## 6 Conclusions  
In this study, the Guanyin Mountain Tunnel of the Chong-Ai expressway was taken as a case study. Based on the FLAC 3D numerical analysis software, the whole construction process of the tunnel portal was simulated, and the viability of the plan was verified from the surface settlement, surrounding rock deformation, and slope stability. The analysis results and construction suggestions of this research can be generalized as follows:  
(1) By monitoring the model surface, it can be observed that the ground subsidence mainly occurred after excavating the upper part of the left tunnel and the right part of the right tunnel. The main subsidence area was located on the right side above the left tunnel and above the center of the right tunnel. The settlement values were 10.467 mm and 10.740 mm, respectively, with a settlement range of 100 m. The surface deformation does not exceed the standard limit, so it meets the safety standards. (2) By monitoring the surrounding rocks of the tunnel model, due to eccentric pressure, the deformation of the surrounding rock in the left tunnel showed an asymmetric distribution, the maximum deformation was located at the arch shoulder, and the settlement value was 14.314 mm. Therefore, the support of the right arch shoulder of the left tunnel needs to be strengthened. The deformation of the surrounding rock of the right tunnel was symmetrically distributed, and the maximum drop value of the arch crown was 17.952 mm. After the excavation face advances to 30 m, the deformation around the tunnel can all stabilize. It should be noted that, in the part of the tunnel in the area of strongly weathered rock, the uplift value of the arch bottom is larger. The tunnel arch bottom and arch foot easily appear to have a large area of the shear-damage plastic zone, and this area should be reinforced. Figure 16: FoS data of slope models. (3) The excavation sequence of the tunnel in the shallow-buried eccentric-pressure section was suggested to adopt the method of excavating the left tunnel first and then the right tunnel. If the right tunnel was excavated first, the slope stability would continue to decline and be in a critical safety state when the left tunnel was excavated due to the disturbance of the rock mass on the right. If the left tunnel is excavated first, the support measures can be changed in time according to the change of the slope, and in the process of excavating the right tunnel, it was safe for the slope because it was not affected by the eccentric pressure. The initial support is applied in time after the tunnel excavation so that the support forms a closed structure, which can improve the safety of the construction environment  
(4) The rock mass and environment are complex, and it is difficult to simulate exactly the same geological conditions as reality through numerical software. This study did not consider the influence of rock-joint and fracture development and water-richness on the surrounding rock deformation; subsequent studies can set the Joints unit in 3 DEC or the interface unit in FLAC 3D to simulate the existence of rock joints and fractures, and also perform the calculation of fluid-solid coupling. Conceptualization, Z.H. ([PERSON]); Data curation, Z.H. ([PERSON]); Formal analysis, H.Z. ; Investigation, C.L. and H.Z. ; Methodology, Z.H. ([PERSON]) and Z.H. ([PERSON]); Project administration, X.L. and Z.H. ([PERSON]); Resources, C.L. ; Software, Z.H. ([PERSON]); Supervision, H.Z., X.L. and Z.H. ([PERSON]); Validation, X.L. ; Visualization, Z.H. ([PERSON]); Writing--original draft, Z.H. ([PERSON]); Writing--review & editing, C.L. and Z.H. ([PERSON]). All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. The experimental data used to support the findings of this study are available from the corresponding author upon request.