THE INFLUENCE OF PROTECTIVE PILLARS ON THE STABILITY OF HOISTING SHAFTS AND ON THE CHOICE OF THEIR LOCATION WHEN OPENING DEPOSITS WITH THICK STEEPLY DIPPING SEAMS AT GREAT DEPTHS

V. V. PROSKURIN

(Presented by Professor D. A. Strelnikov)

Coal deposits with thick steeply dipping seams in the USSR began to be developed more intensively during the years of the first Five-Year Plan. By the present time their coal-bearing areas have been developed to a considerable extent. The overwhelming majority of hoisting shafts sunk for the development of the upper 3–4 levels are located within the productive deposits of the field. Protection of hoisting shafts and surface mine structures from the harmful influence of underground workings is carried out mainly by leaving protective pillars. The coal reserves concentrated in protective pillars are enormous. Individual districts and sections with reserves of coking coals have been developed more intensively than others. For example, Mine No. 5–7 named after Kirov in the Anzhero-Sudzhensk district and the Stalin Mine in the Prokopyevsk district had, by 1953, reached the same mining depth—300 m; meanwhile Mine No. 5–7 has existed for 60 years, whereas the Stalin Mine, which extracts coking coals, has existed for only 20 years. In the future, the development of industrial sections with reserves of coking coals is envisaged at undiminished rates. In the perspective of the development of such districts and sections lies the opening and mining of coal seams at great depths. In Kuzbass, the Prokopyevsk-Kiselevsk deposit should first of all be assigned to this category of districts.

The Prokopyevsk-Kiselevsk deposit is represented by closely spaced, predominantly thick, steeply dipping seams with high-quality coals [10]. The seams plunge practically to an “unlimited”¹ depth. The hoisting shafts of the mines now in operation are located mainly within the productive deposits (64.3%)², less often in the rocks of the hanging wall of the suite of seams (25%), and only in isolated cases (10.7%) in the rocks of the footwall of the deposit. Coal losses in protective pillars beneath the industrial sites of mines are exceptionally large, and with depth they increase sharply (Table 1).

At Mine \(N_1\), with a mining depth of 300 m, losses in protective pillars beneath the industrial site reached 8.5 million t; at Mine \(N_4\), with a mining depth of 210 m, losses reached 8.3 million t; and at Mine \(N_6\), with a depth of only 150 m, they reached 9.9 million t.

Table 1

Losses in protective pillars under the industrial sites of certain mines of the Prokopyevsk–Kiselevsk district

The growth of coal losses in protective pillars under industrial sites as a function of mining depth for certain mines of the Prokopyevsk–Kiselevsk district is shown in Fig. 1.

A considerable difference in the absolute figures of losses depends on the location of the hoisting shafts and on the dimensions and configuration of the industrial sites. For example, the hoisting shafts of mine $N_6$ are sunk in a series of predominantly thick seams; the hoisting shafts of mine $N_1$ are in a series of seams of complex anticlinal folding; the shafts of mine $N_2$ are on the hanging-wall side within the zone of rock displacement. In the Prokopyevsk–Kiselevsk district there are only three mines whose hoisting shafts have been driven in such a way that protective pillars need not be left around them.

The rate at which mining operations are extended to depth can be judged at least from the dates when levels were put into operation at certain mines of the Prokopyevsk district (Table 2).

From the data of Table 2 it is evident that the rate at which mining operations are extended to depth at individual mines reaches 10–15 m per year. Consequently, it must be assumed that in the coming decades mining operations in individual industrial sectors of the Prokopyevsk–Kiselevsk district will reach considerable depths.

One of the principal requirements imposed on deep mines is to ensure the stability of hoisting shafts and of the most important surface mine structures for a very long period of their operation.

Table 2

Dates of commissioning of levels at some mines of the Prokop’evsk district [10]

Reliable methods for determining the displacement of rock masses as a result of underground mining operations, despite many years of research, have not yet been developed [4]. This latter circumstance applies primarily to the protection of structures in areas where thick, closely spaced, steeply dipping seams are being worked.

Fig. 1. Losses in protective pillars beneath the industrial sites of certain mines of the Prokop’evsk–Kiselevsk district of Kuzbass.

