Determination of the Volume of Backfill Material Required for Filling the Mined-Out Space Through Surface Subsidence Openings in the Shield Mining System

G. E. Posokhov

(Presented by the scientific seminar of the departments of mining bedded deposits, mining ore deposits, and construction of mining enterprises)

In the mines of the Prokopyevsk–Kiselevsk district of Kuzbass, in recent years the shield mining system with filling of the mined-out space through surface subsidence openings has become widespread. Structurally, this system differs very little from the shield mining system with caving, and it is applied under the same mining-and-geological conditions, usually on the first and, more rarely, on the second level.

Backfill materials are delivered into the mined-out space by means of bulldozers, excavators, dump trucks, belt conveyors, and scraper loaders. The backfill materials used are mainly rocks of alluvial deposits, as well as burnt rock heaps and nonproblematic bedrock.

Experience has shown that, with systematic delivery of backfill materials onto the shield from the surface, the technical-and-economic indicators characteristic of the shield mining system with caving are considerably improved [2 and 5]:

1) operating losses of coal are reduced owing to a decrease in shield accidents and a reduction in the dimensions of inter-shield and protective coal pillars;

2) the productivity of the shield face is increased, chiefly owing to the calm, uniform lowering of the shield during extraction of the entire pillar;

3) shield accidents are reduced because large dynamic loads are eliminated;

4) the ventilation regime is not disturbed, since air intake and leakage through the subsidence openings are sharply reduced;

5) the need for preventive flooding is completely eliminated.

The positive results of applying the shield mining system with backfilling are confirmed by studies [7, 9] and are presented in Table 1.

The data of Table 1 show that the percentage share of the shield mining system with backfilling in the total coal output for the Prokopyevsk–Kiselevsk district of Kuzbass increased in 1952, compared with 1950, by almost 13 times. One of the main reasons for the rapid spread of this mining system is the exceptionally low cost of backfill ma-

output, which is achieved through the use of high-productivity machines and inexpensive backfill materials.

Table 1

Indicators of the use of shield mining systems
in the Prokopyevsk–Kiselevsk district of Kuzbass

Table 2 gives the costs of 1 m³ of backfill material placed in the mined-out space, depending on the method of constructing the backfill mass, taking into account full loading of the installations [4, 12].

Table 2

Actual costs of 1 m³ of backfill material

A significant drawback of supplying backfill materials into the mined-out space through subsidence openings from the surface is the seasonal nature of the work. This work is usually carried out during the period from May through October, inclusive. During the remaining 6 months of the year it is almost not performed at all.

Table 3 gives data on the volumes of work performed by the Kiselevsk special office for backfilling the mined-out space and filling subsidence openings under shield mining systems for the period 1952–54, by month.

From the data in Table 3 it can be seen that the cessation of backfilling work in winter leads to an increase in the volume of work for filling subsidence openings during the warm months of the year. Therefore, the search for methods of backfilling the mined-out space through subsidence openings from the surface in the winter period is of great national-economic importance in terms of mechanizing labor-intensive processes and reducing the cost of coal.

The possibility of solving this problem has already been proven in practice. For example, at the “Ziminka” mine of the Prokopyevskugol trust, in the winter of 1952, an experiment was conducted for the first time on backfilling the mined-out space and filling subsidence openings with uncrushed burnt rock. The burnt rock was delivered from local-

new quarry by dump trucks and dumped onto an area cleared of snow above the site of the future subsidence opening and around it [1].

Table 3

Stowing of the mined-out space and filling of subsidence openings under shield mining systems in the mines of the Kaganovichugol Trust
(as of July 1, 1954)

Such stockpiling of stowing materials is obviously simpler and more economical to carry out in summer, when the work of extracting and transporting them is facilitated. In addition, opportunities remain for the wide use of the cheapest stowing material itself—rocks of alluvial deposits.

Preliminary stockpiling of stowing materials and their storage directly above the site of the future subsidence opening will undoubtedly ensure more reliable filling of the mined-out space and will significantly reduce the difficulty of carrying out stowing operations under winter conditions.

Thus, the seasonality of stowing operations under the shield mining system can be eliminated by creating, during the summer period, stockpiles of stowing material above the funnel of the future subsidence opening. In this case, the most satisfactory results will be obtained when the volume of the prepared stowing material is sufficient to fill the mined-out space.

In view of the foregoing, it is possible to pose and solve the problem of a preliminary calculation of the required volume of stowing material to be stored above the funnel of the future subsidence opening.

