Study On Mechanical Characteristics And Engineering Application Of Steel Frame And Lock Foot Anchor Pipe In Deep Buried Soft Rock Tunnels

The combination of steel frame and lock foot anchor pipe is an effective and economical support measure for controlling tunnel rock deformation in weak rock tunnel initial support structures,which has been widely applied in the excavation support process of soft rock tunnels.However,the study of the stress characteristics and support parameters of the steel frame and lock foot anchor pipe combination structure is still not deep enough.This paper takes the rock pressure of deep-buried soft rock tunnels as the starting point,and first studies the vertical rock pressure time history curve to determine the rock pressure acting on the initial support structure.Combined with the three-step construction method,the internal force and arch displacement calculation formula of the steel frame and lock foot anchor pipe combination structure are derived for each step.Then,under different rock pressure sharing ratios,steel frame models,and lock foot anchor pipe drilling diameters,the optimal drilling angle of the lock foot anchor pipe is analyzed.Finally,the FLAC3 D numerical simulation software is used to study the tunnel rock displacement under the action of different anchor pipe drilling lengths in each step,and the optimal drilling length of the lock foot anchor pipe is determined.The specific conclusions are as follows:(1)The design specifications and assumptions and applicable ranges of the empirical formulas and Platts formula for highway tunnels are analyzed and compared.At the same time,the calculation method of vertical surrounding rock pressure for deeply buried soft rock tunnels is determined based on statistical measured data of surrounding rock pressure.Different time functions are selected to fit the curve of surrounding rock pressure change and vertical displacement change of the crown of the same section,and the corresponding surrounding rock pressure after each step of excavation can be calculated by comparing the fitting results with the proportion of vertical crown displacement after each step of excavation.(2)The working mechanisms of the steel frame and lock foot anchor pipe are analyzed,and the mechanical calculation models of the steel frame and lock foot anchor pipe combination structure for each step of the three-step construction process are established.On this basis,the calculation methods for the stress of the steel frame and the arch settlement formula are proposed for the upper step and middle step.(3)Based on the study of the internal force and arch settlement of the steel frame and lock foot anchor pipe combination structure after the upper step and middle step excavation,the optimal drilling angles of the lock foot anchor pipe at the foot of the arch for the upper and middle steps are determined to be 45° and 65°,respectively,from three aspects: rock pressure sharing ratio,steel frame model,and lock foot anchor pipe drilling diameter,using the example of the Zhonghe Tunnel.Then,the optimal drilling angle of the lock foot anchor pipe at the foot of the arch for the lower step is determined to be 71° based on the fitting formula for the relationship between tunnel excavation height-span ratio and anchor pipe drilling angle.(4)Based on the optimal drilling angles of the lock foot anchor pipe at the foot of the arch for the upper and middle steps calculated by theoretical calculation,the optimal drilling angle of the lock foot anchor pipe at the foot of the arch for the lower step calculated by the fitting formula is verified using finite difference numerical simulation software.Then,the optimal drilling length of the lock foot anchor pipe at the foot of the arch for each step is numerically studied in three steps,and the optimal drilling length of the lock foot anchor pipe at the foot of the arch for each step is determined.