Microseismicity has long been a precursor for underground mining hazards such as rockbursts and coal and gas outbursts. In this research, a methodology combining deterministic stress and failure ...analysis and stochastic fracture slip evaluation, based upon the widely accepted fracture slip seismicity-generation mechanism, has been developed to simulate microseismic events induced by longwall mining. Using the built-in DFN facility in FLAC3D, discrete fractures following a power law size distribution are distributed throughout a 3D continuum model in a probabilistic way to account for the stochastic nature of microseismicity. The DFN-based modelling approach developed was adopted to simulate the evolution of microseismicity induced by the progressive face advance in a longwall top coal caving (LTCC) panel at Coal Mine Velenje, Slovenia. At each excavation step, global stress and failure analysis with reference to the strain-softening post-failure behaviour characteristic of coal, and fracture slip evaluation for microseismicity are conducted sequentially. The model findings are compared to the microseismic event data recorded during a long-term field monitoring campaign conducted at the same LTCC panel. It was found that the released energy and frequency-magnitude distribution of microseismicity are associated with the slipped fracture sizes and fracture size distribution. These features for recorded microseismic events were fairly constant until a xylite rich heterogeneous zone ahead of the working face was approached, which indicates that fractures within the extracted coal seam follow the same size distribution. The features obtained from modelled microseismic events were consistent over the production period, and matched well the field observations. Furthermore, the model results indicate that the power law fracture size distribution can be used to model longwall-mining-induced microseismicity. This model provides a unique prospective to understand longwall coal mining-induced microseismicity and lays a foundation to predict microseismicity, or even rockburst potential in specific geological realisations.
•A modelling approach has been developed to simulate longwall top coal caving coal mining-induced microseismicity.•DFN has been considered as the hypocentre to generate synthetic microseismicity.•The b value of microseismicity is associated with fracture size distribution within the coal seam.•The histogram of released energy of microseismicity is related to slipped fracture sizes and fracture size distribution.•The power law fracture size distribution can be used in longwall coal mining environments to simulate microseismicity.
•A mathematical physical model for the 2D unloading vibration was established.•The influence factors of unloading vibration were verified by numerical simulation.•The effect of in situ stress release ...on blasting vibration signals was evaluated.•Distinct unloading vibration can only be generated under certain conditions.
Excavation unloading under high initial stress is a typical dynamic process subjected to the combined effects of different factors. In this study, a mathematical physics method for a two-dimensional circular excavation was developed to investigate the mechanism involved in unloading vibration, which is capable of providing insights into the quantitative relationships between vibration features and correlative factors such as the initial stress, cross-sectional area of the tunnel, unloading rates and unloading paths. Then the dynamic unloading excavation process was implemented in the discrete element program PFC 2D for numerical analysis after verifications against the theoretical results. In particular, the characteristics of unloading waveform under high initial stress were investigated for various ratios of horizontal and vertical in situ stresses and aspect ratios of rectangular tunnels. The temporal and spatial characteristics of the excavation process can also be illustrated. In a practical project which considered the combined action of both the blast and unloading vibration, the finite difference program FLAC 3D was further adopted to investigate the contribution of unloading vibration to the character of the seismogram. Results are presented which indicate the 2D numerical analysis provides satisfactory approximation to the excavation process. While the peak particle velocity (PPV) is in direct proportion to the in situ stress variation, it decreases significantly along with the increase of unloading time. In addition, excavation by drill-and-blast (D&B) method should be taken as a dynamic process of blast loading before unloading in a period of time, instead of instantaneous unloading. Although unloading vibration can be generated and significant damage around the tunnel can be induced in the process, distinct unloading vibration can only be generated under high in situ stress and unloading rate.
The cyclic wetting-drying phenomenon, which is a part of weathering processes, plays a vitally important role in affecting the properties of rock materials. To investigate the effect of wetting and ...drying cycles on the physical and dynamic compressive properties of rocks, some essential physical properties of sandstone specimens including density, water absorption, porosity, P-wave velocity and slake durability index (SDI) were measured after every 10cycles (for a total of 50cycles). Dynamic compressive tests were conducted using a modified split Hopkinson pressure bar (SHPB) technique for rock specimens. Laboratory tests results showed that, with the increase of wetting and drying cycles, the porosity and water absorption of rock increases while the density, P-wave velocity, SDI, dynamic compressive strength and elastic modulus decrease. In addition, the surface microscopic morphological characteristics of specimens were examined by scanning electron microscope (SEM). It was observed that the micro-cracks grow and expand in rock after cyclic wetting-drying treatments, which is the main cause of the reduction in dynamic compressive strength. Based on experimental results, an empirical equation was established to describe the effect of strain rate and number of wetting and drying cycles on the dynamic compressive strength of rock materials.
