The Influence of Fracturing on the Control of Side Rocks in the Mining of Coal Seams

P. K. Kulikov

Introduction

In the articles by A. F. Zasyadko, A. P. Sudoplatov, V. I. Baranovsky, L. D. Shevyakov, and others that have appeared recently, emphasis is placed on the need to discover and study the objective laws of nature that determine those processes which proceed spontaneously and which people have not yet learned to control reliably. At the same time, it is noted that only by studying the regularities in the manifestation of natural forces can one make correct forecasts and choose effective methods of combating undesirable manifestations of these forces.

In the articles mentioned, the question is raised, in particular, of the need to study the tectonic structure of coal deposits, and especially those elements of the general structure that have a direct influence on the conduct of mining operations.

In this connection, the study of the fracturing of rocks and its influence on mining operations acquires especially important significance, since fracturing is one of the principal, widely distributed elements of the tectonic structure of deposits.

Our work is devoted to a description of the fracturing of rocks and its influence on the control of side rocks in the Prokopyevsk district of Kuzbass. The problem addressed in this work, while very important, is at the same time very complex and labor-intensive, and for its complete solution requires the prolonged and systematic labor of a large collective. Therefore our work should be regarded as a modest contribution to the development of this question.

Brief Description of the Tectonics of the Prokopyevsk District

As a result of a prolonged and complex process of tectonic dislocations at the junction of the Salair and the Kuznetsk Basin, a distinctive structural zone was formed, consisting of a number of slices bounded by major faults of northwestern strike. Along these faults the slices were thrust over one another, predominantly from west to east.

Within the coal-bearing deposits of Kuzbass, five such slices are distinguished: the Prityrgan, Chertinsk, Belovsk, Leninsk-Kuznetsk, and Meret slices. Each of these slices is, as it were, an independent structural element and is characterized by a more or less extensive complex of folds and faults of various orders. The intensity

of the internal structures of each slice is different, and the increase in “stress” is manifested very clearly when moving from east to west from the central part of the basin toward Salair.

The Tyrgan slice, directly adjoining Salair, is one of the most strongly dislocated. As a result of bending, in the vertical plane, of the axis of the principal synclinal fold of this slice, several brachyfolds appear here, among which the Prokopyevsk–Kiselevsk brachysyncline stands out, corresponding to the deposit of the same name.

Thus, in structural terms the Prokopyevsk–Kiselevsk deposit constitutes a single, very large brachysynclinal fold. On the limbs of this fold there is a whole series of smaller, rather stressed folds, elongated in the NW direction, and a comparatively dense network of large and small tectonic faults. In terms of the stress and complexity of its tectonic structure, the Prokopyevsk–Kiselevsk deposit is one of the most outstanding among all deposits of Kuzbass.

The Prokopyevsk district embraces the southern half of the Prokopyevsk–Kiselevsk brachysyncline and, like the entire deposit, has a complex structure. Within it, 8 synclinal and the same number of anticlinal folds are distinguished, having sharp, peak-like outlines in plan. All folds are characterized by very steep dips of the limbs. The strike of the folds is northwestern. The folds plunge in the same direction.

Fault structures in the district are also rather numerous. The large faults alone, traceable for several kilometers, number about fifteen here. Almost all large faults cut the sequence parallel to the strike of the fold axes and dip to the SW at an angle of 75–85°. In addition to large structures, a large number of small structural forms appear in the district, determining the internal structure of the folded sequence—small folds, faults, pinches, and jointing, which are ubiquitous.

Jointing of rocks

The entire diversity of jointing systems is subdivided by A. A. Belitsky into two groups—normal-cutting and oblique-cutting, each of which is characterized by its own features of manifestation and distribution. We also used this subdivision in our work.

Normal-cutting jointing. This group includes systems of joints arranged perpendicular to the bedding and forming an angle of about 90° with one another. One of these systems cuts the rocks along the strike line and is called the longitudinal normal-cutting system. The other cuts the rock sequence along the dip line and is called the transverse normal-cutting system.

