STUDY OF ROCK DISPLACEMENT IN THE MINING OF THICK STEEPLY DIPPING SEAMS IN THE KUZBASS

E. V. KUNYAEV

Rock displacement, as the result of a redistribution of stresses, is one of the manifestations of rock pressure. At the same time, once displacement of the rocks has begun, it itself becomes the cause of pressure on the supports and on the coal pillars surrounding the workings. Rock displacement as a cause of pressure is, in fact, the proper subject of study in the investigation of questions connected with roof control.

The study of rock displacement provides material for selecting the parameters of development systems, selecting the method of roof control, choosing the type of support and calculating its strength, and for devising measures to combat manifestations of rock pressure of an emergency character: rock bursts, gas outbursts, and clay inrushes into workings.

The maintenance of preparatory workings, especially gate roads, near the extraction face is directly connected with roof control. The correctness of ventilation of extraction faces often depends on roof control. For example, when the roof is incompletely seated, ventilation conditions deteriorate, and when an open cave reaches the surface, ventilation of extraction workings is sometimes completely disrupted. Of particular importance in mining self-igniting seams are complete caving of the roof rocks and their good compaction in the worked-out space.

The process of caving and displacement of rocks in the mining of thick steeply dipping seams has features that introduce specificity into the methodology for studying rock displacement. The presence, during caving, of simultaneous discharge from the upper level of previously caved rocks; deformation and destruction of interlevel pillars and of pillars between sublevels; settlement of stowing material supplied from outside into the worked-out space—all this distinguishes the mining of thick steeply dipping seams from all others.

The specific questions to be studied by means of observations of rock displacement are the following.

1. Determination of the dimensions of the zones of coal squeezing at the boundaries of the extraction working and of the zone of abutment pressure.

2. Determination of the dimensions of stable exposures of the roof rocks and of the coal mass.

3. Study of crack formation in the roof rocks of the seam and in coal; determination of the dimensions of the crack-formation zone.

4. Determination of the degree of compaction of caved rocks or stowing in the worked-out space.

1. Determination of the characteristic magnitudes of roof displacement in mined-out workings and of support deformations.

2. Identification of the complete pattern of caving, subsidence, and displacement of rocks.

It must be said that most questions of roof control are, to a sufficiently complete degree, resolved by studying only the displacements of rocks and deformations of the support. The need for pressure measurements arises only when determining the pressure on the support exerted by caved rocks and by packing, i.e., in those cases where the shifting rocks must be regarded as having no cohesion between particles.

In most cases the magnitude of rock displacement may serve as a criterion for the correctness of roof control, i.e., the smaller the displacement of the roof within the face area, the better the mining conditions and the lower the probability of roof collapse. This proposition is valid for roof control by gradual lowering, but it is not valid for systems of extraction with roof support on coal pillars. Under certain conditions, the absence of roof displacement may mean the accumulation of stresses in the pillars, entailing sudden collapses and sometimes even rock bursts.

A more important and correct criterion of the good condition of the roof is the optimum rate of roof displacement in the face area, characteristic of the given geological and mining conditions. The rate of roof displacement is directly connected with the intensity of coal extraction. The relationship of these two quantities has so far been little studied; one can only say that, other conditions being equal, the same rate of roof displacement corresponds to the same rate of advance of the face, and the greater the rate of advance of the face, the greater the rate of roof displacement. In each individual case, the study of rock displacements takes place in a specific techno-economic setting. Naturally, for proper use of the observational results it is necessary to know not only the rock displacements themselves, but also the principal techno-economic indicators, the geology of the section, the organization of work, and the mining system. Without this it will be impossible subsequently to generalize the results of observations of rock displacements.

Theoretical calculations of the magnitudes of rock displacements, more or less developed for gently and inclinedly dipping seams, are too weakly substantiated for steep dips, and especially for thick seams, where, in addition to the side rocks, large masses of coal also take part in the displacement. Therefore, up to the present time, investigations of rock displacements in the Kuzbass have been carried out mainly experimentally. Material is being accumulated for the creation of a theory of rock displacement in the extraction of thick steeply dipping seams.

