## 5 Conclusions  
The collapse of the deep-buried loess tunnel with the largest cross-section in China was examined in this paper. The tunnel collapse scenario was described, corresponding engineering measures were proposed and the collapse treatment measures were comprehensively evaluated. The main conclusions were as follows:  
1. The development of vertical joints in loess makes it highly water sensitive; the strength decreases significantly after encountering water. The special properties of loess and the synergistic effect of groundwater were internal causes of the tunnel's collapse. Atmospheric precipitation contributed to the collapse to a certain extent and limited initial support, which is an external inducing factor of tunnel collapse. Human factors, such as low support parameters, the untimely application of the secondary lining, and untimely measurement, were important reasons for this water gushing and collapse accident.
2. Comprehensive treatment measures inside and outside the tunnel were proposed (advanced pre-grouting of the collapse body, reinforcement of the advance pipe shed, drilling of drainage holes to strengthen drainage, radial grouting in the tunnel, backfill of gravel back pressure, filling in the cavities in the caves with mortar, using the ground surface as a water-sealing layer and setting up drainage and interception ditches).
3. The field monitoring results showed that after the collapse treatment, the maximum settlement of the tunnel vault \(X_{\mathit{vs\ max}}\) was 65.1 mm, the maximum settlement rate \(V_{\mathit{sr\ max}}\) was 12.8 mm/d, the maximum horizontal convergence \(X_{\mathit{hc\ max}}\) was 25 mm, the maximum convergence rate \(X_{\mathit{cr\ max}}\) was 4.3 mm/d, the maximum surrounding rock pressure \(F_{\mathit{sr\ max}}\) was 0.56 MPa and the maximum stress of steel arch frame \(\sigma_{\mathit{sat\ max}}\) was 54.34 MPa. The variation of the curve was relatively stable and the value did not show a large sudden increase or decrease, indicating that the treatment measures effectively controlled the pressure and deformation of the surrounding rock and reduced the construction risk.
4. The numerical simulation results showed that after the collapse treatment, the vertical stress, horizontal stress and shear stress of the surrounding rock were greatly reduced. The safety factors on the left and right sides of the wall foot of the initial support were the smallest, with values of 2.7 and 2.6, respectively, but they also met the requirements of the specifications. During tunnel construction, attention should be paid to the advanced reinforcement of the tunnel face.  
Conceptualization, D.W. and J.L.; software, D.W. and K.S.; validation, D.W. and L.G.; investigation, F.L.; data curation, D.W. and L.W.; writing--original draft preparation, D.W.; writing--review and editing, D.W. and J.L.; visualization, F.L. and K.S.; supervision, J.L.; project administration, L.G. and L.W. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Data Availability StatementData sharing not applicable.  
The author would like to thank all the anonymous reviewers for their meticulous comments and helpful suggestions.  
The authors declare no conflict of interest.