MAIN DIRECTIONS OF FIRE PREVENTION IN THE DEVELOPMENT OF THICK STEEPLY DIPPING SEAMS IN SOUTHERN KUZBASS

P. A. LEONOV

The directives of the Nineteenth Party Congress on the fifth five-year plan for the development of the USSR for 1951–1955 set a number of most important tasks for the coal industry:

1) rapid growth in coal production, especially coking coals; 2) systematic improvement of methods for developing coal deposits; 3) increasing labor productivity; 4) further improvement of labor protection, and others.

Underground mine fires, as is known, disrupt the normal course of operating work, often frustrate fulfillment of the coal-production plan, and are frequently the cause of accidents and casualties. Therefore, preventing the occurrence of underground fires—or, in other words, fire prevention—is of great importance for production.

Since 1949, Kuzbass has begun the transition to systems for developing thick seams with stowing of the worked-out space. Before that year, in the basin indicated, development systems with caving were used almost exclusively. In a number of cases they caused the occurrence of underground fires, the elimination of which entailed considerable expenditure of funds.

Development systems with stowing of the worked-out space reduce coal losses and create safer working conditions. Under these development systems, the number of underground endogenous fires is sharply reduced. However, the use of development systems with stowing does not fully guarantee against the occurrence of underground fires. Fire prevention is also needed for development systems with stowing. Therefore, the questions of preventing the occurrence of underground fires (fire prevention) remain relevant for Kuzbass even after its transition to the development of thick steeply dipping seams with stowing of the worked-out space.

The present work has been carried out on the basis of materials from the Prokopyevsk and Kiselevsk mines, which are the largest in the basin.

1. On the Directions of Preventive Measures in the Underground Development of Seams

As is known, the causes of endogenous fires are accumulations of coal left in the extraction district, an inflow of air, and the accumulation of heat at the place where the coal is located, where heating subsequently begins and an underground fire arises.

An endogenous fire in a seam is completely excluded in the absence of combustible material, i.e., accumulations of coal. In the presence of the indicated accumulations, the occurrence—

the occurrence of an endogenous fire can be prevented if access of air to the coal is stopped. There may further be a case in which there is an accumulation of coal and an inflow of air, but the possibility of heat accumulation has been eliminated. Under such conditions an underground fire does not occur.

The listed causes of the occurrence of endogenous fires depend on many factors. Such factors include: seam thickness, properties of the surrounding rocks, the tendency of coal to spontaneous combustion, the degree of coal fragmentation, depth of mining, ventilation regime, mining system, improper conduct of mining operations, degree of isolation of the worked-out space, volume of coal accumulation, coal losses, accidents in mine workings, and others.

The factors that determine spontaneous combustion of coal may be divided into two groups: 1) those that can be changed and eliminated, and 2) those that cannot be eliminated in the course of underground operations. Figure 1 shows the dependence

Fig. 1. Causes and factors of underground fires

of the causes of endogenous fires on these factors. The first cause—accumulation of coal (coal losses)—is the result of four factors: the mining system, improper conduct of operations, depth of mining, and accident rate of mine workings. Some of these factors can be changed, and some eliminated, and then there will be no accumulations of coal in the worked-out space. In the second cause—inflow of air—the factors are partially amenable to change, for example, the factors of the mining system, the sequence of extraction and its intensity, and the physical state of the coal. However, it is practically impossible to change the factors in such a way as to completely stop the inflow of air to coal accumulations. The third cause of spontaneous combustion of coal—accumulation of heat—depends on many factors, four of which are ineradicable for the given specific conditions. These factors include: the tendency of coal to spontaneous combustion, depth of mining, seam thickness, and the properties of the surrounding rocks. The influence of the other factors of the last cause of endogenous fires may be weakened and, in individual cases, eliminated.

The direction of preventive measures must be determined by the nature of the factors contributing to the occurrence of endogenous fires and by the degree of fire hazard of the seams. With respect to removable and modifiable factors, the indicated measures will consist of a set of measures that eliminate the action of the given factor or reduce its influence on the cause of the fire; for example, the factor “mining system.” It can be changed to one under which coal losses, and consequently its accumulation in the worked-out space, will be very small. With respect to nonremovable and unmodifiable factors, the situation is worse. It is impossible to change these factors, and it is often very difficult to weaken their influence on the process of occurrence of endogenous fires. Thus, for example, the factors of seam thickness, the properties of the side rocks, the depth of mining, and others are not subject to change or elimination during production operations.

