An Experiment in Determining the Strength of Coals at a Mine of the “Kaganovichugol” Trust

G. E. Bakanov and A. N. Postoenko.

During the performance of the present work (at the end of 1937), coal production in the “Kaganovichugol” trust was being carried out by mines 1–2, 3, 4–10, and 5.

At Mine 1–2, seams of three groups were being worked: II, III, and IV Inner seams (eastern wing of the III syncline—the first group); Kharakternyi, I–IV Inner, Gorelyi, Prokopyevskii, and Moshchnyi (anticline $L$—the second group), and seams I–III Inner (anticline $B$—the third group). At Mine No. 1, work was conducted at horizon 296 m, and at Mine No. 2—at horizon 286 m. Stoping was carried out only in seams of the second and third groups, except for the Kharakternyi and Moshchnyi seams.

At Mine No. 3, seams I–VIII Inner, Kharakternyi, Prokopyevskii, Gorelyi, Moshchnyi, and Bezymyannyi were being worked. Seams V–VIII Inner had only been opened up, and no work was being conducted in these seams. In the Kharakternyi, Bezymyannyi, and Moshchnyi seams (in the eastern part), only preparatory work was being carried out. In the western part, stoping operations were proceeding in the last-named seam.

At Mine 4–10 (adits No. 4 and No. 10), coal production was carried out from seams I–III Inner, Gorelyi, Prokopyevskii, and Moshchnyi. Through Mine No. 5 the following seams were being worked: Moshchnyi, Prokopyevskii, Gorelyi, Kharakternyi, Kharakternyi bis, and I–II Inner. All seams, except the Kharakternyi seams, were in the stage of stoping development.

Seams with a thickness of up to 3.5–4 m (predominantly the Inner seams, except Inner IV) at all mines were worked by the “long pillars” system¹, while thicker seams were worked by chambers.

At the mine, the coal of all seams, except the Moshchnyi seam, in terms of strength (in stoping operations) belonged to category 3.5 (according to the KBU classification); Moshchnyi—to category 2.5. Meanwhile, it was entirely obvious that even the coal of the same seams at different mines possessed different strength. As for different seams, here an enormous difference was observed—

...difference in the strength of coals, although they were assigned to the same category. Thus, for example, we established that the coal of the Moshchnyi seam at mine No. 3 has a strength index \(\gamma = 8.3\); at mine No. 4–10, \(\gamma = 6.4\); and at mine No. 5, \(\gamma = 1.8\). At the same time, the coal of the Bezymyannyi, II Vnutrennii bis, and IV Vnutrennii seams (mine No. 3), classified at the mine as category 3.5, had strength indices: for the first seam \(\gamma = 8.8\); for the second, \(\gamma = 0.9\); and for the third, 0.5. Thus, from the figures cited it is evident that, in the absence of sufficiently objective criteria for assessing the strength of coals, at the mine in certain cases stronger coals were assigned to weaker categories and were even sometimes placed in the same category with very weak coals. This circumstance served as the basis for undertaking the present work, in order to carry out a relatively more accurate classification of the seams of the Kiselevsk deposit according to the strength of their coals.

To determine the relative strength of coal, we used the method of blasting standard charges. For this purpose, in individual seams where the strength of the coal was being determined, boreholes were drilled, which in all cases were charged with 12% grizutin. The depth of the boreholes \((l)\), their direction, the weight \((q)\) and length of the charge \((l_1)\), and the quality and length of the stemming \((l_2)\) were strictly defined and constant for all the seams tested. After blasting such charges, the relative strength of the coal was established from the value \(F\)—the area of the base of the resulting crater. By comparing the dimensions of the bases of the craters obtained in testing different seams, it was possible to judge the degree of strength of the coal of a given seam. Indeed, in conducting the experiments, weaker coals showed a considerable increase in the area \(F\) as compared with stronger coals. For example, Moshchnyi (mine 3)—\(F = 0.120\ \text{m}^2\); IV Vnutrennii (the same mine)—\(2.041\ \text{m}^2\).

