Vertical Crater Retreat Stoping as Applied at the Homestake Mine(Chapter44)

VERTICAL CRATER RETREAT STOPING AS APPLIED AT THE HOMESTAKE MINE Chapter 44

Steven T. Mitchell

Bulk Mining Foreman Homestake Mining Company

Lead, South Dakota

Abstract. The introduction of Vertical Crat- er Retreat (VCR) Stoping at the Homestake Mine in Lead, SD, constitutes a major advance in Homestake Mining Company's efforts to improve productivity and reduce costs through underground mine mechanization. The work to date indicates that Vertical Crater Retreat Stoping is one of the safest methods of underground hardrock mining today, and is competitive with all other forms of stoping in terms of cost and productivity. Homestake anticipates that VCR stoping will account for approximately 35% of the total mine production in 1981.

INTRODUCTION

Mill production from the Homestake Mine in 1979 amounted to 1.29 Mt (1.43 million st) of ore containing 5.9 g/t of gold (0.172 oz per st) compared with 1978 production of 1.44 Mt (1.59 million st) containing 6.2 g/t of gold (0.180 oz per st). The average cost per ounce of gold produced in 1979 rose to approximately $250 contrasted with $176 in 1978.

This increase in unit cost is the result of inflation, a diminishing ore grade, and productivity losses related to a shortage of experienced miners as well as compliance with Federal regulatory requirements. Together, these factors prompted the investigation of several large-scale, highly mechanized underground mining methods. This analysis indicated Vertical Crater Retreat mining to be one of the safest underground methods known today, yet competitive with all other forms of stoping in terms of cost and productivity. Therefore, the feasibility of applying the VCR stoping method at the Homestake Mine was explored.

Vertical Crater Retreat stoping is a recent innovation in large-scale underground mining based on C.W. ~ivingston's crater blasting theories. Figure 1 illustrates the method, which consists of drilling 165.1-mm-diam (6.5- in.) holes from a topsill, down-dip to an undercut on the level below. The bottom of each hole is then loaded from above with a predetermined

in a series of steeply dipping and highly fold- ed Precambrian Rocks. Three well-defined Pre- cambrian Formations are exposed in the mine. These formations from oldest to youngest are known as the Poorman Formation (pmf), Homestake Formation (hf), and the Ellison Formation (ef), Intrusive rhyolite dikes and sills believed to be Tertiary in age cut across all the formations. The gold-bearing ore occurs almost exclusively within the Homestake Formation, which has an unconfined compressive strength of about 275.8 MPa (40,000 psi) .

The ore areas are localized along plunging anticlinal and synclinal folds locally referred to as "ledges". Individual ore bodies plunge 30 to 75' from horizontal and usually dip at 50 to 80° to the east. Typical ore zones range from 3.0 to 30.5 m (10 to 100 ft) wide and up to 121.9 m (400 ft) in length with a strike approximately 35O west of north.

Gold values are erratically distributed in a gangue consisting of cummingtonite schist, sideroplesite schist, chlorite schist, quartz, pyrite, pyrrhotite, arsenopyrite, and ankerite. Ore grades are determined by extensive diamond core drilling, dry drilling and secondary wet- sludge sampling for each stoping block.

OVERVIEW OF LIVINGSTON CRATER THEORY

Several mathematical relationships have been derived by C.W. Livingston (1973) des- cribing the behavior of rock during the crater blasting process. These relationships attempt to correlate variables involving the mechanics of rock materials, explosive properties, and shot geometry. A basic understanding of the cratering phenomenon as learned from theory and experimentation is a prerequisite to the successful design and evaluation of full- scale VCR production blasting.

Strain Energy Equation

charge weight and horizontal slices up to 4.27111 Livingston empirically established a (14 ft) in thickness are blasted into the under- strain energy equation relating the energy of cut. Broken ore is extracted from drawpoints the explosive to the volume of material affec- located on or below the undercut level. ted by placement of the explosive charge. This

he expressed as:

MINE GEOLOGY N=EW 1 /3 (1)

The Homestake Mine is situated in the north- where N = Critical Depth or the ern portion of the Black Hills of South Dakota charge depth at which

609

Also,

where

DESIGN AND OPERATION OF CAVING AND SUBLEVEL STOPING MINES

t h e e x p l o s i v e b e g i n s t o f r a c t u r e rock a t t h e f r e e f a c e ( a s measured from t h e f r e e f a c e t o t h e c e n t e r of t h e cha rge )

E = S t r a i n Energy Fac to r - a d imens ion le s s f a c t o r which v a r i e s acco rd ing t o t h e e x p l o s i v e and rock type .

W = Charge Weight

Dc = Charge Depth a s measured between t h e f r e e f a c e and t h e c e n t e r of g r a v i t y of t h e cha rge .

A = Depth R a t i o - a dimension- l e s s number which e q u a l s Dc /N

The cha rge dep th a t which t h e e x p l o s i v e maxi- mizes t h e c r a t e r volume w i t h i n a c c e p t a b l e f r a g - men ta t ion l i m i t s i s c a l l e d t h e Optimum Charge Depth. The re fo re ,

where Ao=op t imum Depth R a t i o Do=Optimum Charge Depth

N = C r i t i c a l Charge Depth

Maximum E f f e c t i v e Charge Weight

Exper imenta t ion h a s shown t h a t a s t h e explos- i v e cha rge weight is i n c r e a s e d i n a b l a s t h o l e of c o n s t a n t l e n g t h and d i a m e t e r , a p o i n t i s r eached where no i n c r e a s e i n c r a t e r volume re- s u l t s f o r a co r r e spond ing i n c r e a s e i n e x p l o s i v e c h a r g e we igh t . I n r e c e n t y e a r s , Lang, e t . a l . (1977) h a s t h e o r i z e d t h a t w i th a c y l i n d r i c a l e x p l o s i v e column, t h e gases of d e t o n a t i o n and r e s u l t i n g bo reho le p r e s s u r e s a r e d i r e c t e d l a t - e r a l l y outward from t h e l o n g i t u d i n a l a x i s of t h e bo reho le . The magnitude of t h e f o r c e v e c t o r s which a r e d i r e c t e d p a r a l l e l t o t h e bo reho le a x i s toward t h e ends of t h e c h a r g e i s r e l a t i v e l y s m a l l .

The f o r c e v e c t o r s produced from a s p h e r i c a l c h a r g e ( length- to-d iameter r a t i o = I ) , however, a r e d i r e c t e d r a d i a l l y outward a l o n g a l l t heo r - e t i c a l p l a n e s p a s s i n g through t h e c e n t e r of t h e cha rge . Gran t , e t . a l . (1964) h a s found t h a t a s l ong a s t h e l e n g t h - t o - d iame te r r a t i o of a c y l i n d r i c a l cha rge does n o t exceed 6 : 1, t h e r e s u l t s of t h e c r a t e r breakage mechanism w i l l approximate t h o s e of a t r u e s p h e r i c a l cha rge .

s i o n l e s s c o n s t a n t dependent on t h e e x p l o s i v e ' s p r o p e r t i e s and r o c k t y p e , i t i s b e n e f i c i a l t o test a l l of t h e e x p l o s i v e s which l a t e r might be used i n f u l l - s c a l e product ion b l a s t i n g . This d a t a i s then i n c o r p o r a t e d i n t o L i v i n g s t o n ' s S t ra in-Energy Equa t ions t o gene ra t e v a l i d b l a s t - i n g r e l a t i o n s h i p s .

F i g u r e 2 d e p i c t s a n i d e a l i z e d curve f o r shock-type rock f a i l u r e expressed a s a f u n c t i o n of t h e c h a r g e d e p t h . On t h i s graph, Energy Lev- e l s (V/W) v e r s u s Depth R a t i o s (Dc/N) a r e p l o t - t e d from c r a t e r test s h a t s t o h e l p i d e n t i f y a r ange of m a t e r i a l behav io r s which can occur i n t h e c r a t e r b reakage p roces s .

A t a A o v a l u e f o r a given exp los ive and rock t y p e , f l y r o c k i s moderate and c r a t e r vol - ume i s maximized. ASA v a l u e s dec rease from A o , t h e amount of e x p l o s i v e energy a v a i l a b l e f o r t h e f r a c t u r i n g p roces s dec reases and is d i r e c t e d i n s t e a d toward more f l y r o c k , n o i s e , and a i r b l a s t p r e s s u r e . A t A v a l u e s g r e a t e r t h a n abou t 0 . 7 , t h e r o c k m a t e r i a l beg ins t o become i s o l a t e d from i t s surroundings by s p a l l - i n g o n l y , and t r u e f r agmen ta t ion does no t occur.

