Search

To investigate the permeability characteristics in the in-situ fractured coal body around the perimeter of gas extraction boreholes, the steady-state permeability of fractured coal bodies with different gradations was tested using the fractured rock permeability test system. By controlling the axial displacement and permeability pressure, the permeability parameters were obtained under different porosities. The interactions between the permeability parameters and the process of permeability destabilisation are discussed. The results show that the permeability characteristics of the broken coal body obey the Forchheimer relationship: As the axial displacement increases, the permeability resistance of the fluid increases and the non-Darcy property of the sample becomes more significant. With the decrease in the porosity of the sample and the increase in the power index n, the permeability k decreases and the non-Darcy factor β increases. The final fractal structure of the sample will be changed by particle fragmentation and migration during the loading process of the sample with different levels, and the internal pore structure of the sample will further affect the penetration of the penetration channel. A critical characteristic value for the seepage instability in broken coal bodies is given, and an expression for determining the seepage instability by permeability and non-Darcy factors is proposed. The results indicate that a negative non-Darcy factor is not a necessary condition for permeability instability, and the critical Reynolds number for the permeability instability in broken coal bodies was determined from the perspective of the Reynolds number. The conclusions of this study can provide theoretical support for the theoretical study of permeability and the permeability of pre-smoking coal seams.

The permeability of crushed coal bodies plays a bottom neck role in seepage processes, which significantly limits the coal resource utilisation. To study the permeability of crushed coal bodies under pressure, the particle size distribution of crushed coal body grains is quantitatively considered by fractal theory. In addition, the parameters of the percolation characteristics of crushed coal body grains are calculated. Moreover, the permeability of the crushed coal body during recrushing is determined by the fractal dimension and porosity. A lateral limit compression test with the crushed coal bodies was carried out to illustrate the effect of the porosity on the permeability, In addition, a compressive crushed coal body size fractal–permeability model was proposed by combination of the fractal dimension and the non-Darcy equivalent permeability. The results show (1) the migration and loss of fine particles lead to a rapid increase in the porosity of the crushed coal body. (2) Increases in the effective stress cause the porosity and permeability to decrease. When the porosity decreases to approximately 0.375, its effect is undermined. (3) The migration and loss of fine particles change the pore structure and enhance the permeability properties of the skeleton, causing sudden seepage changes. (4) At low porosity, the permeability k is slightly larger than the non-Darcy equivalent permeability ke. Thus, the experimental data show an acceptable agreement with the present model. A particle size fractal–percolation model for crushed coal bodies under pressure provides a solution for effectively determining the grain permeability of the crushed coal bodies. The research results can contribute to the formation of more fractal-seepage theoretical models in fractured lithosphere, karst column pillars and coal goaf, and provide theoretical guidance for mine water disaster prevention.

The compaction and re-crushing characteristics of crushed gangue are important factors which affect the quality and effectiveness of the filling of the quarry. To study the compaction and re-crushing characteristics of the gangue particles, continuous grading and intermittent grading of two different structures were designed to study the bulk gangue particle size distribution. By conducting a side-limited uniaxial compression test on the crushed gangue, the compaction deformation parameters and particle re-crushing parameters of the samples under different axial pressure and grading conditions were calculated, the interaction between compaction and re-crushing was determined, and a compaction-re-crushing model of the crushed gangue was established. The following conclusions were obtained, (1) the axial displacement increment and fractal dimension of the graded crushed gangue are closely related to the graded structure of the skeletal particles; (2) the compaction stage of the graded crushed gangue can be divided into a fast compaction stage, a slow compaction stage, and a stable compaction stage—the fast compaction stage is significantly elastic, the slow compaction stage is more plastic, and the stable stage behavior approaches that of the original rock body; (3) the degree of crushing of the graded crushed gangue increases with an increase in axial stress, and the re-crushing of the specimen mainly occurs in the slow compaction stage; (4) the compaction-crushing-fractal evolution of the graded crushed gangue in the lateral limit compression process is established as the line of questioning. The physical significance of the parameters in the equation is discussed in this paper. The study can provide theoretical support and engineering guidance for the precise filling of a quarry and the prevention of later collapse.