•A fuzzy comprehensive evaluation model was developed for the forecasting of rock bursts.•Laboratory AE measurements were performed to select MS indices used as input to the fuzzy model.•The ...application of this model has been successfully demonstrated in a coal mine in China.
Rock bursts have become one of the most severe risks in underground coal mining and its forecasting is an important component in the safety management. Subsurface microseismic (MS) monitoring is considered potentially as a powerful tool for rock burst forecasting. In this study, a methodology for rock burst forecasting involving the use of a fuzzy comprehensive evaluation model was developed, which allows for a more quantitative evaluation of the likelihood for the occurrence of a rock burst incident. In the fuzzy model, the membership function was built using Gaussian shape combined with the exponential distribution function from the reliability theory. The weight of each index was determined utilising the performance metric F score from the confusion matrix. The comprehensive forecasting result was obtained by integrating the maximum membership degree principle (MMDP) and the variable fuzzy pattern recognition (VFPR). This methodology has been applied to a coal mine in China to forecast rock bursts. To select MS indices for rock burst forecasting using the fuzzy evaluation model, laboratory acoustic emission (AE) measurements of coal samples collected from the mine were performed. The model parameters were first calibrated using historical MS data over a period of four months, during which six rock burst incidents were observed. This calibrated model was able to forecast the occurrence of a subsequent rock burst incident in the mine.
The main constrain for effective gas drainage in coal mines is the low permeability nature of coal reservoirs. As coal mining activities are extending to deeper subsurface, the ever-increasing in ...situ stress conditions is anticipated to result in much lower permeability and more challenges for gas emission control in coal mines. In recent years, hydraulic slotting using high-pressure waterjet along underground gas drainage boreholes, as a general solution to stimulate low permeability coal seams, has become increasingly favourable. This paper presents a systematic investigation into the sensitivity of borehole slotting performance to a number of field and operational parameters. A wide range of geomechanical properties, in situ stress conditions, slot geometry and spacing of multiple slots were considered in a series of numerical simulations. The relations between these key parameters and the failure zone size/volume induced by slotting were quantified. The effect of different parameters in improving slotting performance has also been ranked, which provides theoretical base for mine operators to optimise slotting operations.
Coal and gas outbursts have long posed a serious risk to safe and efficient production in coal mines. It is recognised that coal and gas outbursts are triggered by excavation unloading followed by ...gas-driven rapid propagation of a system of pre-existing or mining-induced fractures. Gas-filled fractures parallel to a working face are likely to experience opening first, then expansion and rapid propagation stages under unloading conditions. The fracture opening is driven by the effective stress inside the fracture, while the fracture expansion and rapid propagation is propelled by the pressure build-up of desorbed gas in the vicinity of the fracture. Based upon this understanding, this research aimed to identify the key factors affecting outburst initiation and its temporal evolution during roadway developments. Specifically, the response of pre-set fractures in a thin coal seam sandwiched between rock layers to roadway development is simulated using a geomechanical model coupled with fracture mechanics for fracture opening and propagation. In addition, kinetic gas desorption and its migration into open fractures is considered. During simulations outburst is deemed to occur when the fracture length exceeds the dimension of a host element. The findings of this research suggest that the simulated coal and gas outburst caused by roadway development may be considered as a dynamic gas desorption-driven fracture propagation process. The occurrence of coal and gas outbursts is found to be influenced mainly by the coal properties, fracture attributes, and initial gas pressure and the in situ stress conditions. Furthermore, the model predictions in terms of dome-shaped erupted-zone and layer-by-layer coal breakage are consistent with the field reports. In addition, the model results suggest that delayed occurrence of coal and gas outbursts, especially after sudden exposure of a coal seam or after blasting disturbance, reported in the literature may be related to the gas desorption behaviour.
•A modelling approach coupled with fracture mechanics has been developed to simulate coal and gas outbursts.•Coal and gas outbursts can be considered as a dynamic gas desorption-driven fracture propagation process.•The potential of outburst hazards is influenced by fracture toughness, fracture radius, gas pressure and stress conditions.•Delayed occurrence of coal and gas outbursts may be related to gas desorption behaviour.
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.
