Geotechnical Assessment Deficiencies

7/29/2019 Geotechnical Assessment Deficiencies

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Min ing Science and Technology, 9 ( 1 9 8 9 ) 2 3 - 3 7 2 3E l s e v i e r S c i en c e P u b l is h e r s B .V ., A m s t e r d a m - P r i n t e d i n T h e N e t h e r l a n d s

Geotechnical assessment def ic ienciesin underground miningT . S Z W E D Z I C K I

Inst i tu te o f Mining Research, University of Zimbabwe, P.O. B ox MP . 167, Mo unt Pleasant, Harare (Zimb abwe)( R e c e i v e d O c t o b e r 2 8 , 1 9 8 8 ; r e v i s e d a n d a c c e p t e d J a n u a r y 9 , 1 9 8 9 )

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S z w e d z i c k i , T . , 1 98 9 . G e o t e c h n i c a l a s s e s s m e n t d e f i c i e n c e s i n u n d e r g r o u n d m i n i n g . M i n . S c i . T e c h n o l . , 9 :2 3 - 3 7 .A s s e s s m e n t d e f i c ie n c i e s i n r o c k m e c h a n i c s c a n b e m a d e d u r i n g s i te c h a r a c te r i z a t io n , d u r i n g p l a n n i n g

a n d d e s i g n , a n d d u r i n g s u p p o r t w o r k o r b l a s t i n g .D a m a g e t o u n d e r g r o u n d s t r u c t u r e s r e s u l t s f r o m o v e r - o r u n d e r e s t i m a t i n g t h e r o c k - m a s s p r o p e r t i e s o r

m i n i n g - i n d u c e d s t re s s. T h e a n a l y s i s o f d a m a g e r e p o r t s i s a v a l u a b l e s o u r c e f o r f u tu r e p l a n n i n g a n d d e s i g n .A s s e s s m e n t d e f i ci e n c i es c a u s e d b y i n s u f f ic i e n t s i te c h a r a c t e r i z a t i o n a r e d e s c r i b e d . E x a m p l e s a r e g i v e n o f

a s s e ss i n g th e v a l u e o f m e c h a n i c a l p a r a m e t e r s , t h e i n s t a b i l i t y o f a r a m p c a u s e d b y l a c k o f i d e n t i f i c a t i o n o fa s h e a r z o n e , a n d c h a n g e s o f t h e p r o p e r t i e s o f t h e r o c k d u e t o w a t e r f l o w .

A s s e s s m e n t d e f i c i e n ci e s i n p l a n n i n g a n d d e s i g n a r e a l s o d i s cu s s e d , w i t h e x a m p l e s o f t h e e f fe c t s o fu n d e r m i n i n g o n t h e s t a b i l i t y o f t h e o v e r l y in g se a m s , a n d t h e d e t e r i o r a t i o n o f g r o u n d c o n d i t i o n s w i t h t h es t a n d - u p t i m e o f t h e e x c a v a t io n s .

T y p i c a l d e f i c i e n c ie s m a d e d u r i n g i n s t a l l a t i o n o f r o c k b o l t s , s te e l a r ch e s , t i m b e r p a c k s , s h o t c r e t e a n dc o n c r e t e w a l l s a r e d i s c u s s e d .

In t roduc t i on

G e o t e c h n i c a l p r e d i c t i o n i n u n d e r g r o u n dm i n i n g i s a t t r a c t i n g m o r e a t t e n t i o n b e c a u s em i n e r a l e x t r a c t i o n i s p r o g r e s s i n g d e e p e r a n dl a r g e r e x c a v a t i o n s a r e r e q u i r e d f o r m o d e r ne q u i p m e n t . T h e d i f fe r e n c e b e t w e e n p r e d i c t i o na n d t h e a c t u a l u n d e r g r o u n d s t r u c tu r ep e r f o r m a n c e c a n b e c a l l e d a g e o t e c h n i c a l a s -s e s s m e n t d e f i c i e n c y .

U n f o r t u n a t e l y , t h e r e ar e m a n y g e o t e c h n i c a lp r o b l e m s w h i c h a w a i t f u r t h e r i n v e s t i g a t i o n .D e s p i t e p r o g r e s s i n r o c k m e c h a n i c s , a c c i d e n t sa n d d i s a s t e r s s t i l l h a p p e n , c a u s e d b y o u rl i m i te d k n o w l e d g e o f t h e p h e n o m e n a t a k i n g

p l a c e i n t h e r o c k m a s s . T h e d i f f i c u l t i e s i nm a t h e m a t i c a l d e s c r i p t i o n o f t h e p h e n o m e n at a k i n g p l a c e i n i n _h o m o g e n eo u s , j o i n t e d r o c kf o r c e u s t o a p p l y s i m p l i f i e d c a l c u l a t i o n s . T h e s ec a l c u l a t i o n s a r e b a s e d o n g e n e r a l d e s c r i p t i o n sa n d o n o b v i o u s l y l i m i t e d p r e v i o u s e x p e r i e n c e .B ien i awsk i [ 1 ] g ive s t yp i ca l cos t s o f de s igni n v e s t i g a ti o n s f o r a g e o t e c h n i c a l p r o j e c t i n t h eU S A . T h e t o t a l d e s i g n c o s t a m o u n t s t o o n l y1 .0 5% o f t h e t o t a l p r o j e c t c o s t b u t d a t a a n a l y -s i s a n d d e s i g n a m o u n t t o o n l y 1 1 % o f t h i sde s ign cos t , 0 . 11% o f t he t o t a l p ro j ec t cos t . I ti s su rp r i s i ng ly sma l l , cons ide r i ng t ha t t h i ss m a l l f r a c t i o n o f a p e r c e n t d e t e r m i n e s t h ep r o d u c t i v i t y a n d s a f e t y o f t h e p r o j e c t . C u t t i n g


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24c o s t s o n r o c k m e c h a n i c s i n v e s t i g a t i o n s a n da n a l y s i s m a y l e a d t o g e o t e c h n i c a l f a i l u r e o rd a m a g e .

T h e r e v i e w o f r e a s o n s a n d e x a m p l e s g i v e ns h o w s t h a t f a i l u r e s a n d t h e i r c o n s e q u e n c e sr e s u l t f r o m d e f i c ie n c y i n h u m a n a c ti v it y .A l t h o u g h i n t hi s p a p e r t h e t e r m " g e o t e c h -n i c a l a s s e s s m e n t d e f i c i e n c y " i s u s e d , t h ed a m a g e o r f i n a n c i a l l o s s e s c a u s e d b y m i n i n gi n t h e m a j o r i t y o f ca s e s a re c a u s e d b y l a c k o fr o c k m e c h a n i c s in p u t . T h i s p a p e r s u m m a r i z e sr e a s o n s f o r a n d g i v e s s o m e t y p i c a l e x a m p l e so f r o c k m e c h a n i c s d e f ic i e nc i es .

Reporting and analysis of underground damageT h e r e a s o n s f o r s h o r t c o m i n g s i n ro c k

m e c h a n i c s p r a c t i c e f o ll o w m a i n l y f r o m u n d e r -o r o v e r e s t i m a t i n g t h e p r o p e r t i e s o f t h e r o c km a s s o r m i n i n g - i n d u c e d s tr es s.

