Seepage in a medium packed with particles of variable granular size can be seen in many fields of engineering applications. Due to the relative complex spatial aspect of pore geometry, there are notable differences in the critical parameters of flow transition (Reynolds number and Forchheimer number) between different structures. It is difficult to distinguish the available range of seepage equations and predict the water flux accurately. This work aims to establish the relationship between particle size and flow transition. This is conducted according to the results of flow region division, which obtains the application range for seepage equations. Experiments were carried out in sand columns with nine different particle sizes of sand with mean diameters of 0.0375, 0.1125, 0.225, 0.45, 0.8, 1.5, 2.18, 3.555 and 7.125 mm. Four flow regimes were identified (pre-Darcy regime, Darcy regime, Forchheimer regime and turbulent regime). The experimental data indicate that the permeability increases exponentially and the inertia factor reduces exponentially with an increase in particle diameter. The inertial effect becomes more significant in the medium with larger particles than with finer particles when the flow transition occurs.

A karst collapse column (KCC), a type of vertical structure that is common in the Carboniferous-Permian coal fields of North China, contains many graded broken rocks and often functions as a channel for groundwater inrush. This study used a custom-built apparatus to investigate the effects of the particle-size distribution on the seepage behavior of a sand particle mixture subjected to a high hydraulic gradient. Three different flow regimes were identified: (1) the Darcy regime, (2) the Forchheimer regime, and (3) the turbulent regime. When flow transition begins, the critical flow velocity, critical Reynolds number, and Forchheimer number increase as the nonuniformity coefficient $C_u$ (or the coefficient of curvature $C_c$ ) increases when the porosity remains constant, and the critical hydraulic gradient decreases. The permeability increases linearly, and the inertia factor decreases exponentially as $C_u\times C_c$ increases. Using new experimental data, the validity of five widely used empirical formulas for permeability and the inertia factor were evaluated. The results indicated that the empirically corrected formulas for permeability and the inertia factor yielded high prediction accuracy and can be used to predict the permeability and inertia factor of KCCs in practical engineering.

Water–sand inrush is one of the most serious disasters for mining in China. The evaluation of the occurrence and development of a high-concentration water and sand mixed fluid is an important issue for mining in China. In this study, contraposing to the 3 phases of water–sand inrush, three kinds of experiments are designed for the investigation of initiation, development, and occurrence of the disaster. A new sand–water transport testing system is setup to perform the tests. The results show that there are two key points in the disaster: (1) sand particle incipient motion and (2) porous skeleton structural instability. The incipient motion of sand grains is accompanied with the phenomena of volumetric dilatation and granular fluidization. The critical velocity of the incipient motion of the water–sand mixed fluid is significantly affected by the particle size and external stress. The interaction between water and sand grains is the key factor affecting the motion characteristics of water–sand mixture. When the hydraulic conditions exceed the threshold, the water and sand grains are mutually promoted, and the aquifer skeleton becomes unstable. Furthermore, during the water–sand inrush, the curves of volumetric flow rates of sand and water, respectively, for different samples manifest as two distinct waveforms.

The electrical characteristics of metal pipe and CaCO₃ particles in aqueous solution were investigated and a new method for anti-fouling by connecting the metal pipe with the ground was developed. The results showed that the detected electric current between the metal pipe and the ground decreased with the large-scale CaCO₃ particles adhered on the inner surface of the metal pipe. The thickness of CaCO₃ scale reduced about 44% by connecting the metal pipe with the ground.