Breakage P r o c e s s Equat ion

From test d a t a , L iv ings ton e m p i r i c a l l y de- r i v e d an e q u a t i o n t o b e t t e r d e s c r i b e t h e rock b reakage p r o c e s s . T h i s he c a l l e d t h e Breakage

M a t e r i a l Behavior i n t h e Breakage P r o c e s s and

P r o c e s s Equat ion o r

3 V/W = E ABC

The v a r i a b l e s A , B , and C were in t roduced t o d e s c r i b e t h e u t i l i z a t i o n of t h e exp los ive ene rgy (A), t h e behav io r of t h e m a t e r i a l (B), and t h e e f f e c t of t h e charge shape (C).

The energy u t i l i z a t i o n number can be expres- s ed a s A = V/Vo. I n o t h e r words, A is t h e r a - t i o of t h e volume of a c r a t e r a t any charge dep th Dc t o t h e c r a t e r volume a t t h e optimum cha rge d e p t h Do. A r eaches a maximum va lue of 1 . 0 when Dc = Do and V = Vo. The energy u t i l - i z a t i o n number i s a measure of t h a t p a r t of t h e e x p l o s i v e ene rgy devoted t o t h e f r a c t u r i n g p r o c e s s . For most r o c k m a t e r i a l s , g r aph ic i l l u s t r a t i o n s of A v a l u e s v e r s u s v a l u e s c l o s e l y r e semble t h e cu rve shown a s F igu re 2.

S i n c e N = E W " ~ , t h e Breakage P roces s Equa- t i o n may be r e w r i t t e n s o t h a t

3 V/E W = ABC, (5)

Before f u l l - s c a l e p r o d u c t i o n b l a s t i n g u s i n g v / N ~ = ABC . ( 6 ) t h e c r a t e r i n g phenomenon can b e s u c c e s s f u l l y implemented, a s e r i e s of s m a l l - s c a l e c r a t e r T h e r e f o r e , t h e M a t e r i a l s Behavior Index (B) s h o t s is performed i n o r d e r t o g a t h e r informa- can b e e x p r e s s e d a s t i o n about t h e e f f e c t s of d i f f e r e n t e x p l o s i v e s on t h e m a t e r i a l behav io r of a g iven r o c k t y p e . 3 B = V / N S i n c e t h e S t r a i n Energy F a c t o r "E" i s a dimen- AC

VERTICAL CRATER RETREAT STOPING AT HOMESTAKE MINE

A

Figure 2. V/W vs A curve f o r shock- type rock f a i l u r e .


When a s p h e r i c a l c h a r g e i s p l aced a t D o , V = V o and A = C = 1 . 0 . Then,

The M a t e r i a l s Behavior I ndex , a c c o r d i n g t o L i v i n g s t o n , i s a measure of t h a t p a r t of t h e e x p l o s i v e ene rgy devo t ed t o p r o c e s s e s such a s c r u s h i n g , compact ion , and p l a s t i c d e f o r m a t i o n , a l l of which p r e c e d e t h e f r a c t u r i n g p r o c e s s .

The v a l u e C i s a d i m e n s i o n l e s s r a t i o of t h e e ne rgy l e v e l e x p r e s s e d a s a c r a t e r volume produced under any c o n d i t i o n s t o t h e ene rgy l e v e l of a s i n g l e - s h o t c r a t e r b l a s t unde r p r o t o t y p e c o n d i t i o n s . T h i s number app roaches a maximum v a l u e of 1 . 0 when p r o t o t y p e c o n d i t i o n s p r e v a i l . Obv ious ly , p a r a m e t e r s a s s o c i a t e d w i t h m u l t i p l e h o l e b l a s t s , chang ing g e o l o g i c c o n d i t i o n s , and t h e manner of d e t o n a t i o n of t h e s h o t c an a l l c r e a t e d e v i a t i o n s from s i n g l e s h o t c o n d i t i o n s .

From t h e mechanics of t h e Breakage P r o c e s s Equa t i on , t h e n , i t becomes e v i d e n t why s m a l l - s c a l e c r a t e r i n g e x p e r i m e n t s a r e a p r e r e q u i s i t e t o s u c c e s s f u l f u l l - s c a l e c r a t e r s t o p i n g . The L i v i n g s t o n app roach t o c r a t e r b l a s t i n g c a n b e s u c c e s s f u l l y a d a p t e d t o a g i v e n o r e body once t h e e m p i r i c a l d a t a h a s been e v a l u a t e d i n t e rms of t h e e x p l o s i v e e n e r g y and t h e m a t e r i a l behav- i o r c h a r a c t e r i s t i c s .

SMALL-SCALE CRATER TESTS

T e s t P rocedu re

I n November 1977 , s m a l l - s c a l e c r a t e r t e s t i n g was i n i t i a t e d i n a t y p i c a l Main Ledge orebody on t h e 2073-m (6800 - f t ) l e v e l . T h i s t e s t con- s i s t e d o f d r i l l i n g f i v e v e r t i c a l 101.6-mm-diam (4 .0- in . ) h o l e s of v a r y i n g l e n g t h i n t o t h e back of a s t o p e s i l l c u t . These h o l e s were d r i l l e d w i t h a n Inge r so l l -Rand JR-300 j a c k l e g d r i l l and were s u b s e q u e n t l y measured and i n d i v i d u a l l y b l a s t e d .

The l o a d i n g p r o c e d u r e f o r t h e 101.6-mm-diam (4 .0- in . ) h o l e s began by i n s e r t i n g one 76.2 x 406.4 mm ( 3 x 1 6 i n . ) c a r t r i d g e o f Tovex 650 w a t e r g e l (one-ha l f t h e c h a r g e w e i g h t ) i n t o t h e h o l e . A 0.45-kg (1 .0- lb) c a s t p r ime r w i t h E- Cord Pr imacord a s a d e t o n a t o r was t h e n b u r i e d i n s i d e t h e s econd c a r t r i d g e and i n s e r t e d i n t o t h e h o l e . Cardboard map t u b e s f i l l e d w i t h s a n d were u t i l i z e d a s bo t t om stemming and were i n - s e r t e d i n t o t h e h o l e b e h i n d t h e c h a r g e . T h i s e x p l o s i v e - stemming s equence was t h e n pushed t o t h e p o i n t o f t h e h o l e and tamped w i t h t h e a i d of 50.8 x 50 .8 mrn ( 2 x 2 i n . ) wooden l o a d - i n g p o l e s . The h o l e was t h e n b l o c k e d w i t h two wooden wedges a p p r o x i m a t e l y 0 .30 m (12 i n . ) l o n g . P r i o r t o b l a s t i n g , a s e r i e s of concen- t r i c c i r c l e s were p a i n t e d a round each h o l e s o t h a t t h e r a d i u s of t h e r e s u l t i n g c r a t e r c o u l d b e more e a s i l y and a c c u r a t e l y de t e rmined .

A f t e r f i r i n g , t h e c r a t e r a r e a was b a r r e d down s o t h a t t h e c r a t e r d e p t h , r a d i u s , and v o l -

ume cou ld s a f e l y be measured. Fragmenta t ion , unusua l g e o l o g i c c o n d i t i o n s , and t h e shape of t h e c r a t e r were a l s o recorded . F igu re 3 shows a p l o t of d a t a o b t a i n e d from 101.6-mm-diam (4 .0- in . ) h o l e t e s t . Energy Levels (V/W) a r e p l o t t e d a g a i n s t Depth R a t i o s (DC/N). The r e - s u l t s o f t h i s t e s t i n g i n d i c a t e d t h a t w i th t h e exp lo s ive - rock combina t ion i nvo lved , N=3.35m ( 1 1 f t ) and n o = 0.45.

Once t h i s c u r v e was e s t a b l i s h e d , t h e d a t a was i n c o r p o r a t e d i n t o L i v i n g s t o n ' s S t r a i n Energy Equa t i on t o de t e rmine d a t a u s e f u l f o r s c a l i n g pu rpose s . From t h e 101.6-mm-diam (4.0-in.) h o l e tests t h e n ,

N = 3.35m (11 f t ) ( s h o t #4)

W = 6.94 kg (15 .3 l b ) Tovex 650 ( i n c l u d i n g a 0.45 kg (1 .0 l b ) c a s t p r ime r ) r'

s i n c e

From F i g u r e 3 , A o = 0.45.

Do = 0.45 x 3.35m (11 f t ) Do = 1.51m (4.95 f t )

I n March, 1978 , i t was dec ided t h a t f u r t h e r c r a t e r t e s t i n g shou ld b e performed w i t h 152.4- mm-diam (6 .0- in . ) h o l e s s i n c e t h e 101.6-mm-diam (4 .0 - i n . ) h o l e s o b v i o u s l y would n o t b e p r a c t i c a l f o r p r o d u c t i o n b l a s t i n g . By s c a l i n g t h e 101.6- mm-diam (4 .O-in .) h o l e s u s i n g t h e S t r a i n Energy Equa t i on , i t was c a l c u l a t e d t h a t t h e optimum c h a r g e d e p t h (Do) f o r a 152.4-mm-diam (6.0-in.) h o l e u s i n g Tovex 650 shou ld be 2.23m (7 .3 f t ) .

