7.
8.
L. I. Slepyan, Nonstationary Elastic Waves [in Bussian], Sudostroenie, L e n i n g r a d (1973). G. I. Marc huk, Methods of Computing Mathematics [in Russian], Nauka, N o v o s i b i r s k (1972).
CAMOUFLET Yu. and
BLASTING
IN B U M P - P R O N E
N. M a k a r o v , B. T . A. A. Filinkov
COAL
SEAMS UDC 622.831.32 : 622.235.36
Akin'shin,
Camouflet blasting is a popular local method of countering shock bumps in c o a l s e a m s . Its effectiveness depends on matching the blast p a r a m e t e r s to the p a r t i c u l a r conditions. As a r e s u l t of r e s e a r c h p e r f o r m e d ~t the All-Union S c i e n t i f i c - R e s e a r c h Institute of Mine Surveying on coal and lignite deposits in the USSI~, we have found quantitative relations for the f r a g m e n t i n g power of cylindrical explosive c h a r g e s . In b u m p - p r o n e s e a m s , the blasting conditions a r e governed by the p r e s e n c e of r a d i a l air gaps resulting f r o m intense f r a c t u r e of the borehole walls during drilling, by filling of the air gaps with w~ter stemming, by the formation of a zone of inelastic deformations around the hole with marked d e c o m p a c t i o n of the coal, by the p r e s e n c e of augmented static s t r e s s e s in the zone of action of the charge, and by a change in the type of state of s t r e s s within the abutment p r e s s u r e zone. In our experimental blasts, the weight of the c y l i n d r i c a l c h a r g e s , 36 mm in d i a m e t e r , was 400 g. The l e n g t h - t o - d i a m e t e r ratio of the c h a r g e s was 10, i.e., these were optimum c h a r g e s . We verified experimentally the popular assumption that further i n c r e a s e in the c h a r g e length leaves the radius of the fragmenting action constant. Of c o u r s e , the g r e a t e r the d i a m e t e r of the c h a r g e , the l a r g e r is the o p t i m u m c h a r g e . We estimated the fragmenting action of c h a r g e s in coal s e a m s with strengths of between 70-100 and 200300 kgf/cm 2 under uniaxial c o m p r e s s i o n . We found that in this range of coal s t r e n g t h s , for blasting in u n s t r e s s e d p a r t s of the coal, the radius of fragmenting action of the c h a r g e is n e a r l y independent of the strength of the coal. During drilling in highly s t r e s s e d p a r t s of the s e a m , where the load on the c o a l is t h r e e or m o r e times its strength under uniaxial c o m p r e s s i o n , the borehole walls break up and give a high yield of drilling fines. The d i a m e t e r of the borehole cavity is related as follows to the yield of drilling fines : dbh=},10 p [cm],
(1)
where P is the yield of drilling fInes (in liters) per m e t e r of borehole. The a i r gap which f o r m s r e d u c e s the c h a r g i n g density and the radius of action of the blast (Fig. la). Under these conditions, filling the gaps with w a t e r i n c r e a s e s the efficiency of the blast. We found that when the r e l a tion between the size of the gap and the radius r 0 of the c h a r g e is
--2r------------~~ dbh -;'o
1.6, the f r a g m e n t i n g action
of the camouflet blast is a maximum (Fig. lb). We decided to make a quantitative e s t i m a t e of the influence of air-filled or w a t e r - f i l l e d gaps by means of the weight of the equivalent c h a r g e qe; this was found experimentally for c y l i n d r i c a l c h a r g e s of various d i a m e t e r s (Fig. 2). By "equivalent c h a r g e " we mean a c h a r g e with unit charging density which, when exploded, will give a fragmented r e g i o n of the same dimensions as that from the blast with the gaps. When holes a r e bored in sections with bump h a z a r d s in c a t e g o r i e s I and II, as well as cavities we get zones of inelastic deformations with m a r k e d decompaction of the coal [1] ; the dimensions of the cavities and the d e g r e e s of decompaction i n c r e a s e with the s t r e s s e s . Under these conditions the dimensions of the zone of decompaction a r e much g r e a t e r than the radius of the fragmenting action of the c h a r g e (obtained for c h a r g e s 36 All-Union Scientific-l~esearch Institute of Mine Surveying, Leningrad. T r a n s l a t e d from F i z i k o - T e k h n i c h e s kie P r o b l e m y R a z r a b o t k i Poleznykh Iskopaemykh, No. 1, pp. 114-117, J a n u a r y - F e b r u a r y , 1977. Original article submitted July 3, 1975.
