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Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. Vol. 23, No. 1, pp. 77-83, 1986 0148-9062/86 $3.00 +0.00 Printed in Great Britain Pergamon Press Ltd
8. Increasing the Extraction Ratio at Yanahara Mine and an Associated Programme of Field Measurements T. SAITO* H. ARAKIt Y. KAMEOKA* Y. HIRAMATSU§
In Yanahara Mine, Japan, a large pyrite ore body has been mined for the last 25yr. The natural conditions of the ore body and country rocks are favourable, but surface subsidence must be avoided. It was considered that to achieve a high extraction ratio of mining without causing any surface damage would be difficult because the ore body is massing and large. Towards the end of primary mining, fracture of rock occurred in the walls of excavations at the pillars. Thereafter, to investigate the effects of secondary mining on the stability of the remaining pillars and the ground, a programme of field measurements has been carried out. This Paper describes the progress of pillar recovery and presents details of following measurements: deformation of roof rock: tilt of the floor of a mine level; rock stress states; subsidence of the roof rock; and average horizontal strain in a pillar. By the end of 1984, about 70% of the mineable ore had been successfully mined without any serious trouble.
INTRODUCTION
Yanahara Mine is situated about 38 km from Okayama City in Honshu island, Japan. In this mine, one of the massive pyrite ore bodies called the "Lower Ore Body" which is 500 m long, 500 m wide, 60 m thick at the centre and on average is 370 m deep, has been mined for the last 25 yr. On the surface above this ore body, there are many residences and rice fields filled with water from spring to summer, with the Yoshii river flowing through them. Therefore fracture of the ground above the ore body and/or considerable surface subsidence cannot be allowed.
The basic plan for mining this ore body is that during primary mining ore is mined by leaving systematic rib pillars, while during secondary mining by recovering the pillars each mined space should be filled with waste as soon as possible. It was supposed, however, that to achieve a high extraction percentage of mining without causing any surface damage would be difficult because the ore body is massive and large.
Towards the end of primary mining, recovery of a rib pillar was started tentatively. To investigate the effect of
* Department of Mineral Science and Technology, Kyoto Univer- sity, Kyoto 606, Japan.
t Yanahara Mine, Dowa Mining Co. Ltd, Japan. Japan Construction Method and Machinery Research Institute,
Japan. § Professor Emeritus, Kyoto University, Kyoto 606, Japan.
this secondary mining on the stability of the remaining pillars arid the ground, any occurrence of fracture of rock in the pillars was observed very carefully and several types of field measurements were carried out. Thereafter, primary and secondary mining with the field measurements were continued. By the end of 1978, about 67% of the mineable ore had been successfully mined without the difficulties it had been thought might have been encountered [I].
Through the experience obtained to date, further pillar recovery has been continued. For monitoring the behaviour of pillars and rock, various kinds of mea- surement techniques have been adopted, the main one being the measurement of the subsidence of the roof rock with a network of water tubes--which was chosen due to easy operation and reliability. This Paper de- scribes the details of measurements and the progress of pillar recovery.
77
MINING
The Lower Ore Body and surrounding geology are shown schematically in Fig. 1. The north part of the ore body dips at an angle of 20 ° , while the south part is rather flat. The country rocks are mainly diabase and clay slate. These rocks are strong, their compressive strengths and Young's moduli being greater than 200 MN/m 2 and 80 GN/m 2, respectively. The ore itself is also strong, the corresponding values for the ore being
7~ SAITO et al.: THE EXTRACTION RATIO AT YANAHARA MINE
EO E5
/ I I I I I I I $15
S2O -R1 - - X
S25
S 3 0
$35
E15
I I I
E20
I I t I I
I B
EIO
I. I I
. . . . . . . . . . . . X . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I I I I I I I I I I I I I I I I I I I I I I I
B '
] Pyri te ore [ ] Clay slate
[ ] Diabase [ ] Dyke
[ ] Waste f i l l ing [ ] Low grade ore
Fig. I. Schematic diagram showing the geology and the basic mining
L
F
I . . . . . . . . .
I I I I I ~ I 1 I I ~ o ~<
~D
o 04 O0
O ,',cb _J
] Quartz diorite
] Miscellaneous
".~..Fault
plan of the Lower Ore Body, Yanahara Mine.
tt'3
03
O t,O (/3
greater than 120 MN/m 2 and 100 GN/m 2, respectively. The ground is intruded by dykes of quartz porphyry which is considerably weaker than the other rocks. No distinct fault has been found except the one running through the southwest part of the ore body.
