Haitao Sun

Soft rock has the properties of low strength, poor integrity, and difficulty in core extraction. In order to study the deformation and failure of soft rock, this study used fine river sand as aggregate, cement and gypsum as bonding materials, and borax as a retarder to produce cylindrical rock-like samples (RLS) with a sand cement ratio of 1:1. Uniaxial compression tests were conducted on RLS under DIT (different immersion times) (0, 4, 8, 12, 24, and 48 h) in the laboratory. The mechanical and energy properties of RLS under water-stress coupling were analyzed. The results showed that the longer the IT of the RLS, the higher their water content (WC). As the moisture time increases, the uniaxial compressive strength, elastic modulus (EM), and softening coefficient (SC) of the sample gradually decrease, while the rate of change of EM is the opposite. The fitted sample SC exhibits a good logarithmic function relationship with WC. During the loading process of the sample, more than 60% of the U (total energy absorbed) during the loading process of the sample is accumulated in the form of U_e (releasable elastic energy), while less than 40% of U is dissipated by the newly formed micro cracks during the compaction, sliding, and yield stages of the internal pores and cracks of the sample. The U before the peak and the U_e of the RLS decrease exponentially with the moisture content; the relationship curves of U_e/U (released elastic energy ratio) and U_d/U (dissipated energy ratio) of RLS during uniaxial compression with the σ₁/σ_max (axial stress ratio) can be divided into three stages of change, namely the stage of primary fissure compaction and closure (σ₁/σ_max < 0.25), continuously absorbing energy stage (0.25 < σ₁/σ_max < 0.8), and energy dissipation stage (σ₁/σ_max > 0.8); the D (damage variable) was defined by the ratio of U_d (dissipated energy) to the U_dmax (maximum dissipated energy) at failure time of RLS, the fitting of the relationship between the damage variable and axial strain conforms to the logistic equation.

Underground mining in coal mines causes strong disturbance to geological structures and releases a large amount of elastic strain energy. When the roof is a hard and thick rock layer, it is easy to cause dynamic disasters such as rock burst. To analyze the impact of a deep thick and hard roof fracture on the safe mining of thick coal seams, this paper studied the dynamic evolution process of the stress field, displacement field, energy field, and plastic zone of the coal seam and overlying strata during the mining process using FLAC3D numerical simulation. The results show that as the working face continues to be mined, the concentrated stress in the overlying strata first increases and then decreases, and the support pressure in front of the working face continues to increase. When it advances to 100 m, collapse occurs, and the stress increases sharply; the bottom plate undergoes plastic failure, resulting in floor heave. The overlying strata mass in the top plate exhibits downward vertical displacement, while the rock mass in the bottom plate exhibits upward vertical displacement, with a maximum subsidence of 4.51 m; energy concentration areas are generated around the working face roadway, forming an inverted “U” shape. When collapse occurs, the energy density decreases slightly; the direction of the plastic zone changes from “saddle shaped” to complete failure of the upper rock layer, and the overlying strata is mainly shear failure, which expands with the increase in mining distance. The research results have important practical significance for guiding the safe mining of deep thick and hard roof working faces.

In order to improve the efficiency of gas extraction in coal mines, a data-driven fine-management and control method for gas extraction is proposed. Firstly, the accurate prediction of coal seam thickness and gas content was used to evaluate the gas reserves. Based on the time relationship between mining activities and gas extraction, the calculation model of borehole distance in different extraction units is established, and the differential borehole design is realized. Then, a drilling video-surveillance system and drilling trajectory measurement device are used to control the drilling process and the construction effect. Finally, the model of extraction data-correction and the identification of failed boreholes is established, then the failed boreholes are repaired. The technology method provided in the paper has realized the fine control of gas-extraction borehole design, construction, measurement, and repair, and formed a more scientific gas-extraction borehole control technology system, which provides new thought for efficient gas extraction.

