Study On The Evolution Mechanism Of Disasters Induced By Mining-induced Failure Of N₂ Laterite In Gully Cushion, Northwestern China

Coal mining with water protection is the major project activities and engineering geological process of natural environment interactions.And it is also a major scientific and technological problems of large-scale coal resources development and eco-environment fragile area in northwest of China.In the arid-semiarid area of northwest,the shallow water such as water of rivers and lakes,valley runoff water,phreatic water of Quaternary in sand-layer and so on is precious water resources which maintain regional ecological geological environment and the body of water resource protection in coal mining.According to the regional geological structure of the shallow water aquifer-aquifuge,the N₂ laterite,distributed widely above coal seam in northwest coal field of China,as the direct aquifuge bottom of regional shallow water or valleys runoff cushion,playing a crucial role to the protection of the shallow water,is the key aquifuge soils.Therefore,it is of great significance to study the evolution mechanism of disasters induced by mining-induced failure of N₂ laterite for the coal mining with water protection and ecological environment protection in northwestern China.This paper takes Lucao gully of Shennan mining area in northern Shaanxi as the research background,with N₂ laterite in gully cushion as the main research object.In this paper,based on the analysis of regional distribution features,the basic physical,hydro-physical and the mechanical properties of N₂ laterite,the mining-induced failure characteristics of N₂ laterite and time-denpendent evolution of its impermeability was deeply studied and analyzed by lab experiments,in-situ measurement,numerical simulation,physical simulation,theoretical analysis and field geological survey and so on.The research results reveal the evolution mechanism of disasters induced by mining-induced failure of N₂ laterite in gully cushion.The main results are as follows:?1?Geologic components and engineering geology properties of N₂ laterite was tested and analyzed.N₂ laterite is a regional soil composed of silty clay and clay,with brown red lumps of clay rich.It is rich in irregular calcareous concretions,showing layered or scattered distribution structure,and surface covered with iron and manganese film.Under natural condition,N₂ laterite is plastic,hard plastic or hard clay.In the clay minerals,the content of hydrophilic minerals,such as montmorillonite and illite/montmoduonite interstratified,is high,which is more hydrophilic and has strong expansionary.The level of N₂ laterite permeability is weak permeable and slightly permeable,so it has well impermeability under natural conditions.?2?Mining-induced failure types of N₂ laterite in gully cushion,including mining-induced caving N₂ laterite,mining-induced crack N₂ laterite and mining-induced bending subsidence N₂ laterite,has been proposed,and their essential characteristics were explained.Mining-induced failure characteristics of N₂ Laterite in the study area were analyzed,and the fracture zone of coal mining in the study area is generally developed to the N₂ Laterite layer.Mining-induced Failure types of N₂ Laterite in the study area mainly included mining-induced crack N₂ laterite and mining-induced bending subsidence N₂ laterite.?3?Based on the analysis of hydrogeological and engineering geological conditions of the study area,the mining water disaster types of study area were classified into four zones: water and sand inrush area,water inrush area,leakage area and security area.In the study area,the distribution of water inrush area is larger,accounting for 50% of the whole area.Based on the evaluation of the slope stability of the study area,combined with the evaluation results of the mining-induced water disaster,it is considered that the phenomenon of slope instability in water and sand inrush area located in the northeast of the study area is relatively small,and the sand source is less.Therefore,the water inrush and sand disaster will occur in the face mining,but the disaster will be the main water inrush,and the amount of sand inrush is limited.However,the phenomenon of slope instability in water and sand inrush area located in the southeast of the study area is more serious,which can provide enough sand sources for water inrush and sand disaster.?4?The three axis unloading seepage test and the water pressure test before and after mining were carried out,The results showed that mining-induced bending subsidence N₂ laterite,although not macroscopic through-cracks,had lots of micro mining-induced fractures,which cause that the impermeability of N₂ laterite decreased in different degree.The permeability coefficient was increased by about one order of magnitude,and the level of permeability is changed from natural slight permeability,weak permeability to weak permeability,medium permeability.The creep deformation and permeability change of mining-induced bending subsidence N₂ laterite during the process of stress recovery was studied by the three axial creep tests.The research results showed that the stress recovery creep property of N₂ laterite is nonlinear power function.The permeability of N₂ laterite decreased gradually with the recovery of post mining stress,and it is a power function relation with the stress level.?5?A simulation test device for soil-water interactions during the process of water inrush was developed,and the simulation experiment for soil-water interactions during the process of water inrush induced by mining-induced crack N₂ laterite was carried out.The results showed that macroscopic through-cracks of mining-induced crack N₂ laterite were closed gradually along with the process of water inrush due to the strong soil-water interactions and strong expansion,and the level of permeability is changed from strong permeability to moderate permeability.Its completely damaged water resisting property also showed a gradual recovery trend.?6?Based on the analysis of the evolution characteristics of permeability for mining-induced failure N₂ laterite,the mining water disaster types of study area are classified into six zones: water and sand inrush area,high water inrush risk area,low water inrush risk area,high leakage area,low leakage area and security area.Among them,low water inrush risk area and low leakage area are most affected by the permeability evolution induced by mining-induced failure of N₂ laterite,which show that water inrush volume of the early is large but attenuation is quick in low water inrush risk area,and the leakage of the early stage is obvious but the decline of the water level in the later period decreases or even recovers in low leakage area.