Experimental and Numerical Simulation of Coal Rock Impact Energy Index Based on Peridynamics

In response to the increasing severity of the rock burst phenomenon and its relatively difficult prediction, peridynamics and indoor uniaxial compression experiments were used to calculate the changes of the internal elastic energy (t) and impact energy (c) for different rock masses during a loading process from an energy perspective. Two traditional indices for judging rock burst tendency—the rock elastic deformation energy index (WET) and the rock impact energy index (WCF)—were combined to define a new actual impact energy index (W) to more accurately determine the occurrence tendency of rock bursts. The LAMMPS software was used to simulate the internal energy changes of rock materials under pressure, and the results were compared with experimental results for verification. The results were as follows: (1) in the uniaxial compression experiments of different specimens, fine sandstone had the strongest impact resistance, followed by coarse sandstone, mudstone, bottom coal seam, and top coal seam, and the obtained material properties provide a reference for predicting the rock bursts of various rock types in practical engineering. (2) The values calculated using the actual impact energy index (W) and the simulation value were 1.75 and 1.77, respectively, which corresponded to a lower error than when the rock impact energy index (WCF) and the rock elastic deformation energy index (WET) were used individually. Thus, this index can better predict the rock burst. (3) The simulated specimen was subjected to a gradual increase in the internal stored elastic energy during compression, which gradually decreased after the ultimate compressive strength was exceeded. The degree of impact damage formed after macroscopic crushing occurred depended on its residual energy.

Study on the Formation Mechanism of Rock Burst Caused by Seam Floor Slip under an Ultrathick Conglomerate

A rock burst accident occurred on coalface 13230 of the Gengcun Coal Mine in Henan Province. Through a field investigation, theoretical analysis, and microseismic monitoring, we studied how the rock burst, which was caused by overall seam floor slip and instability, occurring under an ultrathick conglomerate. Because the overlying ultrathick conglomerate in the mined-out zone close to coalface 13230 had been inadequately mined, the leading section of the coalface was under a high level of stress. Combined with the tectonic stresses from the F16 faultage and the soft floor structure, these factors caused the floor of this coalface to trigger the overall slip-type rock burst. In this paper, an estimation model of the ultimate bearing capacity of a seam floor under an ultrathick conglomerate and the advancing abutment pressure on the coalface is presented. This model is used to show that the ultimate bearing capacity of the seam floor on coalface 13230 is 26.3 MPa, and the abutment pressure is far more than the floor bearing capacity. We also present pressure relief and reinforced supporting measures, which can effectively prevent floor slip-type rock bursts from occurring. The results of this study provide a reference for the prevention and control of floor slip-type rock bursts in coal mining under an ultrathick conglomerate.

Mechanism of Rock Bursts Induced by the Synthetic Action of “Roof Bending and Rock Pillar Prying” in Subvertical Extra-Thick Coal Seams

When studying the rock burst mechanism in subvertical extra-thick coal seams in the Wudong coal mine in Xinjiang, China, most studies focus on rock pillars, while the effect of the roof on rock bursts is usually ignored. In this paper, a rock burst mechanism in subvertical extra-thick coal seams under the control of a “roof-rock pillar” is proposed. A theoretical analysis is first performed to explain the effect of roof-rock pillar combinations on rock bursts in coal seams. Numerical modeling and microseismic analysis are implemented to further study the mechanism of rock burst. The main conclusions are as follows: 1) During the mining of the B3+6 coal seam, an obvious microseismic concentration phenomenon is found in both the roof and rock pillar of B3+6. The rock bursts exhibited obvious directionality, and its main failure characteristics are floor heave and sidewall heave, but there will also be some failures such as shoulder socket subsidence in some parts. 2) The stress transfer caused by rock pillar prying is the main reason for the large difference in rock burst occurrence near the vertical and extra thick adjacent coal seams under the same mining depth. 3) Under the same cantilever length, the elastic deformation energy of the roof is much greater than that of the rock pillar, which makes it easier to produce high-energy microseismic events. With an increasing mining depth, the roof will become the dominant factor controlling the occurrence of rock bursts. 4) The high-energy event produced by the rock mass fracture near the coal rock interface easily induces rock bursts, while the high-energy event produced by the fracture at the far end of the rock mass is less likely to induce rock burst. 5) Roof deformation extrusion and rock pillar prying provide high static stress conditions for the occurrence of rock bursts in the B3+6 coal seam. The superposition of the dynamic disturbance caused by roof and rock pillar failure and the high static stress of the coal seam is the main cause of rock burst in the B3+6 coal seam.

