ON THE CHOICE OF THE ORDER AND SEQUENCE OF MINING CLOSELY SPACED STEEPLY DIPPING SEAMS

V. V. PROSKURIN

(Presented by Professor D. A. Strelnikov)

1. The concept of closely spaced seams

Productive deposits in the subsurface are in a natural stressed state of equilibrium. The extraction of coal, associated with the formation of voids, disturbs the natural state of equilibrium and leads to a redistribution of stresses in the surrounding rocks. As the volume of voids increases, the stresses in the rocks increase. When the stresses exceed the stability limit of the side rocks, the latter begin to move. The process of displacement of the side rocks may encompass a considerable thickness of the productive deposits and cause their deformation. The character of the deformations of the productive strata determines the choice of the order and sequence of mining the individual seams composing the series.

From the standpoint of the influence of the extraction of one seam of a series on the extraction of neighboring seams, independent seams and closely spaced seams are distinguished.

Independent seams in a series are those whose extraction is possible in any order and sequence.

If the extraction of one seam of a series complicates the extraction of adjacent seams, and for their proper mining it is necessary to observe a definite order, sequence, and methods of controlling the enclosing rocks, then such seams are called closely spaced.

Fig. 1. Calculation scheme for determining the closeness of seams.

The assignment of individual groups of seams of a series to the category of closely spaced seams depends on the joint consideration of the following principal factors: the distance between the seams, the angles of their dip, the angles of displacement, and the adopted vertical height of a level.

From the calculation scheme (Fig. 1) it is evident that, in level-by-level mining, seams are considered closely spaced if

\[ a < \frac{h}{K}; \tag{1} \]

the seams are independent if

\[ a \geq \frac{h}{K}, \tag{2} \]

where
\(a\) is the normal distance between the seams, m;
\(h\) is the adopted vertical height of a level, m;
\(K\) is a coefficient depending on the dip angle of the seam and the angle of displacement

\[ K=\frac{\sin \beta}{\sin(\alpha+\beta)}, \tag{3} \]

where
\(\alpha\) is the dip angle of the seam, degrees;
\(\beta\) is the angle of displacement, degrees.

2. Sphere of Influence of the Worked-Out Space

The sphere of influence of the worked-out space is determined by the angles of displacement \(\beta\), \(\gamma\), or \(\beta_1\) and \(\delta\), where \(\beta\) is the angle of displacement across the strike, constructed from the lower boundary of the stoping operations on the dip side of the seam; \(\gamma\) is the angle of displacement across the strike, constructed from the upper boundary of the stoping operations on the rise side of the seam; \(\beta_1\) is the angle of displacement of the rocks of the footwall in steeply dipping seams; and \(\delta\) is the angle of displacement along the strike of the seam [12].

Depending on the magnitude of movements and deformations of the earth’s surface, displacement angles are distinguished as follows: limiting angles, displacement angles proper, and angles of caving. The limiting angles define the boundaries of the trough with practically zero movement of points. The displacement angles proper define the boundaries of the trough with movement of points that is the maximum permissible for the preservation of structures. The angles of caving are the angles referred to the outermost cracks.

The magnitudes of displacement angles in the country rocks depend on the physico-mechanical properties of the enclosing rocks, the dip angle of the seam, the methods of controlling the surrounding rocks, repeated undermining, and natural fracturing.

Many years of instrumental observations have established that, with an increase in the strength of the enclosing rocks, the angles \(\beta\), \(\gamma\), \(\beta_1\), and \(\delta\) increase; with an increase in the dip angle of the seam, the angles \(\beta\) decrease; when working with complete and dense stowing of the worked-out space, the angles \(\beta\) and \(\beta_1\) increase; repeated undermining is associated with a decrease in the angles \(\beta\), \(\gamma\), \(\beta_1\), and \(\delta\) (Fig. 2); and, finally, the influence of natural fracturing on the values of the angles \(\beta\), \(\gamma\), and \(\delta\) depends on the position of cracks or folds relative to the displacement zone. Since the orientation of natural discontinuities relative to the displacement zone may be extremely varied, the problem of determining the angles \(\beta\), \(\gamma\), \(\beta_1\), and \(\delta\) assumes a particular solution.

The magnitudes of displacement angles for individual basins are established by the “Rules for the Protection of Structures from the Harmful Influence of Underground Workings” [6, 7, 8, 12].

The angles of displacement \(\beta\), \(\gamma\), \(\beta_1\), and \(\delta\) determine the sphere of influence of the worked-out space, i.e., the region of altered stress state in the rocks surrounding it. According to the character of rock deformations in the sphere of influence of the worked-out space, three zones are distinguished: caving, crack formation, and bending. The zones of caving and crack formation are commonly called the zone of destructive influence of the worked-out space. The magnitude of the zone of destructive influence to some extent predetermines the choice of the order of mining closely spaced seams and depends mainly on the thickness of the seam, its dip angle, and the physico-mechani-

…of the physical properties of the rocks surrounding the mined-out space and of the methods for controlling the enclosing rocks.