The area covering the ore body is divided into longi- tudinal and transverse panels, both 20m wide; the former are denoted by El, E2 and so on, while the latter by Sl, $2 and so on. The depth is indicated by the numbers of the haulage driven at vertical intervals of 15m, as Ll, L2 and so on.
The panel S21 was chosen as the main structural pillar, which divided the ore body into the north and south regions. Primary mining was then conducted in the following manner. In the north region, the ore was mined in the longitudinal alternate panels by sublevel stoping with waste fill. In the south region, however, the ore was mined in the northern half of each transverse panel by the cut and fill method, leaving the southern half a panel as a pillar.
In secondary mining, the pillars left in both the regions were recovered by several methods. In 1969,
when the primary mining was not yet finished, the pillar left in panel $25 was recovered carefully, and continuous field measurements commenced. Thereafter, primary and secondary mining were carried out simultaneously, but since 1973 only secondary mining has been prac- ticed. Through the whole period, the field measurements were carried out. Figure 2 illustrates the progress of pillar-recovery in panel $25.
During primary mining, collapse of the roof of stopes and small rock bursts on the wall of an underground machine-shop took place. Towards the end of the
]E5 1 I I I IEIQ! ! I I E~'bL ~ L TI T2 T3 T4 T5
Fig. 2. The boreholes for measuring dilation of the roof rock and the stations for measuring tilt of the floor at level L24, together with the
progress of secondary mining of panel $25.
SAITO et al.: THE EXTRACTION RATIO AT YANAHARA MINE 79
primary mining, fracture of rock occurred on the walls of excavations in pillars and dykes. However, it was strange to find that during secondary mining no such fracture of the rock has been experienced anywhere.
RESULTS OF AN EARLIER MEASUREMENT PROGRAMME
The measurements described in this article were started from the beginning of the tentative secondary mining in 1969, when the pillar recovery was about 40% of the mineable ore, and were continued until 1978, when the pillar recovery amounted to 67%.
In this Section, the progress of mining in the area around panel $25 will be explained briefly. The pillar left in panel $25 was mined in 1970 to an extent of about 50%, and the remaining part was mined from 1971 to 1973. At the end of 1970, the ore in panel $24 and the southern half of panel $23 existed, but the northern half of panel $24 was completely mined out in 1971 (during primary mining). Besides the pillar recovery and primary mining, other pillars at a distance from $25 were mined in the period from 1971 to 1973.
From 1974, the pillar left in panel $24 and that left in panel $27 began to be mined. Since 1972, the output was decreased year by year to prolong the life of the mine.
Deformation of roof rock
The deformation of the roof rock caused by mining was measured in the haulage level L24 above the stope of panel $25. The vertical distance from the level to the roof of the stope is about 30 m. From the floor of the level, five vertical holes, D1, D2 . . . . . D5, were bored downward, and stainless steel wires of different lengths were anchored at their lower ends in each borehole as shown in Fig. 2.
The variation in distance between the anchor of each wire and the mouth of the borehole was measured with an extensometer in each borehole. These measurements revealed that the roof rock dilated with the progress of mining. It is considered that this dilation of the roof rock may be mainly due to loosening or separation of joints in the rock. The results obtained are plotted against time in Fig. 3. This figure shows that dilation of the roof rock increases remarkably after 3-5 yr from the beginning of pillar recovery in panel $25, and that the dilation of the lower roof region is greater than that of the upper layer.
Tilt of the floor of a level
The tilt of the floor of the same level was measured at the five points, T1, T2 . . . . . T5, near the boreholes for the deformation measurement, as shown in Fig. 2, with a portable precise tiltmeter of pendulum type by placing it on a horizontal base attached to the floor. This measurement was made in two directions, the one in the direction of the axis of the level and the other at right angle to the former.
The results obtained are plotted against time in Fig. 4, which shows that tilt of the floor rock became significant after 1972 and that the floor rock deformed
D1 x-~ x ~x×xx - x ~ ~ x
40 V h = 2 9 . 2 m ~ " /
/ / _.-~ ~ = ' ~ , ~ , - - - °
2 0 ~ h = 3 0 . 4 - - - - - . -~ x. , .~ xX×~ - ~ x / / x~̂ .lh= 21.4
x x _- , .~, ._ .~. . - --.o . . . . .
