2. Strengthening o v e r slip a r e a s T, TI2, T23, and T21 is r e l a t e d only to the m a x i m a l tangential s t r e s s acting on them. 3. The o n s e t of plasticity o n slip a r e a s T'lz and T'21 depends k i n e m a t i c a l l y on strengthening o v e r a r e a s T21 and T12, r e s p e c t i v e l y . 4. This m o d e l of plastic a n l s o t r o p y r e p r e s e n t s quite s a t i s f a c t o r i l y the c h a r a c t e r of p l a s t i c d e f o r m a t i o n under these loading conditions. LITERATURE 1~

2.

CITED

E. I. ~hemyakin, " A n t s o t r o p y of the plastic state, ~ in: Chislen. Metody Mekh. Sploshnoi S r e d y , 4 , No. 4, N o v o s i b i r s k (1973). S. A. Khristianovich, " D e f o r m a t i o n of p l a s t i c m a t e r i a l d u r i n g strengthening," Mekh. T v e r d . Tela, No. 2 (1974).

CONVERGENCE A.

A.

AT

THE

FACES

OF

DEVELOPMENT

WORKINGS

Borisenko

UDC 622.833

Since 1963 we have b e e n c a r r y i n g out investigations in pits of the P e c h o r a coalfield to e s t a b l i s h the g e n e r a l laws of r o o f - f l o o r c o n v e r g e n c e in the face a r e a s of d e v e l o p m e n t workings and t h e i r role in g a s b u r s t s . We also c o n s i d e r e d how v a r i o u s methods of working on the s e a m influence the amount and type of c o n v e r g e n c e . The c o n v e r g e n c e values w e r e d e t e r m i n e d b y mean.q of m e a s u r i n g p r o p s with pen r e c o r d e r s or i n d i c a t o r s ; when drilling and blasting w e r e in p r o g r e s s we used s p e c i a l l y designed m e a s u r e m e n t p r o p s . The p r o p s we re s e t between s u p p o r t f r a m e s 0.5 m o r m o r e f r o m the face, with duplicate p r o p s on the o t h e r side of the working. The o p e r a t i o n s at the face and the advance of the face Were r e c o r d e d . The i n v e s t i g a t i o n s w e r e m a d e outside the zone of influence of e x t r a c t i o n o p e r a t i o n s , in f i a t - l y i n g s e a m s with c o m p l e x s t r u c t u r e (except f o r the Chetvertyl s e a m ) , including m e m b e r s of b r o k e n coal of v a r i o u s t h i c k n e s s e s (see T a b l e 1). Within the deposit the country r o c k s v a r y in c o m p o s i t i o n and s t r u c t u r e ; however, thick s t r a t a of s i l t s t o n e s and sandstones, with s t r e n g t h s of up to 1500 k g f / c m 2 ag~|n~t unlaxial c o m p r e s s i o n , p r e d o m i n a t e [2]. The o b s e r v a t i o n s w e r e m a d e in 20 workings in five pits of Vorkutaugol' Group, cut by c u t t e r - l o a d e r s and b y drilling and blasting at depths between 350 and 600 m ; the c r o s s - s e c t i o n a l a r e a s of the worl~ngs ranged f r o m 3.7 to 12.0 ms. The m a i n d i f f e r e n c e between the convergence o f e x t r a c t i o n a n d development f a c e s is evidently that in development f a c e s the walls of the working e x e r t an influence on the magnitude and type of c o n v e r g e n c e . Our m e a s u r e m e n t s r e v e a l e d that the surrounding r o c k s undergo flexure; the m a x i m u m c o n v e r g e n c e o c c u r s in the middle of the working. F o r a working 3.0 m wide (an incline in Pyatyi seam), the c o n v e r g e n c e at the c e n t e r of the worl~ng was 2-4 t i m e s g r e a t e r than at the walls. We note that in the h o r i z o n t a l workings the c o n v e r g e n c e at the side to the dip of the s e a m was always 10-15% g r e a t e r tb~n on the side to the r i s e ; we also o b s e r v e d different types of c o n v e r g e n c e h e r e . On this account we will deal h e r e only with c o n v e r g e n c e m e a s u r e d at the l o w e r walls of the workings. The d i s t r i b u t i o n and ~mount of c o n v e r g e n c e also v a r i e d g r e a t l y along the working. F o r example, they could be c o n s i d e r e d at a g r e a t distance f r o m the face but totally absent n e a r it. T h i s can be attributed to local d e f o r m a t i o n s of individual b l o c k s of r o c k resulting f r o m d i s c h a r g e of s t r e s s e s in them. Apparently s t r e s s r e d i s t r i b u t i o n in the s u r r o u n d i n g r o c k s ~nd t h e i r sudden f r a c t u r e a r e the c a u s e s of sudden l a r g e c o n v e r g e n c e s (we will call t h e s e " c o n c u s s i v e " [ R u s s l a n " u d a r o o b r a z n y i " ] , even when t h e r e is no advance of the face). As we see f r o m Fig. 1, these c o n v e r g e n c e s extend to a c o n s i d e r a b l e distance. R o o f - f l o o r c o n v e r g e n c e was a c c o m panied by c r u m b l i n g of the face and enhanced gas e m i s s i o n .