To c o r r o b o r a t e t h e r e s u l t s of t h e i n i t i a l test t h a n , a s econd c r a t e r b l a s t i n g t e s t was conducted . T h i s test c o n s i s t e d of d r i l l i n g f i v e 152.4-mm-diam (6 .O-in.) h o l e s up-dip (70" from h o r i z o n t a l ) i n t o t h e back of a s t o p e s i l l c u t i n a 9 Ledge o r e zone on t h e 975-m (3200- f t ) l e v e l . A Gardner-Denver PR-123 l ongho l e d r i l l mounted on a bar-and-column arrangement was u sed t o d r i l l t h e h o l e s . The l o a d i n g pro- c e d u r e f o r t h e s e h o l e s was e s s e n t i a l l y t h e same a s b e f o r e w i t h t h e e x c e p t i o n t h a t Tovex Ex t r a was s u b s t i t u t e d a s t h e e x p l o s i v e and rough-cut wooden 101 .6 mm (4 x 4 i n . ) s p a c e r s were used f o r bo t t om stemming m a t e r i a l ) .

The r e s u l t s o f t h e 152.4-mm-diam (6.0- i n . ) c r a t e r s h o t s a r e p l o t t e d a s shown i n F i g u r e 4 . D a t a from F i g u r e 4 i n d i c a t e s a &

v a l u e e q u a l t o 0.44 and a Do v a l u e equa l t o 2.21m (7 .24 f t ) . I t shou ld b e no t ed t h a t a l t h o u g h s h o t /I4 produced t h e maximum volume of r o c k , t h e d e g r e e of f r agmen ta t i on was un- a c c e p t a b l e . I n a l l i n s t a n c e s , v i s u a l inspec- t i o n showed t h a t t h e s i n g l e - s h o t c r a t e r s t ended


V / W

3 11.75 2.5 7.0 15.3 2.4 3.12 24.5 1.60 0.64

4 14.25 2.5 11.0 15.3 1.0 0.50 0.3 0.02 1.00

5 15.50 2.5 4.0 15.3 2.5 3.50 32.1 2.10 0.36 EXPLOSIVE = TOVEX 650 (1 .35 g/cm3)

A

F i g u r e 3. C r a t e r i n g t e s t s u s i n g 101.6-mm-diameter (4 .0 - in . ) h o l e s .

- - EXPLOSIVE = TOVEX EXTRA ( 1 . 3 3 g/cmJ)

F i g u r e 4. C r a t e r i n g t e s t s u s i n g 152.4-rmn-diameter (6 .0 - in . ) h o l e s .


t o b e e l o n g a t e d p a r a l l e l t o t h e s t r i k e nnd hed-- d i n g p l a n e s of t h e o r e zone .

F i g u r e 5 d e p i c t s a g r aph of t h e Energy U t i l - i z a t i o n Number "A" v e r s u s t h e Depth R a t i o s f o r d a t a o b t a i n e d f rom t h e 152.4-mm (6 .0- in . ) c r a - t e r t e s t s . L i v i n g s t o n d e f i n e d "A" a s a mea- s u r e of t h e ene rgy o f t h e e x p l o s i v e devo t ed t o t h e f r a c t u r i n g p r o c e s s . G e n e r a l l y , a n i n c r e a s e

690 kPa (90 t o 100 p s i ) t o p r e s s u r e 5 as h igh a s 1896 kPa (275 p s i ) . Mine a i r i s Eurnisheo by 101.6-mm-diam (4 .0 - i n . ) l e v e l l i n e s t o a 63.5-mm-diam (2 .5 - i n . ) b u l l h o s e a t t h e com- p r e s s o r . Each Gardner-Denver ATD 3100 C a r r i e r coupled w i t h a n AOL 1000 b o o s t e r compressor package c o n s t i t u t e s one I T H d r i l l i n g u n i t .

Hole Accuracy i n t h e energy of t h e e x p l o s i v e and a n i n c r e a s e i n t h e s c a l e of t h e expe r imen t y i e l d s a h i g h e r M a i n t a i n i n g h o l e a ccu racy i s impor t an t i n A o va lue . VCR s t o p i n g s i n c e d e v i a t i o n s from a d e s i r e d

h o l e p a t t e r n a lmos t a lways produce subs t anda r Conc lu s ions b l a s t i n g r e s u l t s . Improper d r i l l a l ignment

(az imuth and d i p a n g l e ) , t h e a t t i t u d e and f o l d The main c o n c l u s i o n s t h a t were d e r i v e d a s a i n g of t h e o r e zone , t o o much f eed p r e s s u r e ,

r e s u l t of t h e s m a l l - s c a l e c r a t e r t e s t s a r e : and t h e we igh t o f t h e d r i l l s t r i n g i t s e l f a r e t h e most p r o b a b l e c a u s e s of e x c e s s i v e h o l e de-

1. S t r a i n Energy F a c t o r s , Optimum Depth v i a t i o n . Hole d e v i a t i o n r e s u l t i n g from t h e R a t i o s , and C r a t e r Volumes a r e depen- s t a t i c l o a d of t h e d r i l l s t r i n g becomes much I

d e n t on t h e e x p l o s i v e - r o c k combina t i on , more pronounced when d r i l l i n g a t d i p a n g l e s and must b e de t e rmined by exper imenta- l e s s t h a n 70 d e g r e e s f rom t h e h o r i z o n t a l . t i o n .

2. I n h a r d r o c k f o r m a t i o n s , there i s a need t o s e l e c t e x p l o s i v e s which e x h i b i t r e l a - t i v e l y h i g h d e n s i t i e s , h i g h d e t o n a t i o n v e l o c i t i e s , and h i g h b u l k s t r e n g t h s .

3. ANFO b l a s t i n g a g e n t s a r e n o t s u i t a b l e f o r t r u e c r a t e r i n g a p p l i c a t i o n s due t o t h e r e l a t i v e l y low d e n s i t y , d e t o n a t i o n v e l o c i t y , and b u l k s t r e n g t h v a l u e s .

4. Based upon r e s u l t s o f t h e s m a l l - s c a l e c r a t e r tests and known powder f a c t o r s , a 2.74 x 2.13m (9 x 7 f t ) p a t t e r n was s e l e c t e d f o r i n i t i a l p r o d u c t i o n b l a s t - i n g . The Do v a l u e was reduced t o 2.06m (6 .75 f t ) t o improve f r a g m e n t a t i o n .

IN-THE-HOLE DRILLING

D r i l l R ig s Used

Four Gardner-Denver ATD 3100 Crawler Car- r i e r s e ach equ ipped w i t h a TRW Miss ion Mast and R o t a t i n g Head Assembly a r e c u r r e n t l y used a t t h e Homestake Mine. The Gardner-Denver u n i t u t i l i z e s a i r -powered h y d r a u l i c s f o r t h r u s t and r o t a t i o n . T o t a l h o i s t i n g c a p a c i t y f o r t h e mast assembly i s 5670 kg (12 ,500 l b ) . A i r tramming mo to r s a r e used f o r c a r r i e r m o b i l i t y .

A Miss ion M e g a d r i l l B53-15 bot tom-hole d r i l l o r a n Inge r so l l -Rand DHD 360 hammer i s used i n c o n j u n c t i o n w i t h 1.83m (6 .0 f t ) l o n g x 127-mm- d iam (5.0-in.) d r i l l p i p e w i t h 85.7-mm-diam (3.375-in.) Mi s s ion Mod i f i ed B u t t r e s s t h r e a d . O p e r a t i n g a i r p r e s s u r e s a t t h e hammer v a r y be tween 1551 and 1724 kPa (225 and 250 p s i ) .

To complement t h e ITH d r i l l , a skid-mounted b o o s t e r compressor package was de s igned by Gardner-Denver f o r t r a n s p o r t a t i o n and ope ra - t i o n underground. T h i s u n i t c o n s i s t s o f two a i r - c o o l e d D e a r i n g Model AOL 1000 b o o s t e r com- p r e s s o r s d r i v e n by two 460 V , 30-kw (40-hp)

3 e l e c t r i c motors . The 17-m /min(600-cfm) com- p r e s s o r u n i t w i l l b o o s t mine a i r a t 620 t o

D r i l l o p e r a t o r s p o s i t i o n t h e c r a w l e r a l ong s t r i n g l i n e s t h a t d e f i n e a d r i l l i n g g r i d a s marked on t h e s t o p e w a l l s by mine su rvey crews Once t h e c r a w l e r h a s been s i t u a t e d , t h e mast i: plumbed i n t h e d r i l l i n g p l a n e and t hen s e t on t h e r e q u i r e d d i p a n g l e w i t h t h e a i d of a mag- n e t i c a n g l e f i n d e r ( c l i n o m e t e r ) .