From the “theory of blasting” it is known that the weight of the charge

\[ q = c f(w), \]

where \(w\) is the line of least resistance or action (Fig. 1),
\(f\) is the characteristic of the function, and
\(c\) is a coefficient depending on the strength of the material being blasted, the quality of the explosive, the density of charging, the quality of stemming, and the position of the borehole. Suppose that the strength of the material being blasted is characterized by the strength index \(\gamma\), the quality of the explosive by the quantity \(\alpha\), the charging density by \(\beta\), the quality of stemming by \(\delta\), and the arrangement of the boreholes by \(\varepsilon\). Then, for constant values of the quantities \(q\), \(w\), \(\alpha\), \(\beta\), \(\delta\), and \(\varepsilon\), the effect of the explosion will depend only on the strength index of the material being blasted \((\gamma)\). Consequently, by blasting such standard charges in a medium of varying strength, we can establish the relative strength (hardness) of the material being blasted.

On the other hand, it is also known that, for constant values of the same quantities \((q, w, \alpha, \beta, \delta\) and \(\varepsilon)\), the throw index

\[ = n \frac{R_1}{W} \]

(\(R_1\) is the radius of the base of the crater—Fig. 1) is a function of the magnitude \(\gamma\), i.e., \(n = \psi(\gamma)\). But since the area of the base of the crater \(F\) (for \(w = \text{const.}\)) is determined by the value of \(n\), it follows that, other conditions being equal, the value \(F = \psi(\gamma)\), or \(\gamma = \psi_1(F)\). Thus, the size of the base of the crater obtained from the explosion of a standard charge \((q, w, \alpha, \beta, \delta\) and \(\varepsilon = \text{const.})\) completely determines the relative strength (hardness) of the blasted material (coal, ore, rock, building stones, etc.). Therefore, on the basis of the theoretical premises set forth above, we considered it possible to apply the present method for establishing the relative hardness of the coals of the Kiselevsk deposit. The results subsequently obtained on the basis of experimental work fully confirmed the possibility of applying this method for the indicated purposes.

Fig. 1.

The experimental work was carried out in 26 seams. In each seam two charges were blasted, after which careful measurement was made of the bases of the resulting throw craters. When there was a considerable discrepancy between the results of the two explosions, a repeated blasting was carried out. However, there were very few cases of repeated blasting, since in almost all seams sufficiently close results were obtained. For blasting, as indicated above, 12% grisoutine was used; the weight of the charges was taken as 270 g (2 cartridges of 135 g each). Boreholes of depth \(l = 110\) cm were drilled (in all cases) exactly along the strike of the seams. With such a borehole depth, the length of the charge \((l_1)\) was approximately

0.30–0.35 of the value \(l\). Before blasting, the boreholes were carefully filled with a dense stemming of clay, and then an external stemming of inert dust was made.

In seams worked by the “long pillars” system (with a thickness up to 3.5–4 m), the experimental boreholes were drilled in stoping faces. In thicker seams—in crosscuts, rarely in niches, and sometimes in the faces of drifts. The indicated preparatory workings had sufficient dimensions and did not hinder the formation of normal funnels. However, when possible, even in thick seams it is desirable to drill boreholes in stoping faces. When testing seams, the boreholes were arranged as follows. In seams with a thickness up to 4 m—in the middle of the seam (at a small distance one above the other); in thicker seams—one closer to the roof, and the other to the floor. In the latter case, the sometimes observed difference in the magnitudes of the areas \(F_1\) and \(F_2\) (the base areas of the funnels of one and the other borehole), as verification showed, occurred because of the unequal strength of the coal at the roof and floor of the seam. (Bezymyannyi—mine 3, Gorelyi—mines 1–2, etc.).