Prof. V. D. Slesarev [13], proceeding from the idea that rocks, caving into the mined-out space, increase in volume and, forming a cushion—support, ultimately cease caving, recommends determining, for gently dipping seams, the maximum possible height of the caving zone by the formula

\[ h_0=\frac{m_0}{K_0-1}, \tag{4} \]

where
\(h_0\) — magnitude of the caving zone, m;
\(m_0\) — extracted thickness of the seam, m;
\(K_0\) — coefficient of increase in the volume of rocks during caving.

Fig. 2. Influence of undermining, angle \(\beta_{\mathrm{II}} < \beta_{\mathrm{I}}\).

Actual measurements of the magnitude of the caving zone, although very few have been made, showed, for example, the following values for the Prokopyevsk district of the Kuznetsk Basin [12]. At the Voroshilov mine, seam IV Internal was undermined by crosscut No. 106. Seam IV Internal had a thickness of 8 m, a dip angle of 60°, and was worked using the shield mining system. The caving zone observed from crosscut No. 106 at the ventilation level \(+220\) m showed that only the immediate roof, 2.5 m thick, had caved. In addition, in two cases the caving zone was encountered by workings (Table 1).

Table 1

The Prokopyevsk special office for extinguishing underground fires examined 800 boreholes drilled from the surface to the mined-out space of thick steeply dipping seams and obtained the following results (Table 2).

The greatest height of the caving zone detected by the boreholes proved to be 30 m. Therefore, for the Prokopyevsk district it is accepted that the height of the caving zone is equal to five times the extracted thickness of the seam [12].

Table 2

The limiting magnitude of the zone of destructive influence \(h_p\) is recommended [13] to be taken equal to \(1.75 h_0\), where \(h_0\) is the caving zone, m.

In the mining of thick steeply dipping seams, the zone of destructive influence assumes an asymmetric form, and its maximum height should be located at the upper boundary of the worked-out space.

The height of formation of the caving zone is substantially influenced by the physicomechanical properties of the side rocks and by the methods of controlling the enclosing rocks. When work is carried out with stowing of the worked-out space, an artificial cushion—support—is created, and therefore the reduced seam thickness should be used in the calculation. The reduced seam thickness depends on the type of stowing, the completeness of filling of the worked-out space, and the settlement of the stowing material [15].

The reduced seam thickness may be determined by the formula

\[ m_{pr}=m_0(1-\gamma_y), \tag{5} \]

where \(m_{pr}\) is the reduced thickness of the seam being mined with stowing, m;
\(m_0\) is the extracted seam thickness, m;
\(\gamma_y\) is the ratio of the height of the worked-out space after settlement of the stowing material to the extracted seam thickness [15].

If the undermined seam does not fall within the caving zone, and still less within the zone of destructive influence of the worked-out space, then, in order to establish the sequence of mining closely spaced seams, it is necessary to determine the duration of the process of movements and of its separate stages in the sphere of influence of the worked-out space.

3. Character of deformation of the interseam rocks

Under the influence of stoping extraction of a seam, the interseam rocks undergo plastic bending, fracturing, and caving.

The character of deformation of the interseam rocks depends on many causes, among which the most important are: the thickness of the seam being mined, its dip angle, the physicomechanical properties of the interseam rocks, and the method of controlling the enclosing rocks.

Fig. 3. Elastic bending of roof rocks.

If the seam thickness \(m\) is not large (Fig. 3) and is equal to or less than the maximum possible deflection of the roof rocks \(f\), then during extraction of such a seam there occurs a smooth lowering of the interseam rocks without rupture of continuity, i.e., elastic bending takes place. If \(m>f\), then, other conditions being equal, the greater the ratio of the extracted seam thickness to the amount of roof-rock deflection,

\[ \frac{m}{f}>1, \]

the more vigorously the process of fissuring and collapse of the interseam rocks will proceed.

The dip angle of the seam influences the formation of the fissuring zone and the caving zone. If, when a horizontal or gently dipping seam is extracted, the pieces of rock during caving remain in place, then when a steeply dipping seam is extracted they roll downward under their own weight, forming a stowing of the lower part of the worked-out space. Consequently, the shape of the caving zone during extraction of steeply dipping seams will be asymmetrical with respect to the dip line of the seam (Fig. 4); the dimensions of the caving zone at the rise-side boundary will be considerably greater than at the lower, dip-side boundary. The deformation of the interseam rocks within a single production face on steeply dipping seams may vary from plastic sagging at the lower boundary to caving at the upper boundary. The dip angle of the seam may exert a similar influence on the shape of the fissuring zone. In addition, the displacement process during extraction of steeply dipping seams is characterized mainly by a predominance of shear deformations, whereas under conditions of gentle dip it is characterized by bending deformations.