U n d e r e s t i m a t i n g t h e p r o p e r t i e s o r o v e r -e s t i m a t i n g s t r e s s r e s u l t s i n a c o n s e r v a t i v e a p -p r o a c h w i t h t h e a p p l i e d s a f e t y f a c t o r b e i n gg e n e r a l l y t o o h i g h , a n d i t r e s u l t s i n f i n a n c i a ll o s s e s d u e t o o v e r s u p p o r t i n g , l e a v i n g l a r g e rp i l la r s t h a n n e c e s s a r y o r d e v e l o p i n g e xc e s si v ee x c a v a t io n s . T h i s t y p e o f d e f i c i e n c y d o e s n o tg e n e r a l l y r e s u l t i n h a z a r d , a n d t h e r e s u l t i n gp e r f o r m a n c e o f t h e u n d e r g r o u n d s t r u c t u r e i sc o n s i d e r e d s a t i s f a c t o r y . T h e s e e r r o r s a r e n o ti d e n t i f ie d a n d t h e r e f o r e t h e y a r e n o t a n a l y z e d .

O v e r e s t i m a t i n g t h e p r o p e r t i e s o r u n d e r -e s t i m a t i n g m i n i n g - i n d u c e d s t r e s s i n e v i t a b l yr e s u l t s i n d e t e r i o r a t i o n o f g r o u n d c o n d i t i o n sa n d i n e x t r e m e c a s e s i n s a f e t y h a z a r d s . T h i st y p e o f d e f i c i e n c y m a y r e s u l t in d a m a g e v a r y -i n g f r o m m i n o r d e t e r i o r a t i o n t o c a t a s t r o p h i cf a i l u r e s .

M i n o r d e t e r i o r a t i o n o f t e n l e a d s t o e x p e n -s i v e r e p a i r w o r k o r r e h a b i l i t a t i o n o f e x c a v a -t i o n s o r w h o l e m i n i n g a r e a s , o r i t m a y r e s u l ti n c o n s i d e r a b l e l o s s o f o r e d u e t o i n a c c e s s i b i l -i t y . I n g e n e r a l s u c h m i n o r d e t e r i o r a t i o n s a r en o t r e p o r t e d a n d l e s s o n s f r o m s u c h e x p e r i -

e n c e a r e e a s i l y f o r g o t t e n . T h u s , t h i s t y p e o fd e f i c i e n c y i s o c c a s i o n a l l y r e p e a t e d .

S o m e l ar g e m i n e s h a v e i n t r o d u c e d d a m a g er e p o r t s w h i c h a r e u s e d f o r i n t e r n a l p u r p o s e ss u c h a s a d v i s i n g g e o t e c h n i c a l p e r s o n n e l o nc h a n g e s i n s t a b i l i t y o f e x c a v a t i o n s , o r s e i s m i ca c t i v i t y . S u c h r e p o r t s r e s u l t o n l y i n r e m e d i a ls u p p o r t w o r k a n d a r e v e r y s e l d o m a n a l y z e dt o p r o v i d e l o n g - t e r m g u i d e l i n e s .

R o c k m e c h a n i c s d e f ic i e n c ie s w h i c h a r e r e-p o r t e d , u s u a l l y t o m i n e i n s p e c t o r s , i n c l u d eo c c u r r e n c e s w h i c h c a u s e s e v e r e d i s r u p t i o n o fp r o d u c t i o n ( e.g ., r o c k b u r s t s , r o c k f a l ls , a n dc o l l a p s e s ) , o r r e s u l t i n a c c i d e n t s w h i c h a f f e c tp e r s o n n e l o r e x p e n s i v e e q u i p m e n t . U n f o r -t u n a t e l y , i n t h e s e r e p o r t s , d a m a g e o r i g i n a t i n gf r o m r o c k m e c h a n i c s f a c t o r s a r e f r e q u e n t l yo b s c u r e d b y a l a c k o f d e t a i l e d i n f o r m a t i o n .S u c h r e p o r t s a r e o f t e n c o n f i d e n t i a l a n d m a yn o t b e a v a i l a b l e t o r e s e a r c h e r s f o r a n a l y s i s .

F o r t h e a b o v e - m e n t i o n e d re a s o n s , th e r e -f o r e , r o c k m e c h a n i c s a n a l y s i s o f g e o t e c h n i c a ld a m a g e a n d f a i l u r e s t e n d s t o b e l i m i t e d . T h es i t u a ti o n i s a g g r a v a t e d b y t h e l a c k o f r e l e v a n tl i t e ra t u re . N u m e r o u s r o c k m e c h a n i c sh a n d b o o k s p l a c e e m p h a s i s o n c o r r e c t d a t ac o l l e c t i o n , d e s i g n a n d p l a n n i n g a n d i n s t r u -m e n t a t i o n , b u t d o n o t n e c e s s a r i l y d e m o n -s t r a t e h o w b e s t t o a v o i d t h e p i t f a l l s i n r o c km e c h a n i c s i t s e l f .

Assess ing def ic iences resul t ing from geotech-nical site characterization

G e o t e c h n i c a l s i t e c h a r a c t e r i z a t i o n i s i m -p o r t a n t f r o m t h e p l a n n i n g a n d d e s i g n v ie w -p o i n t , b u t s u c h i n v e s t i g a t io n s a re m a d e o n l yf o r l a rg e g e o t e c h n i c a l p r o j e c ts . F o r s m a l l o rm e d i u m - s i z e p r o je c t s , a s w e ll a s f o r e x p a n d -i n g e x i s t i n g m i n e s , o f t e n v e r y l i t t l e g e o t e c h n i -c a l i n p u t is m a d e . T h e a p p l i c a t i o n o f g e o t e c h -n i c a l d e s i g n in m i n i n g h a s n o t p r o g r e s s e d a tt h e s a m e r a t e a s h a s o t h e r e n g i n e e r i n gb r a n c h e s , a n d m a n y p r o j e c t e n g i n e e r s d e -


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25m a n d i n s u f f ic i e n t g e o t e c h n i c a l d a t a . E v e n i ft h e y a r e i n p o s s e s s i o n o f su c h d a t a , t h e y m a ya s s u m e t h a t l a t e r p r o t e c t i v e w o r k ( s u p p o r tw o r k o r w a t e r s e a l i n g ) w i l l s o l v e r o c km e c h a n i c s p r o b l e m s . T h e s i t u a t i o n i s f u r t h e ra g g r a v a t e d b y a l a c k o f s p e c i f i c c r i t e r ia o n t h ea p p l i c a t i o n o f r e s u lt s f r o m g e o t e c h n i c a l in v e s -t i g a t i o n s a n d r e s u l t s f r o m , w h e n e m p l o y e d ,t h e m o r e s o p h i s ti c a te d i n s t r u m e n t a t i o n .

R o c k - m a s s c l a s s if i c a ti o n s c h e m e s h a v e b e e ni n t r o d u c e d r e l a t i v e l y r e c e n t l y a n d h a v e f o u n dt h e i r p l a c e i n g e o t e c h n i c a l e n g i n e e r i n g , p r o -v i d i n g g u i d e l i n e s f o r a n u m b e r o f m i n i n gp r o j e c ts . T h e s e s c h e m e s a s s i g n n u m e r i c a l v a l-u e s t o t h o s e p r o p e r t i e s o r f e a t u r e s o f t h e r o c km a s s c o n s i d e r e d l i k e l y t o i n f l u e n c e i t s b e -h a v i o u r . T h e p a r a m e t e r s a r e d i v i d e d i n t or a n g e s o f v a l u e s a n d a s s i g n i n g t h e v a l u e s i s ,t o a c e r t a i n e x t e n t , a r b i t r a r y . B e c a u s e o f t h en a t u r e o f t h e r o c k m a s s , t h e p a r a m e t e r s o f t e nv a r y f r o m p l a c e t o p l a c e , e v e n i n c l o s e l ya d j a c e n t s e c t io n s . F u r t h e r , s o m e p a r a m e t e r sa r e d i f f i c u l t t o d e s c r i b e i n l i m i t e d u n d e r -g r o u n d s p a c e s , e.g ., m a x i m u m jo i n t s p a c i n g .