To determine the magnitude of the indicated volume, we adopt the following initial assumptions:

1) in the winter period the mined-out space must be filled up to the level at which the seam enters beneath the alluvium [3, 23];

2) the unfilled part of the subsidence opening located in the alluvium is filled in summer by the usual method (with bulldozers);

3) the overall dimensions of the stockpile of stowing material are determined by the dimensions of the contour of the subsidence-opening funnel on the ground surface.

Proceeding from the adopted conditions, let us denote:

\(V_{zm}\) — volume of backfill material required for filling the mined-out space up to the level at which the seam emerges beneath the overburden, \(m^3\);

\(V_{vp}\) — volume of the mined-out space, \(m^3\);

\(V_{ug}\) — volume of coal pillars left in the mined-out space, \(m^3\);

\(V_n\) — volume of the collapse funnel in the overburden, \(m^3\);

\(V_p\) — volume of the part of the mined-out space filled by the volume from deflection of the rocks of the seam roof, \(m^3\);

\(V_c\) — volume of the roof rocks and seam floor collapsing during caving, \(m^3\);

\(\gamma_{ug}\) — coefficient of loosening of coal;

\(\gamma_n\) — coefficient of loosening of the rocks of the overburden deposits;

\(\gamma_c\) — coefficient of loosening of the side rocks;

\(\beta\) — slope angle of the side rocks, deg.

We determine the value \(V_{zm}\) for the case when the rocks of the roof and floor are competent, i.e., when \(V_{zm}\) has the greatest value and \(V_c\) the smallest.

With this formulation of the problem, according to Fig. 1 we have:

\[ V_{zm}=V_{vp}+V_{ug}(1-\gamma_{ug})+V_c(1-\gamma_c)-V_n\gamma_n-V_p. \]

Fig. 1. Calculation scheme for determining the volume of backfill material.

The increase in the volumes \(V_{ug}\) and \(V_c\) during loosening may be disregarded, since it represents a comparatively small quantity. Under this condition,

\[ V_{zm}=V_{vp}-V_n\gamma_n-V_p. \tag{1} \]

Let us denote the initial calculation data:

\(m_{sh}\) — size of the shield across the strike, \(m\);

\(m\) — seam thickness, \(m\);

\(\alpha\) — angle of dip of the seam, deg.;

\(H\) — height of the level, \(m\);

\(m_{n}\) — thickness of the overburden deposits, m;

\(h\) — height of the coal pillar above the shield assembly crosscut, m;

\(h_{u}\) — height of the coal pillar above the drift, m;

\(f\) — maximum deflection of the roof rocks during the extraction of a shield panel, m \([6,37]\);

\(L\) — dimension of the shield along the strike, m;

\(\delta_{n}\) — angle of displacement of the rocks of the overburden deposits, degrees.

Then

\[ V_{vp}=m_{u}(H-h_{u})\frac{L}{\sin \alpha}; \]

\[ V_{n}\gamma_{n}=m_{n}\gamma_{n}\left(m+m_{n}\frac{\sin \alpha}{\operatorname{tg}\delta_{n}}\right)\frac{L}{\sin \alpha}; \]

\[ V_{n}=f(H+h-h_{u})\frac{L}{2\sin \alpha}. \]

Since the difference \(h-h_{u}\) is comparatively small, we assume that

\[ V_{n}=fH\frac{L}{2\sin \alpha}. \]

Substituting the values \(V_{vp}\), \(V_{n}\gamma_{n}\), and \(V_{n}\) into (1), we obtain the design formula

\[ V_{z.m}=m_{u}(H-h_{u})\frac{L}{\sin \alpha} -m_{n}\gamma_{n}\left(m+m_{n}\frac{\sin \alpha}{\operatorname{tg}\delta_{n}}\right)\frac{L}{\sin \alpha} -fH\frac{L}{2\sin \alpha}. \tag{2} \]

To solve the question of placing the backfill material directly above the sinkhole, it is necessary to establish the overall dimensions of the stockpile.

The stockpile will have the form of an obelisk or a wedge; moreover, under the conditions considered, a wedge-shaped stockpile with equal bases provides the greatest volume. The dimensions of the wedge-shaped stockpile are determined from the expressions:

\[ h_{sh}=\sqrt{V_{sh}\operatorname{tg}\varphi}, \]

\[ l_{sh}=2\sqrt{V_{sh}\operatorname{ctg}\varphi}, \tag{3} \]

\[ L_{sh}=nL+h_{sh}\operatorname{ctg}\varphi, \]

where \(h_{sh}\) — height of the stockpile, m;

\(l_{sh}\) — width of the base of the stockpile, m;

\(L_{sh}\) — length of the base of the stockpile, m;

\(V_{sh}\) — volume of backfill material per each meter of the stockpile base along the strike, m;

\(\varphi\) — angle of natural repose of the backfill material, degrees;

\(L\) — dimension of the shield panel being mined along the strike, m;

\(n\) — number of shield panels mined in the winter period.