Fig. 4. Character of deformation of the enclosing rocks during extraction of thick steeply dipping seams.

The presence of plastic rocks (argillaceous shales) in the interseam strata promotes the uniform development of the displacement process in the form of plastic sagging. Conversely, the presence of hard rocks (sandstones, limestones) in the immediate roof promotes a stepwise development of the process in the form of caving. If sandstones or limestones occur in the middle or upper part of the interseam interval, then the process of fissuring and caving proceeds more slowly. A fairly frequent alternation in the interseam strata of layers of hard and plastic rocks favors the smooth development of rock bending, and on steep dips often leads to the development of interstratal displacements. The displacement process of loose and quicksand rocks proceeds in the form of flow of the rock masses.

Dense and timely filling of the worked-out space substantially mitigates the process of displacement of the interseam rocks and more often limits their deformation to plastic sagging or fissuring.

4. On the duration of the process of displacement of interseam rocks

The duration of the displacement process in the subsurface has been little studied. The duration of the displacement process of the earth’s surface has been comparatively better studied and depends mainly on the depth of mining, the physico-mechanical properties of the rocks, and the extracted seam thickness. In Kuzbass the process of displacement of rock masses is conventionally accepted as ...

divide it into three stages: the initial stage, when subsidence velocities increase from 0 to 30 mm per month; the active stage, when subsidence velocities exceed 30 mm per month; and the attenuation stage, when subsidence velocities are less than 30 mm per month. In the Donets and Chelyabinsk basins it is customary to divide the displacement process into two stages: the active stage, when subsidence velocities exceed 50 mm per month, and the attenuation stage, when subsidence velocities are less than 50 mm per month. Displacement velocities of less than 50 mm per year are considered not dangerous for structures on the surface [8].

There are as yet no uniform criteria for establishing the duration of the individual stages of displacement. It is recommended [3] that the following be adopted as the criterion for the duration of the individual stages of displacement: for the active stage—the period during which the subsidence of a point amounts to 90% of its full value; for the initial stage—the period during which the subsidence of a point amounts to up to 50–150 mm [12]; for the attenuation stage—the remainder.

The duration of the initial stage of displacement in the Kuzbass has been observed to range from 0.5 to 3 months; the duration of the active stage—from 6 to 12 months [12].

Subsidence velocities depend on the depths of the workings, the physico-mechanical properties of the rocks, the size of the working, the extracted thickness of the seam, its dip angle, the rate of advance of the extraction face, the method of controlling the enclosing rocks, and the completeness of coal extraction.

Prof. D. A. Kazakovskii [3] recommends, as a first approximation, determining the maximum subsidence velocity by the formula:

\[ V_2 = V_1 \frac{H_1 v_2 \eta_{0(2)} \cos \alpha_2}{H_2 v_1 \eta_{0(1)} \cos \alpha_1} \tag{6} \]

or, when \(\eta_0\) is unknown,

\[ V_2 = V_1 \frac{H_1 v_2 m_2 \cos \alpha_2}{H_2 v_1 m_1 \cos \alpha_1}, \tag{7} \]

where \(V_2\) is the sought maximum subsidence velocity at mining depth \(H_2\), rate of advance of the extraction face \(v_2\), magnitude of the greatest surface subsidence \(\eta_{0(2)}\), extracted seam thickness \(m_2\), and seam dip angle \(\alpha_2\); \(V_1\) is the known (from observation data) maximum subsidence velocity for \(H_1\), \(v_1\), \(\eta_{0(1)}\), \(m_1\), and \(\alpha_1\).

For Kuzbass conditions it is recommended that the value \(\eta_0\) be found from an empirical formula obtained on the basis of numerous instrumental observations [12],

\[ \eta_0 = \frac{25}{25 + \sqrt{n}}\, m_n \cos \alpha_1, \tag{8} \]

where \(n\) is the distance along the normal between the working and the line \(MN\) (Fig. 5); \(m_n\) is the normal thickness of the mined-out space.

To determine maximum subsidence in the undermined seam, one may take \(n = m_m\), where \(m_m\) is the thickness of the interbed along the normal; then formula (8) takes the form

\[ \eta_0 = \frac{25 m_n \cos \alpha_1}{25 + \sqrt{m_m}} . \tag{9} \]

If the relative magnitude of subsidence is adopted as the criterion for the duration of the individual stages of displacement, then for the active stage it will be equal to \(0.9 \eta_0\).