P e c h o r a S c i e n t i f i c - R e s e a r c h pl~nnfng Institute (PechorNTIl>roekt), v o r k u t a . T r a n s l a t e d f r o m F i z i k o Tekhnfcheskie P r o b l e m y R a z r o b o t k i l ~ l e z n y k h Iskopaemykh, No. 4, pp. 24-29, July-August, 1977. Original a r t i c l e submitted O c t o b e r 29, 1974.

362

0038-5581/77/1304-0362507.50

9 1978 Plenum Publishing C o r p o r a t i o n

TABLE 1

Seam

~orrected Hazard of ;trength on dynamic Seam IthimCkness,~roto2i'phenomena r

Moshctmyi

3,6--4,0

0,75

Troinoi

2,3~2,5

0,6--0,65

Pyatyt Pervyi

0,9 1,2

0,6 0,62

Chetvertyi

1,5

0,8---1,0

Gas burst and shock bump hazard Gas bursthazard The same Threatened by gas bursts No hazard

One experimental difficulty in investigating convergence in development faces is the continual and nonuniform r e c e s s i o n of the f a c e f r o m the m e a s u r e m e n t point. To o v e r c o m e this difficulty, in workings in the Moshchnyi seam, as the face advanced we set up new m e a s u r e m e n t props (the ' s l i d i n g prop" method) and s u m m e d the convergence for each distance f r o m the face. However, this method of m e a s u r e m e n t r e q u i r e s the assumption that the p r o p e r t i e s of the r o c k r e m a i n constant over limited sections. Figure 2 is a plot of the r e s u l t s of a 3 - d a y cycle of observations by the "sliding prop" method. Throughout this time the convergence c u r v e s varied continuously (concussive convergence was r e g i s t e r e d four times) and in some c a s e s there is an e x t r e m u m point such that the convergence d e c r e a s e s as we r e c e d e f r o m it toward the face. E x t r e m u m points were not observed when the prop had its initial position near the face (0.5 m), and in such c a s e s this point obviously lles within f r e s h l y exposed p a r t s of the roof. The difference b e tween the shapes of the c u r v e s in Fig. 2 can be provisionally explained as due to changes in the face advance rate - this was 3.0 m in the f i r s t day, 1.5 m in the second, and 2.6 m in the third (a c u t t e r - l o a d e r was used). The bend in the curve was observed with the lowest r a t e of advance. During the period of observations there was no change in the P r o t o d ' y a k o n o v hardness of the s e a m in any of the m e m b e r s . As we recede f r o m the development face the total convergence i n c r e a s e s , following an S-shaped curve. The maximum convergence is usually found 1.5-3.0 m from the face. On the whole, the c h a r a c t e r of the total convergence in a development face is the same as in an extraction face. Observation of convergence is e a s i e s t during drilling and blasting, when c o n v e r g e n c e due to advance of the face is fairly c l e a r l y differentiated f r o m that due to rheological p r o c e s s e s . The convergence graphs (Fig. 3) have three p a r t s . The p r o c e s s e s of blasting and face advance are always accompanied by marked, p r a c tically instantaneous r o o f - f l o o r convergence. The absolute values of this c o n v e r g e n c e a r e different in diff e r e n t s e a m s ; 1-2 m from the face they v a r y f r o m 1 m m (Pervyi seam) to 20 m m (Moshchnyi seam) for app r o x i m a t e l y equal r a t e s of advance of 1.4-1.6 m / d a y . After blasting, the convergence gradually dies out over 2-3 h. Until the next b l a s t it is then p r a c t i c a l l y linear, and in many c a s e s slight or absent. In the total c o n vergence per working cycle, the fraction due to rheological p r o c e s s e s was at m o s t 25% for the Moshchnyi s e a m and up to 80% for the Chetvertyi seam. To elucidate the influence of various natural and technical f a c t o r s on the extent and c h a r a c t e r of convergence, we analyzed 311 dally convergence graphs in 19 work|ngs. In working with c u t t e r - l o a d e r s , three main types of daily convergence g r a p h s w e r e distinguished. The f i r s t and c o m m o n e s t type is undulatory, s i m i l a r to the convergence graphs in e x t r a c t i o n faces with u a r r o w web extraction in s e v e r a l c y c l e s per day. The convergence waves are always confined to face advance p e r iods. In 18% of the c a s e s the daily convergence was p r a c t i c a l l y linear (second type). Here the p r o c e s s of face advance is not reflected in the graphs, even if the props a r e close to the face (0.7 m) and the daily c o n v e r g e n c e is f a i r l y m a r k e d (up to 10 ram). The third c h a r a c t e r i s t i c type of c o n v e r g e n c e is concussive, with a stepwise variety (22% of the cases); the c h a r a c t e r i s t i c feature of this type is the p r e s e n c e of one or m o r e m a r k e d concussive convergence events on the graphs. It m e r i t s detailed discussion, b e c a u s e it may be dir e c t l y related to the nature of dynamic phenomena. We have agreed to use the t e r m "concussive" for those sudden r o o f - f l o o r c o n v e r g e n c e s which have durations less than the resolving capacity of the devices used to m e a s u r e them, i.e., we have reckoned that they a r e p r a c t i c a l l y instantaneous. Concussive convergences are up to 28 m m (Moshchnyi seam, 1.5 m from face) and s o m e t i m e s determine the daily convergence. As a rule such c o n v e r g e n c e s are accompanied by

