Experimental study on migration characteristics of mixed water and sand flows induced by mining under thin bedrock and thick unconsolidated formations

The startup, migration and inrush processes of the mixed water and sand flows in the overburden fractured channel are studied by designing and manufacturing the test model, which can be used to simulate the migration and inrush of mixed flows. The characteristics of the mixed water and sand flows in various types of fractured channels are investigated. Three stages of the mixed-flow migration and three types of the mixed water and sand inrush are brought forward. Based on the caving zone and the water flowing fractured zone channel of overburden by mining, the fracture channel sidewalls of the mixed flows are divided into three types: straight and smooth wall without filling, rough and stepped wall without filling and soft mud substance with filling. The numerical tests on the three types are carried out. The variation characteristics of water pressure at different positions of the fractured channel are revealed considering the variations of widths, inclination angles and sidewall types of the fractured channel under the water pressure of 0.05 MPa and 0.1 MPa. The results show that the water and sand inrush instantly occur in the mined area. Based on the characteristics of water pressure variations, the migration process of the mixed flows in the fractured channel is divided into three interdependent stages: increasing stage, stability stage and outburst stage. According to the characteristics of the water pressure variations during the mixed-flow inrush, the mixed-flow outburst is divided into three basic types: direct outburst type, saltatory outburst type and gentle slope outburst type. Therefore, the migration characteristics and dynamic mechanism of the mixed flows are explored.