The scope and character of preventive measures must encompass the entire complex of causes of fires and the factors contributing to them. Only in this case can the task of preventing endogenous fires be solved.

2. Distribution and Magnitude of Coal Losses in Stoping Areas

The character of fire-prevention measures and their scope depend on the location and magnitude of the expected fire seats, and on the possibility of their spreading along the strike and dip of the seam. The location of fire seats is associated with the presence, in the working area, of crushed pillars and accumulations of broken or collapsed coal. Heating and spontaneous ignition do not arise in all accumulations of coal, but only where these accumulations have a considerable volume. Therefore, in order to prevent fires from internal causes (spontaneous ignition), concentrated coal losses must not be permitted in mine workings. If such losses are present, their locations must be subjected to preventive treatment. Thus, in order to obtain a picture of the location of possible fire seats, it is necessary to know the distribution, in the stoping areas of the seam, of accumulations of coal of all kinds.

A. Coal losses on the upper horizons mined by systems with caving and stowing. At the Prokopyevsk and Kiselevsk mines, the following mining systems were used on the upper horizon: a) with caving: horizontal slices, inclined slices, zones, room-and-pillar system, shield system, long pillars along the strike; and b) on a small scale, systems with stowing: zones, horizontal slices, inclined slices, diagonal slices, room-and-pillar. Coal losses on the upper horizon in areas where coal extraction was carried out by systems with caving consist of (Fig. 2): 1) interpanel pillars 6–10 m wide, usually arranged along the strike at intervals of 200–400 m; 2) coal losses from incomplete extraction of pillars above the main drifts; usually these losses amount to 50% of the reserves in the pillars, and they are distributed along the entire length of the stoping area; 3) losses in emergency chambers in the form of broken coal, from 40 to 70% of the total volume of the stoping chamber; 4) coal losses in the roof of the seam under the room-and-pillar mining system, where usually 1.0–1.5 m of coal remained in the roof; 5) coal loss in the form of a 1 m thick layer left between horizontal slices in order not to make a flooring. After 1938, when mining seams by the system of horizontal slices with caving from bottom to top, the installation of flooring began to be used, which made it poss—

possibility of not leaving a thickness of coal between the seams; 6) losses of coal in “legs,” i.e., in pillars replacing timber supports when seams 4–7 m thick are worked by the long-pillar system along the strike; the width of these pillars (“legs”) was set at 2–2.5 m; 7) losses in pillars left between extraction chambers; these pillars were left 2–3 m wide along the strike for the full height of the level; 8) losses of coal in the form of pillars or broken coal during accidents in stoping operations under all mining systems except the chamber-and-pillar system; 9) coal left in places of geological disturbances; 10) coal losses above the ventilation drift along the entire strike of the seams (1–5 m along the rise); and 11) coal losses in the form of pillars sometimes left at central traveling furnaces.

Diagram labels visible in the figure:
I — coal losses in the chamber-and-pillar system
II — coal losses in the zone system
III — coal losses at the boundary of seams

1 — inter-panel pillar
2 — pillar at the traveling furnace in the center of the section
3 — pillar above the main drift
4 — inter-chamber pillar
5 — pillar at the roof of the chamber

Dimensions in meters.

A. Recovery of the pillar above the main drift by horizontal layers with a layer height of 3 m.
B. Recovery of the pillar above the main drift by horizontal layers with undercutting of the coal in the roof of the layer.

Fig. 2. Distribution of coal losses at the first level when mining seams by caving systems

The indicated coal losses were places where fire seats originated. It may be stated with certainty, on the basis of an analysis of endogenous fires in the Prokopyevsk-Kiselevsk district, that the worked-out space of the upper level contains such a significant accumulation of coal that, under favorable conditions, fire seats arose there from the spontaneous combustion of coal. This applies, of course, to extraction sections of thick seams that were not treated by preventive flooding.

Under mining systems with stowing, coal losses consisted of: 1) inter-panel pillars (6–10 m along the strike); 2) pillars (2–3 m) between zones and chambers (in zone mining systems with stowing and in the chamber-and-pillar system with stowing); 3) pillars at central traveling furnaces; 4) coal pillars and broken coal left in stoping workings as a result of accidents; 5) coal losses during the extraction of pillars above the main haulage drifts, with coal recovery within 50%.

