This study aims to investigate the failure mechanical properties of deep soft rock–coal combinations when subjected to dynamic disturbance. Hence, uniaxial compression and dynamic disturbance experiments were conducted on samples of muddy sandstone, coal, and soft rock–coal combinations using a creep disturbance dynamic impact loading experimental system. This study analyzed the damage properties of soft rock–coal combinations under dynamic disturbance by combining sample failure characteristics, energy evolution, and waveform signals of samples using the acoustic emission system. Results indicate that the peak strength and modulus of elasticity of the soft rock–coal combinations under uniaxial compression and dynamic disturbance are intermediate between those of coal and rock. The strength deterioration effect of the rock–coal combinations under dynamic disturbance is obvious, and the peak strength of the roof–coal and coal–floor combinations are reduced by 6.17% and 10.30%, respectively. Dynamic disturbances increase the degree of damage to the rock–coal combinations, which is accompanied by large amounts of coal fragments and large fragments of the rock–coal combinations. The failure to the sample is primarily caused by the extension of tensile damage, with the development of shear cracks playing a secondary role. The soft rock-coal combinations undergo small-scale damage at the initial disturbance (N1), with large main frequency eigenvalues. Large-scale damage occurs at the time of failure (N2), with smaller main frequency eigenvalues, sharp fluctuations in the acoustic emission signal, and peaks in the absolute AE energy and counts. 

Experimental study on failure mechanical properties and acoustic emission characteristics of soft rock–coal combination under dynamic disturbance

Study on Creep Characteristics and Nonlinear Fractional-Order Damage Constitutive Model of Weakly Cemented Soft Rock

Electric Potential Response Characteristics of Coal Under Stress Wave Loading

To safely extract residual coal from a mine affected by rock bursts, the concept of "roadways under goafs" has been proposed as an effective layout. The pressure relief and shock reduction mechanism of roadways under goafs is elucidated through theoretical analysis, numerical simulation, and engineering verification. The stress model for goafs is established based on elastoplastic mechanics, followed by the construction of a force model for coal beneath the goafs. The stress composition of roof, floor and two sides of roadway under a goaf are analyzed. Using the FLAC-PFC coupling simulation method, the excavation process from top coal mining to bottom coal mining roadways of a No. 17 coal mine in Heilongjiang Province was simulated. The simulation results validate the spatiotemporal evolution of the stress and displacement in the surrounding rock mass of the roadway situated beneath goafs and determine the precise location of the roadway and length of its bottom coal face. A large diameter borehole of the roadway and presplitting blasting of the coal face roof are used to solve the local high stress danger area. Moreover, the roadway support mode of the "steel shed + spray arch support + monomer + π steel beam" is proposed. Finally, through the data of the field micro-seismic and stress monitoring system, the effectiveness of the pressure relief and shock reduction of roadways under goafs, the high efficiency of local danger relief of the roadways and working faces, and the practicability of the "steel shed + spray arch support + monomer + π steel beam" support of the roadways broken by surrounding rock are verified. A good basis for choosing a reasonable location for roadways and surrounding rock control methods for residual bottom coal mining and rock burst mining can be found in the research results. 

Study on stress distribution law of surrounding rock of roadway under the goaf and mechanism of pressure relief and impact reduction

The fracture failure of flawed coal has always been a problem in underground engineering. Therefore, it is necessary to monitor crack propagation and damage evolution during coal fracture. In this study, the split Hopkinson pressure bar (SHPB) and digital image correlation (DIC) were combined to analyze the impact failure electric potential (EP) of flawed coal samples. The multi-dimensional joint response characteristics of coal EP time–frequency-field under dynamic load were studied and the differential expression and multi-fractal characteristics of EP responses at different measuring points were analyzed. Based on the EP signals, the evolution of damage to the coal was calculated. The results show that the time series evolution of EP and stress–strain curves during coal failure is consistent. The generation of fracture zones will excite the high-value area of local EP, and the spectral distribution of EP signals in this area shows the characteristics of low frequency and high amplitude. Each channel's EP signal and the degree of deformation at the corresponding place are perfectly matched. The multi-fractal spectrum of the EP signal close to the fracture zone has a higher Δα and a lower Δf, which demonstrates the dominance of the large-scale EP signal produced in this region by the destruction of sample. Theoretical study indicates that the main causes of the anomalous distribution of the EP field are crack tip potential, diffusion potential, etc. The results can provide theoretical basis for the evaluation of coal fracturing by EP technology. 

The multi-dimensional joint response of the electric potential time–frequency-field in the fracture process of flawed coal under dynamic load

Fracture response and damage evolution features of coal considering the effect of creep damage under dynamic loading

After the stress wave penetrates the fault, it changes the stress direction and destroys the stress balance of the deep underground mining surface and roadway during mining activities, causing the roadway and underground equipment to collapse. To reveal the failure mechanism, we studied the precursor characteristics of unstable failure of rocks in deep roadway groups. With conducting two PFC numerical models based on common roadway layouts, we studied crack development and failure conditions of surrounding rocks near roadway groups by investigating the force chain fracture, coordination number, energy evolution, and acoustic emission moment tensor in the models. The results show that the overall deformation failure of the roadway group first occurs in the upper roadway, and the weak force chain break in the force chain field of the lower roadway is obvious, especially before and after the peak value, accompanied by the coordination number in the direction of 80° with the joint plane significantly reduced. At the same time, obvious force patterns were observed in the roadway group area: irregular gathering (>90°) and swirl (<90°). These findings clarify the mechanism of fault disturbance in roadways surrounding rocks and help to better understand the deformation failure feature and energy evolution process of coal. 

Failure mechanism features and precursor characteristics of coal roadway groups in fault zone under confined conditions

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Fracture response and damage evolution features of coal considering the effect of creep damage under dynamic loading

Failure mechanism features and precursor characteristics of coal roadway groups in fault zone under confined conditions