363

a

r t J

24

~20 2

Eof6

J 8 4

N 0

4

4

ff

~2

b

=" ~

16

/l -1

0

4

8

12

f8

TLme. h

I 20

24

Time. h

Fig. 1. Graphs of r o o f - f l o o r c o n v e r g e n c e (a) and face advance (b) during cutting of roadway in Moshchnyt s e a m by c u t t e r loader. F i g u r e s on g r a p h s in (a) denote initial distance of m e a s u r e m e n t prop f r o m face in m e t e r s . sounds, visible f r a c t u r e s of the edges of the seam, and enhanced methane emission. sive convergence have been r e c o r d e d .

T h r e e types of c o n c u s -

1. Sudden m a r k e d convergence after a long gap o r delay, s i m i l a r in c h a r a c t e r to convergence during blasting. 2.

"Stopwise, convergence, constituting a s e r i e s of small concussive c o n v e r g e n c e s over a long time.

3. Concussive convergence growing out of smooth convergence by an i n c r e a s e in its rate. Concussive c o n v e r g e n c e is usually confined to face advance periods; however, in some c a s e s it m a y also o c c u r in the absence of extraction operations on the s e a m (see Fig. 1). No s t a t i s t i c a l c o r r e l a t i o n can be seen between the concussive convergences and the p r o p e r t i e s of the s e a m o r the c h a r a c t e r i s t i c s of the w o r k ings; however, their magnitudes n e a r l y always d e c r e a s e as the face r e c e d e s f r o m the m e a s u r e m e n t point. In all c a s e s concussive convergence a l t e r s the configuration of the surrounding rocks. It is usually m o s t m a r k e d after long delays and after large advances of the face. Such convergence is not confined only to development faces. In longwall faces in the b u r s t - p r o n e M o s h chnyl and Dvoinoi s e a m s , concussive convergences a r e o c c a s i o n a l l y (in about 1% of the cases) observed, reaching 10 and 5 m m respectively, 1.6-2.0 m from the face. But a distinguishing feature of development faces is delay of convergence (it is a s s u m e d that t h e r e is no convergence if it is not r e c o r d e d by the i n s t r u m e n t during the advance of the face). Complete absence of convergence during a whole day has been observed in 4% of the c a s e s (not including nonworking days) as the face receded 0.5-3.0 m f r o m the m e a s u r e m e n t point, and in 20% of the c a s e s at g r e a t e r distances. C o n v e r gence was absent over a whole day when the face advanced 5.1 m in the Moshchnyi s e a m and 3.0 m in the Troinoi s e a m . B r i e f delays in convergence were observed m o r e often during face advance, in all c a s e s the delays ended in large concussive convergences. In c o n t r a s t with c u t t e r - l o a d e r operations, during cutting by drilling and blasting no delays of convergence were o b s e r v e d during face advance. With this method of cutting there were also no concussive convergences, or if they did occur, they evidently coincided in time with c o n vergence due to blasting. Evidence for this possibility is provided by the following facts. During cutting of a conveyer road in the P e r v y i seam, the m e a s u r e m e n t prop was always set 1.5 m f r o m the face; during one cycle of advance of 1.5 m, the convergence ranged f r o m 1 to 3 ram. In a worktnz in the Moshchnyi seam, the convergence per advance cycle ranged from 1.5 to 10:0 m m at 2.0 m f r o m the face.