Apparen t h o l e wander w i t h 48.8-m (160-f t ) h o l e s a v e r a g e s 3%, b u t h a s approached 6-8% on o c c a s i o n n e c e s s i t a t i n g f i l l - i n d r i l l i n g . Gen- 1 e r a l l y , t h e f i r s t 30.5m (100 f t ) of d r i l l i n g y i e l d s l i t t l e o r no d e f l e c t i o n , whereas most oi t h e d e v i a t i o n a p p e a r s t o accumula te i n t h e l a s t 15m (50 f t ) o f t h e h o l e . Once d r i l l i n g h a s been comple ted i n a s e c t i o n of t h e s t o p i n g are: mine s u r v e y c r ews e s t a b l i s h a t r a n s i t t i e on a l l h o l e s i n t h e u n d e r c u t t o i d e n t i f y any h o l e d e v i a t i o n .

B i t s

E m p i r i c a l t e s t s were conducted t o de t e rmine which t y p e and make of b i t would maximize long€ v i t y and p r o d u c t i v i t y i n ITH d r i l l i n g . But ton b i t s manufac tured by Mis s ion , D r e s s e r , Numa, Ep l ey , G a u l t , Dalby , and Hughes were t e s t e d i n t h e concave ,d rop c e n t e r , and f l a t - f a c e d e s i g n s . P r e s e n t l y , t h e Numa b i t i s p r e f e r r e d i n t h e concave d e s i g n .

Numa b i t l i f e a v e r a g e s 457m (1500 f t ) , b u t v a r i e s be tween 152 and 975 m (500 and 3200 f t ) depending on d r i l l i n g c o n d i t i o n s . B i t c o s t s a l o n e app rox ima te $1.22/m ($0 .40 / f t ) o f 165.1- mm-diam (6.5-in .) h o l e . P e r i o d i c i n s p e c t i o n s i n d i c a t e t h a t most o f t h e b u t t o n b i t s should b e r e s h a r p e n e d a f t e r e v e r y 30.5 m (100 f t ) of d r i l l i n g . However, s imp le c o s t a n a l y s i s shows i t i s more economica l t o s a c r i f i c e 15-20% of t h e p o s s i b l e b i t l i f e i n o r d e r t o e l i m i n a t e up t o one hour o f r a i s i n g and l ower ing r o d s p e r s h i f t . Bu t t on b i t s a r e sharpened underground w i t h a n a i r -powered d i e g r i n d e r u s i n g s i l i c o n - c a r b i d e whee l s of 60 t o 80 g r i t and J o r K \ i n

VERTICAL CRATER RETREAT STOPINC AT HOMESTAKE MINE 61 5

P i g u r e 5 . !n~-rg . i utilization v s . d e p t h r a t i o f o r 152.4 m - d i a m c t e r (6.0-in.) crater t r s t s .

A Gardner Denver ATD 3100 Carrier with a TRW Mission Mast and Rotating Head Assembly is used for ITH drilling of 165.1-mm-diam (6,5-in,)holes.


hardness .

D r i l l i n g Produc t iv i ty

B l a s t h o l e s i n a l l s top ing a r e a s a r e p r e s e n t l y d r i l l e d on a 2.4 x 2.4 m (8 x 8 i t ) p a t t e r n which y i e l d s about 1 9 . 3 t / m (6.5 s t / f t ) of h o l e The m a j o r i t y of a l l h o l e s a r e d r i l l e d t o t h e undercut s i l l a t d i p ang les r ang ing from 45 t o 90" from t h e h o r i z o n t a l . Typ ica l h o l e l e n g t h s vary from 42.7 t o 53.3 m (140 t o 175 f t ) depending on t h e a t t i t u d e of t h e h o l e .

Hole Angle Blocking Height 90-80" 1.2m (4 f t ) 79-57" 1.5m (5 f t ) 56-50' 1.8m (6 f t ) Less than 50" 2.lm (7 i t )

The r e q u i r e d blocking dep th is c a l c u l a t e d f o r each h o l e by s u b t r a c t i n g t h e blocking he igh t from t h e stope-back measurements. These f i g - u r e s a r e e n t e r e d onto t h e d a t a shee t a long wi th t h e proper de lay p a t t e r n . The completed d a t a s h e e t i s then re tu rned t o t h e loading crew so t h a t b lock ing of t h e b l a s t h o l e s can begin.

S ince mid 1979, p e n e t r a t i o n r a t e s have aver- Blocking B l a s t h o l e s aged about 7.3 m (24 i t ) p e r d r i l l hour , o r 47.5 m (156 i t ) per d r i l l s h i f t . Optimum pene- I n i t i a l VCR e f f o r t s a t Homestake made use t r a t i o n r a t e s occur a t 14 t o 22 rpm, depending of 102 x 102 x 203-mm ( 4 x 4 ~ 8 - i n . ) r ec tangu la r on rock cond i t ions . T o t a l ITH d r i l l foo tage wooden b locks suspended from t h e top s i l l by f o r 1980 amounted t o 54,851 m (179,957 f t ) . 6.35-m-diam (0.25-in.) polypropylene rope.

Improvements i n t h e blocking process have pro- LOADING AND BLASTING PROCEDURE gressed t o t h e p o i n t where two wedge-shaped

b locks 102 x 102 x 254 mm (4 x 4 x 10-in.) a r e Measuring Blas tho les suspended wi th 40 g r a i n S t r i p Mine Spec ia l

Primacord, which a l s o a c t s a s t h e i n i t i a t o r f o r The procedure f o r loading and b l a s t i n g begins t h e down-the-hole cap and pr imer .

by measuring t h e l eng th of each b l a s t h o l e us ing e i t h e r a p l a s t i c p ipe o r wooden load ing s t i c k about 0.76 m (30 i n . ) long fas t ened t o a c a l i - b r a t e d rope chain . The rope cha in is separa ted from t h e end of t h e p ipe by a rope 0.38 m (15- i n . ) long (exac t ly one-half t h e l e n g t h of t h e p i p e ) . The "zero" end of t h e rope cha in i s then t empora r i ly f a s t ened t o t h e c e n t e r of g r a v i t y of t h e pipe by a p l a s t i c c a b l e - t i e wire . When t h e stope-back measurement has been determined, t h e rope chain i s given a sha rp j e r k t o break t h e c a b l e - t i e w i r e , t he re - by a l lowing t h e pipe t o hang v e r t i c a l l y so t h a t i t can be pul led back up t h e h o l e .

By incorpora t ing 40 g r a i n Primacord i n l i e u of 6.35-mm-diam (0.25-in.) polypropylene rope, t h e t o t a l load ing time is reduced approximately 20% s i n c e blocking and priming now becomes a one s t e p p rocess . Furthermore, e l imina t ion of t h e rope r e s u l t s i n a n e t c o s t savings of about $0.06/m ($0.02/f t ) of b l a s t h o l e . This sav ings becomes s i g n i f i c a n t consider ing t h e load-b las t c y c l e o r d i n a r i l y r e q u i r e s a minimum 381m (1250 f t ) of Primacord and 381m (1250 f t ) of rope pe r h o l e t o completely mine through a 45.7m (150 f t ) high s t o p e block i n success ive 3-m (10-f t ) l i f t s .

Hole measurements a r e obta ined by lowering Blocking begins by t y i n g one wedge block t h e p i p e and rope cha in down t h e h o l e from t h e onto t h e 40 g r a i n Primacord and lowering t h e top s i l l u n t i l t h e p ipe h i t s t h e muck p i l e . b lock down t h e h o l e s l i g h t l y ahead of t h e This measurement is recorded on a d a t a s h e e t rope cha in . Af te r t h e proper depth has been f o r t h e s p e c i f i e d ho le . The rope cha in i s reached, t h e rope cha in i s pu l l ed ou t of t h e then p u l l e d up u n t i l t h e p ipe lodges a c r o s s ho le and a second wedge block is dropped down t h e bottom of t h e h o l e a t t h e s t o p e back. This measurement is s i m i l a r l y recorded. The amount of r e l i e f a v a i l a b l e f o r t h e subsequent b l a s t , t hen i s t h e d i f f e r e n c e between t h e s e measurements and is a l s o recorded on t h e d a t a s h e e t . T h i s procedure i s repeated f o r t h e remaining h o l e s i n t h e immediate b l a s t i n g s e c t ion .

