Analysis of Mining Crack Evolution in Deep Floor Rock Mass with Fault

To further explore the crack evolution of floor rock mass, the mechanism of fault activation, and water inrush, this paper analyzes the crack initiation and propagation mechanism of floor rock mass and obtains the initiation criteria of shear cracks, layered cracks, and vertical tension cracks. With the help of simulation software, the process of fault activation and crack evolution under different fault drop and dip angles was studied. The results show that the sequence of crack presented in the mining rock mass is vertical tension cracks, shear cracks, and layered cracks. The initiation and propagation of the shear cracks at the coal wall promote the fault activation, which tends to be easily caused at a specific inclination angle between 45° and 75°. The fault drop has no obvious impact on the evolution of floor rock cracks and will not induce fault activation. However, the increase of the drop will cause the roof to collapse, reducing the possibility of water inrush disaster. Research shows that measures such as adopting improved mining technology, reducing mining disturbance, increasing coal pillar size, and grouting before mining as reinforcement and artificial forced roof can effectively prevent water inrush disasters caused by deep mining due to fault activation.

Analytical Solution of Stress in a Transversely Isotropic Floor Rock Mass under Distributed Loading in an Arbitrary Direction

Rock masses with a distinct structure may present a transversely isotropic character; thus, the stress state in a transversely isotropic elastic half-plane surface is an important way to assess the behavior of the interaction between the distributed loading and the surroundings. Most previous theoretical analyses have considered a loading direction that is either vertical or horizontal, and the stress distribution that results from the effect of different loading directions remains unclear. In this paper, based on the transversely isotropic elastic half-plane surface theory, a stress solution that is applicable to distributed loading in any direction is proposed to further examine the loading effect. The consistency between the analytical solution and numerical simulations showed the effectiveness of the proposal that was introduced. Then, it was utilized to analyze the stress distribution rule by changing the Poisson’s ratio and Young’s modulus of the model. The effects of the formation dip angle on the stress state are also examined. The stress distribution, depending on the physical property parameters and relative angle, is predicted using an analytical solution, and the mechanisms associated with the transversely isotropic elastic half-plane surface subjected to the loading in any direction are clarified. Additionally, extensive analyses regarding this case study, with respect to the mechanical behavior associated with changes in the stress boundary, is available. Hence, the proposed analytical solution can more realistically account for the loading problem in many engineering practices.

Stabilization of Rock Roadway under Obliquely Straddle Working Face

A floor rock roadway under an oblique straddle working face is a typical dynamic pressure roadway. Under the complex disturbance of excavation engineering works, the roadway often undergoes stress concentration and severe deformation and damage. To solve the problem of surrounding rock stability control for this roadway type, this study considered the East Forth main transport roadway in the floor strata of the 1762(3) working face of the Pansan coal mine. In situ ground pressure monitoring and numerical simulation calculation using the FLAC2D software were carried out. The influence laws of the surrounding rock lithology, the vertical and horizontal distance between the roadway and overlying working face, the positional relationship between the roadway and the overlying working face, and the support form and strength of the rock surrounding an oblique straddle roadway were obtained. Within the range of mining influence, the properties of the rock surrounding the roof and floor were very different, and the deformation of the rock surrounding the two sides exhibited regional difference. The influence range of the mining working face on the rock floor of the roadway was approximately 30–40 m, and that of horizontal mining was approximately 50–60 m. The mining influence on the rock surrounding the side roadway of the working face is large, but the mining influence on the roadway below is small. Using FLAC2D, the stress and displacement characteristics of the rock surrounding the obliquely straddle roadway were compared and analyzed when the bolt support, combined bolt and shed support, and bolt–shotcreting–grouting support were adopted, the proposed support scheme of bolting and shotcreting was successfully applied. The deformation of the rock surrounding the roadway was satisfactorily controlled, and the results were useful as a reference for similar roadway maintenance projects.

