THE PROBLEM OF COMBATING SUDDEN OUTBURSTS OF COAL AND GAS AND OF DEGASSING IN MINES OF THE EASTERN COAL BASINS OF THE USSR

O. I. CHERNOV

General Information on Sudden Outbursts of Coal and Gas

In the eastern basins and deposits of the Soviet Union, sudden outbursts of coal and gas occur at the mines “Tsentralnaya,” “Severnaya,” “Koksovaya-2,” and Dzerzhinsky Mine No. 9 in the Kuzbass; at Mine No. 2 “Bursunka” and “Klyuchi 1–3” in the Yegorshino deposit of the Urals; at Mines Nos. 1, 2/5, 16, 20, and 21 of the Suchan deposit and at the “Kapitalnaya” mine of the Tavrichanka Mine Administration in the Far East; and also at Mine No. 40 of the Vorkuta deposit of the Pechora hard-coal basin.

During the period from 1947 through 1956, inclusive, 343 cases of sudden outbursts were recorded in the mines of the eastern basins. At present there are 33 hazardous coal seams, which are being worked by thirteen mines (Table 1).

Table 1

Sudden outbursts of coal and gas, if the Suchan deposit is not counted, occur during the driving of development workings. Most often, sudden outbursts are associated with main drifts and raises. Outbursts may also occur in other prepara-

workings and in boreholes. The distribution of sudden outbursts depending on the character of the workings is given in Table 2.

Table 2

The intensity of sudden outbursts is measured by the quantity of coal ejected and gas released. About 60% of all outburst cases that have occurred in the mines of the eastern basins are characterized by a quantity of ejected coal not exceeding 20 t. In some cases, however, the intensity of sudden outbursts reaches 150–200 and even 500 t of coal. The largest outbursts occur when seams are opened by crosscuts. If the average intensity of outbursts in drifts is 20 t of coal, then when seams are opened by crosscuts it is equal to 100 t.

Sudden outbursts are accompanied, as a rule, by intense gas emission. Most often the quantity of methane released per 1 t of ejected coal is from 50 to 400 m³, but in some cases it reaches still higher values.

Combating Sudden Outbursts of Coal and Gas

The struggle against sudden outbursts of coal and gas is conducted by various methods.

Mining practice has shown that, among the known methods of combating sudden outbursts, preliminary extraction of protective seams is the most effective. A characteristic feature for the mines of the eastern basins is the extraction of an overlying seam as the protective seam. The advantage of extracting the upper protective seam is that its protective action is manifested over the entire height of the stage of the protected seam. This method is the principal one in the development of hazardous seams for many mines.

It has been established that the protective action is manifested at a normal distance between the protective and hazardous seams of about 60 m under Kuzbass conditions and up to 70 m under the conditions of the Egorshino deposit. It should be borne in mind that the protective action is possible only if the extraction of the overlying seam is carried out without leaving pillars, or with pillars of minimum dimensions, and also with a considerable lead along the strike of stoping operations on the protective seam relative to the development workings on the hazardous seam.

The most dangerous operation is the opening of seams, since in these cases, as noted above, the largest outbursts occur. At present, in the mines of Kuzbass and the Urals, essentially one method is used for opening seams hazardous with respect to sudden outbursts—opening with the use of frame support. The essence of the method consists in the fact that, beyond the outline of the crosscut, boreholes are drilled that intersect the coal seam and enter the side rocks on the other side of the seam. Into the boreholes are inserted the stringers of the forepoling support, resting at one end on the support of the working and at the other on the massif of the roof-

or the floor of the seam. The erected frame serves as a mechanical barrier capable of interrupting the development of an outburst at its initial stage, when the acting forces are still small.