A f t e r reviewing t h e d a t a s h e e t s , a b l a s t i n g t e c h n i c i a n p repares a delay p a t t e r n according t o t h e stope-back p r o f i l e . A "plug-set" o r b locking h e i g h t a s measured between t h e s t o p e back and t h e blocking l o c a t i o n based on t h e ang le of t h e d r i l l ho le and t h e r e s u l t s of smal l - sca le c r a t e r t e s t s i n s i m i l a r rock types i s then e n t e r e d onto t h e d a t a s h e e t . Although t h e block h e i g h t may d e v i a t e from normal due t o s p e c i a l rock cond i t ions p a r t i c u l a r t o one s top ing a r e a , t h e values shown i n t h e t a b l e below a r e g e n e r a l l y v a l i d f o r most s t o p i n g a p p l i c a t i o n s a t the Homestake Mine:

t h e h o l e a longs ide t h e Primacord. ~ f t e r t h e second wedge block h i t s t h e f i r s t , t h e Prima- cord i s tugged t o ensure t h a t t h e blocks wedge t o g e t h e r a g a i n s t t h e w a l l of t h e hole .

An a l t e r n a t i v e method of b locking involves measuring and recording a l l of t h e holes i n t h e b l a s t i n g a rea . A wooden s t i c k approximately 0.76m (30-in.) long i s t i e d t o one end of t h e Primacord. Next, a wedge block is t i e d t o t h e Primacord a t a d i s t a n c e equal t o t h e blocking he igh t above t h e wooden s t i c k . Th i s assembly i s then lowered down the hole t o t h e s t o p e back. The Primacord i s then p u l l e d upward u n t i l t h e s t i c k ca tches cross- wise a t t h e s t o p e back. This procedureensures t h a t t h e wedge block i s pos i t ioned a t the proper e l e v a t i o n above t h e s t o p e back, even i f t h e h o l e h a s been chimneyed out . F ina l ly , a second wedge block i s dropped down t h e hole t o lock a g a i n s t t h e f i r s t block.


Next, t h e Primacord i s marked a t t h e c o l l a r of t h e h o l e and t i e d o f f t o p r e v e n t b l o c k s l i p - page. Approximately one f o o t of s m a l l rocks and c u t t i n g s i s shoveled i n t o t h e h o l e t o comp- l e t e l y s e a l and l o c k t h e two wedge b l o c k s to- ge the r . The remaining b l a s t h o l e s a r e t h e n blocked i n t h e same manner.

Loading B las tho le s

The amount of water g e l p u t i n t o each h o l e depends upon t h e d e n s i t y of t h e p roduc t . For example, us ing t h e 6 : 1 charge l e n g t h t o hole- diameter r a t i o , approximate ly 27.2 kg (60 l b ) of DuPontts Tovex 650 (1.35 g/cm3) o r T r o j a n ' s WS-7(H) 1.40 g/cm3) i s loaded i n t o t h e ho le . CIL's Aquamex r e q u i r e s approximate ly 31.3 kg (69 l b ) pe r h o l e due t o t h e r e l a t i v e l y h igh d e n s i t y of t h e product (1.54 g/cm3). S ing le Deck B l a s t : I n load ing a s i n g l e deck, t h e exp los ive bag is c u t l eng thwise and each "sausage" i s shucked and dropped i n t o t h e ho le . Experience h a s shown t h a t t h i s procedure en- s u r e s a h igh degree of l oad ing compaction and coupl ing wi th in t h e h o l e which i s a r e q u i s i t e t o t h e c r a t e r i n g p rocess . A f t e r h a l f of t h e charge has been dropped down t h e h o l e , t h e load ing d a t a s h e e t is c o n s u l t e d t o de t e rmine t h e proper down-the-hole de l ay f o r t h a t ho le . P r e s e n t l y , b o o s t e r p r imers manufactured by Aust in Powder Co. (ADP-1 p r imer ) and A t l a s Powder Co. (Deckmaster pr imer) a r e used a s s l i d e r - t y p e p r imers wi th n o n e l e c t r i c d e l a y in - serts.

Once two a d j a c e n t h o l e s have been primed wi th t h e s l i d e r p r imers , t h e Primacord down- l i n e s a r e e i t h e r t i e d o f f s e p a r a t e l y o r perm- a n e n t l y t i e d t o g e t h e r w i th a s q u a r e kno t lo- c a t e d midway between t h e two h o l e c o l l a r s . This procedure s e r v e s a s added i n s u r a n c e i n c a s e t h e wedge b locks f a i l and t h e Primacord must suppor t t h e e n t i r e column l o a d . Next, t h e two h o l e s are loaded w i t h t h e remaining h a l f of t h e e x p l o s i v e cha rge and stemmed w i t h e i t h e r 2.lm (7.0 f t ) of d r y sand o r two l . l m (3.75 f t ) long wa te r bags , fo l lowed by f i l l i n g t h e remainder of t h e h o l e wi th f r e e - s t a n d i n g water . The p rocess , a s shown i n F i g u r e 6 , i s then r epea ted f o r t h e remaining h o l e s . Double Deck B l a s t : The procedure f o r l o a d i n g a double deck b l a s t i s q u i t e s imi lar t o t h a t of a s i n g l e deck. . Genera l ly , i n a m u l t i p l e deck b l a s t , 6.35-mm-diam (0.25-in.) polypropy- l e n e rope i s used t o ho ld t h e wedge b l o c k s s i n c e t h e s t a t i c column load cou ld exceed t h e 136-kg (300-lb) t e n s i l e s t r e n g t h of t h e Prima- cord downline i f t h e b locks weren ' t s e c u r e l y wedged w i t h i n t h e h o l e .

A f t e r t h e wedge b l o c k s have been set, bottom stemming i s added, fo l lowed by t h e bot tom charge and pr imer . Next, 152mm (6.0 i n . ) of c u t t i n g s a r e shoveled i n t o t h e h o l e and one wooden space r 102 x 102 x 762 mm ( 4 x 4 ~ 3 0 i n . ) i s dropped down t h e h o l e f o r deck ing m a t e r i a l a s shown i n F igu re 7. F ine-gra ined sand i s normally p r e f e r r e d a s decking m a t e r i a l and is

used i n l i e u of t h e wooden space r i n a l l a r e a s where b u l k hand l ing i s n o t a problem.

A f t e r adding t h e space r and more sand, t h e t o p c h a r g e and primer a r e loaded i n t o t h e h o l e . By u s i n g t h e s l i d e r - t y p e de lay p r i m e r s , on ly one downline is r e q u i r e d f o r each h o l e regard- less of t h e number of decks t o be loaded. Approximately 1.8 t o 2 .1 m (6 t o 7 f t ) of t h e h o l e i s t h e n stemmed wi th sand and t h e remainder of t h e h o l e i s f i l l e d wi th water up t o t h e c o l l a r . T runk l ine Tie-In: F igu re 8 i l l u s t r a t e s how E- Cord Primacord (25 g r a i n ) i s t i e d i n t o form a t r u n k l i n e between rows. P l a s t i c co rd connec- t o r s manufactured by Aus t in Powder Co. a r e used t o connect t h e downlines t o t h e t r u n k l i n e . Ex- p e r i e n c e h a s shown t h a t u s e of t h e s e c o n n e c t o r s speeds up t i e - i n t i m e and minimizes t h e poss- i b i l i t y of c u t - o f f s due t o improper knot-angle connec t ions . Cross t i e s a r e u t i l i z e d eve ry 9 .1 t o 12.2 m (30 t o 40 f t . ) . Once f i n a l t i e - i n s have been made, two DuPont e l e c t r i c "or' Acudet Delay caps a r e taped onto t h e end of a Primacord p i g t a i l f o r t r u n k l i n e i n i t i a t i o n . Delay P a t t e r n s : S e v e r a l de l ay p a t t e r n s have been t r i e d i n t r u e c r a t e r b l a s t s i t u a t i o n s . The p r e f e r r e d method i s shown i n F i g u r e 9 and c o n s i s t s of s h o o t i n g a burn o u t of t h e middle of t h e p a t t e r n on t h e f i r s t d e l a y s . The remaining h o l e s a r e capped i n a c o n c e n t r i c o r diamond-shaped f a s h i o n s o t h a t each h o l e h a s two f r e e f a c e s t o b reak t o , i . e . t h e ho r i zon- t a l s t o p e back and a r e s u l t a n t v e r t i c a l i n t e r - i o r f a c e . I n t h i s c i rcumstance , each h o l e in - c o r p o r a t e s a combinat ion s l a s h i n g a c t i o n and c r a t e r i n g a c t i o n . T h i s p a t t e r n u s u a l l y y i e l d s t h e b e s t f r agmen ta t ion and maximum volume broken.

