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Geological hazard and the lack of appropriate survey, design and construction instruments, have always made the design and construction of underground works a risky affair, which could not therefore be faced with the same degree of accuracy as other civil engineering works. As a consequence a larger safety factor should always be considered for underground works compared to similar surface constructions. During excavation of a tunnel and by the opening a cavity the flow lines of stress field in a rock mass are deviated and are channelled around it to create a zone of increased stress around the walls of the excavation. The channelling of the flow stresses around the cavity is termed an arch effect. The arch effect ensures that the cavity is stable and will last over time. Depending on the size of stresses created and the strength and deformation properties of the ground, channelling can be produced in three states: 1- close to the profile of excavation, when the ground around the cavity withstands the deviated stress flow around the cavity well, responding elastically in terms of strength and deformation. 2- far from the profile of the excavation, when the ground around the cavity is unable to withstand the deviated stress flow and responds an elastically-plasticising and arch effect formes far from walls of the excavation. 3- channelling doesn’t produce at all, when the ground around the cavity is unable to withstand the deviated stress flow and responds in the failure range producing the collapse of the cavity. That’s how arch effect determines that the cavity is stable for short or long time or it is not stable at all. Therefore tunneling, especially in soft ground, should always be observed since it is always at risk of falling during excavation and the ground geotechnical properties must be studied beforehand. The gradual reduction of horizontal stress from the tunnel face during the excavation process leads to the formation of deformities on the ground surrounded tunnel. Face Extrusion is the first response of environment to the excavation process. This phenomenon depends on the resistance and properties of the advance core, as well as the main stress area exposed to it. Extrusion appears on tunnel face and along the longitudinal axis of the tunnel and it’s geometry could be symmetric or unsymmetric. In the present study with the help of 3D modeling a part of the Tehran Metro Line 7 tunnel (the selected part for modeling locates in (0 + 900 to 0 + 1000 meters from the beginning of the southeast of line 7), the relationship between the stresses of the tunnel face and its extrusions are investigated and the results are presented in the form of confinement stress-extrusion curve or “tunnel characteristic curve”. In the following, the characteristic curve is plotted under unidimensional axes and it is observed that these non-dimensional curves are placed on each other under special conditions for different tunnels. These conditions are also given at the end. All modellings are done in Flac 3D software.

Keywords: Tunnel face extrusion, Tunnel characteristic curve, Tunnel excavation, Tehran metro, Tunnel face confinement stress.

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