A method that has become widely used is the driving of workings in coal with advance drainage boreholes of 200–300 mm diameter. In horizontal workings, when the seam thickness is 1 m or less, 2–3 boreholes are drilled; when the thickness is 2 m or more, up to 5–6 boreholes are drilled. The boreholes are arranged fanwise in the plane of the seam; the length of the boreholes depends on the geological conditions and is usually 15–40 m, with the preparatory face advanced by the boreholes by a minimum of 5–7 m. To prevent the formation of outbursts when driving raises, three drainage boreholes are drilled, the middle one being located along the axis of the working and the other two in the form of a diverging fan at an angle of 4–5° to it. The use of large-diameter boreholes (200–300 mm) is not expedient under all conditions, since in seams composed of friable coal the stability of such boreholes is extremely low. Under such conditions it is necessary to switch to drilling boreholes of approximately 100 mm diameter.

As is known, most sudden outbursts in horizontal development workings occur from the upper corners of the faces. To support the overhanging masses of coal, advance support is erected; it either prevents the occurrence of sudden outbursts or suppresses them at the initial stage, when the active forces causing the outburst are still small. For this purpose, boreholes are drilled in the upper part of the face, into which wooden or metal stringers are inserted, with a spacing of 0.2–0.5 m between them. The length of the boreholes is taken as 3.5–4 m in soft coal and up to 8 m in hard coal. After the advance support has been erected, the working is driven 1.8–2.0 m and is then stopped for drilling the next group of boreholes, with the calculation that the advance support should lead the face by not less than 1 m. Analysis of actual data shows that advance support by boreholes, as a means of combating sudden outbursts, gives a positive effect in coals of medium strength or in soft but stable coals, and is unacceptable for soft and friable coal.

In steeply dipping seams composed of unstable coal, such a measure as drilling advance drainage boreholes when driving raises proves practically difficult to implement; therefore the question arose of applying some other measure for combating sudden outbursts. One such measure is the improved protective shield of the VostNII design (Fig. 1). The shield is moved with the aid of three pneumatic props. The protective shield, installed tightly against the breast of the face, is a mechanical barrier capable of interrupting the development of a sudden outburst at its initial stage, when the acting forces are still comparatively small. The shield supports the overhanging mass of coal and also holds the spilling coal in place, as a result of which safer working conditions are created for the miners.

For the purpose of preventing sudden outbursts, the driving of inclined workings from top to bottom is practiced. With this driving method, the force of the weight of the coal mass not only does not promote, but even hinders, the formation of sudden outbursts.

To increase safety when driving inclined workings from top to bottom, at the proposal of VostNII, in the mines of the Egorshino deposit, as the working advances, side chambers are driven every 20 m. The chambers are equipped with double hermetically密封...

closing doors. A hose with compressed air is brought into the chambers. When clear signs of a sudden outburst appear, the workers leave the face and take shelter in a nearby chamber. In the event of abundant gas emission, fresh air is admitted into the chamber through the hose.

One of the safe and productive methods is the driving of inclined workings from top to bottom along a through borehole drilled to a lower-lying horizontal working. A borehole 250–300 mm in diameter serves as a measure preventing a sudden outburst and is used for passing coal from top to bottom.

Fig. 1. VostNII protective shield

The VostNII Station for Combating Sudden Outbursts of Coal and Gas, together with its supporting stations located in Kuzbass, the Egorshino and Suchan deposits, was organized in 1951. Since that time, systematic work has been conducted to combat sudden outbursts, as a result of which both the absolute and the relative number of sudden outbursts has decreased from year to year. Thus, whereas in 1951 there were 53 sudden outbursts in the mines of the eastern basins of the USSR, in 1956 the number of outbursts was only 13. The relative number of sudden outbursts, on average per mine seam per year, decreased over this period from 3.1 to 0.4, i.e., almost 8-fold.

Despite the significant reduction achieved in the number of sudden outbursts, the problem of combating these phenomena is far from solved and requires the search for new methods and the improvement of existing methods for combating sudden outbursts of coal and gas.

Prediction of Seam Sections Hazardous with Respect to Sudden Outbursts in the Mines of Kuzbass

The work of VostNII established that the division of coal seams on the basis of the “Safety Rules” into hazardous, threatened, and nonhazardous is conditional, and that in nature there are no inherently hazardous seams; rather, there are individual sections of these seams, some of which are hazardous and others nonhazardous with respect to sudden outbursts of coal and gas.