B. Coal losses at the lower (second) level, where caving mining systems were applied. The types of coal losses and their arrangement in extraction sections do not differ substantially

of the position on the upper level. It should be noted that coal losses under the system of mining by horizontal slices on the lower level are smaller than on the overlying one. This occurs because, after 1936, when extracting coal by horizontal slices, the use of a mat began to be widely applied instead of leaving a crust (thickness) of coal between the slices. The additional losses on the lower level will be the coal in the interlevel pillar, which is left on the ventilation level. This pillar extends along the entire length of the extraction section; along the rise it has a size of 4 m. During stoping with caving, the interlevel pillar is destroyed and falls into the worked-out space, where it forms considerable accumulations of coal.

The total losses on the lower level are increased by the release of coal from the worked-out space of the first level. Part of this coal (for example, from intersection pillars) is not oxidized and may become the site of the origin of a fire seat.

B. Coal losses on lower levels where systems of mining with stowing of the worked-out space are used. The largest coal losses here will be the losses in the interlevel pillar. This pillar must be left for the protection of ventilation drifts. The size of the pillar along the rise, according to practical data, is determined as 4 m. In the system of mining by inclined slices, the indicated pillar has a stepped form (Fig. 3).

1—interlevel pillar; 2—intersection pillar; 3—pillar above the drift.
Vertical projection 1:1000

Side stepped pillar  Side flat pillar  Pillar above the main drift  Upper (interlevel) pillar  Extraction of the pillar above the main drift

I. Section mined by a system of inclined slices with stowing (the level is divided into sublevels)
II. Same, but without division into sublevels
III. Section mined by horizontal slices.

Fig. 3. Coal losses on the lower level, where mining systems with stowing are used

In the system of mining by inclined slices, when the level is divided into sublevels, it is necessary to leave a pillar at the sublevel drifts measuring 2–3 m along the rise, and along the strike for the entire extraction section.

On the level of the main drift, in all systems with stowing, there were considerable coal losses due to incomplete extraction of protective pillars at the main drift. The magnitude of these losses is 50% of all reserves—

coal pillars in the solid. The indicated losses are distributed uniformly along the strike. Their accumulation on the floor of the main haulage drift will make it necessary, when mining the next lower horizon, to leave an inter-level pillar. Otherwise the stability of the ventilation drift will be very poor.

In stoping workings under systems with stowing, in the normal course of operations there are no coal losses. Only in the event of accidents does it have to be left in the seam, but these losses are accidental.

The magnitude of losses during stoping operations in seams is, in most cases, small. When extracting the overlying layer (in inclined seams) or the underlying layer (in horizontal seams), these accumulations of coal will be cut off from the subsequent layers by a thickness of stowing made of barren rock.

Figure 3 shows the distribution of coal losses on the lower horizon under mining systems with stowing and the shapes of the coal pillars left behind.

3. Location of possible fire seats

A. In areas mined by caving systems. Under these conditions, fire seats may occur in all places where there are concentrated coal losses (Fig. 4). The practice of mining thick seams by caving systems shows that almost all fire seats are located in the mined-out space and, for the most part, on the horizon above the main haulage drift. Some fires arose during the extinguishing of protective pillars of the main drift.

Fig. 4. Distribution of possible seats of endogenous fires

B. In areas where systems of mining with stowing of the mined-out space were used. There are grounds to assume that seats of possible fires may occur in the following places (Fig. 4).

1) In the inter-level pillar after it has been disturbed as a result of the pressure of the overlying rocks and settlement of the stowing. During settlement of the stowing, this pillar first cracks and then collapses. Collapse of the pillar may

may be considerable. The movement of coal in the pillar along the dip will occur by the amount of settlement of the filling.

With rapid extraction of coal and the performance of filling operations, collapse of pillars in the immediate vicinity of operating longwalls and stopes (for systems of inclined slices and the shield system with filling) will not occur; but if extraction and filling proceed slowly, then collapse of the pillar or its cracking near the active faces is possible. As a result, a fire will arise in the coal of the interlevel pillar or in the area of the main drift of the upper horizon, where air will enter through the cracks.

