Geomechanics and Geophysics for Geo-Energy and Geo-Resources最新文献

Seismic source location is a classic inverse problem in seismology. In mathematical physics, inverse problems have multiple natural solutions. The objective of this study was to develop a generic theory and method of seeking the true solution from multiple solutions for the location of a coal mine seismic source in an idealized velocity structure model of a coal mine with a small scale and complex geological environment. Starting from the simplest velocity structure model, the complexity of the model gradually increased, until it approached the real velocity structure model, i.e., the multilayered horizontal and inclined velocity structure model, in order to find a generic method for solving the multi-solution inverse problem of coal mine seismic source location. Specifically, the wavefront distribution equation in a two-layer horizontal medium was derived and then expanded to any multi-layer horizontal medium. Based on this equation, a positive definite nonlinear equation system was established from the perspective of any observation system. The equation system contained four unknown variables of the spatiotemporal position of the seismic source. To determine the spatiotemporal parameters of the seismic source, nonlinear equations for four stations were required. To solve the nonlinear equation system, an initial iteration value was determined. In order to reduce the difficulty of determining the initial iteration value, the variable substitution method was used to reduce the number of location parameters. By rotating the original geodetic coordinate system of the station to be parallel and orthogonal to the medium interface, the wavefront method was extended to inclined medium. In conclusion, in this study, the problem of coal mine seismic source location in a multi-layer horizontal or inclined medium was effectively solved. The method proposed in this study provides a reference for solving the true solution from multiple solutions for the location of a coal mine seismic source in small-scale coal mines with complex geological environments.

The study of sandstone dilation characteristics under actual mining and excavation conditions promotes the safe and efficient development of underground engineering. Accordingly, this study employs true-triaxial testing to reveal the influence of intermediate principal stress and unloading action in the minimum principal stress directions on rock dilation. In the stress–strain analysis, with increasing intermediate principal stress, the maximum compression of the sandstone volume in the loading and unloading tests increases, while the onset of dilation is delayed under loading conditions and initially delayed and then accelerated under unloading conditions. The energy storage limit of sandstone under the unloading test tends to decrease with increasing intermediate principal stress, contrary to the results of the loading test, and the characteristic point at which the percentage of dissipative energy is greater than that of elastic energy occurs earlier. The energy of sandstone in the unloading test in the intermediate principal stress direction was smaller than that in the minimum principal stress direction, while the loading test showed the opposite trend. Combining these two analyses can elucidate the restraining effect and tensile stress effect of the intermediate principal stress, as well as the weakening and strengthening effects of the unloading action on the two effects. By further combining the average elastic energy and dissipative energy conversion rate under different test conditions, the weakening effect was manifested by shortening the compressive deformation course, and the strengthening effect was manifested by developing the degree of plastic expansion deformation. This study provides important guidance for deep mining projects.

Ground subsidence in Western coal mining areas is characterized by rapid deformation, extensive damage, and a wide range of impacts. The conventional observation methods are inappropriate for surface damage monitoring in high-intensity mining areas of Western China. Therefore, it is a crucial problem to quickly, accurately, and comprehensively monitor the ground subsidence and environmental damage caused by high-intensity and large-scale mining. In this study, we propose a monitoring method for the ground subsidence of high-intensity mining with Unmanned Aerial Vehicle Lidar (UAV-LiDAR) measurement technology. Taking a mine in Ordos of China as an example, the Digital Elevation Model (DEM) is obtained by Kriging Interpolation of the ground point cloud from UAV-LiDAR. Then, the multi-stage DEM differential processing is employed to get ground subsidence. Finally, the median and bilateral filters combine for denoise to obtain the high-precision ground subsidence. The results show that the accuracy of the ground DEM generated by UAV-LiDAR is 15 mm and the mean square error of the ground subsidence basin is 39 mm. UAV-LiDAR technology can quickly obtain abundant surface data and obtain high-precision ground subsidence. Therefore, the application of this technology and method in subsidence monitoring in mining areas is feasible. And it can provide support for ecological environment monitoring, land reclamation, and ecological restoration in mining areas. The research results can provide a useful basis for monitoring the surface damage of coal mining in Western China.

