On the origin and morphology of Z-shaped splitting of coal deposits (on the example of Eastern Donbass)

Статья посвящена выявлению морфологии и происхождения распространенных во многих угольных регионах мира уникальных углепородных структур – Z-образных расщеплений угольных залежей, впервые обнаруженных автором в Восточном Донбассе. Актуальность работыопределяется необходимостью разработки научных основ угленакопления для определения промышленной ценности и минерагенических перспектив территорий. Цель работы. Совершенствование теории угленакопления и первичного структурообразования в осадочных формациях в целом. Методы работы. Вещественно-структурный анализ пересекающихся геологических разрезов угленосной территории западной части Восточного Донбасса известными литолого-структурными методами, сопоставление выявленного Z-объекта с аналогичными структурами других угольных регионов и гипотез о формировании Z-расщеплений. Результаты работы. Обнаружено и исследовано первое в Восточном Донбассе Z-образное расщепление угольной залежи. Залежь k2н каменской свиты С25 каменноугольной системы у г. Красный Сулин асимметрична в разрезе и плане. Высота Z-объекта составляет первые десятки метров, площадь – многие десятки км2, протяженность с севера на юг превышает 20 км. Объект имеет угольные основание и свод в плане, сопряжение их угольным соединением. Z-образность объекта проявляется в субмеридиональных сечениях. Уникальны односторонняя – с запада – замкнутость соединения, наличие угольных пачек-перемычек, неразвившихся до соединения. Выявлена этапность формирования объекта. На конкретном примере определены условия развития Z-объектов: попеременное фронтальное поступление крупных сбалансированных порций в основном растительного или растительно-минерального материала на смежные участки территории с разных сторон с частичным заходом языка минеральных осадков на покрытый углематеринской массой один из них. Нарушение условий приводит к отклонению облика структуры от типовой либо прекращению ее образования. Механизм поступления материала – разливы рукавов палеодельты. Односторонняя замкнутость соединения в плане вызвана поступлением локальных потоков минерального материала. Ниша седиментации создана процессами тектонического относительного опускания смежных участков территории. Исследованная структура возникла в условиях сочетания тектонических и седиментационных предпосылок, главный вклад в ее формирование принадлежит деятельности палеорусел. В этой связи структура отнесена к аккумулятивному типу. Приведены примеры других Z-объектов. Z-структуры развиты на различных масштабных уровнях. Результаты исследований имеют значение для различных регионов угленакопления и решения общих вопросов седиментации. The article is dedicated to the identification of the morphology and origin of unique coal-bearing structures common in many coal regions of the world - Z-shaped splitting of coal deposits, first discovered by the author in the Eastern Donbass. The relevance of the work is determined by the need to develop the scientific foundations of coal accumulation to determine the industrial value and mineragenic prospects of the territories. Aim. Improvement of the theory of coal accumulation and primary structure formation in sedimentary formations in general. Methods. Material-structural analysis of intersecting geological sections of the coal-bearing territory of the western part of the Eastern Donbass using well-known lithological-structural methods, comparison of the identified Z-object with similar structures of other coal regions and hypotheses about the formation of Z-splits. Results. The first Z-shaped splitting of a coal deposit in the Eastern Donbas was discovered and investigated. The k2n deposit of the Kamenskaya suite C25 of the Carboniferous system near the town of Krasny Sulin is asymmetric in section and plan. The height of the Z-object is the first tens of meters, the area is many tens of km2, and the length from north to south is 20 km wide. The object has a coal base and a vault in plan, their conjugation with a carbon connection. The Z-shape of the object appears in submeridional sections. Unique is the one-sided - from the west - closed connection, the presence of coal bundles-bridges that did not develop before the connection. The stages in the formation of the object are revealed. On a specific example of the conditions for the development of Z-objects: the transverse frontal supply of large balanced portions of mainly plant or plant-mineral material in different parts of the territory from different mineral sediments with a partial entry of the tongue onto one of them covered with a source coal mass. Violation of the conditions leads to a deviation of the structure from the typical one or to the termination of its formation. The mechanism of material receipt is the spill of the paleodelta arms. One-sided closedness of the connection is in terms of providing local flows of mineral material. The sedimentation niche was created by the processes of tectonic relative subsidence of additional areas of the territory. The investigated structure arose under the conditions of a combination of tectonic and sedimentation prerequisites; the main contribution to its formation was the activity of the paleochannels. In this regard, the structure is classified as an accumulative type. Examples of other Z - objects are given. Z-structures are developed at various levels. Research results are of significance for different regions of coal accumulation and solution of general sedimentation issues.

