Investigating the dynamics of water–sand mixing inrush in viscous sand layers: insights from laboratory experiments

The geological hazard of water–sand inrush is a matter of concern for infrastructure construction and resource exploration activities in China, due to the complex interplay between groundwater dynamics and the stability properties of sand particles. This phenomenon is characterized by its intensity, hazardous nature, and unpredictable behavior. Following comprehensive analysis, this study identifies the critical factors influencing water–sand inrush processes as fissure width, water stress (waterhead height), in-situ sand ground stress within the sand stratum, and clay content. To investigate these factors experimentally, a custom-designed hydraulically coupled water–sand inrush test apparatus was used. The apparatus was equipped with a cylinder to apply ground stress, a pneumatic diaphragm pump to regulate water stress, and a bottom opening in the sand layer. Tests were conducted to investigate the dynamic response of water–sand inrush events under various combinations of factor levels. The findings revealed that the critical value for inrush is only present in the fissure width, which was observed to be 3 mm for the tested sand material. Unlike fissure width, the other factors do not have definitive critical values but instead modulate the intensity of the inrush process without determining its occurrence. The ‘inrush rate’ serves as a measure of the severity of water–sand inrush disasters and shows a linear increase with both increasing groundwater stress and fissure width, a negative exponential function relationship between the inrush rate and the clay content. Notably, ground stress does not exert a significant influence on the intensity of the inrush process itself. Under constant conditions, the inrush rate remains relatively constant across different levels of sand ground stress, for instance, in the experiments, the inrush rate was measured at 1.606 kg/s when the water stress was 0.1 MPa and the fissure width was 5 mm. Grey relation analysis was used to examine the sensitivity of each factor’s influence on the inrush rate. The results showed that water stress has the greatest impact on the intensity of water–sand inrush, followed by ground or soil stress, clay content, and the width of the fissures in the sand layer.