## 1 Introduction
[29] further explored the attenuation pattern of explosive stress along the radial direction under different coupling coefficients of water medium in charge structures. The results showed that as the decoupling coefficient of water in the charge increases, the peak value of explosive stress along the radial direction first decreases and then increases, and the distribution range of the crack network is closely related to the attenuation of blasting stress. During the axial decoupling charge blasting process, the distribution of explosive stress on the borehole wall and the fractal damage and crack propagation (characteristics of rock fragmentation) within the rock mass are primarily influenced by the coupling medium, spacing length, and spacing position [30; 31]. For instance, [PERSON] et al. [30] analyzed the distribution pattern of explosive stress on the borehole wall in water medium interval charge structures. Similarly, [PERSON] et al. [32; 33] derived a force calculation formula for the initial shock pressure in boreholes with air interval charge structures and elaborated on the pressure distribution characteristics of such structures. [PERSON] et al. [34] formulated a calculation method for axial borehole stress attenuation based on rock debris filling and segmented charge structures, revealing the patterns of explosive stress attenuation in holes with various rock debris fillings and charge configurations. This study also described the influence of the detonation rate and interval pressure-holding length on the internal damage distribution in concrete specimens. [PERSON] et al. [35], [PERSON], [PERSON], [PERSON], and [PERSON] [36], and [PERSON] et al. [31] investigated the effects of different installation positions and lengths of water separation columns on the rock fragmentation characteristics of decoupled charge blasting, and explored the failure evolution patterns in rock blasting. [PERSON] et al. [37] and [PERSON] et al. [38] investigated the effect of the relative position of the interval on rock fragmentation during blasting. Their findings demonstrated that the interval position determines the propagation paths of stress waves and explosive gases, directing energy to extend in specific directions and enhancing the fracture penetration effect in the rock mass. Currently, optimization methods and technologies for charging structures have been extensively implemented in open-pit blasting projects, effectively improving the distribution of blasting energy and significantly enhancing blasting performance. However, the existing research mainly focuses on optimizing charging structures in single rock layers, whereas research on optimizing charging structures for simultaneous blasting and stratified mining in multi-lithologic mining areas remains insufficient; in particular, there is a lack of systematic research on charging parameter optimization under simultaneous blasting conditions of different lithologies. This study initially optimizes the charge structure based on the different mechanical properties of coal and rock and investigates the effects of various charge structures on stress propagation and rock damage. It is essential to develop charge structures that minimize damage to the rock mass at the coal-rock interface to mitigate the mixing phenomenon near the coal-rock interface post-blasting. Therefore, the mixed blasting technology, which combines upper rock layer casting with lower coal seam loosening, is achieved. These findings provide new approaches for optimizing the design of decoupling charge structures and serve as technical references for reducing costs and enhancing efficiency in extracting thin coal seams in open-pit coal mining.