Laboratory Study on Changes in Gas Desorption Properties of Anthracite after Cyclic Loading

Coal mass is subjected to cyclic loading during pulsating hydraulic fracturing (PHF), and changes in its gas desorption properties affect gas drainage. Therefore, it is of great importance to correctly understand the influences of cyclic loading on the gas desorption properties of coal mass. Firstly, loading tests with different frequencies and amplitudes were performed on anthracite from Qinshui Basin (Shanxi Province, China) using a fatigue testing machine. Secondly, gas desorption tests were performed to determine the associated curves for each test group at different equilibrium pressures, and the initial desorption capacity and diffusion coefficient of the gas were calculated. Finally, the influence of different loading conditions on the gas desorption laws were analyzed. The test results demonstrate that a greater loading frequency increases the ratio of the initial desorption capacity so that the desorption rate of coal samples is higher, and the gas desorption properties become increasingly better in the initial stage. However, variations in the amplitude have minimal impact on the ratio of the initial desorption capacity. When the amplitude is too large in the initial stage, the diffusion coefficient decreases and the gas desorption properties worsen. In addition, the above test results are used to discuss the selection of the amplitude and frequency in the PHF process from a macroperspective. The contained research results provide an important theoretical basis for the field application of PHF technologies in coal mines.

Study on Mechanical Behavior and Seepage Characteristics of Coal Mass during Unloading

With the increase of buried depth, the content of gas increases gradually. The gas in the mining process will lead to gas gush and other dynamic disasters, or even coal and gas gushing in front of the working face. Therefore, the study on the permeability distribution of coal and the surrounding rock is the core work of coal and gas mining at the same time. To study the mechanical behaviors and seepage characteristics of coal mass during unloading is to prepare for coal and gas mining in the future, which can not only ensure the safety of operators to the maximum extent but also increase the mining rate as much as possible. Based on the stress-strain curve and seepage curve, the brittleness index and seepage characteristics of coal are analyzed. The greater the brittleness index is, the more likely the coal mass is to produce cracks, and then to form large cracks, or even fracture. Through the study of brittleness index and seepage characteristics of coal mass, the mechanical behavior of coal mass can be easily obtained, so as to guide the mining of coal mass.

Main Roof Breakage and Vibration Induced Coal Burst Occurring in Longwall Roadways

Rock burst is one major threat to mining safety and economy. Rock burst occurring in the longwall mining roadway accounts for 85% of the total amount of burst events. This paper investigates the causality mechanism of rock burst in longwall roadways by establishing a finite elastic beam model in the working face based on the elastic foundation theory. The breakage process of the main roof and related dynamic effects are analysed. The result shows that the movement of the main roof shows free vibration under certain damping resistance. It is also found that the roof dominant vibration frequency increases with the increase in the thickness and elastic modulus of the roof. During roof vibration, the vertical stress applied on the coal mass is unloaded. The destressing of the roof-coal interface causes the coal mass in the roadway rib to slip into the roadway under the horizontal ground stress, resulting in rock burst. The possibility of rock burst increases with increase in the strength and thickness of the roof and horizontal ground stress within the coal mass. This mechanism explains the occurrence of rock burst in the mining roadway; it provides the fundamental theory for the prevention and controlling technologies of longwall roadway rock burst.

Theoretical Study and Experiment Validation on Drilling Cutting Weight during the Whole Process of Drilling

In mining engineering, coal and gas outburst is extremely dangerous dynamic disaster, which will cause serious casualties and property losses. As a method to predict coal burst, the drilling cutting method has been widely used in coal mines. The drilling cutting weight is an important index of the drilling cutting method. In theoretical calculation, scholars usually assume that the coal is isotropic and homogeneous before drilling to deduce the formula of drilling cutting weight. However, in actual mining engineering, drilling cutting is usually carried out in the plastic coal body in front of the working face. Therefore, in the present study, the theoretical formula of the drilling cutting weight in the plastic coal mass is deduced, as well as in the elastic coal mass. The results show that the drilling cutting weight calculated based on the deduced formula increases with the increase of drilling depth in the plastic coal mass, which is consistent with the field measurement results. The fragmentation degrees of coal around the drilling hole are also considered by introducing cohesion, which changes linearly along the radial direction of the drilling hole. The results show that the smaller the cohesion, the greater the drilling cutting weight. The calculation formula for drilling cutting in elastic coal mass is also given. The dilatancy effect of coal around the drilling hole is also considered by introducing expansion coefficient n, which changes linearly along the radial direction of the drilling hole. There is a good match between the theoretical calculation results and the laboratory test results. The obtained results are helpful for the prediction and prevention of coal burst.