2) In the coal mass above the mined-out slice. When a thick seam is developed by the system of horizontal slices in ascending order, or by the system of inclined slices also in ascending order, there may be cases of slow coal extraction and lagging of the filling of the mined-out space. Under these conditions, the coal mass above the slice being extracted is destroyed; cracks appear in it, and falls of a more or less considerable quantity of coal are often observed. Such a situation creates very favorable conditions for the heating and spontaneous combustion of coal in the disturbed parts of the coal mass above the slice being worked. The danger and frequency of fires are so considerable that the mining system with horizontal slices, with extraction of the slices from bottom to top, is considered highly fire-hazardous.

3) In pillars near intermediate drifts. These pillars have the following dimensions: along the rise 2–3 m, along the seam thickness 6–9.0 m. The probability of fires in such pillars, when they are surrounded by barren rock (filling), is insignificant. Heating of the coal and its spontaneous combustion may occur if these pillars, owing to slow extraction of the lower sublevel, remain for a long time under the action of an air current.

4) In pillars at the main drift (the pillar between the main drift and the lower slice drifts). These pillars, being cut by coal chutes, are difficult to extract without coal losses. Usually, when such pillars are extracted, losses reach 50%. If extraction of the pillars lags and there is significant rock pressure in the mined-out space where the protective pillars at the main drift were located, heating of the coal and even spontaneous combustion are possible. This danger will occur only in the case of extraction with caving.

This method of extraction will have to be used under the mining system with inclined slices. When working by the system of horizontal slices, extraction of the protective pillars of the main haulage drift by horizontal slices with filling is possible. Then the mined-out space of these pillars will no longer be fire-hazardous.

5) At the junction of the worked-out extraction area and the adjacent area newly being developed. Under mining systems with filling, it is recommended not to leave fire-preventive protective pillars between adjacent extraction areas. In this case, in the new area the ventilation drift will begin directly from the boundary of the neighboring worked-out extraction area. In the old area, by the time the ventilation drift is driven in the new area, almost complete settlement of the filling will have occurred, and the interlevel pillar will to a considerable degree be destroyed and displaced downward along the dip. Through the ventilation drift, air can freely enter the broken coal of the interlevel pillar at the boundaries of the extraction areas. As a result, heating and spontaneous combustion of coal may be expected at this place, especially if extraction of the first five or six upper slices is carried out with insufficient intensity.

The situation described may occur in mining systems with stowing, in horizontal and inclined slices, and also in the shield system.

The limited experience in applying stowing systems on the thick seams of the Prokopyevsk mine area already provides facts of coal heating in the interlevel pillar: 1) at mine “M,” on seam Vnutrennii IV, in a minus-level drift (beneath the interlevel pillar), a coal temperature of 32° was observed five months after the driving of the said drift; 2) at mine “N,” on seam Gorelyi, in the ventilation drift (of the second level), heating of coal in the interlevel pillar up to 36° was noted; 3) at mine “P,” heating of coal was observed in the pillar at the ventilation drift during the mining of a thick seam by a system of inclined slices with stowing.

4. Fire-prevention measures

All fire-prevention measures may be divided into five groups: 1) measures to prevent coal from being left in the worked-out space; 2) measures to create conditions that hinder the accumulation of heat during coal oxidation; 3) measures to stop and reduce the admission of air to coal accumulations; 4) measures for treating coal left in the worked-out space so as to bring it into a condition that precludes heating and spontaneous ignition; and 5) miscellaneous measures.

All the measures indicated relate to accumulations of coal in the worked-out space. Methods for preventing the occurrence of fires in the coal mass at development and stoping workings are not considered in the present work, since the thick seams of Southern Kuzbass do not produce endogenous fires under these conditions.

A. Measures to prevent leaving coal and timber in the worked-out space. For mining conditions with stowing systems, this group includes the following measures: 1) elimination of various pillars, 2) replacement of coal pillars with artificial concrete or rock pillars, 3) reduction of accidents in stoping operations to a minimum.

Interpanel pillars may be eliminated by the use of dense stowing with lagging of the wall of the adjacent extraction section to prevent the stowing material from spilling out. The interlevel pillar of the lower level, with the upper level mined by caving systems, is irremovable. For the next level, leaving an interlevel pillar should be avoided. This can be achieved if the protective pillar of the main haulage drift is extracted, using a mining system with stowing. Then waste rock (stowing) will lie on the floor of the drift, beneath which the ventilation drift of the lower level is driven. Pillars at sublevel drifts (in the system of inclined slices) can be eliminated by using stronger support of the sublevel drifts and preserving them in the stowing. Pillars at the main haulage drift will become safe if they are extracted completely, using stowing. In the system of horizontal slices this will present no difficulty.