To ensure gravity flow of the backfill material from the stockpile into the worked-out space, the following condition must be observed during stacking: \(l_{sh}=l\), where \(l\) is the width of the sinkhole, m.

From Fig. 1

\[ l=\frac{m}{\sin\alpha}+2m_{n}\operatorname{ctg}\delta_{n}. \tag{4} \]

It follows from expression (4) that, for a known thickness of the seam being mined, the value of \(l\) depends on the thickness of the alluvial deposits.

Let us denote the minimum thickness of the alluvium at which \(l=l_{sh}\) by \(m_{nk}\) (Fig. 3).

From (3) and (4) we obtain

\[ m_{nk}= \frac{\sqrt{\,V_{z.m}\operatorname{ctg}\varphi-\dfrac{m}{\sin\alpha}\,}} {2\operatorname{ctg}\delta_{n}}. \]

Consequently, it is most rational to carry out the preliminary preparation of backfill material when \(m_{n}\geq m_{nk}\).

Example. Determine the values of \(V_{z.m}, V_{sh}, h_{sh}, l_{sh}\), and \(m_{nk}\) for \(H=80\) m, \(\alpha=60^\circ\), \(h_{c}=5\) m, \(f=0.7\) m, \(\delta_{n}=45^\circ\), \(\varphi=40^\circ\), and \(L=1\) m.

We carry out the calculation for two variants:

a) \(m=5.5\) m and \(m_{sh}=4.5\) m;

b) \(m=8.0\) m and \(m_{sh}=7.0\) m.

We shall take the value of \(m_{n}\) as the series of numbers 0, 1, 2, 3, and so on.

The results of the calculations are presented as the graph in Fig. 2. From the graph the following conclusions follow:

1) when \(m_{n}=m_{n\max}\), \(V_{sh}=0\);

2) when \(m_{n}\geq m_{nk}\), \(V_{sh}=V_{z.m}\);

3) when \(m_{n}<m_{nk}\),

\[ V_{sh}=V_{z.m}+2(m_{nk}-m_{n})^{2}\operatorname{ctg}\varphi, \]

i.e., by increasing the stockpile it is possible artificially to increase the thickness of the alluvium to the value \(m_{nk}\), so that in winter the backfill material will enter the mined-out space by gravity.

Fig. 2. Calculation scheme of the stockpile of backfill material.

Fig. 3. Graph of volumes of backfill material and dimensions of the transverse section of the stockpile

1—curve of the volumes of backfill material for \(m=5.5\) m and \(H=80\); 2—curve of the volumes of the stockpile for \(m=5.5\) m and \(H=80\) m; 3—curve of the volumes of backfill material for \(m=8.0\) m and \(H=80\) m; 4—curve of the volumes of the stockpile for \(m=8.0\) m and \(H=80\) m; 5—height of the stockpile; 6—width of the base of the stockpile.

The proposed method of advance preparation and stockpiling of backfill material on the ground surface along the line where the seams emerge beneath the overburden will help resolve the question of the successful application of the shield mining system with backfilling through surface subsidence openings, regardless of the season, which will promote the further development of coal production in Kuzbass.

REFERENCES

1. Averyanov I. P. Uncrushed native rocks as material for filling subsidence openings in winter. Kuzbass Publishing House, 1953.

2. Bakatyn V. A. Mining of thick steeply dipping seams by a system with shield support and backfilling of the mined-out space at the mines of the Kiselevsk deposit. Proceedings of the Scientific and Technical Conference on the Exchange of Advanced Experience in the Kuznetsk Coal Basin. Ugletekhizdat, 1953.

3. Provisional regulations for the mining of thick steeply dipping seams with backfilling of the mined-out space and for the organization of backfilling operations in the Kuznetsk Coal Basin. Moscow, 1952.

4. Problems of mining engineering. VUGI, Collection No. 10. Ugletekhizdat, 1953.

5. Gorbachev T. F. On improving the mining of deposits in the Prokopyevsk-Kiselevsk district of Kuzbass. Moscow, 1954.

6. Guidelines on issues of rock displacement for the conditions of the Prokopyevsk-Kiselevsk district of Kuzbass. Ugletekhizdat, 1951.

7. Strelnikov D. A., Biryukov R. A. New developments in mining systems in the Prokopyevsk-Kiselevsk district of Kuzbass. Ugletekhizdat, 1954.