Study on the Permeability Evolution Model of Mining-Disturbed Coal

Coal permeability plays an important role in the simultaneous exploitation of coal and coal-bed methane (CBM). The stress of mining-disturbed coal changes significantly during coal mining activities, causing damage and destruction of the coal mass, ultimately resulting in a sharp increase in permeability. Conventional triaxial compression and permeability tests were conducted on a triaxial creep-seepage-adsorption and desorption experimental device to investigate the permeability evolution of mining-disturbed coal. The permeability evolution models considering the influence of the stress state and stress path on the fracture propagation characteristics were established based on the permeability difference in the deformation stages of the coal mass. The stress-strain curve of the coal was divided into an elastic stage, yield stage, and plastic flow stage. As the axial stress increased, the permeability decreased and then increased, and the curve’s inflection point corresponded to the yield point. The permeability models exhibited a good agreement with the experimental data and accurately reflected the overall trends of the test results. The results of this study provide a theoretical basis for coal mine disaster prevention and the simultaneous exploitation of coal and CBM.

New Insights of Grouting in Coal Mass: From Small-Scale Experiments to Microstructures

Pre-grouting as an effective means for improving the stability of roadways can reduce maintenance costs and maintain safety in complex mining conditions. In the Guobei coal mine in China, a cement pre-grouting technique was adopted to enhance the overall strength of soft coal mass and provide sufficient support for the roadway. However, there are very limited studies about the effect of grouting on the overall strength of coal in the laboratory. In this paper, based on the field observation of a coal-grout structure after grouting, a series of direct shear tests were conducted on coal and grouted coal specimens to quantitatively evaluate the quality improvement of grouted coal mass. The results showed that the peak and residual shear strength, cohesion, friction angle and the shear stiffness of grouted coal were significantly improved with the increase of the diameter of grout column. Linear regression models were established for predicting these mechanical parameters. In addition, three failure models associated with coal and grouted coal specimens were revealed. According to microstructure and macroscopic failure performance of specimens, the application of the proposed models and some methods for further improving the stability of grouted coal mass were suggested. The research can provide the basic evaluation and guideline for the parametric design of cement pre-grouting applications in soft coal mass.

Dynamic Characterization during Gas Initial Desorption of Coal Particles and Its Influence on the Initiation of Coal and Gas Outbursts

The law of gas initial desorption from coals is greatly important for understanding the occurrence mechanism and predicting coal and gas outburst (hereinafter referred to as ‘outburst’). However, dynamic characterization of gas initial desorption remains to be investigated. In this study, by monitoring the gas pressure and temperature of tectonically deformed (TD) coal and primary-undeformed (PU) coal, we established the evolution laws of gas key parameters during the initial desorption. The results indicate that the gas pressure drop rate, mass flow rate, initial desorption rate, and gas velocity increase with increasing gas pressure, with stronger gas dynamic effect, generating a high pressure gradient on the coal surface. Under the same gas pressure, the pressure gradient formed on the TD coal surface is greater than that formed on the surface of the PU coal, resulting in easily initiating an outburst in the TD coal. Moreover, the increased gas pressure increases temperature change rates (falling rate and rising rate) of coal mass. The minimum and final stable temperatures in the TD coal are generally lower compared to the PU coal. The releasing process of gas expansion energy can be divided into two stages exhibiting two peaks which increase as gas pressure increases. The two peak values for the TD coal both are about 2–3 times of those of the PU coal. In addition, the total gas expansion energy released by TD coal is far greater than that released by PU coal. The two peaks and the total values of gas expansion energy also prove that the damage of gas pressure to coal mass increases with the increased pressure, more likely producing pulverized coals and more prone to initiate an outburst.

Energy Action Mechanism of Coal and Gas Outburst Induced by Rockburst

The essence of both rockburst and coal and gas outburst lies in fast energy release. In order to explore the energy action mechanism of coal and gas outburst induced by rockburst in rockburst and coal and gas outburst combined mines, the split Hopkinson pressure bar (SHPB) experimental device was firstly used to conduct uniaxial impact failure test of coal specimens prone to outburst under different strain rates, and their energy dissipation laws under impact loading were obtained. Next, under the engineering background of coal and gas dynamic phenomena induced by rockburst with different intensities in Xinyi Coal Mine and Pingdingshan Coal Group No. 12 Colliery in Henan Province and Dingji Coal Mine of Huainan Mining Group in Anhui Province, experimental study results were combined with numerical simulation analysis to discuss the energy mechanism of coal and gas outburst induced by rockburst. The study results show that the outburst can be divided into two different processes—critical outburst and outburst—according to the evolution law of outburst energy, and the critical energy conditions for coal and gas outburst are proposed. The minimum destructive energy range for the critical outburst of coal mass is obtained as (5–10) × 104 J/m3. Under some low gas, high stress, or strong disturbance conditions, applied loads can become the main energy sources causing critical failure and even crushing and throwing of coal mass. The coal mass will present an interval splitting structure under dynamic loading, which is obviously different from the failure mode of coal mass under static actions.