A n e x p e r i m e n t w a s u n d e r t a k e n a n d a ro c km a s s w a s c l a s s i f i e d u s i n g t h e m o d i f i e d B i e n -i a w s k i G e o t e c h n i c a l C l a s s i f ic a t io n [ 2 ] . T h ea r e a w a s c l a s s i f i e d b y a g e o t e c h n i c a l e n g i n e e r ,a m i n i n g e n g i n e e r a n d a g e o l o g i s t . D i f f e r e n tr a n g e s o f v a l u e s w e r e a s s i g n e d t o t h e e x -a m i n e d p a r a m e t e r s b y t h e d i f f e r e n t i n v e s t i g a -t o r s r e s u l t in g i n c o n s i d e r a b l y d i f f e r e n t o v e r a llc l a s s i f i c a t i o n r a t i n g s . T h e o b t a i n e d v a l u e sw e r e f r o m 4 0 ( p o o r - f a i r r o c k ) t o 6 5 ( g o o dr o c k ) a n d r e s u l t e d i n s u b s t a n t i a l l y d i f f e r e n ts u p p o r t r e c o m m e n d a t io n s .

T h e r o c k - m a s s b e h a v i o u r m a y c e r t a i n l y b ep r e d i c t e d u s i n g r o c k - m a s s c l a s s i f i c a t i o n , b u ti t s h o u l d a l w a y s b e r e l a t e d t o p r e v i o u s e x p e r i -e n c e o f m i n i n g i n t h e i n v e s t i g a t e d a r ea .

S i t e e v a l u a t i o n p r o b l e m s a r e d e s c r i b e du s i n g t h r e e e x a m p l e s : a s s e s s i n g t h e v a l u e o fm e c h a n i c a l p a r a m e t e r s , i n s t a b i l i t y o f a r a m pc a u s e d b y l a c k o f i d e n t i f i c a t i o n o f a s h e a rz o n e a n d c h a n g e s i n t h e p r o p e r t i e s o f t h e r o c kd u e t o w a t e r f l o w . G e o t e c h n i c a l a s s e s s m e n t

d e f i ci e n c ie s r e s u lt n o t o n l y f r o m i n c o r r e c t b u ta l s o f r o m i n s u f f i c i e n t s i t e c h a r a c t e r i z a t i o n .Assessment of the mechanical parameters of therock

A s s e s s m e n t d e f i c ie n c i e s w h i c h l e a d t o l a t e rf a i l u r e o f t e n o r i g i n a t e f r o m t a k i n g i n a p -p r o p r i a t e v a l u e s o f t h e s t r e n g t h o f t h e r o c kb a s e d o n t h e l i m i t e d p o p u l a t i o n o f s a m p l e s .D e t a i le d p r o c e d u r e s f o r p e r f o r m i n g l ab o r a -t o r y t e s t s a r e a v a i l a b l e [ 3 ] , b u t t h e r e s u l t s o fe v e n t h e m o s t c a r e f u l l y t e s t e d s a m p l e s d o n o tu s u a l l y r e p r e s e n t t h e a c t u a l r o c k - m a s s p r o p -e r t i e s . F o r e x a m p l e , u n i a x i a l c o m p r e s s i v es t r e n g t h i s e s t i m a t e d f r o m a n a v e r a g e o fs e v e r a l t e s t s . B y t h e n a t u r e o f s a m p l i n g , o n l yt h e s t r o n g e s t s p e c i m e n s a r e c o l l e c t e d , a n dd u r i n g t h e p r e p a r a t i o n t h e w e a k e r s p e c i m e n sa r e d i s c a r d e d o w i n g t o b r e a k a g e .

T h e v a l u e o f t h e s t re n g t h p a r a m e t e r s m e a s -u r e d i n - s i t u i s d i s c u s s e d u s i n g a n e x a m p l e o fr e su lt s o b t a i n e d w i t h a h a r d - r o c k p e n e t r o m e -t e r . P e n e t r o m e t e r t e s t i n g i n b o r e h o l e s g i v e sh i g h -p r e c is i o n m e a s u r e m e n t s o f t h e s t r e n g t hp a r a m e t e r s , a s w e l l a s t h e l o c a t i o n o f f r a c -t u r e s . T h e p e n e t r o m e t e r c o n s i s t s o f a p r o b ea n d i n d e n t e r . I n e s s e n ce , t h e t e s t in v o l v e s t h em e a s u r e m e n t o f t h e c r i t i c a l s t r e s s w h i l eb r e a k i n g t h e w a l l o f t h e b o r e h o l e u n d e r t h ep r e s s u r e o f t h e i n d e n t e r [ 4 ] . O n t w o s t r e n g t hp r o f i l e s ( F i g . 1 ) , t h e u n i a x i a l c o m p r e s s i v es t r e n g t h o f t h e s h a l e i s i n t h e r a n g e 3 5 - 6 0M P a a n d t h e s t r e n g t h o f f i n e s a n d s t o n e i sf r o m 4 0 t o 8 0 M P a . ( T h e r e a d i n g s w e r e t a k e na p p r o x i m a t e l y e v e r y 1 0 c m . ) B e c a u s e t h er a n g e o f t h e r e s u l t s o b t a i n e d v a r i e s c o n s i d e r -a b l y , i t i s d e b a t a b l e w h i c h v a l u e o f t h e u n -i a x i a l c o m p r e s s i v e s t r e n g t h s h o u l d b et a k e n - - t h e m a x i m u m , t h e a v e r a g e o r t h em i n i m u m .

T h e a n s w e r is" n o t s i m p le . F o r e x a m p l e , i nt h e c a l c u l a t i o n o f p i l l a r s i z e s i t i s r e a s o n a b l et o t a k e t h e l o w e s t v a l u e , w h i l s t f o r d r i l l i n gp r e d i c t i o n p u r p o s e s t h e a v e r a g e s h o u l d b et a k en , a n d i n d e t e r m i n i n g b la s t in g p a r a m e -


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26

8,0ii!!!iiiii{ . : ~iii~ ' 7,0 i

i!!~.i?? s,oiii':i

-.~- 3.0{.-- 2,0-.-IZ7~- 1,0:::5--

1 20 30 /,,0 50 60 0 80Re [MPa ]

Fig . 1 . Two penet rometer p ro f i l es o f the s t reng th o frock tes ted in a boreho le .

t o t he r a m p . The pos i t i on o f t he r a m p i nre la t ion to the s tope i s i l lus t ra ted in Fig . 3 .The f oo t wa l l s i de o f t he z one wa s c om pe t e n t ,a nd t he ha ng i ngwa l l wa s b l oc ky wi t h d i s t i nc -t ive j o i n t s a nd ope n c r a c ks. The r e l a ti vem o v e m e n t a c r o s s t h e z o n e w a s m o n i t o r e d o c -c a s i ona l l y ( F i g . 4 ) . The obs e r va t i on m a r kss howe d t ha t du r i ng t he l a s t 8 ye a r s t he m ove -m e n t on one o f t he l e ve l s r e a c he d 18 - 25 c m .