Methods for studying rock displacements under mine conditions are based on precise measurements of both relative and absolute displacements of points marked by signs (benchmarks) fixed in the rocks or in the coal.

Measurements of relative rock displacements in accessible places are made with “SU-1” measuring props (universal prop), manufactured by the Kharkov Plant of Mine-Surveying Instruments (KhZMI), and with steel measuring tapes.

The rates of rock displacements may be measured as the magnitudes of displacements over a certain short interval of time (day, month).

In the case of rapid displacements, the velocity can be measured with clock-type indicators attached to the measuring support SU-1. Having obtained the distribution of the velocity of displacement of the roof of the seam in the direction of advance of the longwall face, one can determine, by integrating the velocity curve, the total amount of roof displacement. A method for processing such velocity measurements was proposed by Prof. S. G. Avershin.

The absolute magnitudes of rock displacements may be obtained by means of mine-surveying and leveling, used in those cases when, first, the displacements reach sufficiently large values and, second, when the floor of the workings moves under the influence of extraction. Measurement of absolute displacements must necessarily be accompanied by measurement of relative rock displacements. With small rock displacements it is difficult to obtain the required accuracy by mine-surveying, since the tie-in traverses from fixed points must, as a rule, be several hundred meters long. If there is confidence that the floor of the working is immobile, then the displacements of the roof of the working are more simply and accurately determined from benchmarks set in the floor, by direct measurements. If it is required at the same time to determine the direction of rock movement, measurements may be made from two adjacent points by linear intersections. In inaccessible places, measurements of rock displacements are made with remote instruments.

In the Siberian Branch of VNIMI the following remote instruments are used:

1. Rheostat-type electrical sensors VNIMI RD-1 and RD-2. The sensor set includes a rheostat in a housing, a resistance measuring bridge, a thin metal tape by means of which the displacement is transmitted to the sensor pulley, and a weight suspended from the tape. The sensors are fastened on pins driven into boreholes drilled in the side rocks of the seams. A telescopic protective device was designed by branch staff member A. S. Shalygin; it makes it possible to install the sensor in the goaf and thus to carry out prolonged measurements of goaf displacements. The sensors are described in the literature.

2. String tensometers, consisting of a steel string enclosed in a housing and set in motion by an electromagnet. From the frequency of oscillations the length of the string and changes in it are determined. The tensometers are designed for measuring deformations of rock and reinforced-concrete support.

3. A string measuring support designed by staff member N. I. Kuksov. The support consists of a strong telescopic housing, containing a mechanism that permits measurements of displacements of the roof of a seam (bed) at a distance by means of a measuring string station used for observations of rock pressure. The drawback of such a support is the limited measuring range—250 mm.

4. A telescopic support with a tape measure passed over a pulley; the end of the tape measure is led through a pipe into an accessible working.

5. Signaling devices showing rock displacement when it reaches a specified value (at the testing stage).

6. Deep benchmarks installed in boreholes according to the method of the Ural Branch of VNIMI. It should be noted here that the best results are obtained if the borehole depth is small (up to 10–20 m), and the number of benchmarks in one borehole 40–70 mm in diameter does not exceed four. The choice of the location for installing the bench-

… in workings. If benchmarks are set in weak interbeds or in rocks of the false roof that are prone to heaving, then the general picture of rock displacement is distorted by local stresses and displacements.

In studying the displacements of the side rocks, they are considered separately:

a) ahead of the extraction face;
b) in the near-face space;
c) in the worked-out space.