It can hardly be assumed that such methods will be developed in the nearest years, since the data of instrumental and experimental observations of rock-mass displacements in Kuzbass, where ...

thick seams, is clearly insufficient. Therefore, the dimensions of protective pillars determined by modern methods may prove either extremely overestimated or, conversely, greatly underestimated. But even if sufficiently accurate methods were developed for calculating the displacement of rock strata, even in that case the method of protecting hoisting shafts solely by leaving protective pillars, under conditions of mining thick steeply dipping seams at great depth, would in most cases be unacceptable.

The stability of hoisting shafts in deep mines depends not only on the correct choice of the appropriate dimensions and shape of protective pillars, but also on the presence within the protective pillars of coal seams, and on its properties and quality. Therefore, posing and examining the question of the influence of protective pillars on the stability of hoisting shafts when opening and mining thick steeply dipping seams at great depths is of great national-economic and scientific significance.

Leaving protective pillars as a measure for protecting the hoisting shafts of deep mines intended for the mining of thick steeply dipping seams does not ensure the stability of such shafts over a long period of their operation for the following reasons.

1. Protective pillars leave open, for the entire period of operation of a deep mine, the possibility of undermining the hoisting shafts, regardless of the causes that may lead to such undermining or the purpose it may pursue.

Many years of practice in mining predominantly thin steeply dipping seams in the Central (Gorlovka) district of Donbass has shown that the overwhelming majority of hoisting shafts of mines are deformed [5]. Curvatures of mine shafts more often reach up to two m, more rarely up to three m and more (Table 3); they are characterized by nonuniformity and by the presence of bends at two or more points (Fig. 2).

Table 3

Data on curvatures of hoisting shafts of certain mines of the Gorlovugol Trust of the Donets Basin

The deformations of the shafts were caused, on the one hand, by insufficient dimensions of the protective pillars and, on the other hand, by winning extraction carried out in the protection zone, which not infrequently pursued predatory and other unlawful aims.

Curvature of a hoisting shaft as a result of underground workings and the release of residual stresses in the rock mass near tectonic disturbances also occurred in Kuzbass at mine No. 5/7 named after Kirov of the Anzherougol Trust [7] and at the Tsentralnaya mine of the Kemerovougol Trust.

Locating hoisting shafts in the rocks of the footwall of a deposit, beyond the harmful influence of mining operations, eliminates the conditions for and possibility of their undermining.

1. The presence in safety pillars of seams liable to sudden outbursts of coal and gas does not exclude the possibility of outbursts occurring when mine workings are driven along these seams in the protection zone of structures (hoisting shafts, shaft stations, chambers, crosscuts, drifts, etc.), and of the formation of such cavities as may have a harmful effect on the stability of shafts and of the structures protected on the surface.

Thus, at the Fontane mine (France) in 1921, about 5000 t of coal were thrown out simultaneously [14]; at the Morrissey mine (British Columbia) in 1904, 3500 t of coal were thrown out [8]; in the Donbass the largest outbursts reached 1000 t. Outbursts also occurred in the protection zone of hoisting shafts. For example, at mine No. 1–3 “Kochegarka,” on the 750 m level, on 6.XI.1950, when the southern main crosscut was approaching the “Devyatka” seam, 850 t of coal and 94 t of rock were thrown out; the outburst occurred in the protection zone, 148 m from the hoisting shaft (Fig. 3); there as well, but while approaching the “Tolstyi” seam, on 26.IV.1952, 400 t of coal were thrown out.

Fig. 2. Graph of the curvature of shaft No. 1 of the “Kochegarka” mine of the Gorlovskugol trust (Donbass).

Fig. 3. Site of a sudden outburst of methane and coal when the crosscut approached the “Devyatka” seam on 6.XI.1950.

In both the first and the second cases, it was not possible to clear the outburst cavities completely of fine coal and rock because of their continuous natural flow. The cavities were passed through by the crosscut by driving ahead with steel support and subsequent concreting. Hoisting in the shaft in October 1953 was stopped for several days as a result of rock displacement, deformation of the support, and deformation of the shaft reinforcement. Outbursts of coal and gas occurred in the protection zone of hoisting shafts at the Severnaya mine of the Kemerovougol trust in the Kuzbass, but their intensity was still small. There are grounds to believe that the intensity of outbursts in thick steeply dipping seams will be considerably greater than in thin seams [8].

