Study on mechanical characteristics of pipe umbrella support in shallow buried tunnels

Considering the micro-arching effect between pipe umbrella, the paper investigates the mechanical response of pipe umbrellas during tunnel excavation by simulating pipe umbrellas as Timoshenko beams on a Pasternak foundation. The finite difference method is employed to solve the deflection, bending moment, and shear force of pipe umbrellas. By analyzing and contrasting with real-world engineering monitoring data, finite element computation results, and existing theories of other researchers, the reliability of the proposed method is verified, and good agreements are observed. A new parameter delta(p) is introduced in our method to represent the proportion of overlying load transmitted through the pipe umbrella, and influences of stiffness of the surrounding rock in front of the tunnel face, initial support stiffness, pipe umbrella design schemes, and excavation depth on the load transfer capacity of the pipe umbrella are subsequently investigated. The conclusions reveal that increases of surrounding rock stiffness, initial support stiffness, and diameter of pipe umbrellas would all enhance the stability of both the tunnel and pipe umbrellas. However, such enhancements have certain limitations. Particularly, when the excavation footage c <1 m, the pipe umbrella diameter should be specifically selected based on cases.