Features and regularity of zonal changes in the physicomechanical characteristics of Zakhidnyi Donbas rocks

Ukraine Abstract. This article summarizes the results of numerous experimental studies of the physicomechanical properties of rocks from deposits of Zakhidnyi Donbas. The analysis of the obtained data allowed us to reveal the zonal nature of their change. Regularity has been established for increasing the strength parameters of rocks in the direction from west to east, the mechanism of which is explained, among other things, by the influence of Coriolis forces arising from the daily rotation of the Earth. It has been established that for more durable rocks, such as sandstone, the change in strength characteristics in the roof and floor is insignificant, whereas for weaker mudstones and siltstones, there is a significant difference in the above strength parameters. A generalization of the test results of Zakhidnyi Donbas rocks showed a large variation in their properties under the conditions of the achieved depth of the reservoir under development, which is due to the variability of the composition, structure, texture of the rocks and the degree of epigenetic


Introduction
One of the important factors determining the parameters of mineral development systems, the nature of the various production processes in mining and processing, as well as during mining for various purposes, is the availability of a certain set of data on the physical and mechanical properties of the massif. Knowledge of the rocks properties is necessary not only in the extraction of minerals, but also in the construction of various buildings, the erection of hydraulic structures, tunnels, road construction, etc [1,2].
In this connection, the study of the physicomechanical properties of rocks, which are an important component in calculating the stability of underground structures, roof supports and guard structures of mine workings, acquires great importance. Many scientists, both in our country and abroad, paid great attention to the problem of assessing the stability of the rock massif in their works [3 -5].
The modern state of theoretical developments in rock mechanics makes it possible to qualitatively and, in some cases, quantitatively assess the degree of stability of the massif in the marginal zones. The Ukrainian scientists V.V. Vinogradov, A.S. Kosmodamiansky, S.V. Kuznetsov, A.A. Levshin, N.S. Khapilova and others made a great contribution to the development of this direction.
A.T. Ayruni, A.F. Bulat, V.I. Nikolin, S.I. Skipochka, B.M. Usachenko and others successfully dealt with the development of methods for increasing the stability of the massif in the marginal zones and their effectiveness research in the direction of choosing the most informative indicators to assess changes in the properties of rocks and the degree of the rock mass stability.
To characterize the stress state of rock massif surrounding production, the following rock parameters are used: specific gravity, density, uniaxial compression strength parallel and perpendicular to the bedding, tensile strength, Young's and shear moduli, Poisson's ratio, as well as rock cutting resistance and abrasiveness of rocks.
The purpose of this work is to summarize the obtained results of experimental studies to determine the physicomechanical properties of Zakhidnyi Donbas rocks to identify regularity of their zonal change.
Roof rocks of coal layers are represented by low-strength mudstone and siltstones, with a small strength from one lithological difference to another and rather weak contact between the layers. The direct roof of the C 1 and C 10 в layers, whose thickness varies from 0.5 to 2.5 m, is mainly represented by alternating layers of mudstones and siltstones, in some areas it is composed of sandstones. Describing the direct roof of the C 2 , C 2 1 , C 4 b , C 5 layers, it is necessary to note the presence in it of a large number of thin layers of mudstones and siltstone. The thickness of the layers varies from 0.15 to 0.5 m, but layers with a thickness of 0.15 to 0.25 m prevail. The average value of the direct roof thickness of the C 2 , C 4 b and C 5 layers varies from 2 to 2.7 m, for the C 2 1 formation its value is 1.2 m. The main roof of the indicated layers is composed of large layer mudstones and siltstones.
The direct roof of the C 6 , C 6 1 layers is distinguished by a large variability of the lithological composition and structure of the massif. Within the developed areas, the structure varies from thin-layer to layered-block. The main roof of the C 6 and C 6 1 layers is represented by dense layered mudstones and siltstones.
The peculiarity of the direct roof structure of the C 8 b stratum is the presence of weak interlacing mudstones and siltstones with a sometimes lumpy texture. In the main roof of this layer, as a rule, more dense mudstones lie.
The coal layer floor is represented by mudstones and siltstones, which are characterized by the so-called curly texture.
The coal content of Zakhidnyi Donbas is confined to the sediments of the lower and middle carbon. Coal deposits of the region have natural limitations in the south and north: in the south -the Ukrainian crystalline shield, in the north -the Mykhaylovskyi reset. The data of geological exploration and field geophysics indicate that the coal content of the lower and middle Carboniferous spreads to the north-west, north and northeast to a depth of 3000 m.
To determine the indicators of the physicomechanical properties of coal formations rocks, the material selected in the mines of DTEK Pavlogradvuhillia PASC was used.
Samples for determining the strength and elastic characteristics of rocks were made by dry cutting on a stone-cutting machine, followed by grinding and fine-tuning of the working surfaces on grinding machines.
Physicomechanical parameters of rocks, such as: density , porosity p, humidity W, dynamic elastic modulus E and Poisson's ratio μ were determined by methods regulated by DSTU BV 2.

