Improved Longitudinal Displacement Profile and Initial Support for Tunnel Excavation

The ground support system has a significant impact on tunnel safety while the installation time of initial support is commonly designed by empirical method. The three dimensional numerical model, based on the engineering practice, is analyzed by FLAC(3D). In this paper, an improved LDP (longitudinal displacement profile) equation is proposed, which is more accurate in describing the convergence of tunnel before excavation face reaching. Then, a methodology is presented to determine the installation location of initial support in a more efficient and effective way. By simulating the process of excavation load releasing, the appropriate time for installing initial support is when the evolution of surrounding rock displacement has a tendency to increase rapidly, namely that the stability of rock mass arrives at the critical state. The mathematical relationship between the convergence of tunnel and the relative location to tunnel face can be established by fitting monitoring data with the improved LDP equation. Substituting the critical tunnel convergence into the LDP relationship, the corresponding installation location of initial support will be calculated. Meanwhile, several schemes with diverse geological conditions are carried out to validate the improved LDP and the methodology.