All these joint systems, intersecting in space, divide the rock sequence or coal seam into more or less regular blocks, giving rise to cubic, platy, or columnar separateness. The sizes of the faces of the separate blocks vary greatly, showing a close dependence on the composition of the rocks, the thickness of the beds, and the stress of the tectonic structure.

The dependence on rock composition is the most distinct. In coal, normal-cutting jointing is very abundant. The distances between individual joints sometimes reach here 0.5–1 cm. However, in some seams the joints are much more widely spaced. For example, in the Moshchnyi seam, the coal of which is distinguished by increased hardness, the distance between individual noticeable joints reaches

0.2–0.5 m. Argillites are also distinguished by a high density of jointing; in this respect they approach coals. In aleurolites and sandstones, jointing is developed much more weakly. Thus, in massive sandstones the distances between adjacent joints reach 1.0 m and more.

The dependence of the density of jointing on the thickness of beds is clearly manifested when beds of the same lithological composition but sharply different thickness are compared. As a rule, in thick beds jointing is less abundant than in thin beds.

Oblique jointing. This group includes joints that form an acute angle with the bedding. All oblique joints are subdivided, according to the classification of A. A. Belitsky, into six systems. Two of them are oriented longitudinally with respect to the axial planes of folds and dip toward one another at an angle of 45–60° to the horizon, forming an angle of 90° between them.

Two other systems, arranged with respect to one another in exactly the same way as the first two, cut the folded structures across their strike. Both systems of the third pair are oriented diagonally with respect to the fold axes and occur at an angle of 60–70° to the horizon. This classification is at present generally accepted in Kuzbass, although it is to a certain extent conventional.

Oblique, as well as normal-oblique, jointing is characterized by varying lengths of individual joints. In this respect all joints are divided into three groups. The first group includes the most widespread joints, with a very small extent—from several centimeters to several tens of centimeters. The second group comprises larger, but also rarer, joints. These joints cut across a seam or bed and terminate near the floor and roof or at their contact. The third group includes very rare joints that cut an entire series of beds, the distance between them sometimes reaching several meters. These joints are, in all probability, a transitional link from ordinary jointing to ruptures.

As has been established, the density of oblique jointing does not remain constant. It varies from place to place and reveals a dependence on the lithological composition of the rocks, on the thickness of beds, and on the intensity and complexity of the structure of folded forms. The nature of the dependence of jointing density on the first two factors remains the same as in the case of normal-oblique jointing.

The dependence of joint density on the intensity of structures, on the general tectonic setting, is manifested more clearly. This dependence determines the character of the manifestation of oblique jointing not only in individual parts of the Prokopyevsk region, but also in various regions of Kuzbass. Thus, oblique jointing is almost unknown in the Leninsky, Belovsky, and Baidaevsky regions, whose tectonic structure is very simple, and is strongly developed in the Prokopyevsk-Kiselevsk, Kemerovo, Bachat, and Bunguro-Chumysh regions, which have a complex tectonic structure. Taking this into account, it may be assumed that this group of joints arises as a result of the action of mechanical loads on the rock mass.

In space, oblique jointing has the most varied orientation; therefore, as factual material accumulates, the points corresponding to individual joints fill the entire field of the circular diagram (Fig. 1), so that the separation of individual

systems of fracturing with the aid of this diagram becomes almost impossible.

It is also necessary to note bedding-parallel fractures. The latter are located parallel to the bedding, sometimes deviating from it somewhat.

The intensity of the manifestation of bedding-parallel fractures in rocks depends mainly on two factors: on the stratification of the rocks and on the stress of the folds. Since bedding-parallel fracturing is, by its origin, most probably due to the process of fold formation, it is natural that both of these factors manifest themselves in interaction. It is known that the rocks participating in the formation of fold closures are most subject to delamination. In this case, thin-bedded rocks are split by bedding-parallel fractures much more strongly than thick-bedded and massive rocks.

Fig. 1. Circular diagram of the fracturing of rocks in the western part of the Prokop’evsk district.