Results and discussion
The complex of physicomechanical parameters of rocks, defined by both mechanical and geophysical methods, is presented as an example for the mine named by Geroiv Kosmosu in the Table 1.
We have previously obtained a pattern of zonal distribution around the working of increased stresses and fracturing, is as which is analytically expressed as follows: where  0 -the initial stress on the working contour, MPa; r -the distance from the working contour deep massif, m; t -the current time, s; τ -the relaxation time, s; R 0radius of working, m; α -attenuation coefficient of a quasi-stationary wave, s -1 .
The zonal nature of the changes in the physicomechanical parameters of Zakhidnyi Donbas rocks as a result of the analysis of the experimental data has also been established. The regularity of increasing the strength parameters of rocks in the direction from west to east has been established. The mechanism of increasing the strength parameters of coal and host rocks, in our opinion, is due to the influence, including Coriolis forces, arising from the Earth's daily rotation. Besides, for the most important rocks indicators the limits of change are obtained, namely: the compressive strengths, the resistance of rocks to cutting and their abrasiveness for both the roof and the floor of mine workings (Table 2).  52 -344 Analysis of the results presented in the table shows that for more strong rocks, such as sandstone, changes in the strength characteristics in the roof and floor are insignificant. Whereas for weaker mudstones and siltstones, there is a significant difference in the above strength parameters in the roof and in the floor.
It should be noted that the compressive strength can be determined by geophysical methods: ultrasonic and shock pulse methods [6]. For these methods, the following correlation relations are defined: -for ultrasound method where V l -the velocity of longitudinal ultrasonic waves, m/s; a and b -the coefficients determined experimentally for each type of rock; -for the shock pulse method where τ -the duration of the shock pulse, s. Rock strength also changes with increasing depth. Using the results of our own research, we evaluated the variability of the physicomechanical properties of rocks at the reached depth. The importance of such an assessment is determined by the ambiguity of opinion on the change in strength and intensity of strength growth with depth and its effect on the stability of rocks. In the general case, the strength of rocks increases with increasing depth of working, since the influence of the softening factor decreases, i.e. the degree of epigenesis. As the depth increases to 400 m, the strength of the rocks grows rapidly, then from a depth of 400 -600 m, the intensity of the growth of rock strength remains constant, and from a depth of 900 m and more it rises again.
The strength of the roof or floor rocks is defined as the weighted average, including layers of rocks (coal) in the roof -to a height of 1.5b, and in the floor -to a depth of 1b, where b is the width of working, m.
There is a correlation between the density of the rock and its strength for uniaxial compression, expressed by the formula: Analysis of the data shows that only sandstones at a depth of up to 300 m and siltstone up to 160 m can be classified as stable rocks in the Zakhidnyi Donbas. The unstable rocks become from the following depth: mudstones -230 m, siltstone -300 m and sandstones -450 m. Considering the structural features of the productive stratum, related to the fact that almost 33 % of it is composed of weak clay lithological varieties, the justification of the maximum depth of the stability state of the rocks must be made according to the most common and least then lasting -mudstone.

Conclusions
1. The zonal nature of the changes in the physicomechanical properties of the rocks of Zakhidnyi Donbas has been revealed.
2. Regularity has been established for increasing the strength parameters of rocks in the direction from west to east, the mechanism of which is explained by the influence, including Coriolis forces, arising from the daily rotation of the Earth.
3. It was found that for more durable rocks, such as sandstone, the change in strength characteristics in the roof and floor is insignificant, whereas for weaker mudstones and siltstones there is a significant difference in the above strength parameters. 4. Generalization of the test results of the Zakhidnyi Donbas rocks showed a large variation in their properties under the conditions of the achieved depth of the reservoir under development, which is due to the variability of the composition, structure, texture of the rocks and the degree of epigenetic transformations. 5. On the basis of the data obtained, the planned rates and methods of preparatory workings are determined, when the workings are carried out using the mine combine method, the cutting tool consumption is planned, the technique and methods of securing the workings are selected.