Evaluation Method of Floor Heave Damage Degree and a Case Study in Zaoquan Coal Mine, China

With the continuous increase of mining depth and complex mining geological conditions, the mileage of roadways in underground engineering such as coal mine is increasing year by year. Complex conditions lead to different floor heave failure laws, and the control technology and strategy should be changed accordingly. How to evaluate the damage degree of floor heave under different conditions has become an urgent problem. Firstly, this paper makes a statistical analysis on the main evaluation indexes of the damage degree of roadway floor heave. Then, the fuzzy comprehensive clustering method is used to establish the classification method of floor heave damage degree, taking the floor heave amount, floor rock fragmentation degree, coal pillar size, buried depth, and floor lithology as evaluation indexes. The damage degree of floor heave can be divided into five types: light type, obvious type, severe type, destructive type, and extremely severe type. Finally, the rationality and accuracy of the method are verified by the measured value and evaluation value of No. 130203 roadway in the Zaoquan coal mine. The results can provide reference for the evaluation of the damage degree of the floor rock in similar condition mine and provide guidance for the design of the support and stability control of the failure of the roadway floor heave.

Variation Law of Infrared Radiation Temperature of Unloading Fracture of Composite Coal-Rock

For composite mining coal-rock dynamic disaster, combining the theory of thermodynamics, damage mechanics, and other disciplines, the thermodynamic coupling mathematical model of composite coal-rock under an unloading condition is deduced, and the simulation model of composite coal-rock is established for numerical simulation. And the variation law of thermal infrared radiation under triaxial loading and unloading of composite coal-rock is analyzed and verified by experiments. The results show the following findings: (1) The distribution of thermal infrared radiation temperature of composite coal-rock is different in different stages of stress. The overall temperature of the temperature field is lower than the initial temperature field in the three-dimensional stress loading stage and the stage of stress-keeping pressure, but the internal temperature of the coal body is the highest. In the first stage of “loading and unloading,” the temperature of a coal seam increases slightly, and the temperature of other parts of the rock layer increases except for the circular low-temperature zone. In the second stage of “loading and unloading,” an alternating zone of high and low temperatures appears in the rock mass, and the temperature field is enhanced, among which the temperature field reaches the strongest after unloading the confining pressure. After jumping over the maximum stress, the temperature field decreases as a whole at the instability and rupture stage. (2) The variation of surface average thermal infrared radiation temperature ( T ave ) of composite coal-rock can be divided into the initial fluctuation stage, the linear heating stage, the local decline stage, the temperature sudden increase stage, and the fracture decline stage. At three different unloading rates of 0.003 MPa/s, 0.03 MPa/s, and 0.05 MPa/s, the T ave of coal body, floor rock, and roof rock reach the maximum before composite coal-rock instability and fracture, and the temperature change of the coal body is the most obvious. (3) Under different confining pressure unloading rates, the T ave of roof rock, coal body, and floor rock shows a strong linear relationship with stress after linear fitting. And the correlation between simulation and experimental results after fitting is above 0.89. The larger the confining pressure unloading rate is, the shorter the peak time of T ave arrives, and the larger the peak value. The comparison between the experimental results and the simulation analysis shows that the two results are consistent, and the research results can provide a theoretical basis for the prevention and control of dynamic disasters in coal and rock mining.

Mechanism and Control of Roadway Floor Rock Burst Induced by High Horizontal Stress

According to the characteristics of rock burst of high horizontal stress roadway floor, the rock burst mechanism of roadway floor was studied with the background of south track roadway Xing’an mine. Based on the deflection theory and energy principle of the slab, the mechanical model of the floor of the roadway under high horizontal stress was established, the stress and energy criteria of rock burst occurred in the floor of the roadway were deduced, the prevention and control measures of the floor pressure relief with large diameter borehole and concrete-filled steel tube pile support were put forward, and the key parameters were determined. By establishing a numerical model, the evolution law of plastic zone, horizontal stress, and elastic strain energy density of roadway floor with or without support is contrastively analyzed. The results show that the effective means to prevent and control the floor rock burst is to cut off the stress transfer path by weakening the hard floor to reduce floor energy accumulation so as to reduce the floor rock burst risk. Based on the above research, field tests were carried out, and the microseismic monitoring results showed that the floor pressure relief of large diameter boreholes and concrete-filled steel tube pile support effectively relieved the floor rock burst and guaranteed the safety and efficiency of roadway excavation. This technology can provide a reference for the prevention and control of floor rock burst of similar roadways.