364

0

Distance from face, m 2 J

4

5

0

L) JO

4O

Fig. 2. Graphs of daily convergence, m e a sured in c o n v e y e r road in Moshchnyi s e a m after 1, 2, and 3 days of observations (numb e r s on curves). During nonworktng days, the convergence was linear in all the faces and did not exceed 1-2 mm. The aggregated data on daily convergence values was analyzed b y the multiple c o r r e l a t i o n method with the aid of a computer. The aim of the ~n~lysis was to elucidate the influence of six f a c t o r s on the daily convergence values: the depth below the surface, the c o r r e c t e d seam strength, the c r o s s - s e c t i o n a l a r e a of the working, the initial distance f r o m the face to the m e a s u r e m e n t prop, the daily advance, a n d the thickness of the seam. The combined c o r r e l a t i o n coefficient was r a t h e r low - 0.49 with a r e l i a b i l i t y of 9.13. The g r e a t e s t influence on the convergence values is exerted by the c r o s s - s e c t i o n a l a r e a and by the distance f r o m the face (the partial c o r r e l a t i o n coefficients being 0.281 and 0.310, respectively), and l e s s e r influences are exerted by the depth below the s u r f a c e and by the c o r r e c t e d strength of the s e a m (partial c o r r e l a t i o n coefficients 0.164 and 0.178); the influences of s e a m thickness and daily face advance a r e v e r y slight. The conclusion that the daily face advance has hardly any influence on the daily convergence was r a t h e r unexpected; however, it is confirmed by analysis both of the aggregated data and of data on individual workings. But the influence of the face advance rate is m o s t probably obscured by the effects of other m o r e influential factors. In the individual s e a m s the influence of the c r o s s - s e c t i o n a l a r e a on the c o n v e r g e n c e values can be quite c l e a r l y traced, especially in workings in the Moshchnyi s e a m . For m e a s u r e m e n t props starting 1-2 m f r o m th~ face, with a daily advance of 3-4 m, in workings with c r o s s sections of 3.7, 7.0-7.3, and 12.0 m 2 the mean daily convergence values were r e s p e c t i v e l y 4.0, 11.0, and 17.8 ram. The multiple c o r r e l a t i o n r e s u l t s indicate that a v e r y g r e a t influence is exerted by d i s r e g a r d e d factors, among which the m o s t important are undoubtedly the p r o p e r t i e s of the surrounding r o c k s . On the basis of our investigations, the m e c h a n i s m of r o o f - f l o o r convergence in the faces of development workings can be r e p r e s e n t e d as follows. As the face advances, s t r e s s e s a r e concentrated in the s u r rounding r o c k s ; these s t r e s s e s r e s u l t in the deformation of the rocks toward the working, and are observed in the f o r m of r o o f - f l o o r convergence. However, if the advance of the working is slight, and if the s u r r o u n d ing rocks are not liable to plastic deformations and are f a i r l y strong, then the d e f o r m a t i o n of the rocks during each individual advance m a y be m e r e l y elastic and will be slight. Near the face of the working this is manifested as a t e m p o r a r y absence or delay of convergence. F u r t h e r face advance leads to an increase in the s t r e s s in the surrounding r o c k s until they b r e a k over the planes of weakening (joints) and move in blocks. In the working this appears in the f o r m of large sudden or concussive c o n v e r g e n c e s . The plane of f r a c t u r e may be ahead of the face o r in the face a r e a . In the f o r m e r case, the deformations of the r o c k s exert a dynamic action on the face zone of the s e a m ; In the latter case, they appear in the f o r m of local r o o f - f l o o r c o n v e r gences. F r o m this viewpoint we can easily explain concussive convergence during stoppages of the face. These result from sudden f r a c t u r e of r o c k s which are o v e r s t r e s s e d and are in a state of unstable equilibrium a f t e r they have been weakened by c r e e p deformations. Occasionally we observed "divergent" deformations between the floor and roof; these are small but quite observable, and can be attributed to random movements of individual rock blocks. Evidence that it is the p r o p e r t i e s of the surrounding rocks which determine the convergence is afforded