The existence of seam sections hazardous and nonhazardous with respect to sudden outbursts is confirmed, first, by the distribution of outbursts on hazardous seams, since outbursts occur not continuously and not-

uniformly, being grouped only in particular parts of hazardous seams and, secondly, by differences in the properties of the coal in hazardous and nonhazardous sections.

As a result of the work carried out, a number of regularities have been established which make it possible to forecast sections of seams that are hazardous with respect to sudden outbursts. The forecasting method is based not on any single property of a seam, but on a set of properties. We believe that only a set of properties can characterize the possibility or impossibility of the occurrence in coal seams of such complex phenomena as sudden outbursts of coal and gas.

The principal factors and properties by means of which it is possible to forecast hazardous sections of seams in the Kuzbass are: the depth of mining from the earth’s surface; the character and orientation of major tectonic disturbances; the coefficient of coal strength; the index of the initial rate of gas emission from coal; hydrogeological conditions; the tectonic structure of the coal and the tectonic texture of the seams.

Coal seams gathered into anticlinal folds are characterized by increased gas content, since gas freely enters from depth but practically cannot migrate to the surface. Sections of seams occurring under such conditions may be hazardous with respect to sudden outbursts, beginning at a depth of 150 m.

Disjunctive disturbances, by rupturing coal seams, facilitate or impede the migration of gas to the surface.

Disjunctives are divided into two groups. The first formed under compressive stresses, and the fractures of these disturbances are closed. These include reverse faults, thrusts, and strike-slip faults. The fractures of the displacers of these disjunctives are not pathways for gas movement and, consequently, in specific cases they contribute to the creation of sections whose coal has an increased hazard with respect to sudden outbursts.

Seams ruptured by disjunctives are divided into three wings (Fig. 2): wing I—the seam is cut by the displacer of the disjunctive only

In the figure:
Wing I is very hazardous with respect to sudden outbursts (75% of outbursts occurred)
Wing II is less hazardous with respect to sudden outbursts (25% of outbursts occurred)
Wing III is nonhazardous with respect to sudden outbursts (no outbursts have occurred)

Fig. 2. Diagram of the distribution of sudden outbursts of coal and gas in the different wings of disjunctives in Kuzbass mines

from above (gas migration to the surface is impeded; outburst hazard is increased); wing II—the seam is cut by the displacers of disjunctives from above and below (gas migration to the surface and from depth does not take place; outburst hazard is reduced); wing III—the seam is cut by the displacer of the disjunctive only from below (gas migrates freely

to the surface, but does not come from depth, the wing is nonhazardous with respect to outbursts).

The hazard of sections with respect to sudden outbursts is to a considerable degree predetermined by the orientation of the cutoffs of seams by the displacers of disjunctives. If the cutoff of a seam by the displacer of a disjunctive is located along the strike, then such a position is the most hazardous with respect to sudden outbursts. When the cutoff is inclined, the position is less hazardous. The location of the cutoff along the dip of the seam in all cases indicates that the sections are nonhazardous with respect to sudden outbursts.

In disjunctives of the fault type, all wings of seams are nonhazardous with respect to sudden outbursts, since gas can migrate unimpeded to the surface through open fractures.

The results of the study of disjunctive disturbances are summarized in Table 3, from which it is evident that, out of 18 different combinations of disjunctive wings and orientations of the cutoffs of seams by the displacers of disjunctives, sections of seams can be hazardous in only one case and less hazardous in two cases. In the remaining 15 cases, sections of seams are nonhazardous with respect to sudden outbursts.

Table 3

Major tectonic disturbances influence the distribution of sudden outbursts. If such disturbances are present, outbursts may occur at a depth of 150 m from the surface. If, however, the seam is not cut by disjunctives and does not have additional folding, while having a direct outlet beneath alluvium, outbursts may occur only at a depth of 340 m. On this basis, taking into account the data of Table 3, a preliminary delineation of sections hazardous with respect to sudden outbursts is carried out.