Displacements of the rocks and deformations of coal pillars ahead of the extraction face characterize the magnitude of the abutment-pressure zone and the degree of activity with which abutment pressure is manifested. Displacements in the near-face space determine the required yielding capacity of the support and the width of the collapse or stowing step. Here it must also be noted that, for calculating the strength of the support in the extraction space, its yielding capacity is the most important indicator. As joint measurements of the compression of support joints and of rock pressure on the support show, the strength of wooden near-face support can be specified with sufficient accuracy on the basis only of linear measurements of the yielding of wooden support elements working in compression across the grain. As is known, props loaded along the axis are extremely rigid, permitting compression of only a few millimeters, after which destruction of the support occurs. In this case, in the overwhelming majority of cases the rock pressure is greater than any wooden support can withstand, and therefore the calculation must take chiefly the yielding capacity of the support into account. Displacements of the roof in the worked-out space (in the goaf) characterize the method of seam mining from the point of view of the completeness of isolation by the coal pillars left in the worked-out space, the effect of undermining on overlying seams and on the earth’s surface, and air leakage during ventilation.

Below are given several examples of the application of the results of observations of side-rock displacements to the solution of practical problems under various mining systems.

Shield system of mining

Observations of side-rock displacements under the shield system of mining are made difficult by the fact that the side rocks in the extraction space and ahead of it (down the dip) are almost not exposed. In certain cases it was possible to observe the mining by shields of rocks of the immediate roof. Thus, at mine No. 5–6 in 1945 a crosscut was mined by shields from seam IV Internal to seam VI Internal, driven specially for observations on the ventilation horizon, at a depth of 80 m from the surface. Seam IV Internal had a thickness of 9.5 m and a dip angle of 62°. During the extraction of the shield pillar, directly beneath the crosscut, the displacement of the roof of seam IV Internal was: vertically 107 mm, horizontally 131 mm, and total—167 mm. It may be assumed that the displacement of the seam roof in the near-face space did not exceed 160 mm. In 1955, at the same mine, postgraduate student of TPI G. E. Posokhov, with the aid of instrumental observations of the displacements of the side rocks in a ventilation heading, was able to establish that the influence of the shield face begins to appear at a distance of 20–25 m ahead of it.

In 1956, at the mine “Koksovaya-1,” the Siberian Branch of VNIMI conducted observations of the displacements of the rock strata during the mining of seam VI Internal, having a thickness of 2.5–2.8 m and a dip angle of 65°,

with the use of the KVKP shield support. In this case the size of the zone of influence was equal to 25 m, and the rate of displacement of the seam roof along the normal ranged from 0 at the edges to 18 mm per day in the middle of the shield pillar.

If one takes into account that the rate of advance of the KVKP shield was 1 m per day, then the total amount of displacement of the seam roof during the time the shield advanced 25 m was of the order of:

\[ h = \frac{25 \times 18}{1 \times 2} = 225 \text{ mm}. \]

The observed magnitudes of displacement of the roof directly beneath the shields, within the range of 160–225 mm, and displacement rates of not more than 20–30 mm/day cannot have harmful consequences under continuous advance of the shields. But if the working face is stopped, only a few days are sufficient for considerable loads to be created on the shield canopy in the direction normal to the seam. With a large number of rows of the canopy, its resistance to the movement of the rocks will be very great, and therefore the possibility arises of the shield becoming wedged between the side rocks. It follows from this that the smaller the number of canopy rows, the less resistance the shield experiences when moving under the pressure of the side rocks.

System of mining by horizontal slices in descending order with filling

The most complete observations of the movements of the side rocks were carried out in 1950–52 in one section of seam IV of the Vnutrennyaya mine “Koksovaya-I,” which had a thickness of 8.5 m and a dip angle of 55°. The strength of the coal was medium; the lower band of the seam, 0.5–1.0 m thick, consisted of weak coal. At 2.5 m above the seam lay the Provodnik seam, 1.2 m thick. The roof rocks of the Provodnik seam were very stable.