The average subsidence velocity for the Kuzbass may be found from the formula

\[ V_c = \frac{0.03 + V_2}{2}\ \text{m/month}, \tag{10} \]

then the duration of the active stage for \(m_2\) will be determined by the formula

\[ t_2=\frac{0.9\eta_0}{V_c}\ \text{months}. \]

Bearing in mind that \(m_n=m_2\), in general form we have

\[ t_2=\frac{45\,m_2\cos\alpha_1}{\left(25+\sqrt{m_M}\right)(0.03+V_2)}\ \text{months}, \tag{12} \]

where \(t_2\) is the required duration of the active stage of displacements in the undermined seam, in months.

Fig. 5. Construction of the line \(MN\) and the value \(n\).

Taking into account that in formula (7) the value \(H_2\), for determining the duration of the active stage of displacements in the undermined seam, may be taken equal to \(m_M\), ultimately, as a first approximation, we have

\[ t_2= \frac{45\,m_2\cos\alpha_1} {\left(25+\sqrt{m_M}\right) \left(0.03+V_1\frac{H_1\,v_2\,m_2\cos\alpha_2}{m_M\,v_1\,m_1\cos\alpha_1}\right)}. \tag{13} \]

Following the principles adopted in formula (7), it is possible, as a first approximation, to find the duration of the initial stage of displacements in the closely spaced lower seam from the equation

\[ t_1=t_1'\, \frac{m_{M2}\,v_2\,\cos(\alpha_2-\beta_1)} {m_{M1}\,v_1\,\cos(\alpha_1-\beta_1)}, \tag{14} \]

where \(t_1\) is the required duration of the initial stage of the displacement process in the closely spaced lower seam, for an interseam thickness \(m_{M2}\), rate of advance of the stoping faces \(v_2\), dip angle of the seam \(\alpha_2\), and angle of displacement from the footwall side \(\beta_1\), in months;

\(t_1'\) is the known (from observation data) duration of the initial stage of displacements under analogous conditions for \(m_{M1}\), \(v_1\), and \(\alpha_1\), in months.

5. Grounds for Dividing a Level into Sublevels

The grounds for dividing a level into sublevels in the mining of closely spaced steeply dipping seams are: the adopted order of mining the seams, mining systems, and methods of controlling the enclosing rocks.

The choice of the order of mining closely spaced seams is considered below (Section 8).

The principal element of the mining system that determines the need to divide a level into sublevels is the direction in which the stoping front advances. With a shield mining system, horizontal seams, and transverse-inclined seams, the level is usually not divided into sublevels. In pillar mining systems along the strike, and also in inclined seams, depending on its height, the level is customarily divided into 1–3 sublevels. A number of sublevels in a level greater than three encounters technical difficulties and is often not economically justified. Practice has established that between the setting increment, the length of the longwall, and the rate of advance of the stope face there is a definite relationship, characteristic only of the particular conditions in which the seam occurs. For the time being, the setting increment is a quantity established practically. It may vary depending on changes in the longwall length or the rate of advance of the stope face: with an increase in the rate of advance of the stope face, the setting increment will increase; with an increase in the longwall length, the setting increment will decrease. With a short longwall length the setting increment reaches 15–25 m; with a great longwall length, on the contrary, the setting increment is extremely small and equals 3–4 m.

Working with stowing considerably reduces the dependence of the longwall length on the stowing increment; in such cases the longwall length is selected depending on the stability of the face or according to an organizational factor—the average progressive labor productivity of the face crew of workers, while observing safety measures (a limited number of workers in the stope face, normal ventilation, combining stowing operations with stoping operations, etc.).

6. Procedure for Mining Closely Spaced Seams

The presently existing procedure for mining closely spaced seams may be covered by the following classification (Table 3).

Table 3

Here the following concepts are taken into account:

1. Ascending order—when the lower seam is mined ahead of the upper one;

2. Descending order—when the upper seam is mined ahead of the lower one.

3. Simultaneous mining of closely spaced seams—when, within the limits of one extraction section, stoping operations are conducted on two or more seams.

4. Sequential mining of closely spaced seams—when, within the limits of one extraction section, stoping operations are conducted only on one seam, and after completion of stoping operations on it, immediately or after some interval of time, stoping extraction is begun on the adjacent seam.

1. Joint preparation of closely spaced seams—when the preparatory or development workings are common to two or more seams.

2. Separate preparation of closely spaced seams—when each individual seam has its own separate system of preparatory and development workings.

