Applicability of stability criterion for tunnels

The stability criterion for tunnels is established based on modified Fenner formula. This criterion expresses the relationship between the support pressure of a tunnel and the radial displacement of its boundary using a rotationally symmetric model. The assumptions underlying rotational symmetry are a circular tunnel, a hydrostatic and uniform initial stress field, an isotropic and homogeneous ground and uniformly distributed support pressure. The applicability limits of the stability criterion for tunnels are sounded out by systematically investigating the effect of deviations from some of the important assumptions. The classical problem of tunnel excavation in a linearly elastic, perfectly plastic ground obeying the Mohr-Coulomb yield criterion is studied, and the effects of anisotropy and non-uniformity of the initial stress field and a non-circular tunnel geometry are analyzed. The results show that the stability criterion provides a reasonably accurate approximation of allowable convergence for different ground conditions that violate the rotational symmetry.