Abstract

To comprehend the stress state and response characteristics of materials under complex conditions, researchers have decomposed stress states into fundamental paths and investigated diverse path combinations. To ensure comparability, four identical samples were carefully selected from a pool of 100 samples using ultrasonic tests based on the wave speed and waveform characteristics. These samples underwent specially designed stress paths to analyze the combined effects of linear loading and perturbation. Our result analysis centred on the perturbation amplitude and stress levels during composite action, revealing intricate relationships between the stress levels, strain, and nonlinear/linear energy evolution under complex stress paths. Simultaneously, 3D surface fractures were precisely reconstructed using the YOLOv5 and FAST feature point detection algorithms, elucidating the evolving patterns of the fractures. As a result of our study, the rotation trend of the main fracture was validated by integrating mechanics and P-wave reflection rules. Notably, our experimental results closely aligned with the theoretical predictions, showing the reliability of our study. These findings can significantly contribute to guiding safety protocols in the field of underground engineering.

During the process of close-distance seam group mining, the coal pillar in the upper coal seam is a stress-concentrated area, which leads to a loss of stability of the roadway during mining of the lower coal seam. This lack of stability introduces great safety hazards to coal mines. To solve the problem of stress concentration of coal pillars, the method of gob-side entry retaining by roof cutting is proposed to remove the coal pillar. In this study, FLAC3D was used to analyze the depth and angle of pre-split blasting. LS-DYNA was used to analyze the spacing of the blasthole. Using the methods of theoretical analysis and numerical simulation, we determined that the optimal depth of the pre-split blasting was 6 m, the optimal angle for pre-split blasting was 15°, and the optimal spacing of the blasthole was 500 mm. A field test was carried out in the 1010201 ventilation roadway of the Yuwang Coal Mine, China. The on-site peeping results showed that when the spacing of the blasthole is 500 mm, connecting cracks can form under the action of blasting stress. After the working surface is mined, the roof strata could collapse and fill the gob over time when the depth and angle of the pre-split blasting are 6 m and 15°, respectively.

The laws of acoustic emission (AE) before and during rock failure are different under different stress states. In this article, a new multi-functional true triaxial geophysical (TTG) apparatus was applied to analyze the AE law of sandstone under different stress paths. The results show that (1) with the increase of Lode angle, the tensile fractures in the sandstone increase initially, followed by a decrease. The number of AE decreases initially, followed by an increase, while the average energy of AE signal increases initially, followed by a decrease. (2) During the loading process, the IB values of rock can be divided into wave type, band type and mixed type, which represent crack propagation process driven by external force, self-driving and mixed driving. It can provide a basis for early warning of underground engineering construction disasters. (3) The variation characteristics of RA and AF in rock failure process show the corresponding relationship with IB value. The RA value corresponding to the IB value of band, wave and distribution type distribution mainly concentrated around 0.05, 0.03 and widely distributed, respectively. According to the value of RA, the types of cracks show different characteristics under different driving forces. (4) With the increase of Lode angle, the failure types of rocks change from single oblique fracture (− 30°) to double-X-type fracture (10°), and finally changes to single-X-type fracture when Lode angle is 30°. The fracture angle of rock decreases initially, followed by an increase with the increase of Lode angle. Therefore, it is important to explore the AE law of rock failure process under different stress states for the early warning of underground engineering construction disasters, and can provide a guidance for the application of human underground space.

