## 1 Introduction  
To realize the net-zero carbon pledge, urgent reforms must be carried out to optimize the energy structure by building a low-carbon, clean and efficient energy system, taking robust renewable energy substitution actions and boosting the development of clean energy such as hydro and solar energy. In the new power system with new energy, the pumped storage power station plays an extremely important role [1]. In 2019, the global installed capacity of pumped storage was 158 million kW, accounting for more than 94% of the global installed energy storage capacity, and pumped storage dominated energy storage. Pumped storage power stations are often located in complex geological conditions. It is a concerning issue how the mechanical behavior of the underground cavern is affected by construction, environment and other factors [2; 3; 4].  
Geophysical methods, such as seismic wave methods like Time-Gated Probing (TGP) and Time-Shift Profiling (TSP), are commonly used for geological exploration [5]. These methods, especially TGP and TSP, are widely applied in tunnel geological exploration due to their advantages of high energy and long propagation distances. Currently, TGP or TSPis primarily used for advanced geological forecasting [6; 7; 8]. After the excavation of the cavern, the side wall rock mass bears the load transmitted from the arch rock mass. If the estimation of the side wall rock mass is insufficient, it can lead to engineering disasters caused by an inadequate initial support-bearing capacity, resulting in side wall extrusion and overall settlement of the arch rock mass. TGP side wall forecasting for the location of joint fissures in the side wall rock mass can complement the limitations of geological exploration reports and enhance the safety of tunnel construction.  
It is well known that using reasonable rock mass mechanical parameters is crucial for studying the construction mechanics of underground caverns. While field tests for obtaining rock mass strength parameters are the most direct and reliable, they are not suitable for extensive use in engineering due to the consumption of significant human and material resources [9]. Elastic wave testing has economic advantages, rapidity, flexible layout of measurement points, and a large number of measurement points compared to in situ tests. The relationship between wave velocity and rock mass parameters has been gradually established by many researchers [10], laying the foundation for predicting rock mass parameters based on wave velocity and numerical simulation.  
Drill and blast methods are flexible and cost-effective, while Tunnel Boring Machine (TBM) methods have the advantages of safety and speed compared to drill and blast methods [11]. TBM tunneling involves the initial construction of a pilot tunnel and subsequent enlargement using drill and blast methods. This method has received widespread attention from scholars, and various studies have discussed the advantages and disadvantages of TBM tunneling compared to drill and blast methods [12; 13; 14; 15; 16]. Some research has focused on the impact of the TBM pilot tunnel position and dimensions on tunnel deformation [17]. In this study, based on TGP side wall forecasting to detect the geological conditions of the excavation section, rock mechanics parameters were predicted using measured wave velocities according to the Hoek-Brown criterion. A three-dimensional simulation model of the entire excavation process for both full-section excavation and TBM pilot tunnel enlargement was established based on the predicted rock mechanics parameters and the actual conditions of tunnel construction. The study investigated the mechanical behavior of the surrounding rock at the interface between full-section excavation and TBM pilot tunnel enlargement as the construction progressed and proposed corresponding design and construction suggestions.