1. The increase of rock pressure with depth may lead to crushing of the protective pillar, especially its edge at the boundary on the dip side; to the spilling of coal from the pillar into the mined-out space when seams are worked with caving, or with stowing where filling is not dense and settlement occurs (Fig. 4). Crushing and spilling of coal from protective pillars into the mined-out space during the working of particularly thick steeply dipping seams may substantially weaken the pillars and lead to deformation of hoisting shafts [1].

2. The presence in protective pillars of seams prone to spontaneous combustion may lead to the outbreak of endogenous fires in the pillars or near them. Such cases occurred at mine No. 3—3-bis in Prokopyevsk. Fires, in addition to enormous general complications in mine operation and the expenditure of labor and resources for their extinguishing, lead to the formation of voids. The volume of voids may be large and may exert harmful effects on structures. For example, instrumental observations have established [1] that during underground gasification of a coal seam 2 m thick at a depth of 60 m in the Moscow Basin, surface subsidence after 5 months reached 1.2 m; in foreign practice, the formation of cracks at the surface was observed as a result of the burning of coal in a seam 2.1 m thick at a depth of about 24 m [9, XXXVII, fire No. 17].

The presence of a pyritic-coal band in the structure of a seam contributes to spontaneous combustion of the coal and to the occurrence of underground fires [9, XXXII and XXXV].

Fig. 4. Diagram of crushing of a pillar under the influence of rock pressure.

The most dangerous from the fire standpoint are thick seams. For example, of the total number of fires that occurred during the period 1930—1950 in the Prokopyevsk district, thick seams accounted for 96%, while seams of medium thickness accounted for only 4%; in the Kiselevsk district all fires occurred only in thick seams [6].

From the foregoing it follows that, when hoisting shafts are located in areas where they will intersect productive deposits, the conditions and possibilities remain for the occurrence of endogenous fires in protective pillars. The danger becomes quite obvious when, at great depths, thick steeply dipping coal seams prone to spontaneous combustion are worked. The formation of voids as a result of fires may lead to deformations of the protected structures (hoisting shafts and the surface mine complex).

1. The presence in a protective pillar of thick and other categories of steeply dipping coal seams contributes to the occurrence of sliding along weakened bedding planes of the upper part of the pillar together with

shafts and surface structures. For example, observations have established that at the contact of Carboniferous rocks the subsidence of a shaft reached 1100 mm [1]. In this connection, the placement of hoisting shafts in productive steeply dipping measures must be recognized as entirely inadvisable. At the same time, taking into account the fissuring of the rocks and coals, especially of thick seams [3], it can hardly be considered permissible to sink the hoisting shafts of deep mines in areas where these shafts will intersect thick steeply dipping seams.

As a consequence of the five reasons indicated above, not only distortions of hoisting shafts may occur, but also movements on the surface within the limits of the location there of highly important structures. These movements underground and on the surface will lead to disruption of the operation of the surface complex of hoisting installations (hoisting machines and headframes); to an increase in danger or even to stoppage of man hoists; to a reduction in the speed of freight hoists [5]; to stoppage of hoisting operations for the period of repair of the deformed part of the support of the hoisting shaft; and to stoppage of hoisting operations for the period of intensive deformation of the hoisting shaft and the equipment in it until the movements of the rocks underground or on the surface in the protection zone of the structures of the hoisting complex have ceased.

Previously, when selecting the site for sinking hoisting shafts intended to open a series of steeply dipping, predominantly thick seams to a depth of up to 300–400 m, one of the most important factors taken into account was the increase with depth in the length of the main level crosscuts. The latter was often due to the low level of drifting equipment and haulage equipment of that time, as well as to the high cost of maintaining the main crosscuts, which were usually supported with timber. Therefore, formerly the more frequent practice was to sink mines within the productive measures of the deposit, and not in the rocks beyond the limits of the suite of seams. At present, when the technology of drifting operations has reached a sufficiently high level, mechanized haulage by powerful electric locomotives and metal support are used in mines, the increase in the length of level crosscuts with the depth of mining has lost the significance it formerly had. Therefore, when selecting the site for sinking hoisting shafts intended to open a suite of steeply dipping, predominantly thick seams at great depths, taking into account shaft stability and more complete extraction of reserves, the expenditures for driving crosscuts and for hauling loads along them will not be decisive. This is easy to verify by using the graphical method [2] and taking into account losses in protective pillars.