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•Some essential physical properties of sandstone were determined after every 10 wetting and drying cycles.•The dynamic compressive strength of rock specimens at the same strain rate decreased with wetting and drying cycles.•The micro-cracks induced by wetting and drying cycles were observed using SEM.•An empirical equation was established to consider rate and cyclic wetting and drying effects on dynamic strength.
•The porosity effects on dynamic response of water-saturated sandstone was obtained.•Stress distribution characteristics on the fracture surface and inter-particles were analyzed.•Dynamic mechanical ...model was established considering pore water pressure and Stephen effect.•Effect mechanism of the porosity and water saturation on water-saturated sandstone was revealed.
The dynamic response of sandstone to porosity and the amount of water are crucial to understanding the role of these physical parameters on the dynamic behaviour of rock. This study used four different types of sandstone, selected based on a microscopic analysis, for experimental tests and an analytical evaluation. During the experimental tests, dynamic compressive examination was employed on these types of sandstone under both water-saturated and dry states. The ratio of dynamic mechanical parameters of water-saturated sandstone to those of dry sandstone was focused on to eliminate the effect of the self-properties of the sandstone. The results indicate that the effect of water saturation on the dynamic mechanical properties of sandstone significantly affected by the porosity and the weakening degree of water saturation to sandstone positively correlates with the porosity. This effect is intuitively expressed in the features of the fracture propagation as recorded by a high-speed camera and dominated by the micromechanics between the pores and microfracture. This is microscopically attributed to the effect of the pore water pressure and the Stefan effect between the fracture surface and water on the fracture initiation and propagation. In addition, the dynamic mechanical model was further established and applied to discuss the effect mechanism of porosity on the dynamic behaviour of water-saturated sandstone. These insights can contribute significantly to improving the safety and cost-effectiveness of water-saturated rock engineering.
Discrete fracture and matrix modeling of coupled flow and geomechanics is instrumental for understanding flow in fractured media for various geoengineering applications such as enhanced geothermal ...energy systems and groundwater remediation. We employ the governing equations for two-phase flow in deformable fractured porous media with a stress-dependent porosity and permeability model of matrix, and variable fracture aperture and the corresponding permeability. A finite element framework is presented, in which fractures are regarded as low-dimensional objects, to discretize the coupled two-phase flow and geomechanics in fractured porous media. A hybrid method, combining discontinuous Galerkin (DG) and continuous Galerkin (CG) finite element methods (FEM), is utilized to solve for the two-phase flow, while the solid deformation is approximated using a discontinuous Galerkin FEM approach. Several benchmark cases are utilized to examine the accuracy of the proposed hybrid DG-CG FEM method. Further validation is performed using more complex, realistic fracture configurations. As the mechanical characteristics of the fracture and the surrounding matrix differ, the simulation results demonstrate that displacement and stress are discontinuous on both sides of the fracture. While the case with low permeability fractures exhibits pressure jump across the fractures, the pressure changes are reasonably smooth for the conduit fractures.
•Dynamic test of soft coal samples with different water contents and porosities were conducted.•Dynamic failure process and evolution mechanism of soft coal samples were analyzed.•Water-bearing ...unequal spheres model was established.•Mechanism of water content and porosity on dynamic strength was discussed.
Determining the effect of water content and porosity on the dynamic mechanics of soft coal is a key problem in reinforcing soft coal seams by water injection. In this study, soft coal specimens with different water contents and porosities were successfully prepared and tested using a Split Hopkinson Pressure Bar (SHPB). The results show that the effect of water content on the strength of soft coal can be divided into two stages based on the critical point of optimum water content. With the effects of increasing porosity, the optimum water content gradually increased, while the peak strength corresponding to the optimum water content gradually decreased. In addition, based on the analysis of the dynamic failure process of the coal recorded using a high-speed camera, the dynamic mechanical model varies with the distance, and the liquid bridge volume between particles was established according to the water-bearing unequal spheres models. Moreover, the mechanism of the effects of porosity and water content on the fracture expansion and dynamic strength of soft coal were further discussed.
Porosity significantly affects the dynamic response of water-saturated coal and the safety of coal mine engineering in water-rich areas. For mastering the dynamic response of water-saturated coal to ...the porosity, this study used the coal with four levels of the porosity for dynamic tests. In this process, the dynamic compressive tests were operated on these coal specimen in both of water saturation and dry states, and the reduction factor of dynamic mechanical parameters of the coal in water-saturated state to these in dry state were focused to eliminate the effect of self dynamic properties of the coal. The results indicate that the dynamic mechanical properties non-linear positively correlate with the porosity, and its sensitivity to porosity gradually weakens as with the increase of the porosity. This effect is intuitively expressed in the dominant fracture propagation features recorded by a high-speed camera, and microscopically attributed by the size and number of microfractures around the dominant fracture. The dynamic stress intensity factor model of the dominant fracture was further established with consideration of total cohesive force for the equivalent particle on the fracture surface. It well reveals the effect mechanism of porosity on the meso-deterioration and macroscopic mechanical properties of the coal around the roadway. On this basis, the applicable conditions and suggested methods were put forward to improve the dynamic stability of water-saturated coal with various porosity.