365

Time. h

E o

2

4

5

Fig. 3. Graphs of r o o f - f l o o r convergence per shift during cutting of workln~s by drilling and blasting. 1) Chetvertyi s e a m ; 2) Moshchnyi seanl.

by the influence of the c r o s s - s e c t i o n a l a r e a of the working on the nature of the c o n v e r g e n c e . On the other hand, although the strength of the seams, a c c o r d i n g to c o r r e l a t i o n analysis, has an o v e r a l l influence on the convergence values, this is not evident in some p a r t i c u l a r c a s e s . In a l m o s t all the workings the c o n v e r g e n c e m e a s u r e m e n t s were accompanied by determInations of the s t r e n g t h of the s e a m s by the method of M. M. Protod'yakonov or by means of a P-1 strength m e t e r ; in s e a m s of constant strength t h e r e was much nonunlfortuity In the convergence and concussive convergence. In p a r t i c u l a r l y weak parts of the s e a m s (In zones with thick fissured m e m b e r s ) there was no o b s e r v e d a n o m a l y in the convergence values. The role of the p r o p e r t i e s of the surrounding rocks is also confirmed by the fact that the convergence values have p r a c t i c a l l y no c o r r e l a t i o n with the s e a m thickness, although the latter r a n g e s over wide limits. The dally convergence is largely governed by the c o n c u s s i v e c o n v e r g e n c e s . Thus in the s o u t h e r n haulage road in pit 1~o. 18 in the Moshchnyi seam with the mean Initial prop position 1.5 m f r o m the face and an a d vance of 3-4 m / d a y , the mean dally convergence during days in which t h e r e was no c o n c u s s i v e c o n v e r g e n c e was 7.4 ram; but In the p r e s e n c e of concussive c o n v e r g e n c e it was 17.8 ram. The m a r k e d nonuniformtty of the r a t e s of convergence during advance of development f a c e s is evidently due to the o c c u r r e n c e of delays In convergence and concussive convergence. On c o m p a r i s o n the convergence values a r e found to be s m a l l e r in development f a c e s than in e x t r a c t i o n faces, but they a r e far f r o m negligible. No dynamic phenomena were observed In the Chetvertyi and P e r v y i s e a m s ; the c o n v e r g e n c e In these s e a m s differed somewhat f r o m that in s e a m s liable to dynamic phenomena. The workings in the Chetvertyi and P e r v y i s e a m s w e r e cut by drilling and blasting. On c o m p a r i n g the r e s u l t s of o b s e r v a t i o n s of c o n v e r g e n c e in workings in these s e a m s with the results o b ~ i n e d In the h a z a r d o u s Moshchnyi s e a m cut by drilling and blasting, we find that the convergence per cycle of advance in the Chetvertyi and P e r v y i s e a m s is l e s s than that for the Moshchnyi seam, the respective figures being 3.9 m m , 1.2 mm, and 7.5 mm on a v e r a g e . However, in the nonhazardous s e a m s there is a c h a r a c t e r i s t i c large convergence not a s s o c i a t e d d i r e c t l y with advance of the face. Whereas in worlrtngs in the Moshchnyl s e a m these c o n v e r g e n c e s constituted an o v e r a l l a v e r a g e of 15% of the dally value, in workings in the P e r v y i s e a m the c o r r e s p o n d i n g figure was 57%, and in workings In the Chetvertyi seam, 70~. Convergence In these s e a m s o c c u r r e d o v e r a long time f r o m the m o m e n t of blasting; in workings in the C h e r t v e r t y t s e a m the r a t e of convergence was 0.1 m m / h for two days f r o m the m o m e n t of blasting. Unfortunately it proved impossible to c o m p a r e the c o n v e r g e n c e s of h a z a r d o u s and safe s e a m s with c u t t e r - l o a d e r working. In a number of c a s e s we succeeded in m e a s u r i n g the c o n v e r g e n c e during gas b u r s t s [2]. All the b u r s t s o c c u r r e d after convergence delays of various durations; the nature of gas b u r s t c o n v e r g e n c e was s i m i l a r to that during blasting (see Fig. 3). In e v e r y case the extent of the convergence during a gas b u r s t was not g r e a t e r than the o r d i n ~ concussive convergence. On 1he whole, In workings where b u r s t s o c c u r r e d the c o n vergence had the same extent and nature as that in workings in the same s e a m in which no b u r s t s o c c u r r e d . Our investigations have thus led us to the following conclusions. 1. R o o f - f l o o r convergence in development working f a c e s m a y be f a i r l y extensive and m u s t be taken into account in d i s c u s s i o n s of the dynamtcs of the state of s t r e s s of the face zone. 2. The convergence In a development face does not depend appreciably on the thickness of the seam, but is governed by the depth below the surface, the width of the working, and the p r o p e r t i e s of the surrounding rocks.