Coal is the mainstay of China’s energy supply. With the gradual progress in China’s policy of phasing out backward coal production capacity, the intensive and deep mining of coal has gradually become the new norm. The current mining depth is increasing at a rate of 10~15 m/year. The high crust stress, high gas pressure, high ground temperature, and engineering disturbance stress in deep coal mines can lead to the occurrence of coal–rock–gas dynamic disasters that are complex and show the characteristics of compound dynamic disasters. It is important to understand the evolution and mechanism of deep coal and rock dynamic disasters accurately for the safe development of deep resources. To study the mechanism of occurrence and the evolution of impact–protrusion compound dynamic disasters, we herein analyzed the apparent characteristics of coal–rock–gas compound dynamic disasters in deep mines and obtained the mechanical and acoustic emission characteristics of coal–rock composites through indoor experiments. Then, we conducted in-depth analysis on the non-uniform deformation behaviors and non-uniform stress field of the coal–rock composite and clarified the generation mechanism of local tensile cracks at the coal–rock interface. Subsequently, we established the energy transfer model of the rock–rock–gas composite specimen in the process of dynamic destabilization in the engineering scale mining field and revealed the mechanism of nonlinear energy evolution and release of the coal–rock–gas composite, which has been less reported in previous studies. In this paper, we further clarified the occurrence and development mechanism of coal–rock–gas compound dynamic disasters in the engineering scale mining environment to guide the prevention and control of coal–rock–gas dynamic disasters in deep mines.

In order to study the mechanical failure characteristics and energy evolution law of gas-bearing coal–rock composites under different gas pressures, a uniaxial mechanical loading experiment was carried out on an upper-rock lower coal binary coal–rock assembly under different gas pressures. The changes in parameters such as compressive strength and elastic energy of the coal–rock combination were analyzed, and the energy transfer in the failure process of the gas-bearing coal–rock assemblage was studied. The results showed that the compressive strength of the combined body decreased linearly with the increase in gas pressure, and the decreasing rate of compressive strength was 6.4%, 16.3%, and 21.4%. The elastic modulus of the combined body decreased with the increase in gas pressure in a power function relationship. The energy accumulated before the peak of the rock part of the composite body and the elastic energy released after the peak, the energy accumulated before the peak of the composite body, and the energy dissipated after the peak of the coal body part all decreased with the increase in gas pressure. The variation range of the indicators K₁ and K₂, which reflect the influence degree of the partially accumulated elastic energy of the rock on the failure of the assemblage, were 5.85~6.68% and 7.34~9.46%, respectively.

The stability of the coal-rock composite structure is of great significance to the safety production of deep mines, and the different interface connection modes of coal and rock have an important influence on its stability. Therefore, the following work was done in this article: Firstly, the mechanical structure model of coal-rock was established, and the influence of different interface connection modes on coal rock was analyzed. Secondly, the mechanical characteristics (plastic zone, stress, and displacement) of coal-rock composite structure under different interface connection modes were studied by numerical simulation, and the energy was quantitatively analyzed by FISH language in FLAC3D. The results were as follows: (1) The interface reduces the strength of rock and increases the strength of coal in the coal-rock composite structure. (2) In the loading process, the coal body is destroyed first and the destruction range increases gradually with the increase of stress. The failure mode is mainly a plastic shear failure, and the deformation of coal is much larger than that of rock in the composite structure. (3) The interface contact mode affects the mechanical behavior of coal and rock structure. The strong contact interface influences the strength, displacement, and energy accumulation of coal and rock structure, among which the influence on energy and displacement is greater, which is helpful to the improvement of strength. Therefore, it is suggested to adopt the strong contact interface in the study of coal and rock structure.