Fa c t o r s wh i c h a f f e c t e d t he s t a b i l i t y o f t her a m p wh i l e s t op i ng we r e ( 1 ) t he v i c i n i t y o ft he de s i gne d s t ope ( < 4 m ) , ( 2 ) t he e x i s t e nc eo f t he s he a r z one wh i c h i n t e r s e c t s t he r a m pa nd t he e xpe c t e d s l i p a l ong t he s he a r p l a ne ,( 3 ) unde r c u t t i ng t he r a m p by t he f oo t wa l ld r i ve s i t ua t e d a bou t 2 .5 m be l ow t he r a m p ,

t e r s , t he h i ghe s t va l ue s hou l d be a dop t e d .Thus , a c c e p t i ng t he va l ue o f s t r e ng t h ob -t a i ne d i n t he l a bo r a t o r y f r om on l y a f e ws a m pl e s m a y r e s u l t i n s i gn i f i c a n t a s s e s s m e n tde f i c i e nc y .Effe c t o f a faul t on the s tabi l i ty o f an excava-tion

W e a s s um e a de s i gn s t a ge a t wh i c h a f a u l twi t h a n a s s oc i a t e d s he a r z one wa s no t i de n t i -f i e d a nd whe r e t he a c c e s s ha u l a ge wa s de s i g -ne d r e l a ti ve l y c l o s e t o t he o r e bo dy . The r oc km e c h a n i c s p r o b l e m t o b e c o n s i d e r e d w a s t h ee f f e c t o f s t op i ng on t he s t a b i l i t y o f a r a m p .T h e p r o b l e m w a s c o m p o u n d e d b y t h e e x -i s t e nc e o f t he s he a r z one wh i c h i n t e r s e c t e dt he r a m p ( F ig . 2 ). The r a m p s i t ua t e d in t hef o o t w a l l f o r m a t i o n w a s a n i m p o r t a n t a c c e s st o t he l owe r l eve ls a nd i t s m a i n t e na n c e wa s o fp r i m e i m por t a nc e . I t wa s e xpe c t e d t ha t whe ns t o p i n g t h e o r e b o d y , t h e g r o u n d c o n d i t i o n swou l d de t e r i o r a t e a nd t ha t a s l i p a l ong t hes he a r z one m a y ha ve r e s u l t e d in s e ve r e da m a ge F i g . 2 . Gro u n d d e t e r i o r a t i o n d u e t o m o v e m e n t a c ro s s ashear zone .


5/15

o r e b o d y

N

n ~ " ' ~ Ramp

Area of expectedground movemen!

N

U q

, i NF i g . 3 . S h e a r z o n e i n t e r s e c t i n g a r a m p i n t h e v i c i n i t y o ft h e o r e b o d y .

a n d ( 4 ) p o o r g r o u n d c o n d i t i o n s o n t h eh a n g i n g w a l l s i d e o f t h e z o n e .T h e e f f e c t o f a l l t h e s e f a c t o r s w a s e x p e c t e dt o c u l m i n a t e i n th e m o v e m e n t o f t h eh a n g i n g w a l l s i d e o f th e s h e a r z o n e i n t o t h es t o p e , a n d i n t h e d e t e r i o r a t i o n o f th e s t a b i l i tyo f t h e r a m p . I n s u c h c o m p l i c a t e d g e o t e c h n i -c a l c o n d i t i o n s , t h e d e v e l o p m e n t o f a n e wa c c e ss r a m p w a s r e c o m m e n d e d s o t h a t th ea r e a o f t h e e x p e c t e d g r o u n d m o v e m e n t c o u l db e i s o la t e d .

27H a d t h e f a u l t a n d s h e a r z o n e b e e n m a p p e d

o n t h e u p p e r l e v e l s , t h e r a m p w o u l d h a v eb e e n s i t u a t e d f u r t h e r i n t h e f o o t w a l l .Failure due to changes in geotechnical condi-tions

I n t h e d e s i g n s t a g e , t h e l o n g - t e r m b e -h a v i o u r o f t he r o c k m a s s s h o u l d b e p r e d i c t e da n d r e l e v a n t t e s t s c o n d u c t e d . A n e x a m p l e i st h e c o l l a p s e o f a s h a f t h e a d g e a r d u e t o c h a n g e si n g e o t e c h n i c a l c o n d i t i o n s .

A s h a f t o f 8 m i n d i a m e t e r w a s s u n k f o r ad e e p u n d e r g r o u n d c o a l m i n e f r o m t h e s u r f a c et h r o u g h 5 m o f s o i ls a n d i n t o a l t e r n a t i n gs a n d s t o n e a n d m u d s t o n e s e q u e n c e s in c l in e da t a b o u t 1 0 ( F ig . 5 ) . T h e u p p e r m o s t s a n d -s t o n e s t r a t u m w a s 1 8 m t h ic k , w a s c h a r -a c t e r iz e d b y l o w s t r e n g th a n d w a s w a t e r -b e a r i n g . W a t e r d r a i n e d t h r o u g h t h i s s a n d -s t o n e a n d t h r o u g h c r a c k s a n d w a s h o u t s i n t h ec o n c r e t e l in i n g t o t h e s ha f t. T h e a m o u n t o fw a t e r s e e p a g e w a s c o n s i d e r e d n e g l i g ib l e a n dw a s n o t c o n t r o l l e d a n d n o w a t e r t e s t i n g f o rc h e m i c a l c o n t e n t w a s c a r r i e d o u t .

A f t e r 2 5 y e a r s o f s u c h c o n d i t i o n s t h eh e a d g e a r c o l l a p s e d . T h i s w a s a f a i rl y g r a d u a lp r o c e s s : T h e t i m e d u r a t i o n f r o m t h e f i rs tm o m e n t t h e m o v e m e n t w a s n o t ic e d u n ti l th et im e o f c o l la p s e w a s 3 0 m i n u t e s. S u b s e q u e n ti n v e s t i g a t i o n r e v e a l e d t h a t t h e w a t e r w h i c hd r a i n e d i n t o t h e s h a f t w a s s l i g h t l y a c i d i c a n dd i s so l v e d t h e s a n d s t o n e b o n d i n g a n d w a s h e do u t t h e f i n e s a n d , c r e a t i n g a l a r g e c a v e r n .T h i s c a v e r n w a s l o c a t e d b e n e a t h t h e s oi l d o w nt o a b o u t 2 0 m b e l o w t h e s u r f a c e. T h e h e a d g e a rs u b s e q u e n t l y b e c a m e u n s t a b l e a n d c o l la p s eo c c u r r e d . T h i s s e r i o u s f a i lu r e c o u l d h a v e b e e na v o i d e d i f t h e w a t e r h a d b e e n c h e m i c a l l yt e s t e d t o e n a b l e t h e c o r r e c t l i n i n g p r o t e c t i o na n d o t h e r m e a s u r e s t o b e d e s i g n e d .

A s s e s s m e n t d e f i c i e n c e s i n p l a n n i n g a n d d e s ig nT h e a n a l y s i s o f g e o t e c h n i c a l f a i lu r e s s h o w s

t h a t d e f i c i e n t p r e d i c t i o n s , b a s e d o n n o t t a k i n g


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Fig: 4. Measuring plates installed across the shear zone.

account of all the factors, may result in seri-ous disasters. Mining history has clearly dem-onstrated disasters involving water or tailingsinrush into mines as a result of the incorrectsiting of surface water reservoirs and tailingdumps. Additionally, shafts have been aban-doned because of damage to linings as aresult of deformation caused by the unsatis-factory design of the shaft pillars.