- 4 0 V D5 h = 27"4 x
+ 20 / ] / h= 24.3
- o '
-x,,~
2 0 F D5 / h = 2 2 . 7 h=19.2
1969 70 71 72 73 74 75 7'6 77 1978
Fig. 3. Dilation of the roof rock above panel S25.
irregularly, possibly in accordance with the quantity of mined ore and the relative position of each mining operation.
Stress variation
The variation in stress was measured with hollow cylindrical glass gauges buried tightly in holes on the rock surface of tunnels and with a portable photoelastic apparatus of reflection type developed by Hiramatsu et al. [2]. This measurement was made at a number of locations in the tunnels excavated in the pillars of $24 and $26. The gauge is made of borosilicate glass and silvered on the back, and is 35 mm in diameter, 30 mm long, and has a hole, 6 mm in diameter at the centre. The photoelastic apparatus is furnished with a compensator and weighs about 500 g.
Some typical results are shown in Fig. 5, from which it is seen that indeed stress first increases, but decreases several times and again increases at each time, and that the direction of the greatest increase varies in most cases.
~o
o
tw -g
E E
8
o
tw
Fig.
1
- 2
- 3 1 ] I I I } ] I I
2
0,4 2 I
-3 I I I I I I I I 1971 72 7s 74 75 76 77 19;'8
4. V a r i a t i o n in t i l t o f t h e f l o o r o f level L 2 4 .
~(i S A I T O et al.: T H E E X T R A C T I O N R A T I O A T Y A N A H A R A M I N E
Rock stress
The rock stress was measured, in August 1978, at the four points, RI, R2, R3 and R4, on level L27 as shown in Fig. 1, By the stress relief technique initiated by the authors [3]. The point R2 is in the middle of the structural pillar of $21. RI is in the rock near the west end of the pillar, and R3 in the rock at a small distance from the east end of the pillar. The point R4 is in the rock at a considerable distance to the south of the ore body. All the points are at a distance of 5-7 m from the wall surfaces of the tunnels.
From a theoretical study, the authors found that rock stress can be determined by measuring at least six strain components on the bottom of a single borehole. Using this principle, a strain cell with eight gauges in it, as shown in Fig. 6, was developed. The relation between strain components on the bottom of a borehole e~, e2, ea , . . . , e8 and the components of rock stress G, a,. . . . . . zx, is represented by equation (l).
el-) a ' a ' c' d ' - d ' - b '
~2 / - a b e d 0 0
e~ a' a' c' d' d' b"
e~ 1 - b a c 0 d 0 (l)
~, = E a' a' c ' - d ' d ' - b "
e~ - a b c - d 0 0
e-, a' a' c ' - d ' - d ' b'
~ - b a e 0 - d 0
The strain coefficients Poisson's ratio v. For v values:
a =0.478, b = 1.712, c = -0 .305, d = 1.223
a ' = 0 . 5 1 1 , b ' = - 1 . 8 5 8 , c ' = - 0 . 4 2 5 , d ' = - 0 . 0 9 5 .
During overcoring, the variations in strain were mea- sured at intervals of 5-10 mm without interrupting the overcoring. The results obtained at point R4 are shown in Fig. 7, as an example.
The states of rock stress at the four points determined by this measurement are shown in Table l, the directions of principal stresses being plotted on the lower hemi- sphere polar nets with dots.
O'x I
17). I
"[ :x
~rv
a ,b . . . . . d' vary a little with = 0.25, they take the following
INFORMATION DERIVED FROM THE EARLIER MEASUREMENT PROGRAMME
A close examination of the measurement results in combination with a knowledge of the mining operation
20 ~ L 2 7 - S 2 6 - E 9 i
15 ~-- i ~ i - ° - o ' ° ~e_i_o_l. e /
Z o i I t i I i i I I I ix . ._ : i : .
L 2 7 - S 2 6 - E l3 ~5~- . - .
