## 3 Numerical Simulation  
In order to study the influence of groundwater in the karst area on tunnel construction when crossing the karst cave and selecting appropriate backfill materials, the 279 + 923 section with the lowest design elevation of tunnel was selected as the research object, and a model was established using the finite difference software FLAC3D for a simulation. Figure 6a is the grid element division diagram, and Figure 6b is the distribution diagram of surrounding rock in the karst cave. So that the tunnel width on both sides is five times that of the width of the karst cave, the calculation range of the model was set to be 50 m in the longitudinal direction, 1300 m in the transverse direction and 700 m in the vertical direction, including 270 m from the tunnel crown to the model top surface, 120 m from the tunnel bottom plate to the karst cave bottom and 400 m from the model bottom. The simulation range was relatively large, and a total of 1 million elements were generated. Only the upper surface is free; all other surfaces are constrained. Therefore, the surrounding and bottom boundaries were set to be impervious. Groundwater rise was simulated by using the seepage module that exists in the FLAC3d software. When the groundwater flow rises, the seepage calculation mode is turned on, and a stable pore pressure field is obtained through single seepage calculation, and then the fluid modulus \(\text{K}_{\text{f}}\) is set to 0 to achieve the mechanical equilibrium calculation [24,25]. During the supplementary survey of karst caves and underground rivers, 11 holes were drilled in the karst cave hall, with a total length of 575.6 m. A total of nine groups of rock samples and two groups of underground river water samples were obtained on site for the experiment. In addition, TEM, geological radar, seismic imaging, the active source surface wave method and the natural source surface wave method were also used to carry out geophysical exploration on the basement of the karst hall and the excavated tunnel. The properties of surrounding rocks inside the karst cave hall were determined by integrating various test methods, as shown in Table 2.  
Figure 5: Site picture of tunnel crown reinforcement. Figure 6: The numerical model and its construction. (**a**) Unit division. (**b**) Stratigraphic distribution. In order to study the impact of the rise of groundwater on the tunnel's construction, this paper uses orthogonal experiments to study the different effects of the mechanical properties, the degree of compaction and the softening coefficient of water on surrounding rock of backfill materials on the tunnel project. In this paper, six main factors are selected for analysis: the elastic modulus, density, cohesion, internal friction angle, porosity and reduction coefficient. The standard orthogonal table L25 (5\({}^{6}\)) is used to design the experimental team, and each parameter takes five levels, as shown in Table 3. The variation amplitude of the elastic modulus and cohesion is 20%, the variation amplitude of the internal friction angle and porosity is 10% and the variation amplitude of the density and reduction coefficient is 5%. Counting the numerical calculation results of nine sections which are at intervals of 5 m from Y = 5 m to 45 m, a total of six average parameters were taken as an evaluation index, including the displacement of 0-20 m under the bottom plate, the crown settlement, the convergence of both sides and the internal force and bending moment of initial lining. In order to make the orthogonal experimental results comprehensively reflect the influence of groundwater on tunnel construction, according to fuzzy mathematics, after taking the absolute value of experimental results in each group, the average value \(\overline{x_{i}}\) and standard deviation \(\sigma\) in the group can be calculated according to Formulas (3) and (4). In practical engineering, it is always required that the average level of each evaluation index is small to ensure the safety of tunnel excavation.