Study on the optimization of the inclination angle of the locking anchor pipe in the excavation of the upper and middle steps of the deep buried soft rock tunnel

In the initial support of soft rock tunnel, the combined support of steel frame and lock foot anchor pipe is an effective and economical support measure to control the deformation of tunnel surrounding rock. Among them, the setting angle of the lock foot anchor pipe is an important parameter affecting its support effect. Based on the existing mechanical analytical model of steel frame + lock foot anchor pipe after upper bench excavation, the mechanical analytical model of steel frame + lock foot anchor pipe after upper and middle bench excavation is derived by using the method of structural mechanics, so as to obtain the theoretical calculation formula of the setting angle of lock foot anchor pipe and the settlement of steel frame vault after upper and middle bench excavation. At the same time, based on the measured analysis of the vertical surrounding rock pressure, the relationship between the angle of the locking anchor pipe after the excavation of the upper and middle steps and the settlement of the steel frame vault is obtained, and the Zhonghe Tunnel is used as the engineering basis to determine the optimal angle of the locking anchor pipe after the excavation of the upper and middle steps. The results show that the tunnel vault settlement obtained by the established mechanical model of steel frame + lock foot anchor pipe after excavation of upper and middle steps is basically consistent with the field measured value. According to the calculation and analysis of the settlement of the steel arch crown, the optimal setting angle of the lock foot anchor pipe after the excavation of the upper and middle steps of the tunnel is 60 ° ~ 70 °. The research results of this paper can provide a theoretical basis for the optimization design of the dip angle of the lock foot anchor pipe in the step excavation of deep buried tunnel.