- . - - -
~, 5 "0. "; c o i I I I ] I I i I I I I ! I i ! i I l r I ; ~ :
10 L. L 3 0 - S 2 4 - E 4 , " C ~ I • t - °" \ ° / * - ° \ / * ' ~ ' ~ ° - ~ ' ° - °
. . *N / I ° " •
. - Oi i i i i i i i i i i I~'~l i i i I j I i i ~ ~
oA
10 F- L 3 0 - S 2 4 - E 9 ~ "o,. -~\,.
, ' " O I I t Plf I l ,~ I I ,t.l,.,+,,#,,4,~l,.f"q i I ' i ' i ' ! ~ Jan July Jon Juf) Jon
1970 1971
Key 2.f,,m--Direction in which the greotest stress N-~,I~-S increase in norrncll OCcurred
Vertical
Fig . 5. V a r i a t i o n in s t ress m e a s u r e d by a p h o t o e l a s t i c s t ress me te r .
has provided information concerning the behaviour of the pillars and the rock as described below.
In the initial period when there are many pillars remaining, the rock is affected little by recovering a pillar, even an adjacent one; but after many pillars have been recovered, the state of the rock is greatly affected by mining even a small portion of a remote pillar.
This phenomenon should be compared with the ex- perience that, though collapse of rock took place several times in the primary mining stage, no such collapse has occurred during secondary mining. This experience seems to be contradictory to the information obtained from measurements as mentioned above, and inter- pretation of this contradiction is necessary.
It might be supposed that in the stage of primary mining, since pillars and rock constitute a continuous elastic body, fracture of rock happened to occur at spots of high stress concentration, but that when the second- ary mining progressed extensively, pillars and rock might yield here and there so that greater deformation might be allowed without collapse.
The dilation and tilt of rock became greater since the end of 1972, although even in the summer of 1978 neither the dilation not tilt amounted to a very serious value.
The variation in stress at the wall of tunnels measured by the photoelastic technique may be hard to interpret.
Overcoring ~ .~
/ / / E I E A , J " ~ \ . j Compouno I - ~ - "~1 _ / / / E ~ [ E -"1 z'=-tr-Al~..~ stra in gauge ~,. 6 b w ' L_d o " /
I " 5 ~ 1 0 m - - -~ ' , , - 0 . 5 m - l ' - - 0 . 5 m-- I gouge
F ron t v iew of compound stra in gouge
Fig . 6. P r o c e d u r e o f r o c k s t ress m e a s u r e m e n t a n d a n e n l a r g e d d i a g r a m o f a c o m p o u n d s t r a i n g a u g e .
SAITO e t al.: THE EXTRACTION RATIO AT YANAHARA MINE 81
• o " 0@% 0
- 1 0 0 "
- 2 0 0
- 3 0 0 <3
I t I I I
o 8
':b°~5~oooo~0oooO%oO Ooo@o ~ oo 00% 0% oo
x • 4
o ~
2 0 0 t @ • Oooo ooo o oooOO o ooo / . . , % .
Horizontal | oe ~Fee= 6 . 100 °J~6~ p eoeeo,oeoeoeoeoeoeoe, ee 0% oooooe N
%,0
°, ~,~:e.~__ . . . . ooc,~v ~o~O~eo,~BD ~ I o I
- 100 j-
R.M.M.S. 23/I--F
0
- 1 C 0
xxxxxx.~xx~xxxxxxxxx~xx- -- 2 0 0 " J . i -..J
( t<O)
2 0 0 "
(1>0)
1 0 0 -
- I 0 0
o o ~ o e e = Oo o
I t I I I
°'.% ~o ~
5 o o Q%ooa'a~,ooeoeee e, oeo,l~eeel~ oOOoe • e e l ~ e e o
o o o o 1
°°oo oo¢~0oo 0oo°O OoO0° o Oo000o °° °°OoOOOOooO
•°°Co° 7
o ° °°°°OoocooOooOo o 0o%00 O°%oeoOoO°OooO
o o
o Q qboeoee o o ' ~ 3
o" , "~"'.'r'.o.~-.'- "~ 10 20 30 4 0 50
I ( c m )
- 1 O O
Fig. 7. Variation in strain with progress of overcoring.
Table 1. Rock stress states determined by the stress relief method and direc- tions of principal stresses shown on lower hemisphere stereographic
projections
Stress (MN/m 2) Point o~ a2 ~3 av
RI - 3 0 -51 -101 -45 R2 -37 - 8 0 -91 -78 R3 - 7 -12 -27 -12 R4 - 1 -17 -33 - 1 4
R1 N R2
S
If the measurements can be relied upon, the stress at the wall o f a tunnel driven in a pillar varies in an irregular manner. The results o f measurement o f rock stress will be discussed later.