A variety of deficiencies may arise duringthe planning and design stage and the mostcommon are caused by incorrect siting of thedevelopmen t and by designing excavations ofinappropriate size and shape. Two examplesare discussed here: the effect of underminingon the stability of the over-lying seam, andthe deterioration of ground conditions withstand-up time (the length of time that anexcavation remains open).Effect o f undermining on coal seam stabil i ty

Special geotechnical consideration is re-quired when planning the extraction of one

seam which lies beneath another. Such situa-tions may occur when a group of coal seamsis mined by longwall methods (Fig. 6). Thecaving of the roof of the lower seam in-fluences the continuity of the overlying rock,and may result in the deterioration of groundconditions so that it may be come necessary toabandon the upper seam [5]. The degree ofdamage to the structure Of the underminedseam mainly depends on the value of thecoefficient of undermining, i.e., the ratio ofthe thickness of the waste rock separatingthese seams to the height of the underminedlongwall. In general, the rock mechanics anal-ysis shows that the upper seam is in a cavingzone and would be totally destroyed if thecoefficient of undermining is less than 3. Ifthis value is between 5 and 7, the upper seamcan be taken in certain conditions. When theseparating distance is greater than 7, thenegative effect of undermining is negligibleand does not influence overall stability [6].Despite these guidelines, a geotechnical anal-ysis will be required if coal seams are to beundermined . Thus, geological factors may re-


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I . . . . . . . . : : : : : : : : : i : : : : _ . : , /. . . . . . . . . . . . . : 1 /I i } i i ? i i i i i i i i i i i ! ! : : : : l / /

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . /

n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . s /n

,2:;:2:22222:221J222:2:222:22:~/n

/l i i ? ! i ? i i i i ! ! ~ /~ ~ ////////

~ S o i lConcrete L i n i n g

I

/ l " : " washout cavern . . . . . ./ h ,/ / i i i i i i 2 ; 2 : : i i i i 2 i i i : 2 1 1 2 i i : i l/ t/ ! i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ./ ' / : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ./ , ~ . . . . . . . . . . . . . .. . . . . . . . . . . . . . : . . . ~ ,

/

E X P L A N A T I O NWater bearing sandstoneImpermeable mudstone

Fig. 5. Simplified cross section through a failure area in the substratum beneath the headgear.sult in considerable problems a nd m ay lead tosubstantial losses of coal reserves. Also, in thedesign stage the effect of undermining on thestabili ty of the roof of the undermined seamshould be assessed and a suitable supportsystem selected.

Stand-up time o f excavationsMany geotechnical problems result from

deterioration in ground conditions caused bythe length of time the excavation remainsopen (stand-up time).


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; ~ " " ] l

IFig. 6. Undermining of a coal seam.

O n o c c a s i o n s , s e v e r a l y e a r s m a y e l a p s e b e -f o r e c e r t a i n d e v e l o p e d a r e a s a r e b l a s t e d o r , i nt h e c a s e o f s t o p e s , b e f o r e p r o d u c t i o n c e a s e s .I n a r e a s o f w e a k g r o u n d c o n d i t io n s , b e n e a t hu n b l a s t e d r e m n a n t s o r i n c l o s u r e p o s i t i o n s ,t h e e x c e s s i v e d e l a y c a n c a u s e s e v e r e s t a b i l i t yp r o b l e m s . S t u d i e s [7 ] h a v e i n d i c a t e d t h a t e x -c a v a t i o n s u n d e r s t r e s s d e t e r i o r a t e e x p o n e n -t i a l l y , i . e . , d e t e r i o r a t i o n b e g i n s s l o w l y , b u tt h e n a c c e l e r a t e s t o w a r d s e v e n t u a l f a i l u r e . T oa s s e s s t h e e f f e c t o f s t r e s s a n d t i m e , a c l a s s i f i -c a t i o n o f e x c a v a t i o n f a i l u r e f o r d r i v e s a n dc r o s s c u t s w a s p r e p a r e d . T h e d a m a g e w a s c l as -s i f i e d i n t o s ix g r o u p s :1 - - n o e f f e c t o f s tr e ss o n s t ab i li ty .2 - - i n s i g n i f i c a n t r a v e l li n g a n d s l a b bi n g ; s p al l-

i n g d o e s n o t e x c e e d 0 .5 m .3 - - f u r t h e r d e v e l o p m e n t o f r a ve l li n g a n d s la b -

b i n g ; s p a l l i n g d o e s n o t e x c e e d 1 . 0 m .4 - - s e v e r e s p a l l i n g u p t o 1 .5 m ( F ig . 7 ).5 - - e x t e n s i v e s p a l l i n g o v e r 1 . 5 m ( F i g . 8 ) .6 - - t o t a l c o l l a p s e .T w o c h a r t s w e r e d ev e l o p e d b as e d o n u n d e r -

g r o u n d o b s e r v a t i o n s - - f o r v e r y s t ro n g ro c k( u n i a x ia l c o m p r e s s i v e s tr e n g t h > 2 0 0 M P a )a n d f o r m e d i u m s t r e n g t h r o c k ( u n i a x i a l c o m -p r e s s i v e s t r e n g t h - 1 0 0 M P a ) ( F i g . 9 ). F r o mt h e s e c h a r t s i t i s e v i d e n t t h a t i n a v e r y h a r dc o m p e t e n t r o c k t h e r e a r e n o m a j o r s t a b i l i t yp r o b l e m s u n l e s s a d r i v e i s u n d e r s t re s s f o rl o n g e r t h a n a y e a r . I n a r e a s o f m e d i u ms t r e n g t h r o c k , t h e s t a b i l i t y i s d e p e n d e n t o nt h e d i s t a n c e o f t h e e x c a v a t i o n f r o m t h e s t o p ef a c e li n e. W h e r e t h i s d i s t a n c e i s le s s t h a n 3 0m , s t a b i l i t y p r o b l e m s c a n o c c u r i n l e s s t h a n 6m o n t h s . S e v e r e f a i l u re o r e v e n c o l l a p s e c a n b ee x p e c t e d w i t h i n 18 m o n t h s . D e v e l o p m e n ta r e as w h i c h a r e u n f a v o u r a b l y s i t u a t e d a n dw i ll n o t c o m e in t o p r o d u c t i o n i n t h e s p e c i f i e dp e r i o d o f t i m e a r e l i k e l y t o s u f f e r f r o m m i n -i n g - i n d u c e d s tr e ss a n d w i ll t h u s r e q u i r e h e a v ys u p p o r t ( a n d p r o b a b l y r e - s u p p o rt ) , d e p e n d i n go n t h e l e n g t h o f t i m e t h e e x c a v a t i o n i s s c h e d -u l e d t o r e m a i n o p e n .

T h e f o l l o w i n g e x a m p l e i l l u st r a te s t h e e f f e c to f a l o n g s t a n d - u p t i m e a n d t h e s h a p e o f t h es t o p e o n o r e d il u t io n . I n a h a r d r o c k m i n e ,t h e d il u t i o n in a n o r e b o d y d e t e r i o r a t e d t o


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stope stand-up time and the stope dimen-sions. However, it could be expected that thedilution would rise exponentially with the in-crease in these parameters.

The stope dimensions and the stand-uptime are interrelated and can be set in theplanning stage. For stopes on the next level, itwas recommended that the large stopes withstand-up times of a few years should beavoided and a stand-up time of 6-9 monthswas tested. A rib pillar would protect thehangingwall and footwall from sloughing andcaving and the pillar could be subsequentlyblasted (Fig. 11).

Fig. 7 . Severe spal l ing of the s idewall of a drive.

