Experimental study on initiation mechanism of water and sand inrush in deep buried weakly cemented roof separation type

The Jurassic coal bearing strata buried deep in the northwest have the characteristics of late diagenesis, mudstone cementation and expansion, low bonding strength of sandstone, and disintegration when exposed to water. Under weak and rich water conditions, multiple water sand outburst disasters occur. In response to the problem of unclear starting mechanism and difficulty in predicting disasters caused by water and sand inrush in the top plate separation type of interbedded structure of mudstone and sandstone, a water and sand inrush starting test device is adopted to achieve dynamic control of water pressure through gas injection and pressure supplementation. 25 sets of orthogonal tests are designed for static and dynamic water pressure, and obtained the influence laws of main controlling factors such as water head height, crack width, crack angle, clay layer thickness, and particle size distribution on water and sand inrush. We analyzed the stage characteristics of water and sand inrush initiation. Using response surface methodology, a quadratic polynomial regression equation was established between the membership degree of water and sand inrush and each main control factor, and the influence of each main control factor and pairwise interaction factors on the initiation of water and sand inrush was analyzed. A comparative analysis was conducted on the phenomenon of water and sand inrush starting disasters under static and dynamic water pressure, and the differences in water and sand starting under static and dynamic water pressure were explained. The results showed that the simulation of the separation layer water accumulation process was achieved by injecting gas to supplement pressure, providing a new approach for simulating and studying the separation layer water accumulation process. The initiation of water and sand inrush has obvious stage characteristics, which can be divided into four stages: initial seepage stage, accelerated seepage stage, piping stage, and water and sand inrush stage. The degree of influence of each main controlling factor on the membership degree of water and sand surges is as follows: clay layer thickness>water head height>crack width>crack angle>particle size distribution. Under the action of dynamic water pressure, the occurrence of water sand burst disasters is accelerated, and the order is continuous gas injection pressure>intermittent gas injection pressure>static water pressure. The research results can provide a theoretical basis for the prevention and control of water and sand inrush disasters caused by roof detachment in deep buried Jurassic coalfields in the western region.