7. Factors influencing the choice of the order of mining closely spaced seams

The factors influencing the choice of the order of mining closely spaced seams are as follows:

1. The presence of seams liable to sudden outbursts of coal and gas, and their position among the closely spaced seams. If the uppermost seam in the group is classified as liable to sudden outbursts of coal and gas, then it becomes necessary to mine first the adjacent seam lying below it, as a protective seam. If the seam liable to sudden outbursts of coal and gas in a group of closely spaced seams lies lowest or occupies an intermediate position, then the adjacent overlying seam may serve as the protective seam. Finally, if both seams are liable to sudden outbursts, then the overlying seam should be mined first, as providing more complete protection for the seam [5].

2. The presence and position, among the closely spaced seams, of a seam that must be drained. Mining first the adjacent seam, for the purpose of draining the seam to be dewatered, is dictated by technical necessity. Therefore, in a particular case, the choice of the mining order for closely spaced seams may be determined by the position among them of the seam to be drained.

3. The thickness and physico-mechanical properties of the rocks in the interseam strata. The greater the thickness of the interseam strata, the more grounds and possibilities there are, when working with complete stowing, for applying an ascending order of mining closely spaced seams. Disturbance of the interseam rock mass by discontinuities and especially by cracks parallel to the line of the longwall face does not favor the use of an ascending order. If the interseam strata consist of friable or quicksand-like rocks, this also does not favor the use of an ascending order for mining closely spaced seams.

4. The presence of thick seams within the group of closely spaced seams. The position of thick seams within a group of closely spaced seams sometimes predetermines the choice of the order of seam extraction. If a thick seam lies uppermost in the group, the order may be descending; if it lies lowest or occupies an intermediate position, then, in order to increase mine productivity, the order may be adopted as ascending, with the indispensable condition that complete stowing of the worked-out space be used.

5. The dip angle of the closely spaced seams. Flat and inclined dip of seams is more favorable to the use of a descending order of mining closely spaced seams. In mining steeply dipping seams there are more grounds and possibilities for applying an ascending order (gravity stowing of the worked-out space, self-delivery when working with caving of the lower part of the stoping area, shifting of the caving zone into the upper part of the stoping area, etc.).

6. Qualitative characteristics of the coals of the closely spaced seams. Sometimes, in order to satisfy consumer demand, it becomes necessary to mine first the seams with high-quality coal. In this

in such a case, the location of a seam with high-quality coals among other contiguous seams predetermines the order of extraction and the methods of controlling the enclosing rocks.

1. Strength of the coals. Sometimes, in order to reduce the strength of the coal in individual contiguous seams, with the aim of increasing labor productivity in coal cutting, a seam with strong coals is underworked. Conversely, if a seam consists of weak coals, then applying an ascending order to it is plainly unfavorable.

2. Property of spontaneous combustion of coal. If the overlying seam consists of coals prone to spontaneous combustion, while the underlying seam is to be worked using slice systems of mining with a face line extended along the strike, and if the interseam thickness is small, then there are grounds and possibilities for working such a pair of contiguous seams in descending order. Conversely, in mining contiguous steeply dipping seams there are cases when the coals of the seam along which a group drift has been driven are prone to spontaneous combustion, and this seam is not the lowest in the group of contiguous seams; if the order of extraction of the wing is adopted from the shaft toward the boundaries of the mine field, then the reserves of the seam with the group drift, because of the danger of endogenous fires, must be sealed off; the reserves in the lower-lying seams will also be sealed off; the period of sealing off the reserves will be equal to the period of working the overlying seams from the shaft to the boundary of the mine field along the strike; the undesirable sealing off of opened and prepared reserves leads to extracting first the seam adjacent below the seam with the group drift, i.e., to applying an ascending order of mining contiguous seams, with measures taken to ensure preservation of the underworked seam from the harmful consequences of underworking.

3. Methods of controlling the enclosing rocks. Complete and dense stowing considerably reduces the sphere of influence of the mined-out space. Complete caving, and especially “knife-edge” subsidence, does not favor the use of an ascending order of extraction of contiguous seams.

4. The presence of fire seats in contiguous seams, their gas content, the specific features of the mining systems employed, and in particular the direction of advance of the stoping faces, must in each specific case be taken into account separately.

