Study on adaptability of primary support arch cover method for large-span embedded tunnels in the upper-soft lower-hard stratum

The Provincial Hospital Station of Guiyang Metro Line No. 2 in China is located on the upper-soft lower-hard stratum. In order to analyze the adaptability of primary support arch cover method for the project, the construction process of the main tunnel of the Provincial Hospital Station is simulated by virtue of numerical simulation. Meanwhile, the excavation process of the tunnel is simulated by the double-side drift method for comparison. The numerical simulation results show that the primary support arch cover method has advantages in settlement control, plastic zone distribution, and supporting structure safety. Only the primary supporting structure is relatively weak in the rock-soil interface and on the arch shoulders and vault, and liable to tension failure. However, the application of strengthened primary support can significantly improve this unfavorable situation. Therefore, it is suggested that the primary support arch cover method be adopted for the construction of the main tunnel of the Provincial Hospital Station. The numerical simulation results obtained by primary support arch cover method are consistent with the field data in terms of the change trend of surface settlement, and the final settlement value is also similar. On this basis, the influence of grade variation of underlying surrounding rock on surface settlement and surrounding rock plastic zone distribution caused by the two methods is further simulated, and the grade range of underlying surrounding rock suitable for primary support arch cover method is given in the article.