For extracting the pillars of the main drift in the system of inclined slices with stowing, it is necessary to design a special variant of the mining system with stowing. In the shield system, the greatest difficulty is presented by the extraction of coal over the height of the last two meters from the floor of the main drift. This layer must be extracted cleanly, with stowing. Otherwise, on the lower level it will be necessary to leave an interlevel pillar. In view of the great difficulty of extracting the coal at the floor of the main drift

with dry stowing; it is advisable here to use hydraulic stowing or good silting with sandy material.

When a thick seam is mined by the system of horizontal slices from top to bottom, an interlevel pillar is located in the roof of the first slice. As the slices are extracted and the stowing settles, the indicated pillar shifts down the dip. At the boundary with the first slice there are wooden ties and props of the sets. They are subjected to considerable pressure, split, and, under appropriate conditions, may become a fire seat. Burning timber can easily ignite the crushed coal of the interlevel pillar.

When a thick seam is mined by the system of inclined slices with stowing, attention must be paid to the support of the side of the pillar of each slice. As stoping advances, the stowing settles, the side support is disturbed, and the interlevel pillar is destroyed. The timber of the side sets and ties, being split and heated, may ignite spontaneously and transmit fire to the coal from the pillar. With the shield system, after the coal in a column has been extracted, the shield stops two—three meters from the floor of the main haulage drift. It cannot be lowered further. Practice shows that, in the process of extracting coal under the shield, the shield lagging is strongly deformed and the logs of the lagging are destroyed. Under favorable conditions such split timber may become a fire seat and ignite the coal beneath the lagging.

In order to prevent fires from timber left at the boundary with the interlevel pillar, and also on the main drift (in the shield system), it is necessary in the indicated places to replace timber with noncombustible material. This can be achieved by replacing wooden ties with metal mesh, and the shield lagging with special reinforced-concrete slabs. For economic reasons, it does not appear possible to replace the wooden props and caps at the side sets with metal ones.

The question of replacing wooden shield lagging with fire-resistant slabs has not yet been worked out. If it is not resolved for a long time, then the only remaining means of combating the fire hazard of wooden lagging is good silting of the worked-out space at the shield up to a level one meter above the upper rows of the lagging.

B. Measures to create conditions preventing the accumulation of heat during coal oxidation. Observations show that heat accumulation during coal heating occurs mainly in large accumulations of broken coal or destroyed pillars. The reason for this phenomenon is the low thermal conductivity of coal compared with sandstones, argillites, and clay. Therefore, to prevent heat accumulation it is necessary to eliminate large accumulations of coal, and to surround the coal with waste rock (stowing). In practice such a measure can be carried out with accumulations of coal in horizontal slices. There they must be surrounded with stowing, and when extracting the lower slice, the slice of waste rock must be cut off from the lower mass of coal.

C. Measures to reduce and stop the access of air to accumulations of coal in the worked-out space. Of the measures of this group, it is recommended to apply: 1) ventilation of extraction sections with minimum depression; 2) the suction-and-forcing method of ventilation (if this is permissible for the given category of mine with respect to methane); 3) construction of stoppings on ventilation slice drifts at the boundary with the worked-out extraction section; 4) packing with stowing of the void that forms on the ventilation horizon of the worked-out section between the upper boundary of the stowing and the lower surface of the coal pillar (interlevel), with clay stowing

with stowing, i.e., hydraulic or pneumatic; 5) construction of fire-resistant stoppings in intermediate crosscuts on the haulage and ventilation horizons; 6) installation of stoppings in crosscuts driven from the rock furnace (under the system of mining by horizontal slices) for each pair of slices; these stoppings are erected as the section is worked out; 7) leaving barrier pillars between extraction sections (under caving systems); 8) leaving an interlevel pillar above the ventilation drift in the case of extracting the safety pillar of the overlying main drift with caving; 9) isolation of the goaf (under the shield mining system) by stoppings on the side of the active stoping workings; 10) isolation by the ground surface (sealing of cracks, artificial caving of the roof strata, lagging and backfilling of sinkholes).