G e n e r a l l y , a p a t t e r n c o n s i s t i n g of 20 t o 30 h o l e s can be loaded and b l a s t e d by a n exper- i enced l o a d i n g crew i n one f u l l s h i f t . A t y p i c a l s t o p i n g b lock 61m (200 f t ) l ong and 10.7m (35 f t ) wide i s d iv ided i n t o t h r e e load - i n g s e c t i o n s . O r d i n a r i l y , b l a s t i n g r o t a t e s from s e c t i o n t o s e c t i o n , t he reby m a i n t a i n i n g u n i f o r m i t y i n s topeback e l e v a t i o n s (Crocker , 1978) . Continued b l a s t i n g i n on ly one s t o p i n g s e c t i o n u s u a l l y r e s u l t s i n e x c e s s i v e s l o u g h i n g of t h e brows ( C a r t e r , 1977) .

Once one end of a s t o p e h a s been c r a t e r e d upward one o r two l i f t s , t h e a d j a c e n t s e c t i o n is e i t h e r s l a s h e d i n t o t h e end s e c t i o n o r c r a t e r b l a s t e d downward i n t o t h e u n d e r c u t . A combinat ion s l a s h i n g and c r a t e r i n g d e l a y p a t - t e r n is commonly u t i l i z e d u s i n g one o r more decks of s p h e r i c a l cha rges . However, when mul t ip le-deck s l a s h i n g p a t t e r n s a r e used , a n e x c e s s i v e amount of rock must b e drawn o u t of t h e s t o p e s o t h a t s u f f i c i e n t r e l i e f i s a v a i l - a b l e f o r s l a s h i n g i n t h e a d j a c e n t s e c t i o n . Consequent ly , d i l u t i o n i n t h e form of s lough rock from t h e hanging w a l l a lmost a lways o c c u r s . For t h i s r e a s o n , mul t ip le-deck s l a s h i n g i s seldom p r a c t i c e d . Exp los ive S e l e c t i o n : S u c c e s s f u l c r a t e r b l a s t -

DESIGN AND OPERATION OF CAVING AND SUBLEVEL STOPING MINES LOADING PROCEDURE

1 . T i e d e t o n a t i n g c o r d d o w n l i n e t o wedge b l o c k . Lower wedge b l o c k and c o r d t o b l o c k i n g h e i g h t s p e c i f i e d on t h e l o a d i n g s h e e t .

2. Drop a s e c o n d wedge b l o c k i n t o t h e h o l e . Add a p p r o x i m a t e l y 30 cm (1 .0 f t ) of g r a v e l , r o c k , o r d r i l l c u t t i n g s o n t o p o f t h e wedge b l o c k s . Recheck d i s t a n c e .

3. Shuck o n e 1 3 . 6 - k g ( 3 0 - l b ) bag o f e x p l o s i v e and d r o p i n t o h o l e .

4. Assemble t h e 0.45-kg ( 1 - l b ) d e l a y p r i m e r w i t h a p p r o p r i a t e d e l a y c a p . Feed o n t o d o w n l i n e and s l i d e i n t o t h e h o l e . T i e o f f t h e d o w n l i n e t o a c o n v e n i e n t r o c k .

5. Shuck t h e r e m a i n i n g 1 3 . 6 - k g ( 3 0 - l b ) bag o f c x - p l o s i v e and d r o p i n t o t h e h o l e .

6 . S h o v e l i n approximately 30 cm ( 1 . 0 f t ) o f d r i l l c u t t i n g s o r f i n e s a n d .

7. Lower two w a t e r b a g s i n t o t h e h o l e , o n e a t a t i m e f o r s temming. Note: When l o a d i n g h o l e s s a n d o r g r a v e l w i t h i n 18.3 m ( 6 0 f t ) of t h e t o p s i l l , u s e 1 . 8 m ( 6 f t ) o f f i n e s a n d i n Lieu o f t h e w a t e r b a g s . F i l l t h e r e m a i n d e r o f t h e h o l e w i t h w a t e r .

27.2 kg ( 6 0 - l b )

0.45-kg (1 .0-Lb) p r i m e r and c a p

4 ~7 ' x ' dim.

90' 1.4m ( 4 . 5 f t ) 8 0 n 1.4m (4 .6 f t ) 70' 1.5m ( 4 . 9 f t ) 60" 1.7m (5 .6 E t ) i 0 - 2.0m ( 6 . 6 E t ) 4 5 ' 2.3m ( 7 . 4 f t )

F i g u r e 6. VCR S i n g l e d e c k l o a d i n g p r o c e d u r e .

A u s t i n ADP-1 b o o s t e r s o r A t l a s Deckmaster 0.45-kg (1 .0- lb) b o o s t e r s comple te w i t h n o n e l e c t r i c cap i n s e r t s a r e used a s i n - t he -ho l e d e l a y s w i t h S t r i p Mine S p e c i a l Primacord downl ines .


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t i j u r ~ '. : I S l l u u l ~ l t d < c k l o a d i n g a r r n # ? g m < n t .

Limr . F i q ~ ~ r t *. VCR Lrunkl i n r t i t - i n p r o c c d u r r - .

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I , Run ' t ' c o r d t r u n k l i n r a r o u n d b l a s t i n g p a t t e r n . 2 . T i c cac l i d o w n l i n e t o t r u n k l i n e u s i n g ' p l a s t i c c o r d c o n n e c t o r s .

Keep l i n r s p e r p e n d i c u l a r t o t r u n k l i n e i f p o s s i b l e . . l i t i n 2 e a . "0" t l e c t r i c c a p s t o t r u n k l i n e p i g t a i l a s shown. 4. A f t e r t e s t i n g e l e c t r i c l e a d l i n e s and e l e c t r i c c a p c i r c u i t , t i e

p i b t a i l Lo m a i n t r u n k l i n e and b l a s t i n normal manner a t s p e c i f i e d


i n g a p p l i c a t i o n s a r e dependent on maximizing o r e body. A t op s i l l d r i f t can be used f o r f an t h e amount of exp los ive energy p e r u n i t l eng th d r i l l i n g wi th t h e ITH d r i l l providing t h e d r i f t of cha rge . Th i s requirement is f u l f i l l e d by is s t r i p p e d h igh and wide enough t o f a c i l i t a t e s e l e c t i n g an e x p l o s i v e which e x h i b i t s r e l a t i v e - t h e d r i l l . Typ ica l p a r a l l e l - h o l e p a t t e r n s l y h igh v a l u e s of d e t o n a t i o n v e l o c i t y , d e n s i t y , range from a 2.4 x 2.4 m (8 x 8 f t ) p a t t e r n t o and bu lk s t r e n g t h . Also , t h e e x p l o s i v e should a 3 x 3 m (10 x 10 f t ) p a t t e r n depending on have s u f f i c i e n t cons i s t ency so t h a t t h e c a r - t r i d g e can be shucked t o i n c r e a s e coup l ing a b i l i t y and f i l l t h e e n t i r e c r o s s - s e c t i o n a l a r e a of t h e b l a s t h o l e .

To d a t e , t h e most widely-used e x p l o s i v e s i n VCR a p p l i c a t i o n s a r e TNT-based w a t e r g e l s , em- u l s i o n s , and n i t r o c a r b o n i t r a t e wa te r g e l s . ANFO b l a s t i n g a g e n t s a r e u n s u i t e d f o r t r u e c r a t e r a p p l i c a t i o n s because of t h e low v a l u e s mentioned above. Exper ience a t t h e Homestake Mine i n d i c a t e s t h a t t h e degree of f r agmen ta t ion and volume of rock broken is p r o p o r t i o n a l t o t h e R e l a t i v e Bulk S t r e n g t h of t h e e x p l o s i v e .

Observat ions a t o t h e r mining o p e r a t i o n s have shown t h a t b l a s t h o l e s which do n o t b reak o u t i n t o t h e undercut o r l a t e r become plugged must b e t r e a t e d w i t h c a u t i o n , s i n c e f l a s h e s of p o s t - b l a s t gases may occur . It is t h e o r i z e d t h e s e i n c i d e n t s have occur red due t o t h e gen- e r a t i o n of combust ib le gases i n b l a s t h o l e s which f a i l t o break o u t upon d e t o n a t i o n of t h e e x p l o s i v e charge . It appea r s t h e phenomena can occur wi th any e x p l o s i v e , a l though high- energy a luminized p roduc t s show a more f r e q u e n t r a t e of occurrence .

