Determination of optimal face pressure in EPB shield-driven tunneling in soft ground - A case study of Mashhad metro line 2

S. Gharedashi, A. Mortazavi

Research output: Contribution to conferencePaperpeer-review


Several approaches have been developed to analyze the face stability for TBM tunneling in soft ground. The face stability for TBM is a three-dimensional (3D) problem. Three characteristic zones can be identified during the tunnel advance in a lined tunnel: a) an undisturbed zone where the soil mass is not yet affected by the passage of the face, b) a tunnel face or transition zone corresponding to the radius of influence of the face, c) a stabilized zone where the face no longer has any influence and the situation tends to stabilize. It is important to observe that in passing from the undisturbed zone to the stabilized zone, the medium undergoes from a triaxial to a plane stress state and that the face zone is where this transition takes place. Consequently, the face area is the most important zone from a design point of view. It is in the face that the action of the excavation disturbs the medium and design considerations must be focused on. The TBM-face interaction is a 3D problem by nature. With regard to the problem boundary conditions 3D analyses are the most realistic especially for the modeling of face stability. In this study, a comprehensive simulation of the EPB shield-driven tunnel advance to be employed in the Mashhad metro line 2 project was performed. The FLAC-3D code was employed to construct a 3D model of the TBM-ground interaction. The main objective of the analysis was to determine the optimum face pressure required to prevent the soil collapse/blow-out processes at the face. Moreover, since the tunnel passes underneath the city center some buildings are located along the tunnel path. The appropriate face pressures to avoid ground subsidence were also calculated numerically at critical building locations. The loads acting on the tunnel face were estimated according to the active and passive earth pressure principles. The optimal face pressures for EPB shield was analyzed as a function of, soil cover to tunnel diameter ratio, lateral earth pressure coefficient and soil mechanical and strength parameters. The analyses provided valuable information regarding the ground deformation mechanisms, the cross sectional lining stresses due to soil loading, and loading of the tunnel lining at the Mashhad metro line 2 project.

Original languageEnglish
Publication statusPublished - Dec 6 2011
Externally publishedYes
Event45th US Rock Mechanics / Geomechanics Symposium - San Francisco, CA, United States
Duration: Jun 26 2011Jun 29 2011


Conference45th US Rock Mechanics / Geomechanics Symposium
CountryUnited States
CitySan Francisco, CA


  • Earth pressure balance (EPB) shield
  • Finite difference method (FDM)
  • Mashhad metro
  • Numerical modeling

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geophysics

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