On the basis of the obtained values \(F_1\) and \(F_2\), \(F_{\mathrm{av.}}\) was determined as the arithmetic mean of the areas of both funnels, i.e., it was taken that \(F_{\mathrm{av.}} = \tfrac{1}{2}(F_1 + F_2)\). The relative strength of the coal of a given seam was characterized by the “strength index”

\[ \gamma = \frac{1}{F_{\mathrm{av.}}} = \frac{1}{\tfrac{1}{2}(F_1 + F_2)} . \]

In this way the strength indices were found for all seams on which experimental blasting was carried out. The test results for all tested seams and the strength indices calculated for them are summarized in Table 1.

From the table presented, it is evident that the coals of the Kiselevsk deposit differ extremely widely in their strength—from very strong (seams Bezymyannyi and Moshchnyi—mine 3) to very weak (seams II Vnutrennii bis and IV Vnutrennii—at the same mine). Meanwhile, at the mine, as was reported above, all seams except Moshchnyi, in terms of coal strength, were assigned to one category (3.5). This circumstance, naturally, had an adverse effect on the correctness of standardizing stoping operations in the trust.

Further, from the indicated table it can be seen that only a few seams of the Kiselevsk deposit have very strong or very weak coal. Thus, of 26 tested seams, only three seams have a strength index \(\gamma > 5\), and only two—less than 1; for the remaining ones (21 seams) the value of \(\gamma\) ranges from 3.3 to 1.2. At the same time, 16 seams from the latter group have a strength index even below 2.5; its average value for this group of seams is about 1.6.

Thus, the figures presented show that at this deposit, in general, there predominate

Table 1.

coals of medium strength. The latter circumstance is a sufficiently favorable factor for their successful extraction with the aid of jackhammers, coal-cutting machines, and combines.

The shapes and dimensions of the bases obtained in blasting the funnels are shown in Figs. 2—5, where the most characteristic bases for 15 seams are given. Since coal in different directions offers far from identical resistance to its separation from the general mass, in most cases the bases of the funnels have a somewhat elongated shape, close to that of an ellipse. The longest axes of the funnel bases were located in the direction of the least resistance of the coal to separation, i.e., normal to the direction of the most clearly expressed cleavage planes. In individual cases, in fractured coals, the funnels have a pyramidal shape with triangular and, rarely, polygonal bases (Fig. 2 c). This, in turn, made it possi-

allows, from the position of the long axis of the base of the funnel, determination of the direction of the more noticeable cracks and more clearly expressed cleavage. Thus, the method under consideration not only makes it possible to establish the relative degree of resistance of coal to breaking (its strength), but also the direction in which the coal is more easily separated. This is very important when working with jackhammers.

The adopted charge weight \((q = 270\ \text{g})\) and the borehole depth established from it \((l = w = 110\ \text{cm})\) were chosen with the calculation that, even in the thinnest seams at the mine (1.5–1.6 m) and in relatively weak coals, the bases of the ejection funnels would be completely contained within the seams (by thickness). In the practical use of such charges, very good results were obtained in all cases.

For the classification of all seams according to coal strength, we adopted the following:

a) very strong coal
with \(F \leq 0.20\ \text{m}^2\) and \(\gamma \geq 5.0\);

b) strong coal
with \(F > 0.20\) to \(0.40\ \text{m}^2\) and \(\gamma < 5.0\) to \(2.5\);

c) coal of medium strength
with \(F > 0.50\) to \(0.80\ \text{m}^2\) and \(\gamma < 2.5\) to \(1.25\);

d) weak coal
with \(F > 0.80\) to \(1.33\ \text{m}^2\) and \(\gamma < 1.25\) to \(0.75\); and

e) very weak coal
with \(F > 1.33\ \text{m}^2\) and \(\gamma < 0.75\).