Influence of fracturing on roof control

Driving a mine working in a rock mass disturbs the initial distribution of stresses in the mass, leads to a redistribution of these stresses and to their local concentration. Around the working a new stress field arises. The appearance of a new stress field is accompanied by deformation of the rocks in the sides of the working, which under certain conditions may end in the destruction of the rocks and the collapse of the working. Since these conditions exist everywhere in the district, any mine working is potentially under threat of destruction.

To ensure the preservation of the working and the safety of work in it, a set of measures is applied for the artificial regulation of rock pressure, bearing the general name—control of rock pressure. In stoping workings on gently and inclined dipping seams, deformations of the roof rocks acquire the greatest importance. Therefore, for such workings the term roof control is used, which loses its meaning where the seam dips steeply, when deformations of the floor rocks also become appreciable.

Control of rock pressure in the stoping workings of coal deposits constitutes an especially complex problem; it is therefore quite understandable that roof control is one of the most important questions of mining. Serious attention is paid to this question in the literature. But, despite this, roof control is a problem that is still far from solved.

One of the main reasons for the insufficient development of this problem is that, in studying it, it is very difficult correctly to take into account geological factors, and especially the fracturing of the roof rocks, which are capable of fundamentally influencing the behavior of the roof after its exposure. The importance of this reason is not disputed by anyone. The literature emphasizes the necessity of a detailed study of geological factors, including the fracturing of rocks; however, as practice shows, their study has not yet been begun. As a result, some specialists concerned with questions of control—

of roofs, even now still compare it with an enormous massive beam resting on supports—pillars of coal.

Such an approach to the solution of this problem, which does not take into account the structural heterogeneity of the roof rocks, is incorrect, as a result of which the results of observations often do not correspond to analytical calculations.

As we have already noted above, rocks are not monolithic; they are penetrated by a more or less dense network of fractures, oriented differently in space. The presence of such a network naturally leads to a fundamental change in the physico-mechanical properties of rocks, and this inevitably affects the behavior of the undermined strata.

In this chapter we attempt to show in what the influence of fracturing on the behavior of the roof is expressed, and how taking fracturing into account affects the choice of the method of roof control.

The rocks composing the productive deposits of the Prokopyevsk field are represented mainly by beds of sandstones, siltstones, argillites, and coals. The thickness of the beds varies within very wide limits—from several centimeters to several tens of meters.

The fracturing of rocks is closely dependent on the lithological composition of the rock, on the thickness of the bed, and on the stress state of the general tectonic structure. Since the principal factors determining the degree of expression of fracturing are different at each point of a district, it is entirely understandable that fracturing also manifests itself unevenly over the area of the district. This is precisely what explains the specific character of the behavior of the side rocks accompanying the mining of a coal seam in each particular case.

Since the behavior of the roof undermined by stoping operations in the Prokopyevsk district depends to a considerable degree on its fracturing, any classification of rocks whose purpose is to facilitate the solution of the question of roof control in this district must necessarily take into account the fracturing of the rocks. At the same time, it must also take into account those factors that determine the degree of expression and density of fracturing, for only by knowing these factors can one make a forecast of the fracturing of the roof rocks and of the possible behavior of the roof in each particular case.

As is known, such a classification does not exist at present. The classification of VUGI, according to which all rocks are subdivided into 4 classes characterized with respect to rock stability and the thickness of individual beds, was developed on the basis of observation materials in the Donets Basin, whose geology differs significantly from the geology of Kuzbass. Therefore it cannot satisfy the coal miners of Kuzbass and is almost not used here.

We also subdivide the rocks of the Prokopyevsk district into 4 types, but the basis of each subdivision is the fracturing of the rocks. In addition, to characterize each of these types, the lithological composition of the rocks and the thickness of the individual beds composing the roof are given. As noted above, the lithology of the rocks and the thickness of the beds, along with the general intensity of the tectonics of the area, determine the density and sharpness of the manifestation of fracturing, and therefore are the principal diagnostic features that make it possible to establish the type of roof of any explored, but not yet opened, seam.

Type I—normally fractured roof. The rocks are penetrated by a dense network of fractures; however, because the fractures have insignif-

... considerable extent and intersect very rarely; the stratum preserves its coherence. Such a stratum has the properties of an anisotropic body, possessing sharply different shear resistance in different directions. A roof of type I is composed of layers of rocks of different lithological composition (sandstones, argillites, siltstones), with the thickness of individual layers not exceeding 2–5 m. This type of roof may also be called a composite roof.