G. Measures to bring coal left in the worked-out space into a state that precludes its spontaneous combustion. The essence of these measures consists in isolating accumulations of coal by stoppings and subsequently treating the remaining coal by silting.

At present two methods of silting are known: mechanical silting (water and clay) and chemical silting (water and soda, or water and lime). The first method is widely used in production; the second is at the stage of experimental development.

The role of clay in silting: filling voids and cracks, formation of films and shells on the surface of coal lumps, and considerable transfer of heat from the coal through the clay owing to its greater thermal conductivity. Filling of cracks and voids is incomplete, since the amount of clay (in the pillar) amounts to only 3–5% of the volume of the worked-out space. The role of water in silting is the cooling of the rock and coal during the silting process, and their cooling after completion of silting as a result of evaporation of the water. The amount of water used in silting is 5–10 times greater than the amount of clay. Thus, from 15 to 30% of the total volume of this space enters the worked-out space as water. One more effect of water must also be noted. It wets and impregnates the side rocks; therefore their cooling effect increases. During silting, water enters underground workings at a temperature of no more than 10°. At such a temperature the oxidation of coal is sharply reduced.

The cooling effect of water has hitherto been underestimated in explaining the mechanism of the silting process. The role of water in silting is not secondary, but one of the principal ones. The very high effectiveness of prophylactic silting can in all probability be explained by the good cooling action of water, and not only by the process of filling voids and cracks with clay and forming a shell on the coal surface.

Silting may be of two types: non-pressure and high-pressure. At present in Kuzbass, almost exclusively non-pressure silting is used, by gravity through boreholes. For silting accumulations of coal in the worked-out space, the non-pressure method gives satisfactory results. It should further be noted that this method does not require pumps and is comparatively inexpensive. The pressure method of silting is suitable for producing silting of fractured coal masses. Under systems with stowing there is no reason to expect that the coal pillars will be completely destroyed. They are only partly deformed. Stowing does not create conditions for large movements of coal pillars. Prophylactic silting of incompletely destroyed pillars gives good results if it is carried out under pressure. Such a situation may be expected on the basis of general considerations. Therefore, at present, in mining thick seams, ...

systems with stowing, tests should be carried out of two types of hydraulic filling—pressure and non-pressure—and then the more effective of them should be selected.

For hydraulic filling, two types of filling installations are used: 1) central filling installations with delivery of the pulp from the surface, and 2) underground mobile filling installations (not connected with the surface). The productivity of installations of the first type is usually 50–100 or more \(m^3\) of pulp per hour, or 1000–2000 \(m^3\) per day. Underground installations have a small productivity, on the order of 10 \(m^3\) per hour.

For the lower horizons, where stowing systems were used and which are located below the horizon worked by caving systems, the volume of hydraulic-filling work will be considerable. Much filling under these conditions will be required by the interlevel pillar. The possibility is not excluded of carrying out filling work also in the lower part of the first horizon. The worked-out space of the pillar of the main haulage drift, if the pillar is extracted with caving, will require no small amount of filling. On the subsequent horizon (after the first, worked by stowing systems) there will be no interlevel pillars. The pillar at the main haulage drift will in all probability be extracted with the use of stowing. Under these conditions, hydraulic filling will be required only for small accumulations of coal (because of an accident, etc.).

It is therefore recommended: 1) for preventive hydraulic filling of the first horizon, worked with stowing, to use a central filling installation with delivery of the pulp from the surface; 2) for the second and subsequent horizons, worked with stowing, to carry out preventive hydraulic filling by means of an underground mobile filling installation. Such a small installation will also be required for individual operations on the first horizon.

Preventive hydraulic filling, i.e., filling to prevent the occurrence of endogenous fires, has been used in Southern Kuzbass since 1937. At present it is widely used as the principal fire-prevention measure and gives good results. Extraction areas treated by preventive hydraulic filling almost do not produce endogenous fires. These circumstances give grounds for recommending, in the development of thick steeply dipping seams, preventive hydraulic filling after systems of mining with stowing as one of the principal means of preventing fires from internal causes.

