Rock mechanics aspects caving chrysotile asbestos orebodies at Shabanie mine, Rhodesia

Abstract

ive s u b l e v e l c a v i n g ar.d b lo c k c u v i n j o p e r a t i o n s v.ere i n v e s t i g a t e d a t Shabani e l i n e t o e s t a b l i s h ground b e h a v i o u r c h a r a c t e r i s t i c s , th e m ech an is es o f c a v i n g , t h e c o n d i t i o n s r e q u i r e d f o r c v i n g , and t h e e f f e c t s o f c a v i n j on t h e r ock euid development openi ngs b e 1 r>V/ a ’ in th e p e r i p h e r i e s o f a caved b l o c k . T h i s work i n c l u d e d th e te s t i > or development o f a r an ^ e o f i n s t r u m e n t s to I n v e s t i g a t e and m o n ito r g ro i i d i s p la c e m e n t ar.d d e f o r m a tio n above t h e cave back, i n t h e p e r i p h e r i e s a I below a cave b l o c k . Tne c h i j ' s o t i l e a s b e s t o s nii.ed at S haoanie l i n e occu i n uoa eroua o r e b o d i e s l o c a t e d i n an u l t r a u a f i c s i l l which has been f ded ar.d f a u l t e d . The f i b r e o c c u r s as s u b p a r a i l e l seams i n b o d ie s o f par - a l l > s e r p e n t i n i s e d d u n i t e o c c u r r i n g between t a l c - c a r b o n ^ t e zones and c a r b o n a t e d s e r p e n t i n e zones which a r e d ev elop ed a l o n g t h e f a u l t s . The o r e b o d i e s a r e i r r e g u l a r i n shape and v ary i n s i z e and competency. The ore and s u r r o u n d i n g r ock s a r e c u t by numerous, mo stly s t e e p d i p p i n g s l i p s (minor f a u l t s ) . S t r e s s measurements t o det er m in e t h e p r e - m i n i n g r o c k s t r e s s e s a e r e made a t s i x s i t e s a t d e p t h s o f between 200 and 400m, u s i n g p h o t o - e l a s t i c b i a x i a l s t r a i n g a u g e s , and t h e CSIR t r i u x i a l c e l l was a l s o u s e d a t one s i t e . Some o f t h e s t r e s s e s measured were a f f e c t e d by p r e v i o u s mi ning as i t was p r e f e r r e d t o measure t h e s t r e s s i n t h e same ro ' type t h a t would be mined, r a t h e r th an i n th e f o o t w a l l , ou t o f t h e i n f l u e n c e o f p r e v i o u s m i n in g . The r e s u l t s i n d i c a t e d t h a t the l a t e r a l s t r e r u e s exceeded t h e v e r t i c a l s t r e s s e s by up to 50/' a t th e dep th o f minin g. These h i g h e r l a t e r a l s t r e s s e s a r e p ro b a b ly r e s i d u a l t e c t o n i c s t r e s s e s from th e p e r i o d o f f o l d i n g . R epea te d measurements a l o n g ti.e l e n g t h o f a b o r e h o le i n d i c a t e d l a r ue v a r i a t i o n s i n s t r e s s magnitude ar.d o r i e n t a t i o n a t t r i b u t e d t o th e e f f e c t o f v a r i a t i o n s i n ro ck p r o p e r t i e s and p r o x i m i t y o f s t r u c t u r a l f e a t u r e s . From o b s e r v a t i o n s o f s m all d i s p l a c e m e n t s on j o i n t s i n th e p e r i p h e r i e s o f Caved b lo c k s i t i s c o ncluded t , a t t h e r ock mass us a whole behaves as a quasi-Bingham s u b s t a n c e , th e r e b y g i v i n g any t h e o r e t i c a l s t r e s s a n a l y s i s a p a t h dependui.cy. For t h e p r e s e n t , t h e problems o f a d e q u a te e q u a t i o n s