8. Selection of the order of mining contiguous seams

The ascending order of extracting contiguous seams is recommended when work is carried out with complete, dense, and promptly installed stowing of the mined-out space in the following cases: a) if the uppermost seam in the group of contiguous seams is dangerous with respect to sudden outbursts of coal and gas; b) if the uppermost seam in the group of contiguous seams is watered; c) when $\alpha > \beta_1$ and the overlying seam is mined using the shield system, while the underlying seam is mined by long pillars along the strike (Fig. 6); and d) when the time required to develop the production capacity of the mine is reduced, and in other cases as a result of which greater economy may be achieved. Economy may be considered achieved if the costs of repairing and maintaining preparatory workings during the period of their underworking, and the costs associated with carrying out additional measures for controlling the enclosing rocks, are equal to or less than the amount saved as a result of increased productivity

the mine against an additional increase in the operating line of stopes, i.e., provided that the condition is observed:

\[ (r_p-r_n)lt+(a_z-a_o)Z_v \leq \left(\frac{B}{A}-\frac{B+b}{A+a}\right)(A+a)t, \tag{15} \]

where

\(r_p\) — costs of repair and maintenance of 1 linear m of development workings when they are undermined, per year, in rubles;

\(r_n\) — costs of repair and maintenance of 1 linear m of development workings under conditions when they are not undermined, per year, in rubles;

\(l\) — length of the section mined during the time period \(t\) years, in m;

\(t\) — period of mining a section of length \(l\) m, in years;

\(a_z\) — costs of roof control when the seam is undermined, in rubles per m;

\(a_o\) — costs of roof control under conditions when the seam is not undermined, in rubles per m;

\(z_v\) — industrial reserves of the seam section being undermined within boundaries equal to \(l\) m, in t;

\(B\) — total mine-wide costs per year, in rubles;

\(A\) — annual output of the mine before applying the ascending order of extracting seams in the series, t;

\(b\) — additional total mine-wide costs caused by an increase in the mine’s capacity by the amount \(a\), in rubles;

\(a\) — increase in the mine’s capacity from the additional increase in the operating line of stopes due to the application of the ascending order of extraction of closely spaced seams, in t.

Fig. 6. Diagram for determining the order and sequence of mining closely spaced seams: extraction sequence—I, II, III.

Example. Given: \(r_p=100\) rubles per 1 linear m per year; \(r_n=50\) rubles per 1 linear m per year; \(l=300\) m; \(t=1\) year; \(a_z=25\) rubles per 1 m; \(a_o=18\) rubles per 1 m; \(z_v=100000\) t; \(B=5\) million rubles; \(A=500\) thousand t; \(b=100\) thousand rubles; \(a=100\) thousand t (figures adopted for illustration).

We have:

\[ (100-50)\,300\cdot1+(25-18)\,100000 \leq \]

\[ \leq \left( \frac{5000000}{500000} - \frac{5000000+100000}{500000+100000} \right) (500000+100000)\cdot1 \]

\[ 715000<900000. \]

As can be seen, mining with the use of an ascending order of extraction of closely spaced seams is more economical.

In the remaining cases, except those indicated in items a, b, c, d, it is, as a rule, recommended to mine closely spaced seams in descending order.

9. Factors influencing the choice of the sequence of mining closely spaced seams

The factors influencing the choice between simultaneous or sequential mining of closely spaced seams will be the following:

1. The presence, in the group of closely spaced seams, of seams prone to sudden outbursts of coal and gas, or of watered seams, or seams with high gas content. The presence of such seams in a group of closely spaced seams does not favor the use of their simultaneous extraction; time is required for degassing or drainage; for this purpose, the use of sequential mining will be the most favorable.

2. The thickness and physico-mechanical properties of the interseam rocks. A small thickness of the interseam rocks, composed of rocks that cave readily, favors the use of simultaneous seam extraction; conversely, if the interseam rocks are hard, do not possess plastic properties, and cave with difficulty, then the use of sequential seam extraction will be more favorable. When an ascending order of seam mining is used, as the thickness of the interseam increases, the sphere of influence of the mined-out space increases, the duration of the active stage of the movement process increases over time, and there are more grounds for choosing sequential extraction of closely spaced seams.

3. The property of coal to undergo spontaneous combustion. The tendency of coal to spontaneous combustion does not favor the use of sequential mining of closely spaced seams. Simultaneous mining of closely spaced seams, provided that the leads are kept to the absolute minimum and that a preventive regime is observed, is less hazardous with respect to fire.

4. Methods of controlling the surrounding rocks. Work with complete stowing favors the use of simultaneous seam extraction. Work with caving, when an ascending order of seam extraction is used, is more favorable for the sequential mining of extraction sections in closely spaced seams. When the working face advances on the dip (shield system), allowing the rock to pass onto the shield favors the use of simultaneous mining of closely spaced seams while observing the appropriate leads (see item 12).

5. Mining systems and their parameters. More often it is necessary to take into account the direction of advance of the stoping front and the dimensions of extraction panels along the strike. Slice mining systems with extraction of slices within a storey along the dip or rise (horizontal slices, transverse-inclined slices) promote the use of sequential mining of closely spaced seams.