55% compared to the planned dilution (due toirregularity of the orebody) of 22-25%. Theapplied stoping method was sub-level openstoping. The height of the stope was 60 mwith the width dependent on the orebodythickness. The length of the stope wasdeter mined by the strike length of the ore. Atgiven pulling requirements, the real stand-uptime of the stope was about 2-3 yrs, with ahorizontal open span of more than 100 m.Due to stress changes around the stope, andloss of the integrity of the country rock withtime, the rock mass around this huge voiddeteriorat ed and sloughed into the open stope(Fig. 10). No investigations were carried outto correlate the percentage dilution with the

Effect of substandard support workThe principal objective of underground

support is to assist the rock mass in support-ing itself. Incorrect design, delayed installa-tion or substandard work by contractors mayresult in instability. Support work not com-plying with ac cep ted standards, however, canbe a waste of money at best and a disaster atworst.

A vast amount of support is installed inplaces which may not require it. When therock mass is of inherently good quality andwhen sufficient care is taken to minimizedamage to excavation walls, by careful blast-ing, there should be no need for a supportsystem. As a result of inadequate supervision,poor access and poor visibility, however,scaling may be incomplete and small blocksmay be detached due to rock mass deforma-tion and weathering across the joint planes,etc. Such scaling may be taken as evidence ofinstability and may lead to unnecessary sup-port.

In horizontally laminated strata, the instal-lation of bolts in the roof will ensure fullstability. Based on previous experience ob-tained in different geotechnical conditions,and using approved universal bolting pat-terns, excavations are bolted in the roof and


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Fi g . 8 . Ex t ens i ve de t e r i o ra t i on o f a d r i ve s i t ua t ed benea t h a p i l l a r .

also in the walls. Correc t rock mechan ics (a)analysis prior to the recommendation of the 36support and proper supervision of support -30work should permit a substantial reduc tion inthe num ber of installed rockbolts and will ~ 24considerably increase the stability of under-ground excavations. ~ leDesign principles are well established and ~ lzonly some examples of incorrect installationare discussed below. ~: 6

00Rockbol ts

Rockbolts are used extensively to controladverse ground conditions caused by bothstress and unfavourable geological structures.This type of support has proved to be verysuccessful and will remain the principal sup-port technique. In many underground excava-tions, a percentage of the bolts which should

Fi g . 9 . S t ab i l i t y p red i c t i on char t s showi ng t he r e l a t i on -s h i p b e t w e e n d a m a g e t o d r i v e s f o r d i f f e r e n t e x p o s u r et i mes and t he d i s t ance t o t he s t ope f ace l i ne [7 ] . a .Dr i ves s i t ed i n med i um hard rock . b . Dr i ves s i t ed i nv e r y h a r d r o c k.

(b)42

36~ 3oE,~ 24

/= I6

Tofa[ :o[!apse/

10

Extensive / /.... / upeVteor~pm~:~irnags///ISpaltingmetre less /than

/ /

No effect of stressI i

20 30 40 50 60 70Distance to the stope face fine(m )

,,t.,,,v. / / /:Ima:l~ge Ve/SeSeve! spaliing / '%" /

/ / t No ,.e ~, o, st..../

/

10 20 30 40 50 60Distance to thestope face line(m) 70


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33

( - ~ . . . . -, ~ / ~ . \ t i / I /I ~ . . - f / 7 / / / Y \ \ .. ..I

1 1 i / , " - -It

Besioned stopeCaved in rock

Fig. 10. An example of a stope with high ore dilution.

h a v e b e e n i n s t a ll e d i s m i s s i n g ( m o s t l y b e c a u s eo f u n d r i l l e d , d o s e d o r s h o r t h o l e s ) , a n d s o m eb o l t s a r e n o t c o r r e c t ly i n s ta l le d . T h e i n c o r r e c ti n s t a l l a t i o n o f g r o u t e d b o l t s c a n b e t h e r e s u l to f i n s u f f i c i e n t f il li n g o f h o l e s o r p o o r q u a l i t yg r o u t .

I n t h e c a s e o f m e c h a n i c a l l y a n c h o r e d b o l ts ,i n c o r r e c t i n s t a l l a t i o n m a y r e s u l t f r o m i n a d e -q u a t e t i g h t e n i n g o f a n e x p a n d i n g s h e ll . I np o o r q u a l i t y r o c k , v e r y h i g h b o l t t e n s i o n s ,a p p r o a c h i n g t h e y i e l d s t r e n g t h o f t h e b o l t ,c a n c a u s e l o c a l c r u s h i n g o f t h e r o c k a t t h ea n c h o r p o i n t s a n d l e a d t o a n c h o r s l i p . A l s o ,

a t a h i g h e r l o a d , t h e u n c e r t a i n t y i n t h e r e -l a t i o n s h i p b e t w e e n t h e t o r q u e a p p l i e d t o t h en u t , a n d t h e t e n s i o n i n d u c e d in t h e b o l t, c a ng i v e r i s e t o v e r y l a r g e v a r i a t i o n s i n b o l t t e n -s i o n . A p a r t f r o m p o s s i b l e i n a c c u r a c i e s i n t h et o r q u e m e a s u r e m e n t , th e t e n s i o n i n t h e b o l t isi n f l u e n c e d b y s u c h f a c t o r s a s t h e r u s t i n g o ft h e b o l t t h r e a d s [ 8 ] .

S p l it s e ts a r e i n c o r r e c t l y in s t a l le d w h e n t h e ya r e p u s h e d i n t o h o l e s l a r g e r t h a n 3 8 m m i nd i a m e t e r w h e r e t h e y d o n o t d e v e l o p s u f f ic i en tf r i c ti o n t o t r a n s m i t t e n s i o n .

T h e r e s i n c a r t r i d g e s y s t e m g i v e s a f u l l - t e n -f ~

,I I ~ 1 1 ~ i

I ~ R ib pillarCaved in Pock

Fig. 11. Proposed solution to reduce dilution from the stope.


12/15

34sion, high-capac ity bolt in one simple andrapid operation. The disadvantage of this sys-tem occurs when rotating the bolt to mix theresin and catalyst. If the rotation time ex-ceeds the setting period of the resin, it willresult in destroying the structure of the al-ready hardening bond.

In the case of missing bolts, or where boltsare not fulfilling their function, the requiredcohesion in the rock mass will not be devel-oped and a volume of the rock will loosen,the reinforcement becoming inadequate. Theself-supporting arch will not be created asdesigned, and the structure will not be stable(Fig. 12). In the case of a large open spanexcavation, omitting a few bolts can have adetrimenta l effect, especially when supportinga potentially unstable roof in laminated rocks.The designed beam will not be fully devel-oped and would be liable to fail.

Late installation is another factor influenc-ing poor performance of rockbolt systems.Such a manner of installation allows excessivedeformation to occur before the bolt reactionis fully mobilized. Untensioned anchored andgrouted steel dowels can only accept a load asthey are strained by deformation in the sur-rounding rock, and if they are installed toofar behind the advancing face, most of therock deformation will have taken place andthe dowels will not be as effective.

The hazard resulting from substandardwork is greatly increased when dowels areplaced with relatively large spacing, or wherethey are installed in holes drilled in the wrongdirection.Steel arches

Steel arches are used where high load-car-rying capacity is required to support road-ways. Their efficiency depends on the qualityof blocking provided to transmit the loadsfrom the rock mass to the steel sets. Whenarches are installed in strongly deformingareas, poor blocking and protuberance of the

Fig. 12. Effect of a missing bolt on the establishment ofa self-supporting arch (modified after Hoek and Brown[ 8 1 ) .

rock results in eccentric loading and distor-tion of the arches (Fig. 13). The arches alsoreadily deform under sidewall pressure.Deformation of steel arches may result frompoor packing, from not bracing them in avertical position, and from reduced frictionon clamp joints.

