Material source and disaster model of mud sand inrush in coal mining under extremely thick Cretaceous in Ordos basin

The Ordos basin is a coal rich area with the largest coal reserve and good coal quality in China. The central part of the basin is covered by a huge water bearing Cretaceous sandstone layer with a thickness of hundreds of meters to thousands of meters. In recent years, as coal mining around the basin has gradually extended inward, some serious mud sand inrush accidents have occurred in some of the Jurassic era coal mines in the thick Cretaceous coverage area, posing a serious threat to the safety of coal mining. Through the analysis of geological theory and regional outcrop geological survey, the authors found that the paleoweathered crust at the bottom of the Cretaceous and the top of the Middle Jurassic (K1/J2) has the mud sand inrush structural composition conditions. Further testing and analysis of paleoweathered crust rock mass structure, material composition, grain size composition, water physical property, point load strength test and in situ large scale shear test have been carried out. The result shows that the lithology of the paleoweathered crust is dominated by argillaceous sandstone and mudstone,and the rock mass structure has the zoning characteristics of fully weathered soil layer, strongly weathered gravel layer, and medium weathly weathered fractured rock layer in the vertical direction. The composition is dominated by breccia, and the content of mud sandy particles increases significantly after immersion in water. When encountering water, there are different degrees of argillization, disintegration, and fragmentation. The paleoweathered crust is generallya soft and broken rock mass with high mud and sand content, the rock mass has low strength, and the overall properties are similar to loose argillaceous sandy soil. It is the main material source of mud sand inrush in coal mining in the Cretaceous covered area. This paper has analyzed the comprehensive hydrological engineering geological conditions of mud sand inrush under the K1/J2 paleoweathered crust, provided the identification method for the concealed paleoweathered crust, and proposed an engineering geological model for the evaluation of mining mud sand inrush disaster under the K1/J2 paleoweathered crust.