10. Choice of the sequence of mining closely spaced seams

Sequential mining of closely spaced seams, under any order of their extraction, is recommended: a) if the adjacent seam is prone to sudden outbursts of coal and gas; b) when it is necessary to drain an adjacent watered seam; c) if there is a need to degas an adjacent seam with high gas content; d) when working with caving in cases where the duration of the period of the active

the stage of the displacement process is equal to or greater than the period of extraction of the extraction section on the adjacent seam, and d) in cases where different directions of advance of the longwall faces are used in adjacent seams (horizontal slices on one seam and long pillars along the strike on another).

In the remaining cases it is recommended to use simultaneous working of closely spaced seams.

11. Conditions for the Joint Preparation of Closely Spaced Seams

The conditions favoring the use of joint preparation of closely spaced seams for extraction are, in aggregate, the following: a) interseam thickness up to 10 m; b) identical grade composition of the coals; c) the presence, in the group of seams, of thin seams or seams of medium thickness; and d) when it is technically more convenient, and economically more expedient, to drive and maintain a gathering drift along only one of the seams of the group.

The use of joint preparation and working of closely spaced seams is economically expedient if the costs under separate preparation are greater than or equal to the costs under joint preparation, i.e., if the condition is satisfied

\[ n_n(K_{\mathit{shtr}} + r_{\mathit{shtr}}t) \geq \frac{l_n(n_n - 1)}{S} K_n + K_{sb} + r_{sb}t, \tag{16} \]

where
\(n_n\) — number of closely spaced seams worked jointly;
\(K_{\mathit{shtr}}\) — cost of driving 1 running meter of drift along the seam, in rubles;
\(r_{\mathit{shtr}}\) — cost of repairing and maintaining 1 running meter of drift along the seam per year, in rubles;
\(t\) — time of maintaining the drift, years;
\(l_n\) — length of the coal chute, m;
\(S\) — distance between coal chutes along the gathering drift, m;
\(K_n\) — cost of driving 1 running meter of coal chute, in rubles;
\(K_{sb}\) — cost of driving 1 running meter of gathering drift, in rubles;
\(r_{sb}\) — cost of maintaining 1 running meter of gathering drift per year, in rubles.

Example. Given: \(n_n = 2\); \(K_{\mathit{shtr}} = 500\) rubles; \(r_{\mathit{shtr}} = 50\) rubles; \(t = 1\) year; \(l_n = 15\) m; \(S = 10\) m; \(K_n = 100\) rubles; \(K_{sb} = 700\) rubles; \(r_{sb} = 60\) rubles. (The values are adopted for illustration.)

We have

\[ 2(500 + 50 \cdot 1) > \frac{15}{10}100 + 700 + 60 \cdot 1, \]

\[ 1100 > 910. \]

It is expedient to use joint preparation and working of closely spaced seams.

In the remaining cases, except those indicated in items a, b, c, d, it is recommended to use separate preparation and working of closely spaced seams.

12. Limit-Minimum Leads Between Longwall Faces in Closely Spaced Seams

Between the longwall faces of closely spaced seams, leads of not less than the limit-minimum values must be maintained.

The limit-minimum leads of longwall faces along the strike in adjacent closely spaced seams must be:

a) when using an ascending order and sequential mining—no less than the duration of the active stage of displacements;

b) when using a descending order and sequential mining—no less than the duration of the period of degassing or drainage of the adjacent seam, and in the case where \(\alpha > \beta_1\), no less than the duration of the active stage of displacements;

Fig. 7. Diagram for determining the limiting-minimum lead when mining seams with the use of a longwall system along the strike.

c) with an ascending order and simultaneous mining—no less than two increments of stowing and a reserve for the irregular advance of the extraction faces;

Fig. 8. Diagram for determining the limiting-minimum lead when mining seams by the shield mining system and the longwall system along the strike.

d) with a descending order and simultaneous mining in the following cases:
1) if the underlying seam does not fall within the sphere of influence of the worked-out space, i.e., when \(\alpha < \beta_1\) (Fig. 7),—no less than two increments of stowing (subsidence), a reserve for the irregular advance of the extraction faces, and

\[ m \frac{\operatorname{ctg}\delta}{\cos \alpha}, \]

where \(m_m\) is the normal thickness of the interseam rock, \(\delta\) is the angle of displacement along the strike, \(\alpha\) is the dip angle of the seam; the same, but when using the shield mining system (Fig. 8),—no less than two shield ...

pillars, a reserve for the nonuniform advance of the stoping faces, and \(m_m \dfrac{\operatorname{ctg}\delta}{\cos\alpha}\); and 2) if the lower seam falls within the sphere of influence of the worked-out space of the overlying seam (Fig. 9), i.e., in the case when \(\alpha > \beta_1\), then, when shield mining systems are used, the advance along the strike must be equal to zero, and along the dip it must be within the zone determined by the displacement angles \(\beta_1\) and \(\beta\); when mining systems with advance of the stoping faces along the strike are used, the advance must be within the limits of not less than two setting (seating) steps, a reserve for the nonuniform advance of the stoping faces, and \(m_m \dfrac{\operatorname{ctg}\delta}{\cos\alpha}\), and in time not more than the duration of the initial stage of the displacement process \((t_1)\); otherwise it is necessary to use consecutive mining of the closely spaced seams (item 6).