The meso-structural changes of rocks during repeated cycles of water invasion are the fundamental cause of macroscopic physical property damage. In this paper, based on the computed tomography scan images of rock samples under different numbers of water invasion cycles, a three-dimensional pore network model was constructed to analyze the changes in pore structure under the action of water invasion. The damage variable was introduced to quantitatively characterize the parameter damage of each pore and reveal the evolution of rock meso-damage. The results show that 81% of the pore radius is less than 10 μm under 0 water invasion cycles and that 76% of the pore radiuses are less than 10 μm after 10 water invasion cycles. After 10 water invasion cycles, the peak range of the pore radius distribution enlarged from the initial range of 2–4 μm to that of 4–6 μm and the proportion of pore throats with a radius less than 10 μm decreased from an initial 82–72%. With an increase of water invasion cycles, the proportion of large pores increased and the connectivity among pores enhances gradually. The damage variable of each pore parameter changed the most during 2–5 water invasion cycles. After 10 water invasion cycles, the maximum degree of damage that the pore volume reached was up to 41.44% and the minimum degree of damage of the pore coordination number was 5.80%. The test results helped to reveal the pore structure changes and the damage of rock samples during water invasion cycles.

With the large scale mining of coal and the increase of abandoned goafs under weakly cemented aquifer strata in Western China, it is urgent to study the time dependent mechanical properties of water sensitive aquifer strata. In this paper, creep behavior of dry and saturated medium-grain sandstone, which represent two limit states affected by water, were studied and compared. The results showed that water greatly weakens the compressive strength of medium-grained sandstone, but the difference in axial strain between dry and saturated rock samples decreases with the increase of confining pressure. The creep compression volume of rocks decreases with the increase of deviatoric stress, and only under low confining pressure does the creep volume of rocks exhibit expansion. There is an order of magnitude difference in creep strain between medium grained sandstone and other common rocks. The instantaneous elastic modulus has a negative exponential relationship with deviating stress. The creep rate has a positive exponential relationship with deviating stress. Based on Burges model with exponential damage variables, the law of the influence of confining pressure on creep model parameters has been discussed. The linear relationship between elastic modulus of medium-grained sandstone and parameters of Burgers model with damage was found. The research results are conducive to the convenient prediction of creep behavior of medium grained sandstone engineering and the long-term stability control of the surrounding rock.

Gravity heat pipe system has previously been proven as an environmental and efficient technology in exploiting hot dry rock. However, it is unclear what geothermal reservoir siting is more favorable for the system’s heat extraction performance. Herein, we analyzed the influences of geothermal field siting (rock property and reservoir environment) on heat extraction performance of gravity heat pipe systems through a 3D thermal–hydraulic coupled model. It is found that rock properties have huge influences on heat compensation, heat extraction ratio and heat compensation ratio. Low rock density, low rock specific heat capacity and high thermal conductivity could increase heat compensation, heat extraction ratio and heat compensation ratio. It is also found that geothermal reservoir environment affects the heat extraction rate seriously. High initial temperatures and low temperature gradients increase heat extraction rates. Geothermal reservoir pressure affects the heat extraction performance slightly, and low initial pressures increase heat extraction rate. The study results would provide suggestions on deep geothermal exploitation locations.

Coal mine galleries and construction tunnels are commonly excavated using roadheaders. Estimating the performance of roadheaders is crucial for planning and cost estimation when planning tunnel or tunnel projects. The aim of this study is to derive generalized performance prediction models including Schmidt hammer value, needle penetration index, and volumetric joint count for roadheaders used in coal mines. The performance measurements of axial and transverse type roadheaders were carried out in six different coal mines. Schmidt hammer tests, needle penetration index tests, and of volumetric joint count measurements were also performed at the locations where the performance measurements were conducted. The extensive data were evaluated using multiple linear and nonlinear regression analysis. The developed formulas were evaluated using statistical tests. The Equations that include the Schmidt hammer value or needle penetration index value in addition to cutter head power have been shown to be unreliable. However, the equations that include Schmidt hammer value or needle penetration index value in addition to cutterhead power and volumetric joint count have been shown to be reliable. This study concludes that the developed reliable equations can be used for the performance assessment of roadheaders used in coal mines.