The long-term stability of water-immersed coal pillars in proximity to urban areas is critical for ensuring the safety of urban development. This study investigates the creep damage evolution in strip coal pillars through a comprehensive approach integrating in-situ investigations, laboratory experiments, and numerical modeling based on a real-world case study. The analysis focuses on the evolution of stress distribution, creep damage propagation, and the mechanisms driving continuous damage in coal pillars. Findings indicate that in the studied case, concentrated stress around the elastic-plastic boundary progresses toward the central region of the coal pillar as creep damage expands. Initially, the propagation of creep damage is rapid but decelerates due to the residual strength of damaged coal, which bears the overburden pressure and provides lateral constraint. The state of creep damage expansion in a coal pillar depends on the evolution of triaxial creep strength of coal induced by the stress conditions of the intact coal body, which will be changed by the transferred overburden pressure and increased confining pressure.

pdf/investigation-on-the-long-term-damage-evolution-of-a-strip-ete9jcpjm4cz.pdf

Investigation on the long-term damage evolution of a strip coal pillar

Underground coal mining of gas-bearing coal seams is accompanied by the emission of large amounts of methane, which increases with depth. Coal seam methane is not only a major cause of major accidents in coal mines, but is also a greenhouse gas that has a significant negative impact on the Earth’s atmosphere. Analysis of the efficiency of underground coal mining suggests that as the depth of mining increases, the productivity of a longwall decreases by a factor of 3–5 or more, while the specific volume of methane emitted increases manifold and the efficiency of methane management decreases. Effective management of coal seam methane can only be achieved by monitoring its content at key points in a system of workings. Monitoring of methane not only eliminates the risk of explosions, but also lets us assess the effectiveness of using methane management techniques and their parameters to improve efficiency and reduce the cost of methane management (including a methane drainage) for ensuring sustainable underground coal mining. The aim of this article is to develop a software and hardware complex for monitoring methane in a coal mine by creating a simulation model for monitoring methane. The Arduino Uno board and the methane sensor MQ-4 were used for this purpose. In this article, the causes of methane emissions in coal mines, gas control systems, the structure of the mine monitoring system, and the causes of risks and occurrence of accidents in coal mines are considered. As a result of the work, the mathematical model of the methane measurement sensor was developed; the Arduino Uno board developed a simulation system for methane monitoring; and the numerical results of the research are presented in the graphs.

Methane Emission Estimation Tools as a Basis for Sustainable Underground Mining of Gas-Bearing Coal Seams

During coal mining operations, the coal will be deformed and damaged due to multiple mining disturbances (MMD), often resulting in disasters, like rock burst. To understand the evolution rules of coal deformation under MMD and its final fracture characteristics after impact dynamic load loading, reduce the adverse effects of mining disturbances, and improve disaster prevention and control capabilities, quasi-static uniaxial cyclic loading-unloading (L-U) and dynamic axial compression tests were conducted on large-sized coal-like samples. During the tests, three-dimensional (3D) laser scanning and acoustic emission (AE) monitoring technology were utilized to accurately capture the full-field deformation and AE response data, facilitating a systematic analysis of deformation and fracture characteristics. The results show that: (1) Under the cyclic L-U effect induced by MMD, each loading cycle causes compression deformation with partial recovery during unloading, presenting an overall "wavy" variation trend. (2) The maximum load is the most critical factor affecting the damaged coal deformation, with smaller load resulting in less overall sample deformation. (3) After the impact dynamic loading, the damaged samples suffered large-scale impact splitting failure, with the compressive-shear layer failure mainly occurred inside the holes. (4) Lower loading during cyclic L-U process correlate with reduced damage degree, and smaller debris particles with a higher fractal dimension when impact failure occurs, indicating a more severe impact failure. (5) With multiple cycles of L-U, the cracks inside the sample gradually extend and expand from around the hole to the outside. The greater the load and the number of cycles, the more serious the crack damage will be. (6) In the practical mining process, it is crucial to reinforce roadway interiors while minimizing low-loading cyclic disturbances induced by MMD. The study has obtained the deformation evolution rules and failure characteristics of coal under MMD, providing a theoretical basis for the prevention and control of corresponding engineering disasters. 

Study on the evolution rules of coal deformation and failure characteristics caused by multiple mining disturbances

Coal has remained the primary component of China’s energy structure, and high-intensity extraction has continued in the central and western coal-producing regions of China. In contrast to the abundant coal resources, water resources have become extremely scarce in these regions, creating a conflict between coal resource extraction and water resource conservation. The coal mine underground reservoir (CMUR), as a typical technology for combined coal and water extraction and water-preserving coal mining, has been applied in numerous mines in central and western China. This effectively alleviates water resource shortages and achieves the goal of water resource conservation. The CMURs utilizes the goaf created by longwall mining as the water storage space. The reservoir dam structure comprises coal pillars, which serve as protective coal pillars in the mining area, and artificial dam structures that filled the gaps between these coal pillars. The stability of the dam structure under the complex stress effects of hydraulic coupling has been identified as the key to maintaining the safe operation of the CMUR. The mechanical properties, stress field, fracture field, and seepage field (“three fields”) change mechanisms, as well as the research results on size optimization of coal pillar dams and artificial dams in CMURs, were systematically reviewed. The core content included the instability and failure mechanisms of dam structures under the comprehensive coupling effects of factors such as dry–wet cycles of mine water, long-term immersion, chemical effects of high-salinity water, dynamic and static loads, and cyclic loads. This paper is considered to have certain reference value for the study of the stability of dam structures in CMURs and to provide some guidance for the safe operation of CMURs. 

