Estimation of Minimum Heterogeneous Face Support Pressure in Mechanized Tunneling Using Numerical and Analytical Methods, Case Study: Tehran Metro Line 7

Determination of the optimum face support pressure is one of the fundamental tasks in a excavation using tunnel boring machines (TBM). The optimum face-stabilizing pressure ensures the safety of the excavation, helps to minimize a surface settlement and ultimately prevents the collapse of the entire tunnel. Existing methods have been presented only for homogeneous soil at the tunnel face. The heterogeneity or boundaries of the face layers have a significant effect on the stability of the tunnel face. Practical experiences show that the tunnel face stability in heterogeneous soil is relatively more difficult than homogeneous soils and it cannot be easily solved by the models proposed for homogeneous soil. This study investigates the influence of heterogeneity of the soil on the tunnel face instability in EPB TBM . Therefore, the numerical softwares (Plaxis 3D Tunnel and Phase2) and analytical methods (Broere and Carranza-Torres) were used for the stability analysis of tunnel face in heterogeneous soil. The results of comparing values of safety factor showed that analytical methods are conservative. Moreover, the numerical modeling results showed that when the soils with poor material properties are located in the upper part of the face, they can significantly increase the ground pressure and partial slide. From comparing the face support pressure for one layer (assumption of homogenous soil and averaging of physical and mechanical properties of soil) with multilayers of tunnel face, it can be concluded that assuming of tunnel face as a one layer, causes incorrect estimation of tunnel face support pressure.