Because a furnace had been driven along the Provodnik seam, it was possible to make periodic surveying measurements in this furnace and thus to observe the absolute displacements of the immediate roof of seam IV Vnutrennyaya. These data, together with data on the displacement of the roofs of the slices, made it possible to establish the character of the movement of the roof rocks of the seam and the movement of the filling of the overlying slices.

The zone of abutment pressure below the floor of the slice being extracted was equal to 10 m; the zone of loosened filling extended over a height equal to the thickness of the seam, above which lay the zone of compacted filling.

When seams are mined by horizontal slices in descending order, the filling mass moves downward along the dip as the slices are extracted; therefore support for the interlevel coal pillars and the surface is not ensured (Fig. 1). The free movement of the filling mass is hindered by the movement of the roof rocks of the seam, which creates, at some height above the active slice, a zone of compaction. The faster the extraction operations advance along the dip, the smaller the roof displacement will be during the extraction of one slice, and the later the moment will arrive when the filling mass is compacted under the pressure of the roof rocks. Therefore the height of the column of the filling mass that determines the pressure on the support (and, consequently, the displacement of the roofs of the slices in the face area) will be greater with rapid advance of the extraction operations than with slow advance. This conclusion is valid.

at such rates of advance of the extraction face, which are of practical significance. With the face stopped, the accumulation of displacements will occur owing to the influence of the time factor, and over a sufficiently long interval of time the displacements may reach a large magnitude.

Displacement of the roof of the 5th layer during the extraction and backfilling of one 7 m long step.

Displacement of the roof of the 9th layer at a distance of 6 m from the face.

Fig. 1. Displacement of the roof of horizontal layers during the mining of seam IV Internal at the “Koksovaya-1” mine with pneumatic backfilling

Compression of the seam at the floor level of the working layer reached 340 mm, which caused strong pressure on the layer drifts. The presence of increased pressure on the layer drifts may be expected if they are located within the zone of abutment pressure.

The observation results presented indicate that the backfill does not fully perform its functions under a system of mining by horizontal layers.

System of mining by inclined layers with backfilling

Observations were carried out during the mining of thick seams IV Internal and Moshchny with a descending order of layer extraction and with gravity backfilling of the worked-out space, and of seams IV Internal and Gorely with an ascending order and hydraulic backfilling. The Gorely seam was mined in strips along the strike; seams IV Internal and Moshchny were mined by longwalls 25 to 70 m long. The seams were mined in 3 layers, and only the Moshchny seam in 4 layers; the thickness of each layer was 3 m. The dip angles of the seams were within 36–41°. The side rocks of the seams consisted of argillites and sandstones of medium stability and stable sandstones.

Numerous observations of the deformations of the near-face timber support, consisting of three-meter props, caps, and sills, established that the average magnitude of roof displacement of inclined layers (except for the first), at which the props still remain unbroken, is 250–300 mm. This value includes crushing of the caps and sills, penetration of the support into the coal, into the side rocks and into the backfill, and bending of the caps and sills in irregularities of the roof and floor.

During observations in the seams of the Moshchnyi bed, the magnitude of the limiting yielding of the roof support was established: in the 1st seam, 60–85 mm, and in the 2nd and 3rd seams, 210–250 mm. When the roof displacement reached this magnitude, the props began to break. In the first seam the support was more rigid than in the lower seams, owing to rigid bearing against the roof and floor of the seam and the absence of packings.

The magnitude of the total yielding of the support, divided by the rate of roof displacement, is equal to the service life of the support. Therefore, in the 1st seam of the Moshchnyi bed the service life of the support was 3–4 days, and in the lower seams, 6–7 days. This determines the width of the stowing interval and the duration of the work in it.

By comparing the rates of roof displacement in inclined seams with different leads of the working face, it was established that, when three or more seams are mined simultaneously in one extraction panel, with the seams leading by no more than 30–40 m, rapid displacements of large rock masses of the main roof occur, hindering mining operations; therefore the leads for the 3rd and 4th seams must be greater than for the first two (counting the seam numbers in the order of extraction).

