3D Modeling of Intersection between line 6 and 7 Tunnels of Tehran Subway to determine the Best Excavation Sequence for Non-Level Crossing Tunnels

Summary: There are many factors which affect the interaction between non-level crossing tunnels such as excavation sequence, excavation method, lining thickness, the distance between tunnels, the length of unsupported span, soil properties and etc. The excavation sequence is one of the most important factors which should be determined before construction of their intersections, especially in urban areas as it has many effects on surface subsidence and other negative aspects of tunneling. The main purpose of this study is determining the best excavation sequence for non-level crossing tunnels in urban areas. For this purpose, the intersection between lines 6 and 7 tunnels of Tehran subway has been considered and the best alternative for a sequence of their excavation was determined by 3D numerical models by MIDAS GTS NX, numerical finite element software. Both tunnels have an equal diameter and are excavated by TBM-EPB machines. This software has good capabilities for modeling of mechanized excavation of tunnels. To achieve the above-mentioned objective, three scenarios, and five alternatives were considered and in each scenario, the effect of excavation sequence on the results of the model such as surface subsidence, total displacement, axial and shear forces on lining has been investigated by numerical models. The results show that the best scenario is the excavation of upper/shallow tunnel at least 10 times of diameter earlier than lower/deeper tunnel.
First of all, assumptions considered are intended. Then, literature was reviewed and the project is introduced precisely. Finally, the 3D model by finite element software was made, and all scenarios and alternatives of excavation sequences were investigated. According to results the best scenario is the excavation of upper/shallow tunnel at least 10 times of diameter earlier than lower/deeper tunnel.
Methodology and Approaches: Because there are various factors affecting the interaction of non-level crossing tunnels, so the best method for investigating these, are numerical models. And because the medium that tunnels are constructed in is a continuum, the finite element method for solving the problem is used. And due to 3D nature of the problem, 3D modeling is used.
Results and As mentioned before, the best scenario is the excavation of upper/shallow tunnel at least 10 times of diameter earlier than lower/deeper tunnel. Other researchers also believe to the results obtained. So suggestion for feature studies is: the investigation should be focused on only the scenarios that shallower tunnel is excavated first.