## 1 Introduction
Additionally, [PERSON] and his team [14] calculated that during the instantaneous excavation of a long tunnel, the transient unloading of initial stress in the rock mass leads to the occurrence of dynamic tensile stress in the surrounding rocks. The magnitude of this stress is closely related to the unloading rate. The investigation of this issue entails a host of complex engineering challenges. Current research has primarily emphasized the numerical modeling and analysis of a specific issue. Obtaining an effective blasting load is critical in the numerical analysis of the dynamic response of an adjacent tunnel. There are two main strategies. In the numerical analysis of adjacent tunnels, converting the impact load into a pulse that is applied to the tunnel excavation faces is a typical approach for obtaining an effective blasting load [15; 16; 17]. The research conducted by the [PERSON] team [18] demonstrated the importance of incorporating dynamic parameters in rock mass modeling for an accurate prediction of response under the explosion. By correcting the triangular pulse load and employing a mixed format of AUTODYN and UDEC, the team achieved prediction results that closely aligned with the on-site test results. This highlights the significance of including dynamic parameters to ensure the reliability of prediction outcomes. To obtain an effective blasting load for the numerical analysis of adjacent tunnels, another approach is to directly simulate the complete process of the blasting construction [6; 19; 20]. In their study, [PERSON] et al. [20] utilized LS-DYNA finite element software to simulate the complete blasting process. A systematic analysis and study were conducted on the safety and stability of an underground cavern group under dynamic loads. While simulating the entire blasting process yields a high simulation accuracy, for more efficient engineering analysis, utilizing an equivalent impact load calculation is favorable and effective. This approach is supported by previous research [21]. As mentioned above, abundant research has been carried out to explore the influence of the blasting load on existing tunnel structures. However, these existing studies mainly focus on the scenarios where the blasting load is applied to the outline of the new tunnels. In this scenario, the existing tunnel is parallel to the new tunnel. A scenario where the excavation face that acts on the blasting load is perpendicular to the new tunnel is seldom investigated. In light of this, the latter scenario is particularly investigated in this work, where the blasting construction of a horizontal adit for vehicles towards the existing Yonghe tunnel, located on the Huangpu Open Avenue in Guangzhou, is referenced as a background project. Based on the equivalent blasting load model, a 3D numerical model was established to analyze the dynamic responses of the existing tunnel lining when a perpendicular horizontal adit is constructed using the blasting method. Additionally, the threshold value of the distance from the excavation face of the horizontal adit to the existing tunnel is proposed to guarantee the safety of the existing tunnel based on the numerical analysis results.