Study on a mathematical model and experiments for nonlinear seepage in complex crushed rocks with mud-rock slurry

Faults and collapse columns are some of the main hidden dangers affecting the coal mining above confined aquifers, which structure is composed of crushed stones and argillaceous fillings (mud-rock slurry), and thus is very complex. Therefore, the determination of their permeability is usually based on qualitative analysis, lacking quantitative models. Seepage experiments were conducted to study the nonlinear seepage law in order to study the quantitative calculation method of the permeability of complex crushed rocks with mud-rock slurry in this study. A mathematical model for nonlinear seepage in the crushed rocks was established. The influence of clay composition and mud filling effect on the permeability was considered in the model of calculating permeability for the crushed rocks. The model is based on the fractal theory of porous medium and nonlinear seepage theory. The accuracy of the mathematical model was validated by comparing the calculation results from the mathematical model with the experimental results. The results show that: ①The particle size distribution of the crushed zone of collapse columns has a fractal law. The fractured rock mass containing clay particles undergoes a process of argillization when encountering water, resulting in an increase in the fractal dimension. The process fills the large pores in the large particles, reducing the permeability of the crushed rocks. ②The critical pressure gradient at the onset of nonlinearity increases as the permeability of crushed rocks decreases, indicating that the nonlinear critical pressure gradient can be used to quantify the water barrier capacity of the crushed rocks; ③The experimental results of the critical pressure gradient at the onset of nonlinearity are consistent with the calculated results from the mathematical model, which verifies the accuracy of the model. The mathematical model first tries to consider the influence of clay composition and mud filling on the permeability, which provides a new quantitative method for the permeability of complex crushed rocks with mud-rock slurry, and a new research approach for the water barrier performance of geological structures under special mining conditions such as the coal seam under and above confined aquifers.