Type II—an unfractured roof, or one in which fracturing is in an incipient state. A roof with well-developed longitudinal, normally cutting fracturing may also be assigned to this same type in cases where the stratum dips steeply and the fractures are located at less than the angle of natural repose and do not contribute to destruction of the roof after its exposure. A roof of type II is usually composed of thick layers of homogeneous sandstones or coarse siltstones, as well as layers of very strong sandstones of medium thickness. The degree of stress in the general tectonic structure of the area is moderate or weak. This type may be called a massive roof.

Type III—the roof consists of a thick layer of unfractured or weakly fractured rock, corresponding to the main roof, and of a moderately or strongly fractured layer of any lithological composition constituting the immediate roof. The thickness of the immediate roof does not exceed 4–6 times the thickness of the seam. The general tectonic setting is the same as in the case of a type II roof. In lithological composition and properties the main roof is similar to a type II roof.

Type IV—a strongly fractured roof. The fracturing is developed to such an extent that the entire stratum is broken into a whole series of blocks of various shapes and sizes. The physicomechanical properties of a type IV roof approach those of loose bodies. The lithological composition of the stratum may be anything from argillite to sandstone. The general tectonic structure of the area is characterized by increased stress and complexity.

From the standpoint of roof control, the composite roof is the most favorable; therefore it is considered that extraction of seams with such a roof takes place under normal mining-and-geological conditions. Since the fracturing of the roof in this case is sufficiently abundant and uniform, the load on the support is distributed uniformly. The roof weighting interval¹ is established quickly and, as the seam is cut, does not change, which makes it possible to conduct mining operations in accordance with the support plan. Roof weighting is carried out easily, simply, and quickly, regardless of the type of breaker support. The average value of the weighting interval, depending on the seam thickness, is 8–12 m. Collapse of the roof occurs successively, layer by layer, and the falling blocks have comparatively small dimensions.

As a result of the collapse of part of the rocks, arch-shaped depressions are formed in the roof; these increase as individual domes coalesce while the longwall face advances. If the fracturing of the rocks in the layers is sufficiently uniform, then the walls of the arches are relatively smooth; if it is nonuniform, the walls of the arches have stepped outlines, with projections forming along layers of stronger rocks. Since the weighting of longwalls on inclined and steep seams usually begins from above, the roof collapses in the upper part of the worked-out space, while the lower part of it is undercut at the expense of

of the rocks that had collapsed above, as a result of which roof displacements are prevented here.

In seams where the movement of rocks into the excavated space is impossible because of the small dip angle of the seam, roof collapse occurs over the entire excavated space, with the exception of a narrow near-face strip supported by powered supports. The picture described was observed, in particular, in the Prokopyevsk, Sporny, and Podsporny seams of the Maneikha mine.

The behavior of the roof, which we have assigned to the second type, is directly opposite. As an example, let us describe the case of working the Kharakternyi seam from crosscut No. 52 of the same mine. In this area the seam has the following characteristics: seam thickness 2.20 m, dip angle 40–50°. The coal is strong and fissured.

Argillites occur in the floor of the seam, with a thickness of 1.5–2.0 m, below which lies a thick stratum of sandstones. The immediate roof of the seam is represented by a thin layer of conglomerate, gradually passing into sandstone, which composes the main roof and has a thickness of up to 40 m. The sandstone stratum of the roof is only rarely intersected by normal-secant fractures. Oblique-secant and bedding-plane fractures are almost absent.

The seam is worked by a system of long pillars along the strike, with caving of the roof. The longwall length is 35 m. The intended initial caving interval was 20 m. The very first attempts to cave the longwall showed that the magnitude of the caving interval had been chosen incorrectly. It was not possible to cave the longwall even at 30 m. The collapse of the roof occurred suddenly, after the face had advanced 35 m; moreover, the longwall support was completely destroyed, and the roof was sheared off right at the face itself. On the surface above the longwall a subsidence funnel formed. The longwall was buried and went out of operation.