Permeability models developed for coalbed reservoirs are generally not suitable for describing permeability response under fixed confining pressure in the laboratory test conditions where the core ...samples are allowed to expand with adsorption of gas. This paper presents a novel modelling approach to analyse the laboratory permeability data by seeking to isolate the impact of swelling strain on the permeability, in the form of a swelling strain term, from the test data for different gases. Two sets of comprehensive permeability and swelling data for three pure gases on two different (low rank) coals reported in the literature were analysed and it was found that the swelling strain terms thus obtained for the two samples can be described using a common empirical equation of the free swelling strain. The empirical equation (with two fitting constants) was then used to predict permeability response of the same cores to the flow of a binary gas mixture of N2 and CO2 based on the measured free swelling strain data. Good agreement with the measured permeability data was achieved for one of cores over the entire test pressure range. Comparison between the empirical swelling strain equation and the corresponding theoretical strain equation under uniaxial strain conditions reveals that the ratio of the two constants determined for the two cores can be related to their elastic properties. It is further shown that the establishment of the empirical swelling strain equation allows for the estimation of the part of the swelling strain (and thus the partition ratio) that contributes to the reduction in the permeability under fixed confining pressure conditions.
•A new modelling approach for analysing laboratory permeability data is proposed.•Applied to permeability data for different gases on two coals reported in literature•Established an equation describing how swelling strain affects cleat permeability•The equation allows for estimation of swelling strain partition ratio.
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In recent years, health monitoring systems (HMS) have aroused great interest due to their broad prospects in preventive medicine. As an important component of HMS, flexible force ...sensors (FFS) with high flexibility and stretch-ability can monitor vital health parameters and detect physical movements.
In this review, the novel materials, the advanced additive manufacturing technologies, the selective sensing mechanisms and typical applications in both wearable and implantable HMS are discussed.
We recognized that the next generation of the FFS will have higher sensitivity, wider linear range as well as better durability, self-power supplied and multifunctional integrated. In conclusion, the FFS will provide powerful socioeconomic benefits and improve people's quality of life in the future.
Artificial meniscal implants can be used to replace a severely injured meniscus after meniscectomy and restore the normal functionality of a knee joint. The aim of this paper was to design porous ...meniscal implants and assess their biomechanical properties.
Finite element simulations were conducted on eight different cases including intact healthy knees, knee joints with solid meniscal implants, and knee joints with meniscal implants with two types of triply periodic minimal surfaces. Compression stresses, shear stresses, and characteristics of stress concentrated areas were evaluated using an axial compressive load of 1150 N and an anterior load of 350 N.
Compared to the solid meniscal implant, the proposed porous meniscal implant produced lower levels of compression and shear stresses on the cartilage, which facilitated the cartilage to retain a semilunar characteristic similar to the natural meniscus. Moreover, both compression and shear stresses on the artificial cartilage were found to be sensitive to the pore properties of the meniscal implant. The meniscal implants with primitive surfaces (porosity: 41%) showed a better performance in disseminating stresses within the knee joint.
The present commercial meniscal implant has the problem of equivalent biomechanical properties compared to natural menisci. The main advantage of the proposed porous structure is that it can be used to prevent excessive compression and shear stresses on the articular cartilages. This structure has advantages both in terms of mechanics and printability, which can be beneficial for future clinical applications.
A probabilistic risk assessment framework was developed to mathematically represent the complex engineering phenomena of rock bursts and gas outbursts for a heterogeneous coal seam. An innovative ...object-based non-conditional simulation approach was used to distribute lithological heterogeneity present in the coal seam to respect their geological origin. The changing mining conditions during longwall top coal caving mining (LTCC) were extracted from a coupled numerical model to provide statistically sufficient data for probabilistic analysis. The complex interdependencies among abutment stress, pore pressure, the volume of total gas emission and incremental energy release rate, their stochastic variations and uncertainty were realistically implemented in the GoldSim software, and 100,000 equally likely scenarios were simulated using the Monte Carlo method to determine the probability of rock bursts and gas outbursts. The results obtained from the analysis incorporate the variability in mechanical, elastic and reservoir properties of coal due to lithological heterogeneity and result in the probability of the occurrence of rock bursts, coal and gas outbursts, and safe mining conditions. The framework realistically represents the complex mining environment, is resilient and results are reliable. The framework is generic and can be suitably modified to be used in different underground mining scenarios, overcoming the limitations of earlier empirical indices used.
