Chapter 15
NUMERICAL MODELING OF BLOCK CAVING AT THE GRACE MINE
Giovanni B. Barla
Department of Structural Engineering Politecnico di Torino
Torino, Italy
Stefan H. Boshkov
Henry Krumb School of Mines Columbia University New York, New
York
ABSTRACT
The block caving method is examined in this paper on the basis
of experi- mental results and observations in the field, and
through the use of numerical modeling by the Finite Element Method.
The Grace Mine, developed by a panel caving method, served as a
reference case. However, the results obtained and the simulation
techniques which have been developed may find useful application in
more general conditions as encountered in other mining operations
by block caving. Following a brief description of the Grace Mine
and of the mining method, the problems associated with the behavior
of underground drifts at the production levels, during undercutting
and caving, are addressed. The most important results obtained
through monitoring of strains and loads in the supports of a runway
are discussed, while giving relevance to the sequence of mining
operations in the immediate vicinity. The most important
geomechanical data, as provided for subsequent modeling, are
described with reference to: (a) rock mass characterization and (b)
in situ state of stress. Then, the numerical modeling procedures,
developed by the Finite Element Method in order to predict the
caving phenomena and the behavior of underground openings during
undercutting, are discussed. Finally, an attempt is made to compare
the pre- dictions of the numerical analyses with,the measurements
carried out at the instrumented site. Suggestions as to how to
improve the results both quali- tatively and quantitatively are
also given.
NUMERICAL MODELING OF BLOCK CAVING AT GRACE MINE
INTRODUCTION
A s i g n i f i c a n t problem i n mining by BZock Caving i s t
h e evaluat ion of t h e e f f e c t s o f undercu t t ing and
caving on t h e behavior of underground develop- ment and e x t r a
c t i o n openings and t h e i r supports (Merr i l and Johnson,
1964) . I n recogni t ion of t h e need t o provide r e l i a b l e
s o l u t i o n s t o such a prob- lem, a research program was c a
r r i e d ou t from 1974 through 1979, with t h e Grace Mine, near
Reading (Pennsylvania) , being considered a s a u s e f u l exem- p
l i f i c a t i o n of t h e method ( ~ a r l a and Boshkov,
19791
This program encompassed i n s i t u s t r e s s de te rmi na t
ion , monitoring of equipment i n s t a l l a t i o n (Agac wal e t
a l . , 1973) , and subsequent accumulation o f d a t a from 1974
through 1976. Subsequently, t h e fol lowing ob jec t ives were
posed: 1. t o r e l a t e f i e l d observat ions and experimental
d a t a t o f i e l d experience, and 2 . t o a t t empt t o f i n
d a l inkage between t h e two, through t h e des ign o f a
numerical model based upon the F i n i t e Element Method. The r e
s u l t s of t h e i n v e s t i g a t i o n s c a r r i e d ou t a
r e de- sc r ibed i n t h e p resen t paper .
MINE DESCRIPTION
The Grace Mine i s s i t u a t e d approximately 1 0 km south of
Reading ( ~ e n n s ~ l v a n i a ) . Up t o t h e end of 1978, a
magnetite orebody, wi th an average th ick- ness of 100 m , about
400 m wide, has been mined a t a depth of 700 m by t h e panel
caving method.
?'he o r e v a r i e s from moderately weak t o moderate l y s t
rong . The foo twal l , d ipp ing approximately 25 degrees t o
North-East, i s d iabase , a f i n e grained and r a t h e r
competent rock . A moderately s t r o n g q u a r t z i t e form; t
h e hanging w a l l . The mine p lan is shown i n Figure 1, where a
completely dep le ted a r e a i n t h e orebody i s r e p r e s e n
t e d , t o g e t h e r with t h e runways used f o r o r e p roduc
t ion . Figure 2 il- l u s t r a t e s t h e method of mining.
Runways (4.5x4.0 m) a r e d r iven along t h e s t r i k e ,
approximately 20 m below t h e ore-diabase c o n t a c t . E n t r
i e s (draw p o i n t s ) a r e turned o f f a t 60 degrees each 15
m from t h e runway. Concurrent ly a d r i f t (2x3 m) (undercu t t
ing d r i f t ) i s dr iven p a r a l l e l t o t h e runway, 11 m
t o t h e down-dip s i d e . Access t o t h i s d r i f t i s
secured by d r i v i n g a p i l o t d r i f t through a t every t
h i r d e n t r y . D r a ~ - ~ o i n t s a r e c rea ted by en
eleven-*,ole i n c l i n e d f a n p a t t e r n d r i l l e d on
1.5 m cen te rs throughout t h e l e n g t h of t h e undercut t
ing d r i f t . A f t e r a s u f f i c i e n t number of e n t r i
e s a r e completed i n suppor t requirements , t h e fan holes a r
e b l a s t e d . Both t h e runways and e n t r i e s a r e heav i
ly supported by r e i n f o r c e d con- c r e t e and s t e e l s
e t s .
A l l openings experienced l a r g e s t r e s s and de-
formation changes during u n d e r c u t t i n g and caving. This r
e s u l t e d i n supports f a i l u r e , c o s t l y r e p a i r
o r replacement. I n o rder t o a l l e v i a t e t h e s e prob-
lems, p rogress ive ly heav ie r s t e e l s e c t i o n s and t h
i c k e r concrete l i n i n g s have been used a t t h e mine w i
t h apparent small improvement i n r e s u l t i n g condit ions.
No f ixed p a t t e r n t o t h e f a i l u r e o f supports could
be defined. Also, t h e f a i l u r e i n
no case was v i o l e n t , b u t gradual and unre len t ing
.
FIGURE 1 - Mine p lan .
FIGURE 2 - Method of mining. S e c t i o n through t h e runways
showing f o o t w a l l and undercut .
IXSTRUMENTED RUNWAY
With t h e purpose o f determining t h e load on s t e d s e t s
and s t r a i n s i n t h e concre te l i n i n g , it was de- c
ided t o instrument an o r e haulage runway (Runway 6 0 9 ~ , 6 t h
l e v e l ) , a t tvo a d j a c e n t e n t r i e s t o t h e caved
o r e , a s shown i n Figure 3.
This runway was instrumented between November 1973 and October
1974 (Agarwal e t a l . , 1973). The equipment i n s t a l l e d
included concre te s t r a i n t r a n s - m i t t e r s and load c
e l l s . The s t r a i n t r a n s m i t t e r s