I
material is protected by copyright ,egi~ered In the name of Plenum Publi~.h. In, Corporation. 227 West 17th Stre.et, New. York: N..Y.,10011. No ~. rt [Of thi~ publication may be reproduced, xtored In a retrieval system, or tranomtted, In any form or by any. means, etec~omc,.mectu~ntT; I photocopying. I microfilming, recordIng or otherwise, without written permi.,Mon o f the publisher. A copy of this article is available lrom the pumzsner l'or ~ .so. j
94
a
601
"
2oI
! o
I
\!
b
12R
t
_i
r"
l'F-t=
,
f
2
f
3
2
3
o
;
50
Size of gap, (dbh -2rt)/r 0 Fig. 1
~
foo
/so 200
dbh, mm Fig. 2
Fig. 1. Graphs of radius of action of blast, a) Versus size of radial air gap; b) vs size of r a d i a l w~ter-filled g~ps. Fig. 2. Weight of equivalent c h a r g e vs sizes of air-filled and water-filled gaps. Charge d i a m e t e r s : 1) 36 mm; 2) 45 mm; 3) 60 m m ; 4) 80 m m ; 5) 100 ram. Solid c u r v e s , air gaps; dashed curves, water-filled gaps. mm in diameter). T h e r e is then a severalfold reduction (to 0.1-0.3 g/cm 3) explosion itself hardly i n c r e a s e s the size of the zone of decompaction.
in the
charging density, and the
To elucidate the influence of the magnitude of the s t r e s s e s and the type of s t r e s s state on the efficiency of camouflet blasting, we c a r r i e d out investigations in the b u m p - p r o n e lignite seam V of the Shu~abskii deposit. The n o r m a l s t r e s s e s were determined by means of the change in natural moisture content of the coal [2]. Charges were laid in the edge part of the seam up to the zone of maximum loads and directly at the abutment p r e s s u r e maximum, i.e., in the region of bulk c o m p r e s s i o n with unequal components, and also outside the zone of influence of the working, i.e., in the hydrostatic c o m p r e s s i o n region. In lignite s e a m s the borehole walls do not break up, and t h e r e f o r e it was possible to c a r r y out investigations in sections with different s t r e s s e s but with the same blasting conditions. We found that under hydrostatic p r e s s u r e , an i n c r e a s e in s t r e s s leads to a d e c r e a s e in the radius of fragmenting action of the charge, whereas under bulk c o m p r e s s i o n with unequal components it leads to an increase. In Fig. 3 as ordinate we have plotted the coefficient K e r e p r e s e n t i n g the ratio of the radius of the fragmenting action of the c h a r g e in the given blasting conditions to the radius in the u n s t r e s s e d rock. The following relation has been suggested for the effective distance between boreholes (C) in camouflet blasting: C = 0.8K..Ko,
(2)
where 0.8 m is the distance between boreholes in the load-relieved sections for the detonation of charges of ammonite P Z h V - 2 0 or T-19, 36 m m in diameter, with unit charging density; K R o is a coefficient representing the type of explosive, the diameter of the cylindrical charge, and the size of the air-filled or water-filled gaps in terms of the corrected radius of a spherical charge of T N T ; and K o is a coefficient representing the acting stresses (degree of b u m p hazard) and the type of stress state. To determine K R o from the experimental data, we plottedthe graph in Fig. 4. The radius of the spherical charge of T N T
is given by
Ro = 0.06~//~'vQ "qe,
(3)
where Q and QT a r e the specific heats of explosion for the given explosive and for TNT, r e s p e c t i v e l y , in kcal/kg. The optimum spacing between boreholes in lignite s e a m s with PZhV-20 (or T-19) c a r t r i d g e s 36 mm in d i a m e t e r ranges f r o m 0.7 to 1.5 m according to the blasting conditions. The distance between boreholes in nonlignitic coal s e a m s is 0.8 m for the same c h a r g e s . We found that the efficiency of camouflet blasting can be improved by increasing the radius of the cylindrical charge, as we c l e a r l y see from Fig. 2. It is possible to use c a r t r i d g e s 45-60 mm in d i a m e t e r , or a l t e r natively to c h a r g e the holes with placer explosives, or to use single charges in capron sheaths with pneumatic charging [3]. The distance between holes can be much l a r g e r when the charges are of large diameter:" Thus
