## 1 Introduction  
Karst areas are widely distributed in my country, and urban rail transit in my country inevitably encounters karst-developed strata. Under the condition of improper control of blasting and rock breaking, it is easy to cause geological disasters such as water and mud intrush, fissure seepage and dangerous rock shedding to rock underground engineering [1, 2, 3]. The existence of cavities has a significant effect on the development of rock cracks under blasting loads. Since the cavity is a free surface, stress wave reflection and stress concentration will occur. Due to its low tensile strength, the rock near the cavity is more likely to be damaged under the action of reflected tensile waves, which obviously promotes the development of rock cracks near the cavity [4, 5]. The existence of a cavity in the rock mass makes the direction of the resistance line between the local cavity and the blast hole to be the smallest. Before the rock is completely destroyed, the explosion energy accumulates in the direction of the cavity, resulting in the release of blasting energy. As a result, cavities can cause localized energy release phenomena in blasting charges, which can lead to problems such as flying rocks and uneven fragments. Grouting [6, 7] is a kind of engineering remediation measure, which is widely used in karst engineering [8]. It can prevent water seepage in karst caves [9], enhance the stability of surrounding rock [10], prevent water gushing in tunnels [11] and so on, achieving good application. However, the effect of filled karst voids on rock blasting is still unclear. At present, the blasting research of foundation pits [12] mainly focuses on the vibration and impact of foundation pit blasting on existing underground pipelines [13], nearby subway tunnels [14], buildings adjacent to the ground's surface [15] and people on the surface. At the same time, some scholars have studied other aspects of foundation pit blasting. [PERSON] [16] analyzed the impact of blasting disturbance on the evolution of water inrush disasters in confined water-bearing karst caves. [PERSON] [17] considered the effect of blasting shock waves against the minimum outburst prevention thickness of rock mass against water. [PERSON] [18] studied the calculation method of anti-water-inrush thickness of karst foundation pits considering the number of single blasting. The above article focuses on the influence of dynamic blasting load on the stability and seepage field of underground engineering rock mass in karst cave areas. However, studies on how karst and cavern media affect the rock-blasting process are still scarce. In recent years, smooth particle hydrodynamics (SPH) and the finite element method (FEM) are two of the most commonly used numerical simulation methods in explosion engineering and blasting engineering. SPH method is often used for analysis of large deformation simulation,, while finite element calculations are more stable on model boundaries. In order to accurately simulate the blasting action process in all directions in space, and to realize the stability of rock mass rupture and large deformations near the blasting area and the calculation of the boundary position of the model, the SPH-FEM coupling calculation method is gradually applied to numerical analysis [19; 20; 21], such as projectile penetration into concrete [22], slotted charge blasting [23] and other research directions. For karst engineering, some scholars have used the SPH-FEM coupling calculation method to study the flow of karst aquifers [24], small-scale infiltration dynamics in fractures [25], and inrush water in karst areas [19]. However, no scholars have used the SPH-FEM coupling calculation method to study on the effect of the variation of filling medium in karst cavity on the rock breaking effect of blasting.