The evaluation of the hydraulic properties evolution of granular sandstones in relation with groundwater inrush within faults is an important issue for mining engineering applications. This paper presents the results of an experimental investigation of small particle migration from granular sandstone samples under different original porosities, particle size compositions and water flow pressures. A new rock testing system has been setup to carry out the tests. Based on the results, it is observed that the overall permeability evolution during the tests can be divided into four different phases, including i) re-arrangement of large rock fragments, ii) water inrush with substantial particle migration, iii) continued moderate particles seepage, and iv) steady state water flow. The crushing of edges and corners of large rock fragments, and the evolution of the fracture network has mainly been observed in the first two phases of the tests. The results indicate that the migration of small particles has an essential effect on permeability and porosity increase during water inrush through fractured sandstone. The samples with higher original porosity, higher percentage of fine particles in their formation and under higher water flow pressures, achieve higher permeability and porosity values when the test is complete. Furthermore, using the measured data, the performances of a number of empirical models, for permeability evolution in fractured porous media, have been studied. The prediction results indicate that not all of the fractures in a sample domain contribute in small particle migration. There are parts of the fracture network that are not effective in particle flow, a sample with less original porosity, more fine particles and under lower water pressure shows less ineffective fractures. Therefore, using the concept of the effective porosity (fracture) is sufficient enough for the flow calculation.Keywords: granular sandstone; groundwater inrush; particle migration; permeability; porosity.
A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT
Water inrush is one of the most dangerous disasters in coal mining. Due to the large-scale mining and complicated hydrogeological conditions, thousands of deaths and huge economic losses have been caused by water inrush disasters in China. There are two main factors determining the occurrence of water inrush: water source and water-conducting pathway. Research on the formation mechanism of the water-conducting pathway is the main direction to prevent and control the water inrush, and the seepage mechanism of rock mass during the formation of the water-conducting pathway is the key for the research on the water inrush mechanism. This paper provides a state-of-the-art review of seepage mechanisms during water inrush from three aspects, i.e., mechanisms of stress-seepage coupling, flow regime transformation and rock erosion. Through numerical methods and experimental analysis, the evolution law of stress and seepage fields in the process of water inrush is fully studied; the fluid movement characteristics under different flow regimes are clearly summarized; the law of particle initiation and migration in the process of water inrush is explored, and the effect of rock erosion on hydraulic and mechanical properties of the rock media is also studied. Finally, some limitations of current research are analyzed, and the suggestions for future research on water inrush are proposed in this review.
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