Load transferring mechanism of pipe umbrella support in shallow-buried tunnels

• Finite difference method used to simulate load sharing characteristics of pipe roof construction.
• Systematic method to analyze and quantify load carrying mechanism of pipe roof system.
• Quantified the relationship between pipe roof deflection and its design characteristics including stiffness.

A double-parameter elastic foundation beam model with variable subgrade coefficients was established to simulate for pipe umbrella support mechanism in disturbed shallow-buried tunnel sections. Through finite difference formulation, the deflection and internal forces of the pipe umbrella system during excavation were analyzed in this study. The results showed that (1) the range of disturbance in surrounding rock from excavation was largely limited to approximately −0.5H to 0.5H (H is the height of the excavation) away from excavation face with the pipe umbrella support; (2) the primary purpose of pipe umbrella is to provide a load transferring mechanism to distribute the load to the primary support system before the excavation face and the surrounding rock ahead; (3) the load on pipe umbrella increases when surrounding rock is weak and load transferring efficiency increases when stiffness of the primary support system increases; (4) there is a logarithmic relationship between the load carried by the excavation face and the ratio of surrounding rock stiffness to primary support system stiffness.