A 3D rotational silo-torus model for face stability analysis of circular tunnels

To avoid tunnel collapse during construction, a critical support pressure needs to be determined and applied on the tunnel face. This paper presents the development of a new 3D limit equilibrium model including geometries of a lower torus and an upper silo. The torus geometry was derived using a newly developed spatial discretization technique and subdivided by a wedge slice scheme. The critical support pressure was determined iteratively using the equations given in this paper. The model adopted different lateral stress coefficients for the silo and the torus geometry under different arching levels. The interface behaviour was also considered between the sliding torus and the surrounding soils. Validating with centrifuge tests, analytical and numerical tests demonstrated the accuracy of the proposed 3D model which is based on a realistic side rotational mechanism. The shear force distribution on the rotational surface was presented at the state of limit equilibrium. Finally, the effects of geotechnical and geometrical parameters (c’, φ’ and Tm) were discussed on the face stability of circular tunnels.