Subsequently, instead of general caving of the roof, which proved impossible, weakening of the excavated space was undertaken by blasting the roof and the upper part of the excavated space, in the immediate vicinity of the entries, where the rocks are strongly weakened by weathering. In working the second sublevel, weakening of the excavated space was carried out by allowing rocks to flow from the upper sublevel¹.

An entirely different picture was revealed when working the southern part of this same seam, near the boundary with the Zenkovskie Inclines mine, where the roof was strongly broken by fractures and as a result had lost its solidity. In connection with the different nature of the development of fracturing in the roof of the seam, two sections, each 100–150 m in length along the strike, are distinguished here. In one of them, weakening of the roof occurred as a result of undermining of this section by the Gorely seam, located below the Kharakternyi seam.

In the second section, weakening of the roof was caused by the development in it of a tectonic fault and the associated network of fracturing. Both sections were worked by longwalls with the caving interval reduced to 6–8 m. Caving of the longwalls proceeded without particular difficulty; no hanging roof² was observed. Collapse of the roof was accompanied by the appearance of sinkholes at the surface. The faces worked normally, without unexpected complications. In working the second of these sections, in certain places continuous cutting-through of the roof in the longwall was necessary in order to—

to prevent the spilling of rock fines formed as a result of the soaking of the destroyed sandstone. The seam roof in both of the latter sections may be assigned to the fourth type of our classification, which includes disturbed, strongly fissured rocks of any lithological composition.

The behavior of the roof of the third type may be characterized by the example of the Udarnyi seam, eastern limb of the II syncline of the “Manenkha” mine, where observations were carried out in connection with the testing of the Polyakov–Arkhangel’skii movable roof support. The thickness of the seam in this section was 0.7–0.9 m. The dip angle of the seam is 65°. The coal of the seam is fissured, of medium strength. Aleurolites and sandstones occur in the floor of the seam. The roof of the seam is represented by sandstone, aleurolite, and argillite. A thin interbed of argillite 0.1–0.15 m thick, constituting a false roof, is strongly fissured and unstable. Above it lies a layer of aleurolite 1.0 to 1.5 m thick with well-developed normal jointing. This layer constitutes the immediate roof. The thickness of the monolithic sandstone layer forming the main roof reaches 20 m and more.

The seam was worked by a system of long pillars along the strike with caving of the roof, equipped with the Polyakov–Arkhangel’skii sectional metal support. Extraction of the seam encountered very serious difficulties, as a result of which testing of the support was suspended.

The main obstacle to carrying out stoping operations arose from the roof. It turned out that all the layers composing the roof behave differently. First of all, at the moment of breaking the face, the false roof collapsed together with the coal. The immediate roof began to move immediately after the sections were taken out from under load, even before they were moved.

Along the existing normal joints, the aleurolite layer broke up into blocks reaching 2–3 m in cross section; as they settled, these blocks crushed and jammed the support sections (Fig. 2), owing to which some of the sections had to be left in the collapse. The main roof, in contrast

Fig. 2. Operation of a Polyakov–Arkhangel’skii support section on the Udarnyi seam of mine No. 11.

to the immediate and false roof, proved to be exceptionally stable and was not made to settle, despite the attempts undertaken.

The question of roof control here could have been resolved in the same way as on the Kharakternyi seam, i.e., the working space of the longwall could have been secured by packing the worked-out space with rock ...

the roof, blasted in its upper part near the drifts, as well as by the drifts themselves. In this case, however, extraction of the seam would have been characterized by certain specific features, because of which carrying out drilling-and-blasting work to cave the roof in the upper part of the worked-out space would have been associated with very serious danger to the workers.

Because of the increased danger of the work, this method of roof control had to be rejected. The only remaining possibility was to use gravity stowing of the worked-out space, but this method too, because of the absence of a stowing operation and transport, also proved unacceptable. The question of roof control thus remained open.

