## 1 Introduction  
The main common coal-bearing strata in China are the Jurassic and Carboniferous-Permanian strata. It is rare to encounter the combined mining of Jurassic and Carboniferous-Permanian coal seams in the same coalfield. However, the Datong coalfield is the most typical one with the characteristics of \"Jurassic and Carboniferous-Permanian and multi-coal seams\". At present, the shallow coal resources in the Datong Jurassic coalfield are increasingly depleted, and the deep Carboniferous-Permanian coal is the main coal seam. The mining of the upper Jurassic coal seam left the goat and coal pillar, forming a wide range of fracture development areas. The secondary failure of the rock caused by the mining of the Carboniferous coal seam is characterized by the coexistence of multiple structures formed by the combined mining of the Jurassic and Carboniferous coal seams, which makes the mining conditions more complicated (Figure 1). The overlying rock movement and the development of cracks caused by mining are more likely to cause the occurrence of various coal mine disasters [1,2]. Water disaster is the main problem faced by the combined mining of the Jurassic coal seam and Carboniferous coal seam. The goat water left by the mining of Jurassic coal seam and surface water are likely to enter the underground through the water-conducting fracture zone formed by deep coal seam mining, which seriously threatens the safe mining of coal resources and also causes severe impacts on the environment and roof water bodies [3,4]. China is vigorously developing safe energy and green energy [5,6]. Therefore, it is important to study the occurrence conditions of mine water inrush, accurately predict water inrush and formulate prevention and control countermeasures. Understanding the evolution characteristics of a water-conducting fracture zone in the combined mining of Jurassic and Carboniferous coal seams is critical to achieving this goal. Scholars have carried out theoretical analysis, experimental research, and on-site measurement to solve the problem of roof water inrush. Approaches mainly include key strata theory [7], upper three zone theory (caving zone, fracture zone, and bending zone) [8,9], thin plate model theory [10], empirical formula method [11], similar simulation experiment [12,13], numerical simulation [14,15,16], and field test methods [17,18,19,20]. At present, the determination of the height of the water-conducting fracture zone of the coal seam roof in China is mainly based on the formula in the 'Buildings, water bodies, railways and main roadway coal pillars and coal mining specifications' [21]. This formula was based on the empirical formula established by the measured data of the shallow coal seam mining in the Permian system in the North China type coalfield, and only the influence factor of mining thickness was considered [22]. A large number of studies have shown that the development height of the water-flowing fractured zone is affected by many factors, such as coal seam burial conditions, overburden structure, and mining technology. The above reasons cause that the height of water flowing fractured zone in many coal mines is not completely consistent with the calculation results of the empirical formula. To understand the mechanism of water-conducting fracture zone formation, scholars put forward the surface failure characteristics of shallow buried coal seam with large mining height [23] through the key layer theory; using the rock fracture process analysis (RFPA) system, the relationship between rock movement and goat time and space was studied [24]. The deformation and migration patterns of overlying strata in coal seam mining were studied by using high-precision microseismic monitoring and distributed optical fiber [25]; other scholars have studied the mechanical properties and engineering response characteristics of rock mass under the interaction of slope rock mass and underground mining, and revealed the deformation and failure process and evolution rules of fractured rock mass [26].