Such a division is, of course, to a certain extent conventional, but it was adopted on the basis of studying the structure of the seams and the degree of resistance of the coals to breaking. In particular, the coal of the Moshchnyi seam (mines 3 and 4–10), assigned to the group of very strong coals, is distinguished (at these mines) by great hardness and toughness, by the absence of clearly expressed cleavage and noticeable cracks, and also by comparatively high resistance during breaking (the shape of the base of the ejection funnel is close to a circle—Fig. 2a); that of the IV Vnutrennii is strongly crushed and caves easily.

The relative correctness of the division of coals into categories that we have made is also confirmed by the following fact. The coal of the Gorelyi seam, tested under ordinary conditions, fell partly into the group of “strong” coals and partly into that of “medium-strength” coals. Meanwhile, at mine No. 3, where at that time extraction of near-entry pillars (“legs”) was being carried out in this seam, according to the testing results the coal of the Gorelyi seam belonged to the group of “weak” coals. This is quite understandable, since in the pillars the coal had been weakened more than in unworked seams. Therefore, taking into account the explanations given, we believe that the classification of the coals of the Kiselevsk deposit by strength that we have outlined fully corresponds to their physico-mechanical properties.

Fig. 2

Fig. 3.

Fig. 4.

Fig. 5.

The coal-strength indices obtained on the basis of experimental work, and the accepted division of them into groups, enabled us to construct a sufficiently simple classification of the seams worked by the mines of the Kaganovichugol Trust. In the present classification (Table 2), the seams are divided according to their thickness and the strength of the coal.

Table 2.

Classification of the seams of the Kiselevsk deposit by thickness and coal strength.

As was to be expected, in this classification (Table 2) the principal place is occupied by seams whose coal has medium strength. Only four seams entered the first, fourth, and fifth categories. Since the coals of the Prokopyevsk deposit, in their physicomechanical properties, are very close to the coals of the Kiselevsk deposit, it must be assumed that a similar picture will be observed there as well. At the same time, since the coals of the indicated deposits in the Kuzbass are considered the strongest, one may conclude that the largest part of the seams of the Kuznetsk Basin will belong to the third and fourth categories. This circumstance is a very favorable factor for the fairly broad application in Kuzbass of mechanized and hydraulic methods of winning.

The method we used for determining the relative strength of coals in the mines of the Kaganovichugol Trust, so far as we know, is being applied for the first time. Meanwhile, the practical results obtained by means of this method show its complete suitability for the stated purposes (regardless of the method of coal extraction). In view of the foregoing, we believe that the method considered here for determining the relative strength of coals can quite well be applied in all hard-coal basins of our Union. Using the present method, it is possible to compile a unified classification of coal seams by coal strength for all mines of the Soviet Union. This will greatly simplify the standardization of mining operations in the coal industry and will eliminate the arbitrary division of seams by coal strength that currently exists at many mines.

As for the magnitude of the charge, the type of explosive, and the length of the shot holes, they may be taken the same as those used in carrying out the present work, or other values may be established that are more suitable for the thinnest seams with very weak coals. In any case, this question can readily be resolved, since testing of seams can be carried out even with different explosives and different charges (with \(\frac{q}{w^3}\) and, as far as possible, \(\frac{l_1}{w}\) held constant). In the latter case, the actual coal-strength index can easily be found from the equations:

\[ \gamma = \frac{1}{F_0} \quad \text{and} \quad F_0 = \frac{\alpha W^2 F_{\mathrm{avg.}}}{\alpha_1 W_1^2}, \]
where

\(\alpha\) is the relative strength of the explosive composition adopted as the standard; \(W\) is the established line of least resistance used in the testing; \(W_1\) is that adopted for the given seam; and \(F_{\mathrm{avg.}}\) is the average area of the bases of the ejection craters formed.

The same method can also be applied to the classification of ores, but with a somewhat different relationship between the quantities \(q\) and \(w\).

1. In fact—by short pillars (G. B. and A. P.). ↩