Continuing the tests of the movable support under these conditions would have been pointless, since there was a danger of sudden roof collapse. It was therefore decided to divide the longwall into two sublevels and to mine the upper sublevel with ordinary timber supports, which would make it easier to undercut the longwall by the drifts and weathered rocks, while in the lower sublevel the tests of the sectional support would be continued, after first modifying somewhat the procedure for conducting work in the longwall so as to ensure the passage of rock from the upper sublevel.

Thanks to the introduction of these changes into the seam-mining system, it was possible to solve the problem of roof control in the longwall of the upper sublevel and to ensure the safety of work in the longwall of the lower sublevel. But this did not solve roof control in the longwall of the lower sublevel, for the immediate roof continued to behave exactly as before, i.e., its destruction coincided with the moment when the sections were brought out from under load, thereby causing their constant jamming. This explains why the Polyakov–Arkhangel’skii support did not find application on the Udarnyi seam.

We may confine ourselves to the examples cited, since they sufficiently characterize the influence of the degree of fracturing of roof rocks on its behavior. Indeed, massive, unfractured or weakly fractured rocks prove less capable of destruction under the stresses arising when the stratum is undercut than fractured rocks. Thus, control of the roof of the Kharakternyi seam proves to be very difficult, and its complete caving is often impossible. At the same time, this seam, when undercut, was mined without particular difficulty by long pillars with roof caving, and roof caving was carried out comparatively simply. An even clearer dependence between the ability of the stratum to cave and the degree of its fracturing is established when comparing the mining features of undisturbed and disturbed sections of the same seams.

Fracturing causes a weakening of the rock stratum. The more strongly it is developed, the greater the tendency of the rocks to cave and, conversely, the less fractured the stratum, the more difficult it is to destroy and the greater the probability of its hanging up, with all the ensuing consequences.

This regularity has a definite practical significance, which consists in the following. Until recently, owing to the lack of forecasting capability, the choice of a method of roof control was made by successively testing all known methods and selecting the one that, under the given conditions, was the most convenient and safe. This procedure has a number of serious shortcomings, as is easily seen by recalling the case of mining the undisturbed part of the Kharakternyi seam, when during such a test a sudden collapse of the roof occurred, causing complete blockage of the longwall and putting it out of operation.

With the establishment of the noted regularity, we obtain the possibility of a scientific approach to solving the problem of finding the method of roof control most suitable for working a particular section of a seam. This scientific approach consists in determining, even before the start of seam extraction, the degree of fracturing of the roof rocks, which is associated with the lithology of the roof, the thickness of the beds, and the tectonics of the area—factors sufficiently well known by that time. Knowing the degree of fracturing, it is already possible to predict with relative accuracy the behavior of the roof when it is exposed and, on this basis, to select the appropriate method of roof control and determine the magnitude of the setting step.

In conclusion, as an example of forecasting the physical and mechanical properties of the roof and using this forecast to select a method of roof control, we shall make several observations concerning the working of certain coal seams in the district.

1. Seams with a massive, unfractured, and unbedded roof are dangerous with respect to roof hanging. They are dangerous at any dip angle of the coal. In addition, seams with a slightly fractured homogeneous roof dipping more steeply than 45—50° are dangerous with respect to roof hanging. At such dip angles of the seam, the longitudinal normal-secant fractures are positioned more gently than 45°, i.e., more gently than the angle of natural repose, owing to which the destruction and sliding of the roof rocks along them is precluded.

With a gentler occurrence of the strata, the longitudinal normal-secant fractures assume a steeper position, which, undoubtedly, facilitates roof failure.

Among the seams dangerous with respect to roof hanging may be included the Kharakternyi, Podlutuginskii, and in places the Prokop’evskii, Dvoinoi, Udarnyi, and Slozhnyi seams. For these seams the most acceptable methods of roof control are stowing, and also partial or complete caving of the worked-out space by allowing alluvium or collapsed rocks from the upper horizons to pass through. In addition, in the case of an especially stable roof, when stowing or caving cannot be applied, it is possible, as an exception, to recommend undermining the seam as a measure for weakening the roof and preventing its hanging. This last recommendation is based on the study of an example of undermining the Kharakternyi seam by the Gorelyi seam, which took place at the Maneikha mine, where the thickness of the Gorelyi seam is 7 m, and the distance between these seams reaches 46 m.