After this, samples are selected to determine the coal-strength coefficient and the index of the initial rate of gas emission from the coal. The coal strength is determined on the apparatus of Prof. M. M. Protodyakonov, and the rate of gas emission on the Ettinger–Zupakhina instrument reconstructed at VostNII.

The main indicator of the hazard of sections is the index “K,” which is determined from the strength coefficient and the index of the initial rate of gas emission from the coal.

If the index “K” is greater than 1, then the seam areas are hazardous with respect to sudden outbursts; when “K” is less than 1, the seam areas are nonhazardous with respect to sudden outbursts.

It should be borne in mind that, when assigning seam areas to the hazardous category, it is necessary to take into account the hydrogeological conditions of their occurrence. In hazardous areas the coal of the seams is always dry and contains no visible moisture. When any appreciable amount of moisture appears in the coal, the seam area is considered nonhazardous with respect to sudden outbursts.

According to the degree of tectonic disturbance, we distinguish five types of coals (Fig. 3). Sudden outbursts can occur only in those areas of seams whose coal is crumpled and has a breccia-like, fine-lens-like, earthy-granular, or earthy tectonic structure. In areas composed of undisturbed coal, sudden outbursts do not occur.

Undisturbed coal structure

Breccia-like coal structure

Lens-like coal structure

Earthy-granular coal structure

Earthy coal structure

Fig. 3. Tectonic structures of coals

The degree of disturbance of coal seams is also characterized by tectonic textures¹. Of the four types of tectonic textures distinguished (Fig. 4), sudden outbursts are possible when the seam has an intensely folded, folded, or weakly folded texture. Seam areas with an intensely folded tectonic texture are the most susceptible to sudden outbursts.

Assignment of seam areas to the category hazardous with respect to sudden outbursts is carried out with allowance for the comprehensive linkage of the factors and properties indicated above. In this connection, on the basis of the depth of mining and the character of major tectonic disturbances, a preliminary idea is formed of

the danger of a given section, and, on the basis of the coal strength, the index of the initial rate of gas emission from the coal, the hydrogeological conditions of seam occurrence, and the tectonic structures of the coal and the textures of the seams—the final conclusion.

If, during mining operations in a section of a seam, warning signs are observed that are caused by redistribution of rock pressure due to the driving of workings, this must also serve as a basis for classifying such a section as hazardous with respect to sudden outbursts of coal and gas. This category of signs includes: sound effects (noise, hissing, whistling, cracking, impacts); increased pressure on the supports (bending, breaking, floor heave); phenomena on the exposed surface of the face (coal scaling, coal crumbling, squeezing-out of coal, pieces of coal rebounding); increased gas emission (uniform or sharply increasing).

Fig. 4. Tectonic textures of coal seams

The presence in a coal seam of small plicative disturbances (flexures, swellings, pinchings, floor steps), manifested in changes in the shape of the seam, must serve as a sign indicating possible sites of sudden outbursts within a hazardous section.

The work carried out by the institute made it possible for the first time to raise the question of the need to divide seams into hazardous and nonhazardous sections and to propose a method for their division. The developed method for forecasting hazardous sections was made the basis of a specially compiled instruction. The instruction is currently at the stage of introduction in the mines of Kuzbass and the Egorino deposit.

It should be noted that the method of identifying hazardous sections, besides increasing the safety of mining operations, also gives a rather significant economic effect. At present all development workings in hazardous seams are driven with the mandatory use of some measure for combating sudden outbursts. The identification, in seams, of hazardous and nonhazardous sections makes it possible to apply outburst-control measures differentially, i.e., such measures need be used only in those sections of seams where distinctive signs actually confirm that sudden outbursts of coal and gas may occur there.