VCR STOPE DESIGN

LHD Versus S lushe r E x t r a c t i o n

VCR e x t r a c t i o n systems a r e v a r i a b l e depend- i n g on t h e s i z e of t h e o r e body, s t a b i l i t y of t h e surrounding coun t ry r o c k , and a v a i l a b i l i t y of equipment. Exper ience a t t h e Homestake Mine h a s shown t h a t e x t r a c t i o n sys tems u t i l i z i n g Load-Haul-Dump equipment can a c h i e v e two t o t h r e e t imes t h e p roduc t ion r a t e s o b t a i n e d by s l u s h e r s . With m u l t i p l e d rawpo in t s , l a r g e rock can b e s o r t e d by LHD equipment f o r b lock- h o l i n g , t he reby minimizing t h e amount of l o s t mucking t ime due t o o v e r s i z e rock .

F igu re 10 shows an i d e a l i z e d VCR s t o p e u s i n g s l u s h e r e x t r a c t i o n , and i l l u s t r a t e s how ground c o n t r o l can be a c o n t i n u a l problem due t o t h e r e q u i r e d c l o s e p rox imi ty of t h e s l u s h e r d r i f t t o t h e drawpoint hoppers . A LHD hau lage d r i f t , on t h e o t h e r hand, can b e d r i v e n a g r e a t e r d i s t a n c e from t h e s t o p e , and i s l e s s a f f e c t e d by b l a s t i n g . The equipment needed f o r LHD e x t r a c t i o n r e q u i r e s a r e l a t i v e l y h igh i n i t i a l c a p i t a l i nves tmen t , b u t t h e a s s o c i a t e d h i g h p roduc t ion r a t e s e n a b l e t h e equipment t o b e cyc led among s t o p e s which o r d i n a r i l y would n o t s t a r t producing u n t i l a l a t e r d a t e .

D r i l l i n g Cons ide ra t i o n s

t h e rock type and e x t r a c t i o n system used.

F igu re 11 shows how c e r t a i n s tope b locks can b e brought i n t o product ion sooner by u t i l i z i n g a combination VCR and Blasthole-Longhole method. I f , f o r example, manpower i s n o t a v a i l a b l e or ground c o n d i t i o n s w i l l not a l low a f u l l under- c u t f o r a VCR s t o p e , a r i n g d r i l l i n g d r i f t can be d r i v e n i n s t e a d of c u t t i n g out t h e f u l l under- c u t . The bottom 15.2 m (50 f t ) of t h e o r e body i s t h e n d r i l l e d ou t u s ing longhole d r i l l s and p roduc t ion b l a s t i n g begins wh i l e ITH d r i l l i n g p r o g r e s s e s a t t h e oppos i t e end of t h e s tope b lock .

E a r l y Crown P i l l a r Tes t Stopes

By mid 1978, a d e c i s i o n was made t o exper i - ment w i t h t h e VCR method ona l a r g e r s c a l e . The method could e a s i l y be adapted t o e x i s t i n g Shr inkage Stoping a r e a s by c u t t i n g t h e top s i l l t o f a c i l i t a t e a n ITH d r i l l . This was t r i e d wi th some degree of success i n 66-67 D Stope 9 Ledge, on t h e 1707-m (5600 f t ) l e v e l where some f o r t y - t h r e e 165.1-mm-diam (6.5-in.) ho le s were d r i l l e d on a 2.1 x 2.7 m (7x9 f t ) p a t t e r n i n a 15.2-m (50- f t ) t h i c k o r e a r e a over t h e s h r i n k a g e s t o p e . A f t e r making two c r a t e r b l a s t s , an a t t empt was made t o double-deck the remaining 7.6-m (25-f t ) t h i c k crown p i l l a r u s i n g a combination of e l e c t r i c and non-elec- t r i c i n i t i a t i n g systems. Unfor tunate ly , t h e e l e c t r i c c i r c u i t s were no t p rope r ly balanced, and s e v e r a l of t h e e l e c t r i c caps f a i l e d t o d e t o n a t e caus ing t h e round t o f r e e z e up.

I n October 1978, a second crown p i l l a r approximate ly 7.3-m (24-f t ) t h i c k was d r i l l e d ou t w i t h 41 h o l e s on a 2.1 x 2.7 m (7x9 f t ) p a t t e r n , a s shown i n Figure 12 . This sho t was double-decked wi th Tovex Ex t ra wa te r g e l us ing 6.1-m (20- f t ) long None1 MS Delay caps a s i n - t he -ho le d e l a y s i n con junc t ion wi th HDP-1A c a s t p r imers . ANFO was used where necessary t o b r i n g t h e cha rge h e i g h t of t h e top deck up t o a 1.2-m (4-f t ) stemming d i s t a n c e . Approximate- l y 4717t (5,200 s t ) were broken i n t h e 68-69 D Stope w i t h e x c e l l e n t f ragmenta t ion (Er icksen, 1978) .

By e a r l y 1979, Aust in Powder Co. was mark- e t i n g t h e ADP-1 b o o s t e r primer wi th a 4 g r a i n P r i m a l i n e Primadet MS Delay b l a s t i n g cap. This pr iming sys tem r e q u i r e s only one de tona t ing cord downline, r e g a r d l e s s of t h e number of d e l a y s employed i n each h o l e . Aust in ADP-1 p r imers were used i n con junc t ion wi th 1 r e c o 1 s I r e g e l 382 Emulsion i n a t r i p l e - d e c k shot i n a 9.1-m (30-f t ) t h i c k crown p i l l a r a s shown i n F i g u r e 13 .

The top s i l l of a VCR s t o p e b l o c k i s u s u a l l y Exper ience i n o t h e r crown p i l l a r b l a s t s in- a s i l l c u t s l a s h e d o u t t o t h e f u l l w id th of t h e d i c a t e d t h a t a c o n c e n t r i c o r diamond shaped

VERTICAL CRATER RETREAT STOPING AT HOMESTAKE MINE 621

Wagner ST-2B loade r s move o r e from the s t o p e drawpoints t o the o r e pass .

A Kent KB 999-HD Rock Breaker wi th a pedestal-mounted PM-15-HD boom i s used t o break o v e r s i z e on t h e g r i z z l y a t the o r e pas s ,


Longi tud ina l > L C t i o n Lross S e c t i o n

l i gu rc l I . Li'R s t o p i w i t l i r i n g - d r i l l e d bol tol~l s i l l .

No te : MS P e r i o d Upper, Deck/MS P e r i o d Lower b e c k

( 6 / 5 ) 41 h o l e s a t 56 ' and 8.5m ( 2 9 f t )

F i g u r e 12 . Doub le -deck c rown p i l l a r V(:R b l a s t i n 68-69 SD s t o p e 9 l e d g e , 5900 l e v e l .

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delay p a t t e r n most o f t e n yie lded t h e g r e a t e s t c r a t e r he igh t s and b e s t f ragmentat ion. Be- cause of the l i m i t e d number of MS de lays a v a i l - a b l e , two "breaks" were incorporated i n t h e 42 D crown p i l l a r b l a s t a s i l l u s t r a t e d i n Figure 14. Decking m a t e r i a l cons i s t ed of a combina- t i o n of sand and wooden 102 x 102-m (4x4-in.) spacers . The es t imated powder f a c t o r f o r t h i s 7620t (9400st) b l a s t was 0.75 k g / t (1.5 l b / s t ) .

Two unsuccessful crown p i l l a r b l a s t s were made i n mid-1979 (74-75 B 9 L 6050 and 38-43 F 11 L 5000), both of which l e f t a ledge of s o l i d ground between t h e c r a t e r e d a r e a s of t h e top and bottom charges. The main conc lus ions reached from these double-deck crown p i l l a r b l a s t s were:

1. A l l ho les i n a double-deck c r a t e r i n g p a t t e r n should measure l e s s than 7.3 m (24 f t ) between t h e top s i l l and t h e s tope back.

2 . The e f f e c t of p l a c i n g charges a t a d i s t ance l e s s than t h e optimum charge depth w i l l reduce t h e depth and volume of t h e r e s u l t i n g c r a t e r s .

3. In terdeck stemming i n a t r u e c r a t e r de- l a y p a t t e r n should no t exceed 1.2 m (4 f t ) .

S ince November 1979, s u c c e s s f u l double-deck crown p i l l a r b l a s t s have been made i n 10-15 EA Stope 9 Ledge, 2750 Level and 1 3 PEA Stope 9 Ledge, 2750 Level incorpora t ing t h e above con- s i d e r a t i o n s .

B las t ing Problems - Solu t ions

I n 38-43 F 11 L 5000, i t became necessa ry t o d r i l l ou t a 2.1 x 2.7-m (7x9-ft) drop r a i s e a f t e r a double-deck loaded crown p i l l a r b l a s t f a i l e d t o break through i n an a d j a c e n t b l a s t - ing s e c t i o n . The 19.8-m (65-f t ) drop r a i s e was c r a t e r e d through t o s i l l i n about f i f t e e n b l a s t s . Considerable d i f f i c u l t i e s were en- countered with plugged ho les .