1. For seams with a complex roof—for example, the Spornyi, Sadovyi, Lutuginskii, and others—provided there is no danger of soil sliding, any method of roof control may be used, in particular roof caving, namely its self-subsidence.

2. The working of seams in which the main roof, unlike the immediate and false roof, is prone to hanging must be carried out with consideration of the same observations as in the working of seams dangerous with respect to hanging. The only acceptable methods of roof control when working seams under such conditions are complete stowing of the worked-out space and the passing of alluvium or collapsed rocks from the upper horizons. It should be remembered that the danger of sudden roof collapse in such seams is much greater than in all others, since it is usually not taken into account because the hanging of the main roof is masked by the collapse of the immediate roof.

3. When working seams with a strongly disturbed roof, as in the case of a complex roof, any method of roof control may be used, with the exception of smooth bending, provided that a reduced setting step or stowing step is used in comparison with the normal one.

The Effect of Fracturing on the Stability of the Floor of Coal Seams

Sometimes, during the mining of coal seams, displacement of rocks is observed not only in the roof, but also in the floor of the seam; moreover, the consequences of such displacements in the floor, since as a rule they are not taken into account in due time, are far more dangerous than the consequences of roof displacements.

A case characteristic of such phenomena occurred during the mining of the Prokopyevsky seam, western limb of the II syncline, at the Maneikha mine. The Prokopyevsky seam here consists of two benches, each 2.3–2.5 m thick. The dip angle of the seam is 75°. The coal benches of the seam are separated by an interbed of argillite, whose thickness varies from 1 to 1.5 m.

This interbed, because a bedding-plane rupture had passed along it, was slickensided and greatly weakened, and the surfaces of the cracks had undergone rubbing and polishing. The roof of the seam is represented by siltstone of medium stability. The floor of the seam is composed of similar siltstone and argillite. The coal of the seam is fractured and partly slickensided. The upper bench of the seam was mined as an independent seam, while the lower bench remained untouched. The mining system was long pillars along the strike with caving of the roof. The length of the longwall panels was 20 m. During the setting of the longwall, the floor slid along a plane inclined to the horizon at an angle of 60–65°. The sliding began approximately from the middle of the worked-out space, involved the rock interbed and the lower bench of the seam together with the minus-level entry previously driven along it (Fig. 3).

Fig. 3. Sliding of the floor during mining of the upper bench of the Prokopyevsky seam at mine No. 11.

Since the caving of the roof was accompanied by sliding of the floor, this method of roof control had to be abandoned, and it was necessary to proceed to stowing the worked-out space with rocks delivered from the upper level. However, this method also proved unsuccessful, since the destruction of the interlevel pillar led only to an increase in the probability of floor sliding. The only acceptable method of controlling the side rocks under these conditions proved to be stowing of the worked-out space.

Displacement in a seam bench, expressed in its sliding, is generally encountered only under certain conditions and is connected primarily with fracturing of the floor rocks. It was observed in seams where the floor is represented by bedded, strongly fractured argillite or siltstone, and is completely unknown in seams with a massive, strong floor. In all known cases, the planes of floor sliding dip toward the worked-out space at an angle of 55–70°.

Since longitudinal diagonal cracks, along which floor sliding usually occurs, attain such a dip angle in seams dipping more steeply than 70°, it is natural to suppose that all seams dipping at an angle greater than 70°, in whose floor longitudinal diagonal and bedding cracks are developed, are dangerous with respect to floor sliding.

Mining of seams (on which floor sliding is possible) by the system of long pillars along the strike with caving of the roof or with undercutting—

… caving of the mined-out space by transferring rock from the upper horizons is, unquestionably, impermissible. The only acceptable method of controlling the side rocks when mining such seams is complete stowing of the mined-out space; moreover, depending on the degree of disturbance of the floor, the stowing interval may be reduced to 4–5 m.

Kuzbassugol Combine

1. The flow of rocks is carried out by destroying the inter-sublevel pillar, which forms a bridge between the sublevels. ↩↩

2. A hanging roof is a roof that has not yielded to destruction and retains its original position above the worked seam. ↩