Steel arches are widely used to supportroadways in coal mines where they are oftenrequired to sustain quite large deformations,but from the author's experience they havelimited application in very hard rock mining.It has been reported that they have beenrejected in some South African mines [2].Shotcrete

The properties of shotcrete depend on suchfactors as the quality of the cement, aggregategradation, type of accelerators, mix propor-tion, etc. All these parameters can easily becontrolled and good supervision generallyensures high-quality work. From the perfor-


13/15

35

Fig. 13. Distortion and damage to steel arches under eccentric loading.

mance v i ewpoin t , t he on ly f ac to r which i s no tusua l ly adequa t e ly con t ro l l ed i s t he t h i cknessof t he sprayed l aye r . The t h i ckness o f t hel aye r i s genera l l y e s t ima ted f rom the vo lumeof t he ma te r i a l p l aced wi th some a l l owancefor l os ses due t o r ebound . When spray ingvery i r r egu la r su r f aces i t i s a lmos t imp oss ib l eto ach i eve a un i fo rm th i ckness . Espec i a l l y i n

pa r t s o f t he excava t ion where access i s d i f -f i cu l t t he t h i ckness i s usua l ly l e s s t han wasdes igned . In such p l aces , because o f p rob l emsw i t h s p r a y in g a n d c o n s e q u e n t l y p r o b l e m s w i t hcont ro l o f t he t h i ckness o f t he c r ea t ed l ayer , i ft he sho t c re t e cover s t he sur f ace o f t he rock o rins t a l l ed wi re mesh or we ld mesh th i s i s con-s idered sa t i s factory. In the case of a thin

Fig. 14. Fai lure of concrete walls under high stress.


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36s h o t c r e t e l i n i n g , a b r u p t c h a n g e s i n t u n n e lp r o f i l e c a n c a u s e h i g h s t r e s s c o n c e n t r a t i o n ,c a u s i n g c r a c k i n g a n d s e r i o u s r e d u c t i o n i n t h el o a d - b e a r i n g c a p a c i t y o f t h e l in i n g .Concrete walls

C o n c r e t e w a l ls c a n b e u s e d t o c o n t r o l s p a ll -i n g b u t c o n t r a r y t o p o p u l a r b e l i e f t h e y r a r e l yp r e v e n t d e f o r m a t i o n . B e c a u s e o f t h e b r i t t l eb e h a v i o u r o f c o n c r e te , s u c h w a ll s h a v e p r o v e dt o b e r e l a ti v e l y i n e f f e c t iv e i n h i g h - s t r e s s a r e a sw h e r e t h e y f r a c t u r e e a s i l y a n d a r e u s u a l l yd i s p l a c e d ( F i g . 1 4 ) . T h e u n i a x i a l c o m p r e s s i v es t r e n g t h o f c o n c r e t e , i n p r a c t i c e , i s n o m o r et h e n 2 0 M P a , i.e ., m u c h l e ss t h a n t h e u n i a x i a lc o m p r e s s i v e s t r e n g t h o f t h e a v e r a g e u n d e r -g r o u n d r o c k w h i c h t h e c o n c r e t e r e p l a c e s .Timber packs

I n a re a s o f h i g h c lo s u r e o r u n d e r d y n a m i cl o a d i n g c o n d i t i o n s , t i m b e r p a c k s h a v e f o u n dw i d e a p p l i c a t i o n . O v e r t h e y e a rs a g r e a t n u m -b e r o f d i f f e r e n t p a c k d e s i g n s h a v e b e e n t r i e dw i t h v a r i o u s d e g r e e s o f s u c c e s s .

I n t h e m a j o r i t y o f c a s e s , p a c k s a r e i n s t a l l e dc o r r e c t l y , i . e . , t i m b e r s l a b s a r e p l a c e d p r e -c i s e l y o v e r o n e a n o t h e r , a r e c o n s t r u c t e d i n av e r t i c a l p l a n e a n d a r e w e d g e d a g a i n s t t h er o o f . O n o c c a s i o n s , e s p e c ia l ly w h e n t h e r o o fs u r f a c e i s ir r e g u l a r , t i m b e r s l a b s a r e l a i d t o f i tc a v it ie s , w h i c h u n d e r m i n i n g - i n d u c e d st re s sr e s u l t s i n t h e b r e a k i n g o f t h e t i m b e r w h i c hc a u s e s u n c o n t r o l l e d r o o f l o w e r i n g . T i m b e rp a c k s m a y b e c o m e i n e f fe c ti v e w h e n t h e irh e i g h t i s g r e a t e r t h a n 2 - 2 . 5 t im e s t h e i r w i d t h .

I n p r a c t i c e , m a n y t i m b e r p a c k s d o n o ts h o w a n y s i g n o f s t r e s s . T h i s i m p l i e s t h a t t h es a f e t y f a c t o r w a s t o o h i g h a t t h e s u p p o r tr e c o m m e n d a t i o n s t a g e .

E f f e c t s o f s u b s t a n d a r d b l a s t i n g p r a c t i c eT h e e f f e c t o f b l a s t in g d a m a g e o n t h e s ta b il -

i ty o f u n d e r g r o u n d m i n i n g s t r u c tu r e s is n o t

w i d e l y r e c o g n i z e d . H o e k [ 9 ] a s s e s s e d t h a t b l a s td a m a g e c o u l d e a s i l y e x t e n d s e v e r a l m e t r e si n to t h e r o c k w h i c h h a d b e e n p o o r l y b l a s te d ,r e s u l ti n g i n s e r io u s i n s t a b i li t y p r o b l e m s i n t h er o c k s u r r o u n d i n g th e o p e n i n g .

C o n v e n t i o n a l b u r n c u t p e r i m e t e r b la s t in go f t e n r e s u l t s i n e x c e s s i v e o v e r b r e a k i n g a n ds h a t t e r i n g o f t h e r o c k m a s s . G e o t e c h n i c a ls u r v e y s , c a r r i e d o u t b y t h e a u t h o r i n a h a r dr o c k m i n e , h av e s h o w n t h a t o v e r b r e a k i n g c a nb e a s h i g h a s 2 5 % o f t h e c r o s s - s e c t i o n a l a r e a .S id e w a ll s a n d r o o f s o f n e w d e v e l o p m e n t s a rev e r y o f t e n h e a v i l y f r a c t u r e d a n d d a m a g e d .M a n y r o c k f a l l s a n d o c c u r r e n c e s o f g r o u n dd a m a g e a r e a t t r i b u t e d t o g r a v i t y a n d a r es t r u c t u r a l l y c o n t r o l l e d , b u t s o m e o f t h e mw o u l d n o t o c c u r if t h e d e v e l o p m e n t s w e r es m o o t h l y b l a st e d . T h e h i g h d r i ll i n g a n d b l a s t-i n g c o s t s m a y b e o f f s e t b y a l o w e r s u p p o r tco s t .

S o m e d e v e l o p m e n t s s u f f e r u n p r e d i c t e dm i n i n g - i n d u c e d s tr e ss r e s u lt in g f r o m i n c o m -p l e t e b l a s t i n g i n c h a i n o r c r o w n p i l l a r s . I t i so f t e n a s s u m e d t h a t a f t e r b l a s t i n g a l l t h e r o c km a t e r i a l i s b r o k e n . D e v e l o p m e n t s w h i c h a r es i tu a t e d b e n e a t h s u c h i n c o m p l e t e ly b l a st e dr e m n a n t s s u f f e r f r o m t r a n s f e r r e d s t r e s s a n da r e o f t e n p r o n e t o r o c k f a il u re . I n s u c h c a s es ,i f a d d i t i o n a l b l a s t i n g i s n o t f e a s i b l e , s u p p o r tr e c o m m e n d a t i o n s f o r t h e a f f e c t ed e x c a v a t i o n ss h o u l d b e i s su e d . T h i s i n t u r n w i l l r e d u c e t h ed a m a g e a n d e n s u r e s a fe ty .