The method set forth by VostNII, for the first time not only in the USSR but also abroad, makes it possible to divide seams into sections that are hazardous and nonhazardous with respect to sudden outbursts.

Degassing of Coal Seams in Mines

Preliminary degassing of coal seams, as a method of controlling gas emission, has been used in our country relatively recently. The first degassing installations were built in 1951 in the Kuzbass at the Severnaya and Koksovaya-2 mines. Beginning in 1952, degassing work was started in the Karaganda Basin and in the Donbass. The organization of degassing work was prompted by the need to reduce the gas content of mine workings, which in recent years has increased greatly owing to the increasing depth of mining. At present, the relative gas abundance of a number of Kuzbass mines amounts to tens of cubic meters. The release of enormous quantities of methane disrupts the normal operation of mines and constitutes a serious danger to the lives of miners. It is therefore quite natural that preliminary degassing of coal seams has attracted ever increasing attention in the last decade. In the Kuzbass alone, over the past 5 years, 7 vacuum installations have been built and put into operation, serving more than 20 degassing sections. During this period about 200 drainage boreholes have been drilled.

Seams occurring under various conditions have been subjected to degassing. Degassing of coal seams in Kuzbass mines was carried out by boreholes drilled from underground workings up-dip, along strike, and across the strike of the seams, as well as by boreholes drilled from the surface to the axis of the anticlinal fold of the seam series. The greatest effectiveness in degassing seams by boreholes was achieved when making use of the effect of relieving the degassed seams from rock pressure.

As a result of degassing, not only the gas content but also the gas pressure in the massif is sharply reduced. In some cases a decrease in gas content by 2.5–3.0 times and in gas pressure by several times was observed. There were cases when, as a result of degassing coal seams, the gas pressure in control boreholes decreased to atmospheric pressure and even to a vacuum.

A sharp decrease in gas content and gas pressure in the coal seam has a favorable effect on reducing the gas abundance of mine workings. Observations have established that, as a result of degassing, the gas abundance of mine workings may decrease by 2–3 times (Table 4).

Table 4

When degassing is carried out, safer working conditions are created, and the productivity of sections and of mines as a whole increases. The latter is clearly confirmed by the example of the Voroshilov Mine, where

As a result of prolonged stoppages of the longwall on seam IV Internal because of gas contamination of the workings, in August 1956 alone the production section fell short by about 6000 tons of coal. When degassing was carried out, this same section in September of the same year, 1956, produced 8000 tons of coal instead of the planned 7000 tons.

The productivity of vacuum installations in the mines of Kuzbass fluctuates within the range of 2–6 thousand and averages 3–4 thousand m³ of methane per day. At present it is possible to bring methane production at each installation up to 10–12 thousand m³ per day, which can be used to meet household needs. Such a quantity of methane, if the annual gas requirement of one person is taken as 100 m³, is sufficient to meet the needs of a settlement or part of a city with a population of about 36 thousand people.

It has been calculated that for the Prokopyevsk–Kiselevsk district of Kuzbass, in 40–50 years, when the mines will be working seams at a depth of 700–800 m from the earth’s surface, the average gas content for some mines will be about 100 m³/t of daily output. From a mine with an output of 5 thousand tons of coal per day, about 500 thousand m³ of methane will be released. If it is assumed that, by means of degassing, 20% of the total amount of methane emitted can be removed through gas pipelines, then it should be sufficient for 360 thousand people.

From what has been said it is evident that degassing of coal seams at great depth will be an even more effective method of controlling gas emission in mines and a source for obtaining large quantities of high-calorific fuel. The reduction of gas content by 2.5–3 times and of gas pressure in the coal seam to atmospheric pressure gives grounds for considering preliminary degassing of seams by boreholes with the use of vacuum one of the effective methods of combating sudden outbursts of coal and gas.

1. By the tectonic structure of coals is meant the morphology and size of coal particles. The tectonic texture of coal seams reflects the spatial orientation and arrangement of the constituent parts—coal particles. Tectonic structure and texture are predetermined by tectonic processes. ↩