THE CURRENT MEASUREMENT PROGRAMME
F r o m experience gained during these earlier measure- ments , it was found that measurements which can evaluate the deformat ion o f a rock mass will be prefer- able to those which determine the stress or deformat ion at points or in l imited areas. With due considerat ion o f the matter taking into account s implicity and reliability, the measurements described be low have been carried out since 1977. Addit ional ly , rock stress was again deter- mined, this t ime by the hydraulic fracturing technique, to check the former measurements obtained by the stress relief technique.
82 SAITO et al.: THE EXTRACTION RATIO AT YANAHARA MINE
EO 2 ~O 12: ,:70 J I I [ ! i i I ;
- / . . : , : : : : • , . ' , . ............ I
SAITO et al.: THE EXTRACTION RATIO AT YANAHARA MINE 83
,o - , Oo
• ~ ~ o N - 2 °o'~o o × DI Is!
o D1 2ncl ° ' ° " ~ - 3 o D2 oh-..
o o'0"-
- 4 I I 0 1 O0 200 300
Time (hr) Fig. 10. Variation in average rock strain in pillar S2I.
D2 is in the tunnel driven at the level L27 in the rock under the same pillar. Figure 9 shows the positions of D1 and D2 and the mined area at the end of 1984. Measure- ments were conducted for periods ranging from 150 to 300 hr, and the results obtained are shown in Fig. 10. It was found that the rock strain in the pillar changed uniformly at the rate of about - 2 . 4 x 10-7/day, while that in the floor rock is almost constant. At present, interpretation of the results is not complete but further measurement results will be used to obtain useful infor- mation to interpret the stability of this underground structure.
R o c k s t res s s t a t e s
From the previous measurement of rock stress, it was found that the stress in the area remote from the ore body was not high, but that the stress in the pillar of $21 was very high, about 8.5 times the overburden stress, 7.-, in the vertical direction and 9.8 times 7.- in the horizontal direction.
In 1983, Dr Mizuta of Yamaguchi University deter- mined the rock stress by the hydraulic fracturing tech- nique. He obtained results indicating that the stress in the pillar of $21 was smaller than ~: both in the vertical and horizontal directions; however, the stress in the area remote from the ore body was of the same order as the stress obtained by the authors.
The discrepancy between the results of these two measurements is a very serious problem in relation to the
possibility of further pillar recovery. It is urgent, in the first place, to investigate the cause of this discrepancy; and, in the second place, to examine the stability of pillars collectively from observation of all rock pressure phenomena and the results of other field measurements.
CONCLUSIONS
In Yanahara Mine, a large pyrite ore body has been mined for the last 25 yr. The natural condition of the ore body and country rocks are favourable, but subsidence damage must be avoided. Half the ore was mined without difficulty by leaving pillars. Towards the end of primary mining, secondary mining (that is, pillars recov- ery) and several measurements were undertaken. After secondary mining had been achieved no problems have been experienced. From the measurements, a number of observations about the behaviour of pillars and rock were obtained, though no unusual phenomena were detected just by visual observation. In the summer of 1978, 67% of the mineable ore reserves were already mined out, and the possibility of further mining became more important. A new system of field measurement was adopted and more effort has been expended on compact filling. Though mining is progressing smoothly at present, a sudden large collapse of pillars is of concern. The authors believe that maximum recovery will be achieved by paying the greatest attention to detect any anomalous behaviour and by interpreting the results of careful measurements.
Acknowledgements--The authors are indebted to the Yanahara Mine, Dowa Mining Company, for their assistance in the study and wish to thank them for permission to publish this paper.
R E F E R E N C E S
1. Hiramatsu Y., Oka Y. and Kameoka Y. Pillar-robbing supported by field measurements in Yanahara mine. Proc. 4th Int. Congr. ISRM Vol. 2, pp. 235-241. Montreux (1979).
2. Hiramatsu Y., Niwa Y. and Oka Y. Measurement of stress in the field by an application of photoelasticity. Technical Rept. En- gineering Research Institute, Kyoto University, No. 37, pp. 49-63 (1957).
3. Oka Y., Kameoka Y., Saito T. and Hiramatsu Y. Investigation on the new method of determining rock stress by the stress relief technique and application of this method. Rock Mechanics in Japan, Vol. 3, pp. 68-70. Japan NG for ISRM (1979).