Research and Application of Active Support and Pressure Relief Protection in Deep Mines

In recent years, with the extension of the coal mine at depth, rock bursts are more and more frequent. Therefore, the research of roadway support and pressure relief has become the key to prevent rock burst. In this paper, the primary high strength support and hydraulic fracturing pressure relief measures of roadway are studied and analysed. The primary high strength support emphasizes the matching characteristics of support materials, strengthen the protection of the active support to the roadway, and realize an integrated support system. Considering strong burst-prone area, the hydraulic fracturing is used to achieve pressure relief of surrounding rock mass, which can release and transfer the energy within the range of pressure relief. When there is energy release outside the range of protection system, the pressure relief region can absorb the energy so as to achieve the purpose of protecting the roadway. A field engineering case is applicated, the results shown that the stress of roadway surrounding rock and micro seismic energy significantly decreases, reducing the possibility of rock bursts.

General features of strong rock bursts and induced earthquakes in critical-stress areas of the Earth’s crust

The problem connected with rock bursts and induced earthquakes is yet one of the most critical in the mining regions. The manmade nature of disastrous earthquakes induced in the areas of the heaviest impact on the subsoil is being widely discussed. The main argument against the manmade genesis of such earthquakes is their great depths and high energies. The general features of the induced earthquakes are considered. The displacement directions of the walls of large tectonic faults during such events are analyzed. The sizes of focal zones are estimated and related with sizes of geodynamcially active blocks in the Earth’s crust. The location of hypocenters of geodynamic events relative to the manmade impact zones is studied. The found homogeny of strong rock bursts and induced earthquakes is explained by the interaction of local and regional (global) geodynamic processes. The critical stress state of the upper Earth’s crust having hierarchical block structure is considered as the basis of such interaction. When focal zones of rock bursts and induced earthquakes have sizes of hundreds of meters or a few kilometers, the initiation zones of such events reaches many kilometers in size, is commensurable with the Earth’s crust blocks and is larger than the mining impact zone. Therefore, displacements along large faults are the part of a tectonic process, i.e. displacement directions along large faults during strong rock bursts are correlated with the regional stress field.

APPROACHES TO RISKS ASSESSMENT DURING POTASH-MAGNESIUM AND ROCK SALTS DEPOSITS EXPLOITATION

The key segment which is prepared during hazardous facility safety justification de-velopment is the Assessment of Accident Risk. This article deals with the approaches to the assessment of accidents risks, specific hazards of ore and non-ore mines. The most significant hazards at a mine are rockfalls, rock bursts, ignitions and explosions of flammable gases, fires. Certain types of hazards can be assessed through failure rates, the ventilation system failure, for example, can be calculated on the basis of equipment failure rates, while other types of hazards, the rock bursts, for example, cannot be assessed using the existing (author-ized) methods. It is for that reason that this article reviews a principled approach to the as-sessment of mountain hazards, cites the description of the Factor Method, concerning the potash mines. The methodology cited in the article is the basis for the «Safety technologies» LLC standard «Recommended Practices of Assessment of Accidents Risks at Magnesium Mines» Accident Prevention Company Standard 4.21-2019.

Impact Dynamic Properties and Energy Evolution of Damaged Sandstone Based on Cyclic Loading Threshold

Rocks in deep coal mines are usually in varying degrees of damage state before they are destabilized by impact loads such as rock bursts. For the problem of the mechanical properties and energy evolution of damaged rocks under impact loads, the authors use static loads with different cyclic load thresholds to act on sandstone specimens to make them in distinct degrees of damage. Then, the rock mechanics system (MTS-816) and the Split Hopkinson pressure bar (SHPB) are employed to perform uniaxial compression and impact dynamics tests on sandstones with different degrees of damage. The results show that, from the perspective of mechanical properties, the uniaxial compressive strength and dynamic compressive strength of the damaged sandstone gradually decrease with the increase of the upper limit of the cycle threshold and both obey the growth law of the quadratic function, and the dynamic strength increase factor (DIF) also decreases with the increase of the cyclic load threshold. In terms of energy, with the increment of the cyclic load threshold, the number of cracks in the damaged sandstone is large and the scale is enormous. Due to the effect of cracks, when the incident energy is a fixed value, the transmission energy decreases with the increase of the damage degree and the change law of the reflection energy is the opposite. The systematic study of the dynamic mechanical properties and energy evolution law of the damaged sandstone provides some reference for the prevention and mechanism research of rock bursts.