Observations during the mining of the Gorelyi bed by inclined seams with hydraulic stowing in ascending order, with extraction in strips along the strike, established that the stowed mass is compacted mainly during the first three months after extraction, and after a year the process of settlement of the stowing is practically completed, provided that no coal pillars are left in the worked-out space. The settlement of the stowing, at which the stowed mass becomes dense, amounts to 15–17% of the original volume, but the displacement process ends with a settlement of 26–29%. The rather considerable settlement coefficient is explained by the fact that the stowing material consists 30–40% of clayey rocks, which completely disintegrate under the action of water and pressure and fill all the voids between the stowing particles.

Roof displacement under this system of mining occurs freely, since no coal pillars are left in the entire extraction panel; this causes a high roof pressure on the stowed mass, reaching, at the middle of the panel, the weight of a column of rock of a height equal to the mining depth. Dense stowing serves as a good insulating material, protecting the coal in the overlying seams from spontaneous combustion, and this, in turn, makes it possible to mine the seams successively, one after another, without maintaining development workings for the other seams except the one being extracted. In mining the Gorelyi bed at the Koksovaya-1 mine by this system, the best indices in productivity and in the consumption of timber materials were achieved in comparison with other mining systems with stowing.

Longwall mining system along the strike with stowing

In one section of the Koksovaya-1 mine, using this system with gravity stowing, seam VI Vnutrennii, 2.6–3.0 m thick, dipping at an angle of 58–63°, and having stable side rocks, is being mined.

The inclined story, 116 m high, was divided into three substories, with the lower substories being mined ahead of the upper ones, and the substoring drift being the unstowed part of the worked-out space beneath the upper side of the longwalls.

The magnitude of the greatest displacement of the seam roof in the cleared space during coal extraction by one stowing step 27 m long was equal to 280 mm; the displacements were uniform.

The stowing mass substantially improves the conditions for supporting the roof. The maintenance of entries in the stowing mass caused no difficulties. The surface of the stowing mass that served as the floor of the entries remained practically immobile, since the mass was compacted mainly as a result of displacement of the seam roof. The displacement of the seam roof at the level of the ventilation entry of the lower longwall over its entire service life, with an advance of the sublevels of the order of 40 m, was approximately 250 mm, which did not exceed the yield of the entry support.

An important conclusion from the observations under this mining system is that the most expedient sequence for extracting the sublevels is ascending. In this case the stowing mass is effectively used to support the seam roof, losses of coal in the sublevel pillars are eliminated, and the safest working conditions are achieved with respect to supporting the upper edges of the longwalls, as compared with the descending sequence of extracting the sublevels. The latter is explained by the fact that, with the ascending sequence of extracting the sublevels, the upper part of the longwalls is supported by large coal pillars above and in front (from the face side), and the seam roof is displaced at this place only under the influence of coal extraction in the given longwall. With the descending sequence of extracting the sublevels, the upper edges of the longwalls are located in the zone of overlapping displacements from two adjacent sublevels (Fig. 2), and therefore the sublevel coal pillars are crushed and create a danger of collapse.

Fig. 2. Scheme of distribution of compacted stowing during the mining of seams by long pillars along the strike with stowing:
a—descending sequence of extracting sublevels;
б—ascending sequence of extracting sublevels.

Conclusion

1. The results of observations by the Siberian Branch of VNIMI of the movements of surrounding rocks and coal pillars made it possible to identify a number of regularities of substantial significance for the choice of elements of systems for mining thick steeply dipping seams.

2. By using even the simplest instruments and a simple methodology, it is possible to organize large-scale observations under mine conditions, which is necessary for well-founded conclusions in view of the great variability of mining and geological conditions.

3. It is necessary to increase attention to the study of the processes of rock displacement in order to improve roof control, not only on the part of specialized scientific-research organizations, but also on the part of all collectives and private individuals engaged in design and research work in the field of improving methods for mining coal and ore deposits.