## 6 Conclusions  
By using the interface element to connect the backfill body and karst cave, an effective FLAC\({}^{\text{3D}}\) finite difference model is established. The variation characteristics of the surrounding rock deformation and the mechanical response of the support structure before and after grouting are analyzed. The feasibility and rationality of using grouting to pretreat the backfill body to ensure the safety of tunnel excavation are analyzed. The inadequate grouting section was recognized by using the field grouting record analysis and the single hole grouting quantity analysis, and recognition results was verified by the coring observation method. Finally, the grouting effect is evaluated by using field monitoring results of horizontal convergence. The following conclusions are obtained. The position relationship between tunnel and karst cave has a great influence on the displacement development law of surrounding rock and the mechanical response of the supporting structure. The karst cave located on the single and both sides of the tunnel are defined as I-type and II-type tunnel, respectively. Before grouting, the surrounding rock has a large displacement. In the backfill region of the I-type and II-type tunnels, the surrounding rock at the side wall and bottom of the tunnel has a large deformation, while the backfill body in the I-type tunnel away from the side of the tunnel produces a similar landslide phenomenon. Large bending moments are generated in the backfill region and contact region of I-type and II-type tunnels. In the I-type tunnel, the karst cave is located on one side of the tunnel, resulting in large torque in the backfill region and contact region. However, the distribution of karst cave in II-type tunnel is more symmetrical than that in I-type tunnel, the torque is only generated at the bottom of the tunnel. The deformation of the support structure is related to the excavation method. For I-type and II-type tunnels, the large deformation is occurred at the foot of each step and the bottom of the tunnel, while the deformation at the vault is almost 0. Grouting can reduce the displacement of the surrounding rock in I-type and II-type by 70-90%, the range of stress redistribution in the surrounding rock is decreased, and the displacement in the landslide region of I-type is reduced by 43%. The bending moment of the support structure within the backfill body is reduced by 80-90%, the torque is reduced to negligible, and the large deformation of the support structure at the foot of each step and the bottom of the tunnel is effectively controlled. Excepted for D3K279 + 898 section, other sections are sufficient grouted, with small horizontal convergence which is less than 30 mm and short stabilization time which is less than 20 days, good grouting effect is achieved. D3K279 + 898 section is the transition interval affected by groundwater. The grouting material on the section is cement-based single slurry, and the grouting quantity is 0.92 m\({}^{3}\)/m, which is less than the designed value. The single hole grouting quantity is characterized by large grouting quantity in the middle hole and large end pressure of surrounding hole. Core observation reveals that the rock core is broken and has large moisture content on D3K279 + 898, D3K279 + 898 is an inadequate grouting section. The hydraulic connection should be sealed with cement-sodium silicate slurry and then reinforced with the cement slurry. Conceptualization, P.P. and F.P. ; methodology, Z.S. ; software, P.P. and Z.S. ; validation, Z.S., D.Z. and F.P. ; formal analysis, P.P. and Z.S. ; investigation, F.P. ; data curation, F.P. ; writing--original draft preparation, P.P. ; writing--review and editing, P.P. and Z.S. ; visualization, Z.S. ; supervision, D.Z. ; project administration, D.Z. ; funding acquisition, D.Z. All authors have read and agreed to the published version of the manuscript. This research was funded by National Natural Science of China, grant number 51738002. Not applicable. The authors declare no conflict of interest.