B l a s t h o l e s w h i c h p e n e t r a t e s t o p e w a l l s ,d u e t o in a c c u r a c y i n d r i ll i n g o r p o o r p r o s p e c -t in g , re s u l t i n b r e a k a g e o f c o u n t r y r o c k w h i c hc a u s e s i n s ta b i l it y a n d u l t i m a t e l y o r e d i l u t io n .I n t e n s e lo c a l f r a c tu r i n g a n d d i s r u p t i o n o f t h ei n t e g r it y o f t h e i n t e r lo c k e d , j o i n t e d r o c k c a nb e p r o d u c e d b y p o o r b l a s t i n g d e s i g n . M o r ee x c e s s i v e a d v e r s e e f f e c t s c a n b e i n d u c e d b yt h e t r a n s m i s s i o n o f e n e r g y b y e x p l o s i v e a c -t i o n . I n h i g h - s t r e s s e n v i r o n m e n t s , s u c h a st h o s e w h i c h o c c u r a t d e p t h , d i s t u r b a n c e s a s -s o c i a t e d w i t h b l a s t i n g m a y t r i g g e r e x t e n s i v ei n s t a b il i ty i n t h e m i n e s t r u c tu r e .


15/15

ConclusionsD e s p i t e t h e v e r y r a p i d p r o g r e s s i n t h e o r e t i -

c a l r o c k m e c h a n i c s , g e o t e c h n i c a l d a m a g e ,a c c i d e n t s a n d d i s a s t e r s s t i l l o c c u r . T h e y a r ec a u s e d b y l i m i t e d k n o w l e d g e o f t h e p h e n o m -e n a t a k i n g p l a c e i n t h e r o c k m a s s u n d e r m i n -i n g - i n d u c e d s t re s s a n d b y i n s u f f i c i e n t r o c km e c h a n i c s a n a ly s is .

G e o t e c h n i c a l a s s e s s m e n t d e f i c i e n c i e s - - t h ed i f f e r e n c e b e t w e e n p r e d i c t i o n a n d t h e a c tu a lp e r f o r m a n c e o f t h e r o c k m a s s - - c a n o c c u rd u r i n g s it e c h a r a c te r i z a ti o n , d e s i g n a n d p l a n -n i n g , a n d m a y b e r e l a te d t o s u b s t a n d a r d s u p -p o r t w o r k a n d s u b s t a n d a r d b l a s ti n g p ra c ti ce .

T o e n s u r e t h a t s a t i s f a c t o r y g e o t e c h n i c a lp e r f o r m a n c e o f u n d e r g r o u n d s t r u c t u r e s i sa c h i e v e d i n t h e l o n g t e r m , t h e f o l l o w i n g m e a s -u r e s s h o u l d b e t a k e n :

( 1 ) S i t e c h a r a c t e r i z a t i o n s h o u l d b e c a r e -f u ll y p l a n n e d , w i t h t h e u n d e r s t a n d i n g o f h o wa l l p a r a m e t e r s c o n t r o l t h e b e h a v i o u r o f t h er o c k m a s s .

( 2 ) G e o t e c h n i c a l a n a l y s i s i s e s s e n t i a l b e -f o r e a n y m i n i n g a c t iv i ty . T h e a n a l ys i s s h o u l db e b a s e d o n g e o t e c h n i c a l s i t e e v a l u a t i o n , t e s t -i n g o f p r o p e r t i e s , e t c . , a n d a l s o r e p o r t s o f t h et y p e s o f g e o t e c h n i c a l d a m a g e e x p e r i e n c e d i ns i m i l a r c o n d i t i o n s . T h e c h a n g e i n m i n i n g - i n -d u c e d s t r e s s a n d t h e c h a n g e o f r o c k - m a s sp r o p e r t i e s o v e r t i m e s h o u l d b e c o n s i d e r e da n d p r o p e r p r e c a u t i o n s t a k e n.

( 3) T o a c h i e v e a g o o d s t a n d a r d o f s u p p o r tw o r k g e o t e c h n i c a l s u p e r v i s i o n o f i n s t a ll a t io n si s r e q u i r e d t o e n s u r e t h a t s u p p o r t c o m p l i e sw i t h a c c e p t e d s t a n d a r d s .

( 4) G e o t e c h n i c a l i n p u t i n b l a s t in g p r a c t i c em a y r e s u l t in i m p r o v e m e n t s in t h e s t a b i li ty o fe x c a v a t i o n s a n d i n t h e r e d u c t i o n o f t h e r e -q u i r e d s u p p o r t .

A c o n s i d e r a b l e a m o u n t o f d a m a g e t o u n -d e r g r o u n d e x c a v a t i o n s c o u l d b e a v o i d e d i fc o r r e c t g e o m e c h a n i c a l a n a l y si s is c a r r i e d o u t .

37I t i s t o b e s t r e s s e d t h a t i n t h e m a j o r i t y o fc a s e s o f t h e f a i l u r e o f r o c k m e c h a n i c s n e g a -t i v e e f f e c t s r e s u l t f r o m a f e w s u p e r i m p o s e dg e o t e c h n i c a l f a c t o r s .

AcknowledgementsT h i s p a p e r w a s c o m p i l e d w h i l st a t th e I n -

s t i t u t e o f M i n i n g R e s e a r c h , U n i v e r s i t y o fZ i m b a b w e , a n d t h e g e n e r o u s c o n t r i b u t i o n o ft h e i n s t i t u t e ' s t i m e i s g r e a t l y a c k n o w l e d g e d .T h e a u t h o r i s g r a t e f u l t o P r o f e s s o r K . A .V i e w i n g , t h e C h a i r m a n o f t h e I n s t i t u t e , f o rh is c o m m e n t s a n d s u g g e st io n s .

References1 Bieniawski, Z.T., Rock Mechanics Design in Miningand Tunnelling. Balkema, Rotterdam (1984).2 Laubscher, D.H., Design aspects and effectiveness ofsupport systems in different mining conditions. Trans.Inst. Min. Metall., (1986) AT0-A81.3 Brown, E.T. (Editor), International Society for Rock

Mechanics. Rock Characterization, Testing andMonitoring. ISRM Suggested Methods. Pergamon,Oxford (1981).

4 Kidybinski, A., Principles of Mining Geomechanics.Slask, Katowice (1982) (in Polish).5 Szwedzicld, T. and Lojas, J., Penetrometer measure-

ments of the effect of undermining on the overlyingroadways. In: Proc. Int. Syrup. Field Measurementsin Geomechanics (Zurich, Switzerland, 1983). Bal-kema, Rotterdam (1984), pp. 1229-1236.6 Lojas, J. and Konopko, W., Criteria for nondestruc-tive undermining of coal seams. Min. Rev., 7-8 (1981)409-414 (in Polish).7 Szwedzicki, T. and Kurzeja, A., Planning of newdevelopment work. (1986) (Unpubl.).8 Hock, E. and Brown, E.T., Underground Excavations

in Rock. Inst. Min. Metall., London (1980).9 Hock, E., Impact of blasting damage on the stabilityof rock structures. In: Proc. Workshop Mine Systems

Design and Ground Control, 2nd (Reno, Nevada,1984).

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