366

3. Faces in development workings with strong surrounding rocks have nonuniform convergence, which ranges from nearly zero to rapid extensive concussive convergence. 4. The nonuniformity of the convergence in development workings is due to periodic sticking and breaking of the country rock strata, and is governed by the variability in the properties of the rocks and by the position of the face in each case relative to the planes of weakening of the rocks. LITERATURE 1. 2.

M . F . Kuntysh, O. M. Chumachenko, and G. S. Senatskaya, "Combined investigations of the physicomechanical p r o p e r t i e s of rocks in the Vorlmta deposit," Tekhnol. Ekon. Ugledobychi, No. 5 (1966). A . A . Borisenko, "Rock p r e s s u r e manifestations in b u r s t - p r o n e s e a m s , " Ugol' Ukrainy, No. 4 f1969).

RESULTS OF

CITED

O F AN I N V E S T I G A T I O N

STRESS

(BY T H E

OF THE

ELECTROANALOG

N. P. Erofeev, I. V. Mtleteako, and

ROCKS

V.

V.

OF THE

AROUND

MINE

STATE WORKINGS

METHOD)

B. K. Balykbaev, T. M. Mukhamediyarov,

UDC 622.28.1

Kosov

Electroanalog modeling is being increasingly used to solve problems in r o c k m e c h a n i c s . Let us examine a p a r t i c u l a r case of d e t e r m i n a t i o n of the strength of a field, E, at the i n t e r f a c e of two media with different conductivities ~/1 and ~2 (a single c i r c u l a r working). The apparatus consists of a flat bath filled with e l e c t r o l y t e I, d i e l e c t r i c II (circular working), b u s b a r s A and B, and a double probe N, attached to a p o t e n t i o m e t e r P (Fig. 1). The p r o c e d u r e f o r calculating the e l e c t r i c field is s i m i l a r to that for determining s t r e s s e s in the plane t h e o r y of elasticity. F r o m the given strength e0, for a uniform external field we calculate the field modified by the p r e s e n c e of a c i r c u l a r plate. The c o m p a r i s o n r e v e a l e d that the e x p e r i m e n t a l values a r e close to those calculated according to Lekhnitskii [1]. Solution of the p r o b l e m s for a multiply connected region r e q u i r e s f u r t h e r substantiation. We may r e f e r to a paper by Ugodchikov [2], who used e l e c t r i c modeling for conformal r e p r e s e n t a t i o n of a c i r c u l a r ring onto a given doubly connected region. We have p e r f o r m e d e x p e r i m e n t s with r e g a r d to the conditions at the Dzhezkazgan deposit, which is worked by s e v e r a l different v e r s i o n s of the r o o m and pillar s y s t e m . The r e s u l t s w e r e c o m p a r e d wtth the data of observations and analytical solutions. The plane e l e c t r o l y s i s bath (Fig. 2) consisted of an audiofrequency g e n e r a t o r 1, a p o t e n t i o m e t e r 6, a A !

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Fig. 1. Scheme f o r calculating an e l e c t r i c field at the i n t e r f a c e of two m e d i a .

Institute of Mining, Academy of Sciences of the Kazakh SSR, Alma-Ata. T r a n s l a t e d f r o m FizikoTekhnicheskie l>roblemy Razrabotki Poleznykh Iskopaemykh, No. 4, pp. 30-35, July-August, 1977. Original a r t i c l e submitted July 3, 1975.

0038-5581/77/1304-0367507.50 9 1978 Plenum Publishing Corporation

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