Conclusions

1. When selecting the site for sinking hoisting shafts intended to open deposits with thick steeply dipping seams at great depths, it is necessary to take into account, in addition to the factors generally known in the mining-engineering literature (the terrain relief, the amount of losses in protective pillars, the water-bearing character of the rocks intersected by the shaft, the presence of old worked-out spaces, the degree of development on the surface, etc.), also the influence of the method of protection.

2. The principal requirements imposed on shafts of great depth are: a) to ensure the stability of the hoisting shafts and of the hoisting complex on the surface for a long period of their operation; and b) to ensure the extremely minimal losses in protective pillars.

3. Hoisting shafts intended for the opening and mining of thick steeply dipping seams at great depths should, as a rule,

it should be laid: a) in the rocks of the footwall or hanging wall of the deposit, outside the limits of the harmful influence of underground workings (Fig. 5),¹ except in cases when other mining-engineering factors in specific areas acquire decisive importance (terrain relief, the location of bodies of water, etc.) or, otherwise, b) in areas where the productive strata are intersected, only if two additional conditions are observed: first, that the boundary of the protective pillar across

Fig. 5. Hoisting shaft in the rocks of the footwall

the strike of the productive strata

Fig. 6. Hoisting shaft intersecting the productive strata.

the strike lines a, b, c (Fig. 6) should extend into the rocks of the footwall or hanging wall no lower than the upper boundary of the occurrence of sudden outbursts (g, d), and, secondly, so that the shafts do not intersect thick seams.

1. The surface hoisting complex (hoisting machines and headframes) must be located on the side opposite from the boundary of displacements (Figs. 5 and 6), with the horizontal axes of the drums of the hoisting machines arranged parallel to the strike of the seams.

2. The possibility of opening deep levels of the Prokopyevsk-Kiselevsk deposit in the vicinity of certain mines now in operation is subject to detailed study; for the majority of the remaining mines, however, the task should be reduced to searching for sites that satisfy the requirements for the construction and operation of deep mines.

The main provisions of the present work were reported by the author at the scientific seminar of the Mining Faculty of TPI on October 2, 1952, at the scientific seminar of the Institute of Mining of the Academy of Sciences of the USSR on November 5, 1953, and at the scientific-technical conference of TPI on March 24, 1954.

REFERENCES

1. Avershin S. G. Displacement of rocks in underground workings, Ugletekhizdat, 1947.

2. Bakanov G. E. On the choice of the location for sinking mine hoisting shafts, taking into account the influence of losses of minerals in protective pillars when opening deposits by level crosscuts, Proceedings of the Tomsk Polytechnic Institute named after S. M. Kirov, vol. 84, 1956.

3. Belitsky A. A. Toward an operational classification of the fissuring of rocks of coal-bearing deposits, collection “Problems of Mining Thick Seams of Kuzbass,” dedicated to the seventieth anniversary of Honored Worker of Science and Technology Prof. D. A. Strelnikov, Ugletekhizdat, 1953.

4. Results of the discussion on the question of methods for studying the process of displacement of rocks above underground workings and methods for precalculating its elements (from the editors), Ugol’, No. 7, 1954.

5. Korotkov M. V. Extraction of coal beneath structures in the Donbass, Ugletekhizdat, 1953.

6. Leonov P. A. Classification of coal seams of Southern Kuzbass by degree of fire hazard, collection “Problems of Mining Thick Seams of Kuzbass,” dedicated to the seventieth anniversary of Honored Worker of Science and Technology Prof. D. A. Strelnikov, Ugletekhizdat, 1953.

7. Manukyan P. A. Protection of the Subsurface, Ugletekhizdat, 1951.

8. Nekrasovsky Ya. E. Mining seams liable to sudden outbursts of coal and gas, Ugletekhizdat, 1951.

9. Underground mine fires and their control (collection of abstracts), under the general editorship of Acad. A. A. Skochinsky, ONTI, 1936.

10. Proskurin V. V. The Prokopyevsk Mine of Kuzbass, Tomsk, Poligrafizdat, 1951.

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

12. Strelnikov D. A. Systems for mining thick seams of the Prokopyevsk Mine in Kuzbass, OGIZ, Novosibirsk, 1931.

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

14. Yarovoy I. M. Guidelines for mining seams hazardous with respect to outbursts of coal and gas, Ugletekhizdat, 1949.

1. The angle $\beta_1$ is taken according to the guide [11]. ↩↩

2. Of the total number of mines. ↩