e s i t e .

S t o r m w a t e r C o n t a i n m e n t P a d d o c k s

W h i l e t h e b u t t r e s s w a s b e i n g c o n s t r u c t e d , w o r k w a s c o m m e n c e don the cons t ruc t ion of a ser ies of s torm-water conta inment paddocks( P h o t o 3 2 ) t o c o n t r o l s u r f a c e r u n - o f f f r o m t h e d u m p a n d b u t t r e s ss i t e s . A t o t a l o f 8 p a d d o c k s w e r e c o n s t r u c t e d , i m m e d i a t e l y u ps l o p e o f t h e s o l u t i o n t r e n c h . T h e p a d d o c k w a l l s w e r e c o n s t r u c t e do f s u i t a b l e m a t e r i a l e x c a v a t e d f r o m t h e s o i l r e p l a c e m e n t t r e n c h e sa long wi th add i t i ona l ma te r i a l excava ted f rom wi th in the paddockst h e m s e l v e s . ( P h o t o 3 3 ) T h e m a t e r i a l w a s p l a c e d i n l a y e r s o f 2 0 0m m m a x i m u m t h i c k n e s s a n d c o m p a c t e d b y m e a n s o f a v i b r a t i n gr o l l e r t o 9 5 % M o d A A S H TO d e n s i t y. ( P h o t o 3 4 ) .

O n c e a g a i n r e g u l a r d e n s i t y t e s t s w e r e c a r r i e d o u t u n t i l a s t a n d a r dcompaction procedure was establ ished and then sampling was reducedt o s p o t c h e c k s o n e a c h l a y e r a s a m o n i t o r i n g p r o c e d u r e . O n c et h e w a l l s h a d a t t a i n e d t h e r e q u i r e d h e i g h t t h e y w e r e s h a p e d t ot h e i r f i n a l p r o f i l e b y a b a c k - a c t o r. ( P h o t o 3 5 )100

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S i n c e t h e c o s t o f s u c h a p r o j e c t i s h i g h a n d t h e q u a l i t y o f w o r kis of utmost importance, the mine appointed a ful l - t ime civi l engineert o e n s u r e a d h e r e n c e t o s p e c i f i c a t i o n s a n d q u a l i t y c o n t r o l . T h et o t a l c o s t o f t h e p r o j e c t w a s R 2 1 0 0 0 0 0 .

C O N C L U S I O N

R e m e d i a l w o r k o n t h e d u m p w a s c o m p l e t e d i n D e c e m b e r 1 9 9 6 .( P h o t o 3 6 & 3 7 ) E i g h t e e n m o n t h s o n , n o f u r t h e r m o v e m e n t o ft h e g r o u n d h a s b e e n e x p e r i e n c e d . T h e d u m p i s c o n t i n u o u s l y b e i n gm o n i t o r e d f o r m o v e m e n t a n d e r o s i o n b y t h e s u r v e y d e p a r t m e n t .T h e s t a b i l i s a t i o n p r o c e s s h a s p r o v e d t o b e a d e q u a t e t o t h e t a s k .The sub - so i l d r a in s a r e s t i l l ope ra t i ona l , w i th no t i ceab l e i nc r ea se sin flow during and after heavy precipitation As yet the final rehabilitationo f t h e d u m p h a s n o t b e e n c a r r i e d o u t b u t t h i s i s s c h e d u l e d f o rt h e f u t u r e , t o c o i n c i d e w i t h t h e n e x t r a i n y s e a s o n . T h e f i n a l r e -p r o f i l i n g o f t h e s u r f a c e a n d i t s c o v e r i n g w i t h s o i l s w i l l b e t h ela s t s t age o f th i s dump ' s r ehab i l i t a t ion . The wa te r d ra inage sys tem,b u t t r e s s a n d s t o r m w a t e r c o n t a i n m e n t p a d d o c k s w i l l s t i l l b e i no p e r a t i o n a f t e r t h e d u m p h a d b e e n r e - p r o f i l e d a n d c o v e r e d w i t hsoi ls . I t wil l be incorporated in the rehabi l i ta t ion. The water drainages y s t e m , b u t t r e s s a n d s t o r m w a t e r c o n t a i n m e n t p a d d o c k s w i l l s t i l lb e i n o p e r a t i o n a f t e r t h e d u m p h a s b e e n r e - p r o f i l e d a n d c o v e r e dw i t h s o i l s . I t w i l l b e i n c o r p o r a t e d i n t h e r e h a b i l i t a t i o n d e s i g no f t h e d u m p .

The following important factors should be considered in the constructiono f d i s c a r d d u m p s :

– D u m p s t a b i l i t y a n a l y s i s c a r r i e d o u t .– G r o u n d s t u d y o f t h e d u m p s i t e– G e o h y d r o l o g i c a l s t u d i e s– G e o l o g i c a l s t u d y ( D y k e s , f a u l t s , e t c )– P o s s i b l e e f f e c t o n n e a r b y i n f r a s t r u c t u r e e g . R a i l w a y l i n e s ,

r o a d s , e t c .– St a b i l i s i n g e f f e c t o f f o r e s t r y i n u n s t a b l e , s a t u r a t e d g r o u n d .– Cost compar i son be tween proper s tud ies done in in i t ia l s tages

a n d r e p a i r p r o j e c t .– Av a i l a b i l i t y o f c a p i t a l– A e r i a l p h o t o g r a p h y– S u r f a c e c o n t o u r s

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– F i n a l c l o s u r e o f t h e m i n e– R e t u r n w a t e r h a n d l i n g– L i f e o f m i n e ( a r e a r e q u i r e d f o r t h e s i t e )– I d e n t i f i c a t i o n o f i n t e r e s t e d p a r t i e s– R e l o c a t i o n o f g r a v e s a n d v i l l a g e s– C o a l d i s p o s a l v s . S l u r r y p o n d s– P o l l u t i o n c o n t r o l m e a s u r e s

The authors would like to thank Duiker Mining Ltd. Senior managementf o r p e r m i s s i o n t o p r e s e n t t h i s p a p e r

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