Two of t h e most p r e v a l e n t problems experienced i n a l l VCR s t o p e s has been h o l e c o l l a r damage and plugged ho les . O r i g i n a l l y , i t was thought t h a t s e t t i n g 152.4-mm-diam (6.0-in.) s t e e l p ipe i n concre te i n t h e top 1.5m (5 f t ) of t h e 165.1-nun-diam (6.5-in. ) h o l e s would e l i m i n a t e most of t h e c o l l a r damage. R e s u l t s of t h e i n i t i a l b l a s t showed, however, t h a t t h e c o l l a r cas ings apparen t ly c r e a t e d too much r e s - t r i c t i o n , s ince a l l of t h e c a s i n g s were blown out of t h e holes .

The occurrence of plugged h o l e s h a s been i n d i r e c t l y a t t r i b u t e d t o excess ive cha rge h e i g h t s i n chimneyed h o l e s and e x c e s s i v e broken rock being l e f t on t h e f l o o r from c u t - t ing-out of t h e top s i l l . It h a s been found, t h a t lowering wooden b locks 101.6 x 101.6 x 762 mm ( 4 x 4 ~ 3 0 i n . ) t o t h e chimneyed a r e a is

an e f f e c t i v e means of blocking chimneyed o r b u l l e d h o l e s . The h o l e is then f i l l e d wi th broken rock and c u t t i n g s up t o t h e p roper charge depth.

Much of t h e d i f f i c u l t y experienced wi th c o l l a r damage and plugged ho les h a s been mini- mized by us ing s u f f i c i e n t amounts of stemming m a t e r i a l . Current p r a c t i c e invo lves shove l ing 0 .3 m ( 1 f t ) of d r i l l c u t t i n g s d i r e c t l y on t h e charge column followed by lowering two 1 . 1 - m (3 .75-f t ) long water stemming bags . P r i o r t o t y i n g i n , a l l of t h e ho les a r e f i l l e d wi th water from a water hose. A l l ho les measuring l e s s than 18.3 m (60 f t ) from t h e top s i l l r e - q u i r e a d d i t i o n a l stemming, and a r e stemmed with 2.1 m (7 f t ) of f i n e sand followed by water from a hose.

COMPARATIVE COSTS AND PRODUCTIVITY RATES

A comparison of 1980 p r o d u c t i v i t y r a t e s a t t h e Homestake Mine shows t h a t on a tons p e r man s h i f t b a s i s , B las tho le Sub-level Stoping y i e l d s 28.8 t o n s p e r man s h i f t (31.8 s t /ms) w h i l e Open Cut-&-Fi l l Stoping averages 1 5 . 1 (16.7 s t / ms). VCR ou tpu t averaged 28.3 tons p e r man s h i f t (31.2 s t /ms) i n 1980 and i s expected t o i n c r e a s e i n 1981 a s more s topes r each t h e produc t ion phase.

Table 1 summarizes d i r e c t s t o p i n g c o s t s by mining type f o r t h r e e d i f f e r e n t methods. Fig- u r e s shown f o r t h e VCR method inc lude development c o s t s a s s o c i a t e d wi th mining of t h e top s i l l and undercut , a s w e l l a s ITH d r i l l i n g c o s t s . The average 1980 s top ing c o s t f o r t h e VCR method is r e l a t i v e l y h igh , mainly because only e i g h t s t o p e s ou t of a t o t a l of twenty-four i n development had reached t h e product ion phase a s of Dec. 31, 1980. A s more VCR s topes r each t h e product ion s t a g e , t h e a d d i t i o n a l tonnage r e - a l i z e d i s expected t o o f f s e t development c o s t s and c o n t i n u e t o s t a b i l i z e t h e o v e r a l l VCR s top- ing c o s t . U l t ima te ly , t h e g r e a t e s t pe rcen tage of t o t a l VCR ou tpu t w i l l be s h i f t e d from rem- nant low-grade blocks i n t o t h e higher-grade 19 and 21 Ledge o r e a r e a s , thereby making t h e t o t a l s t o p i n g c o s t even more a t t r a c t i v e .

CONCLUSIONS

Appl ica t ions of t h e c r a t e r i n g mechanism using L iv ings ton ' s s p h e r i c a l charge t h e o r i e s became a d a p t a b l e t o underground s t o p i n g about s i x y e a r s ago w i t h t h e i n t r o d u c t i o n of In-The- Hole d r i l l equipment capable of d r i l l i n g l a r g e diameter h o l e s a t an accep tab le c o s t .

I n t h e l a t e 19701s , s u b s t a n t i a l i n c r e a s e s i n t h e c o s t of p roduc t ion from t r a d i t i o n a l s t o p i n g methods prompted i n v e s t i g a t i o n of t h e V e r t i c a l C r a t e r R e t r e a t Method f o r p o s s i b l e a p p l i c a t i o n s a t t h e Homestake Mine. Experimentation wi th

626 DESIGN AND OPERATION OF CAVING AND SUBLEVEL STOPING MINES

t h e VCR Methad i n d i c a t e d t h a t i n inany a r e a s 2 . C rocke r , C .S . , 1975, " V e r t i c a l C r a t e r

of t h e mine, i t would be more economica l t o R e t r e a t Mining a t t h e Cen t enn i a l Mine of

mine l e s s s e l e c t i v e l y u s i n g mechanized equip- HBM&S Co., L td . , " F l i n F lon , Manitoba.

ment , w i t h I.ess dependence on t h e a v a i l a b i l i t y of s k i l l e d mine r s . 3. E r i c k s e n , S.D., 1978, " V e r t i c a l C r a t e r

R e t r e a t S t o p i n g a t t h e Homestake Gold

Most of t h e VCR p r o d u c t i o n , t o d a t e , h a s be en d e r i v e d from low-grade b l o c k s i n t h e

Mine, I ' Homestake Mining Company, Lead, SD .

u p p e r l e v e l s o f t h e mine. S u c c e s s f u l a p p l i - c a t i o n s i n t h e s e a r e a s , h a s l e d mine pe r - 4. Gran t , C .H., 1964, "Simplif i e d Explana-

s o n n e l t o p l a n numerous VCR s t o p e s i n t h e t i o n of C r a t e r Method," Eng inee r i ng and

r e c e n t l y deve loped h i g h e r g r ade a r e a s of t h e Mining J o u r n a l , Nov., pp. 86-89.

mine. It i s e s t i m a t e d t h a t o v e r t h e n e x t f i v e y e a r s , VCR p r o d u c t i o n w i l l a c c o u n t f o r 35-50% 5. Lang, R.C., Roach, R.J., and Osoko, M.N.,

of t h e t o t a l mine p r o d u c t i o n . 1977 , " V e r t i c a l C r a t e r R e t r e a t an Impor- t a n t New Mining Method," Canadian Mining J o u r n a l , S e p t . , pp. 69-76.

REFERENCES

1. C a r t e r , G.S., 1977, " V e r t i c a l C r a t e r R e t r e a t Mining a t HBM&S Co., L t d ' s . C e n t e n n i a l Mine," D i s t r i c t 4 C . I . M . Convent i o n .

6. L i v i n g s t o n , C.W., 1973, U.S. P a t e n t 3 ,762 ,771 , Oc t . 2 .

7. Monahan, C . J . , 1979 , he C r a t e r B l a s t - i n g Method Appl ied t o P i l l a r Recovery a t F a l c o n b r i d g e N i c k e l Mines L imi t ed , " Underground O p e r a t o r s Conference , Feb. 19-21.

Tab l e 1. Compara t ive S t o p i n g C o s t s by Mining Type.

OCF - BH - VCR - Actua l 1480 S t o p i n g & F r i n g e ( $ / s t ) $10.65 $ 4.79 $ 9.47

Budgeted 1980 Grade ( o z / s t ) 0 .221 0.143 0.149

Tons Mined i n 1980 ( s t ) 825 ,967 264,144 336,220

Budgeted 1981 S t o p i n g & F r i n g e ( $ / s t ) $16.47 $13.02 $ 6.21

Budgeted 1981 Grade ( o z / s t ) 0 .2172 0 .1406 0.1515

Budgeted 1981 Tonnage ( s t ) 622 ,000 194 ,000 597,500

Note: The above c o s t s a r e d i r e c t s t o p i n g c o s t s o n l y and do n o t i n c l u d e h o i s t i n g , m i l l i n g , a d m i n i s t r a t i v e o r o t h e r mine o p e r a t i n g c o s t s .

Vertical Crater Retreat Stoping as Applied at the Homestake Mine(Chapter44)

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