Considerable accumulations of coal left in the worked-out space, as indicated above, are places where seats of fires from spontaneous combustion of coal arise. Therefore preventive hydraulic filling should be used in extraction areas worked by mining systems both with caving and with stowing. Below is a brief description of the objects of hydraulic filling and the nature of filling work for mining systems with caving and with stowing.

a) Areas worked with the use of a mining system with caving. Coal losses in such areas are located from the ventilation drift to the floor of the main haulage drifts. Therefore preventive hydraulic filling should be carried out throughout the entire worked-out space, delivering the pulp through boreholes drilled to the horizon of the ventilation drift, and, where there is an interlevel pillar, to the horizon of the main haulage drift of the level above. This procedure for carrying out preventive hydraulic filling has been verified by many years of practice. There is a proposal to replace preventive hydraulic filling of the entire level by filling only the lower part of the level—between the main haulage and the first parallel drifts. Such a procedure for filling may give good results only in individual cases, namely,

with reliable isolation of the upper part of the level and the absence of large accumulations of coal in it.

b) Sections where mining systems with stowing were used. The largest volume of stowing work will be at the interlevel pillar and at the horizon of the main haulage drift. Stowing emergency sections in the seams will not require a large amount of work. In addition, in individual cases it is necessary to stow the coal mass above inclined seams if, as a result of slow extraction in the seam, cracks appear in it and heating of the coal is detected. With a section length of 200 m, a seam thickness of 9 m, and an interlevel-pillar thickness of 4 m, the volume of coal in the pillar will be \(200 \times 9 \times 4 = 7200\ \text{m}^3\). To stow this quantity, according to existing standards, 220 m\(^3\) of clay in the pillar will be required, or 2200 m\(^3\) of pulp of composition 1:9. The stowing of the indicated pillar must be carried out before the start of stoping operations in the lower sublevel, by delivering the pulp onto the floor of the main drift of the upper horizon. It is possible that in some cases such stowing will not have the proper effect because the pillar is not intact.

When 2–3 horizontal seams and all inclined seams are extracted along the strike for about 50 meters, the interlevel pillar, owing to settlement of the stowing in its lower part, will be destroyed and fractured by cracks. Such a pillar may produce several centers of heating and even spontaneous combustion. Therefore it is necessary to halt coal extraction in the section for some time and subject the interlevel pillar to preventive stowing. Stowing can be carried out in two ways: through boreholes from the surface and through boreholes drilled from the ventilation drift of the adjacent seam.

If working practice shows that an interlevel pillar not treated by stowing, undercut by horizontal or inclined seams of the extraction section of the lower horizon, will not produce fires during the period of working the section, then the need to stow this pillar disappears. However, there is greater probability of expecting the appearance of centers of heating and spontaneous combustion in the interlevel pillar when this pillar is exposed during development and stoping operations of an adjacent extraction section (Fig. 4). At the moment when the vertical wall of the pillar is exposed by the workings of the adjacent section, the interlevel pillar of the old section will, of course, prove to be significantly deformed. Stowing the flank of the interlevel pillar at the junction with the new extraction section must be carried out along the strike for no less than 20 m. For this it will be necessary to drill one borehole to the upper boundary of the pillar, taking into account the displacement of the pillar along the dip by the amount of settlement of the stowing. The approximate stowing volume will be 20 m\(^3\) of clay in the pillar. Such a quantity of clay can be delivered through a central stowing installation or by means of an underground mobile stower. For preventive stowing of the flank of an interlevel pillar, it is of interest to use chemical stowing. For this it is necessary to drill a borehole from the ventilation drift of the new extraction section and, with the aid of an underground stower (designed for chemical stowing), feed through it a solution of soda or lime. In this case the positive qualities of stowing are that the coal and workings are not contaminated. Coal accumulations in the area of stoping workings and the pillars at sublevel drifts have a comparatively small volume. Drilling stowing boreholes from the surface or from the ventilation drift under such conditions will prove economically disadvantageous. Boreholes from the adjacent seam may be permitted only in two particular cases: a) the accumulation of coal is located at the horizon of the ventilation and first parallel drifts; b) the adjacent seam is not at a considerable distance from the stowing—

… The amount of clay for stowing must be taken such that it fills all the voids throughout the entire length of the section to a height of 4 m.

These stowing operations at the main haulage drift are considerable in volume, are very expensive, and their execution will require a long period of time. Therefore it is recommended to improve the mining systems with stowing so that the extraction of pillars above the main drift is carried out with stowing, and not with caving. The extraction of these pillars with stowing will be much cheaper than with caving and preventive grouting.