Excessive strain energy accumulation in the coal seam is one of the essential conditions for rockbursts to occur. It is imperative to understand the parameters that affect the strain energy ...accumulation in retreating longwall mining to optimise the design and minimise rockbursts occurrences. In this paper, new analytical models were developed to calculate the strain energy accumulation considering the current state-of-the-art longwall machinery being used in the industry. Seven parameters, i.e., mining depth, length of the cantilever roof, coal seam thickness, thickness of the immediate roof, Young’s modulus of coal and the roof, and Poisson’s ratio of the coal seam, were identified as the parameters affecting the strain energy accumulation. A detailed statistical analysis of the parameters was conducted using the orthogonal testing method, which revealed that mining depth, Young's modulus of coal and the coal seam thickness significantly influence the strain energy accumulation within a 99% confidence interval.
Highlights
An analytical model to calculate strain energy accumulation in longwall mining panels.
Excellent agreement between analytical and numerical models.
Parametric dominance was determined using the orthogonal testing method.
Easy to use equations to calculate rockburst risk in longwall mining panels.
Causal mechanisms for fluid injection-induced seismicity are characterised by various stress paths to drive slippage of underlying fractures. Investigation into the role of stress paths on the ...fracture slip behaviour and seismic response is crucial for understanding causal mechanisms for induced seismicity. In this work, novel experimental stress and pressure paths were proposed to induce fracture slip in a fashion that decouples fracture normal and shear stresses, whilst maintaining the same rate to approach slippage (the same Coulomb stress change rate). Laboratory experiments were carried out on a shale sample to simulate slippage along a pre-existing fracture under three fluid injection-related stress and pressure paths, i.e. pore pressure elevation, fracture normal stress relaxation, and fracture shear stress increase. Seismic stable fracture slip characterised by frictional strengthening occurred in these experiments, and the time-varying friction of the fracture was reasonably described by the rate-and-state friction law. The pore pressure elevation and fracture normal stress relaxation paths at the same stressing rate caused similar fracture slip and seismicity behaviour, and can be considered to be equivalent. Results have shown a transition from quasi-static to dynamic fracture slip under both fracture effective normal stress relaxation paths, but not under the fracture shear stress increase path. This indicates that the fracture effective normal stress relaxation tends to cause accelerating slip, whilst the fracture shear stress increase initiates prolonged gradual slip. Accumulative seismic moment scales linearly with fracture slip duration, which implies that fracture shear stress increase causes larger seismic moment than fracture effective normal stress relaxation paths.
Highlights
Novel stress paths that isolate normal and shear stresses under the same Coulomb stress change rate are designed to promote fracture slip.
Fracture effective normal stress relaxation is more inclined to promote slip acceleration than fracture shear stress increase.
Seismic moment scales linearly with slip duration, and shear stress increase causes larger moment than effective normal stress relaxation.
•Active acoustic monitoring was used in hydraulic fracturing experiments on naturally fractured coal.•Recorded seismograms captured both fracture initiation and interaction with natural ...fractures.•The seismograms of S wave diffractions were found to be helpful to identify fracture propagation.•CT scan and seismic velocity tomography located the pre-existing fractures and injection-disturbed regions.
Hydraulic fracturing of coalbed methane wells has been widely practised as an effective method to increase drainage efficiency in low-permeability, low-pressure and low-saturated coal seams. To investigate hydraulic fracture performance and associated seismic response in coal, hydraulic fracturing experiments were carried out on two cubic coal blocks containing a host of natural fractures using a true triaxial rock testing machine equipped with loading, injection and acoustic systems. The acoustic system uses transducers with active sources to repetitively generate and receive ultrasonic P/S wave pulses for characterising mechanical properties of the coal blocks and revealing fracture growth. Silicon oil was injected into the middle of coal blocks to create hydraulic fractures under deviatoric stress conditions, and the stress and displacement, borehole pressure and volume, and seismic response were recorded over the injection process. X-ray computed tomography (CT) was conducted before and after the experiments to identify the location and geometry of hydraulic and natural fractures. Results have shown that the fracturing behaviour, the drawdown period of borehole pressure and the intrusion of fracturing fluid are dominated by the complexity and insulation offered by internal natural fracture networks of coal blocks. In addition, seismic spectrograms captured both fracture initiation and its subsequent interaction with natural fractures, which indicates that the induced fracture and fracturing fluid interfere with the propagation of seismic waves and influence ultrasonic seismic characteristics. Seismic velocity tomography of ultrasonic acoustic signals recorded also provided the spatial information of fractures, such as approximate locations of pre-existing fractures and injection-disturbed regions.
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