Fig. 9. Diagram for determining the advance between shield faces.

13. On the mutual position of development workings

The calculation and substantiation of the mutual position of stoping faces and development workings in a seam are given in work [15]. Additional requirements are imposed on the choice of the mutual position of development workings when mining closely spaced seams: 1) development workings must not be driven or maintained within the zone of the active stage of the rock-displacement process; and 2) as an exception, their driving and maintenance are permissible only within the zones of the initial stage of the displacement process or the stage of attenuation.

Conclusion

The order of mining closely spaced seams is closely connected with the correct choice of mining systems, their elements, and methods of controlling the enclosing rocks; and, conversely, the choice of mining systems, their elements, and methods of controlling the enclosing rocks depends in many respects on what order of extraction of the closely spaced seams has been adopted.

Knowing the rates of displacement and the duration of the initial and active stages of displacement, it is possible, for a specific part of a deposit, to compile a forecast horizontal map of the order and sequence of mining closely spaced seams. Unfortunately, instrumental observations of rock displacements in the subsurface have clearly been carried out insufficiently even in such regions,

as Prokopyevsk-Kiselevsk, where the resolution of this question is especially important. The need to organize instrumental observations of rock-mass movements underground on operating horizons, above all at the mines of the Prokopyevsk-Kiselevsk district, and also to compile, on the basis of these instrumental observations, a forecast map of the order and sequence of mining closely spaced seams on lower, deeper horizons, is dictated by the following considerations: a) with the transition to working with stowing, the height of the level must be reconsidered in the direction of increasing it; b) the timely establishment of the order of mining closely spaced seams will determine horizon-by-horizon planning of mining operations and the coordination of group workings by levels; c) it will serve as the basis for planning stowing operations as a whole for the district, according to the methods of constructing the stowed mass at mines, and, in addition, for determining the locations for lowering stowing materials into underground workings.

A forecast map for the mining of closely spaced seams can serve as a guiding document for design organizations and builders of surface structures in specific mining-industrial districts.

LITERATURE

1. Avershin S. G. Movement of rocks during underground mining operations, Ugletekhizdat, 1947.

2. Gorbachev T. F. Preliminary results of observations of the undermining of coal seams, mine workings, structures, and water sources in Kuzbass, Ugletekhizdat, 1951.

3. Kazakovskii D. A. Movement of the earth’s surface under the influence of mining operations, Ugletekhizdat, 1953.

4. Manukyan P. A. Protection of the subsoil, Ugletekhizdat, 1951.

5. Nekrasovskii Ya. E. Mining of seams prone to sudden outbursts of coal and gas, Ugletekhizdat, 1951.

6. Rules for protecting structures from the harmful influence of mining operations in the Donetsk and Moscow Region coal basins, GONTI, 1939.

7. Rules for protecting structures from the harmful influence of mining operations in the Kuznetsk coal basin, Gostoptekhizdat, 1940.

8. Rules for protecting structures from the harmful influence of underground workings in the Chelyabinsk brown-coal basin and at the Bulanash coal deposit, Ugletekhizdat, 1950.

9. Proskurin V. V. Investigation of the order and methods of mining closely spaced steeply dipping seams of the Kiselevsk deposit of Kuzbass, in the collection Questions in the Mining of Thick Seams of Kuzbass, Ugletekhizdat, 1953.

10. Proskurin V. V. Prokopyevsk ore mine of Kuzbass, Tomsk, Poligrafizdat, 1951.

11. Proskurin V. V. Conditions determining or necessitating and making possible the extraction of closely spaced steeply dipping seams in a suite in ascending order, Proceedings of the Tomsk Polytechnic Institute named after S. M. Kirov, vol. 79, Tomsk, 1954.

12. Guide on questions of rock-mass movement for the conditions of the Prokopyevsk-Kiselevsk district of Kuzbass, Ugletekhizdat, 1951.

13. Slesarev V. D. Mining of a seam suite, Ugletekhizdat, 1948.

14. Strelnikov D. A. Technical problems in the field of mining thick seams of Kuzbass. Proceedings of the First All-Union Mining Scientific and Technical Congress, vol. IV, 1928.

15. Shevyakov L. D. Mining of mineral deposits, Ugletekhizdat, 1953.