95
f / 0
-50
~00
~'ks. mS ~0
/
o Z
Z O O 2~0
Fig. 3
4
6
8
fO
/~o.wc m
Fig. 4
Fig. 3. Graph of K a vs s t r e s s . 1) Under h y d r o static c o m p r e s s i o n ; 2) under bulk c o m p r e s s i o n with unequal components. Fig. 4. Graph of KRo vs c o r r e c t e d radius of s p h e r i c a l TNT c h a r g e . when the charges are i0 c m in diameter and the holes 11-20 c m in diameter, the distance between the latter can be 1.5-2.8 m . Since the use of these methods is at p r e s e n t difficult owing to the combined effects of new types of explosive and new c h a r g i n g techniques, in nonlignitic c o a l s e a m s with marked f r a c t u r e of the borehole walls the blasting efficiency can be increased by using hydroblasting. Experiments r e v e a l that in this c a s e the radius of action of c h a r g e s of ammonite PZhV-20, 36 m m in d i a m e t e r , can be improved by 30-80%. Hydroblasting is simple in descending blast holes; in ascending holes it can be effected with the aid of h y d r o s e a l s made of N I ~ P (Scientific-Research Institute of the Rubber Industry) expanding sleeves [4], which a r e c o m m e r c i a l l y produced for the c h e m i c a l industry. In hydroblasting, the g r e a t e s t efficiency can be attained when the ratio between the borehole d i a m e t e r and the d i a m e t e r of the c y l i n d r i c a l c h a r g e is 1.8 (1.6-2.2). This condition can be satisfied by using three standard sizes of sleeves (50, 70, and 95 mm in diameter), giving dbh/do = 1.6-2.2, in c h a r g e cavities ranging f r o m 80 to 210 mm in d i a m e t e r , i.e., in p r a c t i c a l l y all existing c a s e s . As a r e s u l t of blasting with optimum water-filled gaps, the radius of the f r a g m e n t i n g action of the blast can be 0.8 m for a c h a r g e d i a m e t e r of 50 ram, 1.1 m for a c h a r g e d i a m e t e r of 60 mm, and 1.7 m for a c h a r g e d i a m e t e r of 95 ram. Thus we have established a quantitative relation between the radius of the f r a g m e n t i n g action of a c a m o u flet blast and the principal f a c t o r s determined by the n a t u r a l and technological conditions. This has enabled us to develop a method of calculating the p a r a m e t e r s of camouflet blastIng (spacing between blast holes) for lignite s e a m s . These principles of calculation may in future also be applied for nonlignite coal s e a m s . Our investigations show the way to i n c r e a s e the efficiency of camoufiet blasting - by using hydroblasting, finding the optimum water-filled gap, utilizing the energy of r o c k p r e s s u r e , and using l a r g e - d i a m e t e r c h a r g e s , powerful explosives, and hydrostemming. LITERATURE 1.
2.
3.
4.
96
CITED
I. M. Petukhov, V. P. Kuznetsov, and V. S. Sidorov, "On the possibility of determining the abutment pressure on the face of a coal s e a m from the yield of drilling fines from a borehole," in: Investigation of Rock Pressure Manifestations in Deep Horizons in Pits [in Russian], All-Union Scientific-Research Institute of Mine Surveying, Leningrad (1971). Calculation and Experlmontal Estimation of Stresses in Pillars and Face of a Coal Seam: Methodological Instructions [in Russian], All-Union Scientiflc-Research Institute of Mine Surveying, Leningrad (1973). V. I. M a m a e v and D. I. Nenakhov, "Results of tests on n e w safety explosives in pits in the Karaganda coalfield," in: Improving Safety of Blasting in Coal Mines [in Russian], A. A. Skochinskii Mining Institute, M o s c o w (1973). Sh. G. Gamsakhydriya, "Preliminary weakening of the rock by means of hydroblasting," in: Improving Safety of Blasting in Coal Mines, A. A. Skochinskii Mining Institute, M o s c o w (1973).