Fracture Propagation and Hydraulic Properties of a Coal Floor Subjected to Thick-Seam Longwalling above a Highly Confined Aquifer

The high-pressure and water-rich confined aquifer occurring in the Ordovician limestone sequence poses great threats to the routine production of underground longwall mining. Considering the intense cooperation of mining disturbance and water pressure, water-conducting fractures within a coal seam floor can connect the lower aquifer and upper goaf, and this hydraulic behavior is considered the root of water inrush hazard and water loss or contamination. In this paper, the panel 4301 of the Longquan coal mine serves as the case where the panel works closely above the floor with high water pressure. By the combination of physical and numerical modelling approaches, the variation characteristics of fracture development and volumetric strain of floor rocks subjected to mining disturbance are analyzed. A numerical computation model is constructed based on the volumetric strain-permeability equation obtained by curve fitting, and on such basis, the impacts of different mining parameters on floor rock permeability are studied. The results show that the floor rocks experience fracture generation, extension, and convergence procedures as the workface advances along the longitudinal direction, and fractures appearing in front of the workface are more developed. In the whole process of coal seam extraction, the volumetric strain profile exhibits “Λ” shape and an inverted saddle shape before and after overburden strata collapse. By controlling a single variable, the paper reveals that panel height is of greater impact on floor permeability changes than panel length and panel width.

Study on Broken Floor Rock Mass by Mining Underground Pressure

As the dangerous level of floor water inrush in Chinese coal field is becoming more and more serious annually, the widely used formulas of broken floor rock mass are belonged to nonlinear type or empirical type. However, they are not well conformed to the practical situation and including mining underground pressure. The biggest depth of broken floor rock mass and the length of gob-floor or mining-floor until the maximum broken floor location are expressed by theoretical formulas on integrity theory. Taking a mining face in Chinese Anhui Province as the object, the relationship between broken floor rock mass and mining underground pressure is studied by numerical simulation, the theoretical analysis, and the DC exploration. The peak and scope of broken floor rock mass will enlarge until reaching limit value with the increasingly advanced distance. The mining gob stress contour is saddle-shaped, and its growing speed is becoming slower, so the 180 m coal mining face has reached the sufficient mining stage. Wave velocity of broken floor rock mass from 0 m to 16 m is greatly decreased by the mining disturbance, and it is basically conformed to theoretical formula and practical situation. The results can be relatively better used in the pressure mining of the Ordovician limestone, because it can provide some safe guarantee for mining deep coal seam.

Evolution Law of Floor Fracture Zone above a Confined Aquifer Using Backfill Replacement Mining Technology

Disturbances owing to coal mining result in the movement and failure of floor strata. Mining-induced fractures within the floor may propagate to the confined aquifer, thereby causing water inrush disasters. In this study, we propose using strip mining and backfill replacement mining above the confined aquifer to investigate the failure depth of the floor. The problem is simplified as a distributed force model on a half-plane body. First, the stress disturbance of the floor during strip mining is calculated and the results are combined with the von Mises yield criterion. Then, the destruction of the floor after replacing the remaining coal pillars is explored. The results show that the widths of the strip mining face and coal pillars play an important role in affecting the failure depth of the floor: the greater the width, the larger the failure depth. Based on the parametric study results, the mining face and retention coal pillar width of 20 m is sufficient to prevent the occurrence of water inrush accidents. After the replacement of the remaining coal pillars, the failure area of the floor rock mass increases, but the maximum failure depth remains unchanged. Finally, we employed field measurement techniques at the Bucun coal mine to monitor the shear and vertical strains of the floor. The data collected was compared with the predicted results obtained from the proposed theoretical model. Good agreement was found between the monitoring and calculation results, which demonstrate the effectiveness of the proposed method.