To obtain the similar materials with specific physical and mechanical parameters and adsorption and desorption indexes used in coal and gas outburst simulation tests, pulverized coal was selected as aggregate, and sodium humate was selected as cementing agent and river sand was selected as auxiliary materials. Based on this, orthogonal tests with 6 factors and 5 levels were designed, and the tests of weighing, uniaxial compression, firmness, adsorption and desorption were carried out. The parameters such as density, uniaxial compressive strength, elastic modulus, firmness coefficient and adsorption–desorption index of similar materials with different ratios were obtained, and the sensitivity of each factor was analyzed by range analysis. The influence of various factors on the similar materials was studied, and the ratio model of similar materials was obtained. The reliability of the model was verified, and a complete method for determining the ratio model of similar materials of outburst coal was put forward. The results show that the density of the similar materials increases with the river sand content, and the uniaxial compressive strength and elastic modulus increase significantly with the pulverized coal ratio and sodium humate content, and the firmness coefficient increases linearly with the pulverized coal ratio. The adsorption constant increases linearly with the sodium humate content, while the adsorption constant b decreases linearly with the sodium humate content. The initial elution rate Δp of similar materials increases at first and then decreases with the increase of sodium humate content.

In order to deeply explore the destructive effect of the impulsive airflow on the mine antioutburst door when coal and gas outbursts occur in underground coal mines, a large-scale coal and gas outburst dynamic effect simulation experiment device is used to carry out the coal and gas outburst disaster simulation experiment. The impact load and deformation characteristics of the antioutburst door under the impact airflow during coal and gas outburst are analyzed, and the experimental results are discussed in depth through numerical simulation analysis and field example analysis. Based on this, the internal and external causes of the damage of the antioutburst door on the coal mine site are analyzed, the key technologies that need to be solved in the design of the antioutburst door’s disaster resistance are studied, and the overall design of the structure optimization design of the antioutburst door is proposed. The research results show that after coal and gas outburst, the pressure on the antioutburst door will rise and fall, and the fluctuation will be greater. Under the same installation position, the farther the antioutburst door is from the protruding point, the less pressure it bears. In the middle and late stages of the outburst, intermittent negative pressure of the gas at the antioutburst door appeared. The key technologies for the design of the antioutburst door and its disaster resistance mainly include that strengthen theoretical and experimental research on the formation mechanism of outburst shock waves in mines, the interaction mechanism between disaster shock loads and dampers, and the magnitude of disaster expected shock loads; optimize the structure of the antioutburst door size, the width of the contact surface between the air door and the door wall, the stress distribution of the air door under impact load, the design of the safety hole, and the locking device; and improve the disaster monitoring and alarm capabilities of the antioutburst door and collect changes in antioutburst door pressure in real time. The research results provide a theoretical basis and technical support for the optimization of the antishock performance of underground antioutburst doors in coal mines and have important practical significance for improving the disaster resistance of the ventilation system.

Two kinds of common tunnel shapes, i.e. elliptical opening and square opening were selected for biaxial compression tests, and the influences of two kinds of opening shapes on the mechanical properties, failure characteristics and failure modes of sandstone were compared and analyzed. The complex variable theory and mapping functions were used to obtain the analytical stress solution around elliptical and square openings. The results show that the stability of the specimen containing an elliptical opening was better than that of the specimen containing a square opening under the same lateral stress. Compared with the elliptical opening, the local damage was formed earlier in the square opening which might be caused by a higher stress concentration around the square opening. The stress distributions around openings were influenced by the opening shape and lateral stress coefficient. The top and bottom of square opening were more prone to tensile fracture, and the distribution range of tensile was larger than that of elliptical opening. When the opening failed, the intensity of square opening failure was weaker than that of elliptical opening. On the basis of the average frequency value and the rise angle value, the failure mode of specimen containing elliptical or square opening was distinguished. It was found that the mixed tension and shear failure dominated the failure of specimens with different opening shapes, and the number of shear cracks in the specimen containing a square opening was greater than that in the specimen containing an elliptical opening. The above method of judging failure mode by acoustic emission signals was well verified by the CT images of damaged specimens.