D. Other measures for creating conditions that prevent the occurrence and spread of underground fires. Among other measures, sufficiently tested in practice and recommended by the “Safety Rules,” the following should be noted: a) opening thick steeply dipping and inclined seams by district (intermediate) crosscuts; b) working thick seams in separate extraction districts; c) extracting the district in retreat (from its boundaries); d) limiting the size of the extraction district of a thick seam along the strike to 200–400 m, in order to work out the district in a short period of time (6–8 months); e) intensive conduct of stoping operations; f) a descending order of extracting layers in layered systems of working with stowing of the worked-out space.

Conclusions

1. Preventive measures against underground fires must be determined by the nature of the factors contributing to the occurrence of endogenous fires and by the degree of fire hazard of the seams. The type of fire-prevention measures and their scope depend on the magnitude of the expected fire seats and on the possibility of their spread along the strike and dip of the seams.

2. Heating and spontaneous combustion of coal under conditions of underground mining arise in places of significant coal accumulations (crushed pillars and broken coal). The greatest fire hazard is presented by interdistrict pillars, interlevel pillars, and coal left in the worked-out space as a result of incomplete extraction and accidents.

3. All fire-prevention measures may be divided into five groups: 1) measures to prevent coal from being left in the worked-out space; 2) measures to create conditions preventing the accumulation of heat during the oxidation of coal; 3) measures to stop and reduce the inflow of air to coal accumulations; 4) measures for treating coal left in the worked-out space, bringing it into a state that excludes spontaneous combustion; and 5) other measures.

4. The principal preventive measures for preventing fires in thick seams should be recommended as follows: 1) pe-

transition to mining systems with stowing (to eliminate coal losses); 2) opening seams by intermediate crosscuts; 3) mining seams in separate panels 200–400 m along the strike; 4) extracting panels on the retreat, i.e., from their boundaries; 5) intensive conduct of stoping operations; 6) isolation of the worked-out space from underground mine workings, as well as from the surface; and 7) preventive hydraulic filling of the worked-out space, pillars, and separate accumulations of coal (the filling measures are described in detail below in paragraphs 6–9).

1. The worked-out space of panels on the upper levels, where caving systems were used, must be subjected to preventive hydraulic filling over the full height of the floor. On the lower levels, transition to filling only the lower half of the floor is permissible when experiments confirm the effectiveness of this filling method.

2. After stoping panels of thick seams by stowing systems below a level where caving systems were used, filling must be carried out along the boundaries of the panel along the strike for 20 m, over the entire area of the inter-floor pillar, which is highly dangerous with respect to fire, and also in those places where coal has been left in significant accumulations as a result of incomplete extraction and accidents during mining operations.

3. When stoping panels by stowing systems, complete extraction of coal from the pillars above the main drift should be achieved. In this case, during operation of the underlying level, there is no need to leave an inter-floor pillar above the ventilation drift.

4. For the first level of stowing operations below a floor where caving systems were used, in view of the large volume of work involved in filling the inter-floor pillar, large filling plants should be available, with preparation of the pulp on the surface.

5. It is advisable to carry out filling of separate accumulations of coal when mining by stowing systems with the aid of underground mobile filling devices. In this case, central filling plants with preparation of the pulp on the surface will not be required if the stoping panels have no inter-floor pillar and the coal above the main drift has been taken without losses.

REFERENCES

1. Directives of the XIX Party Congress on the Fifth Five-Year Plan for the Development of the USSR for 1951–1955. Gospolitizdat, 1953.

2. Skochinsky A. A. and Ogieivsky V. I. Mine Fires. Gostoptekhizdat, 1940.

3. Gorbachev T. F. Ways of improving mining systems for thick steeply dipping seams of Southern Kuzbass. Ugletekhizdat, 1949.

4. Buzikov M. L. Prevention of underground fires in thick coal seams of the Southern Kuzbass. Ugletekhizdat, 1949.

5. Maevskaya V. M. Principal indicators of the propensity of coals to spontaneous combustion. Bulletin of MakNII, No. 16, 1947.

6. Kovachevich P. M., Parusimov V. F., and Shchepotiev K. N. Experience in introducing a new method of fire prevention on thick steeply dipping seams. Ugol, No. 7, 1950.

7. Safety regulations in coal and shale mines. Ugletekhizdat, 1953.