Highlights
Parameters causing rockbursts and gas outbursts were linked along with their influences and interdependencies into a probabilistic risk assessment framework.
Dynamically updated system feedback from the numerical model was fed into the framework to represent the current stress state in retreating mining and estimate the probability of the occurrence of rockbursts and gas outbursts.
Statistically significant data were used to quantify the probability of rockbursts and gas outbursts using Monte Carlo simulation.
•Fault reactivation under CO2 injection was simulated using a two-way coupled model.•Strain-dependent permeability was applied to represent the fault hydrological behaviour.•A close history match of ...bottomhole pressure was achieved over the period modelled.•Fault reactivation results in Coulomb stress changes in near-fault areas.•The variation of computed Coulomb stress changes matches the heightened field recorded seismicity.
Stress transfer caused by injection-induced fault reactivation plays a significant role in triggering induced seismicity. This work aims to investigate to which extent the shear slip stress transfer mechanism might have contributed to a 4-month period of heightened microseismicity around one of the horizontal injection wells (KB-502) at the In Salah CO2 storage site. Building upon previous reservoir modelling and history matching work by the authors, coupled geomechanical and reservoir modelling of CO2 injection at KB-502 was carried out, featuring the explicit simulation of injection-induced fault reactivation and stress transfer, and the implementation of a strain-dependent permeability model to represent the fault hydrological behaviour. This approach allows a much-improved overall match to the field bottomhole pressures at KB-502 over the previous results, where fault zone reactivation and associated dynamic permeability behaviour were not considered, especially over the 4-month period of interest. Based upon the coupled modelling results, Coulomb stress changes were used to evaluate the potential for enhanced microseismicity related to CO2 injection-induced fault reactivation at KB-502. Analyses on the potential for microseismicity have shown that seismic events are likely to take place in both hydraulically connected regions and stress transfer influenced regions. The variation of computed Coulomb stress changes in near-fault areas compares favourably with the heightened field recorded seismicity during the period modelled. The integrated interpretation of microseismic monitoring and coupled geomechanics and reservoir modelling have suggested that the shear slip stress transfer mechanism was active and contributed to the occurrence of induced seismicity at In Salah.
Mining-induced microseismicity is widely considered as a result of slippage of pre-existing critically stressed fractures caused by stress perturbations around an advancing face. An in-depth analysis ...of the recorded microseismicity associated with longwall top coal caving mining at Coal Mine Velenje in Slovenia has been previously carried out and reported by the authors. It has been concluded that while microseismic event rate is affected by mining intensity (longwall face daily advance rate) as well as local abundance of pre-existing fractures, spatial and magnitude characteristics of microseismicity are predominantly influenced by the latter. Based upon this improved understanding of fracture-slip seismic-generation mechanism, the current work aimed at establishing a data-driven yet physics-based probabilistic forecasting methodology for hazardous microseismicity using microseismic monitoring data with concurrent face advance records. Through performing statistical analyses and probability distribution fitting for temporal, magnitude and spatial characteristics of microseismicity within a time window, a short-term forecasting model is developed to estimate the probability of potentially hazardous microseismicity over the next time interval in the form of a joint probability. The real time forecasting of hazardous microseismicity during longwall coal mining is realised through regularly updating the statistical model using the most recent microseismic sequence datasets and face advance records. This forecasting methodology is featured by the physical basis which provides a good explicability of forecasting results, and the probabilistic perspective which accounts for the stochastic nature of mining-induced microseismicity. This model has been employed to make time-varying forecasts of hazardous microseismicity around two longwall panels over a one-year coal production period at Coal Mine Velenje, and satisfactory results at both panels were achieved. In addition, the analysis suggested that the energy magnitude distribution of microseismicity is a dominant factor in contributing to the potential of hazardous microseismicity. This statistical model using microseismic monitoring data has important implications in the evaluation of mining-induced hazards and optimal control of longwall face advance in burst-prone deep-level mining sites.
•A probabilistic model was proposed for evaluation of seismic hazard potential.•Time-varying forecasting of hazardous microseismicity was verified.•Local fracture attributes are reflected in recorded microseismicity.•Segmental stationary of seismicity shows spatial continuity of fracture attributes.
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