A Review of Stability of Dam Structures in Coal Mine Underground Reservoirs

Characterization of multi-step creep behavior and fractional derivative modeling for a water-saturated coal

The long-term stability of a coal pillar dam is a serious concern for coal mine underground reservoirs because of the creep behavior of coal in complex water immersion and mechanical environments. In order to investigate the characteristics of creep deformation of water-immersed coal and develop a proper creep model, this paper implemented a series of creep experiments of coal via multistage loading at various water-immersion times. The experiment data were analyzed, in terms of immersion-induced damage, elasto-plastic performance, creep behavior, etc., suggesting obvious mechanical properties’ degradation of coal by water. The elastic modulus and peak strength of water-immersed coal decrease exponentially with the immersion time, while the creep rate of coal shows an upward tendency with the promoted immersion time. According to the remarked relationships of elastic, viscoelastic, and viscoplastic properties versus the stress levels and water-immersion time, a creep model based on conformable fractional derivatives is proposed, considering the influence of the water-immersion time and variable stress level. The proposed model was verified using the experiment data, showing a good capacity of the creep model for reproducing the creep process of water-immersed coal. This paper provides a fundamental model for further studying the stability of coal pillars and their influence on the safety of underground water reservoirs.

A Fractional-Order Creep Model of Water-Immersed Coal

Abstract Coal burst is a serious hazard in deep coal mining. To investigate the coal burst proneness evolution during multiple mining disturbances (MMD), this paper implemented a range of uniaxial cyclic loading–unloading tests coupled with acoustic emission (AE) measurement to simulate the loading–unloading effects of the MMD. The maximum load was used to simulate the highest load that the coal has endured in history, the loading–unloading rate was used to simulate the mining speed, and the loading–unloading cycles were used to simulate the number of mining disturbances. Finally, the burst proneness indexes of the damaged coal sample are measured and compared with the origin coal sample. Analysis on the experiment results suggest that, the burst proneness increases when the maximum load is 30% of the uniaxial compressive strength (UCS), interestingly, when the low loading–unloading rate is applied, 50% of the UCS enlarge the burst proneness as well. However, it decreases when the maximum load is 70% of the UCS. Sensitivity analysis indicates that the maximum load, loading–unloading rate, and loading–unloading cycles exhibit decreasing influence sensitivity to the elastic modulus, as well as to the UCS and duration of dynamic fracture. Similarly, the influence sensitivity of maximum load, loading–unloading cycles, and loading–unloading rate to the elastic strain energy index decreases, as well as to the bursting energy index. To ensure the safety of coal mining, it is crucial to avoid the disturbances that would put the coal under low loading and low loading rate. 

Study on the evolutionary characteristics of coal burst proneness under the action of multiple mining disturbances

Sports Industry Trend Analysis in the Fourth Industrial Era : Focusing on news Topic Modeling Analysis

It is hard to control roadways effectively with a heightened burst risk using conventional support techniques, which may cause burst fatalities in the coal mine. Thus, an in-situ modification method, which involves artificially constructing a cracked zone and a reinforced zone, was proposed to improve the stability of the roadways with heightened burst risk. This paper investigated the efficiency to non of the in-situ modification method in improving burstroadway safety by performing a physical experiment. The results of the experiment indicated that the dynamic stress was obviously reduced in the modified roadway. Accordingly, the influence factors of the in-situ modification method were explored using numerical simulation. It was found that the thickness and the degree of damage to the cracked zone were key governing factors in reducing energy and improving roadway stability. The energy dissipation value increased as the thickness and the degree of damage to the cracked zone increased but showed a slower growth trend when the thickness and the degree of damage to the cracked zone increased to a certain value. In addition, the high sensitivity to dynamic load was found in parts of the roadway side exhibiting a larger stress reduction rate even with thinner cracked zone thickness and a lower degree of damage. This research provides an effective and economical method for coal roadways with heightened burst risk.

https://www.frontiersin.org/articles/10.3389/feart.2023.1128697/pdf

An in-situ modification method for coal roadways with heightened burst risk

This paper implemented the Barcelona basic model (BBM) into the OpenGeoSys (OGS) platform for numerical modeling of the coupled hydro-mechanical (HM) behavior of unsaturated soil. Within the implicit integration approach in the OGS, the integration rule of the BBM was developed first and the integration form of the BBM under a return mapping algorithm was built. The closest point projection method was used for calculating the return mapping directions with the associative flow rule. The numerical simulation results show that the BBM is feasible in fitting the experimental results. The numerical integration algorithm can reflect the elastic–plastic mechanical behavior of materials, and improve the calculation accuracy. The material exhibits obvious elastic–plastic characteristics during numerical simulation and experiment, and the water absorption process can lessen the mixture’s compression stiffness while enhancing its recovery stiffness.

/pdf/numerical-implementation-of-the-barcelona-basic-model-based-25zatlx5.pdf

Numerical Implementation of the Barcelona Basic Model Based on Return-Mapping Integration

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