Lining pressure for circular tunnels in two layered clay with anisotropic undrained shear strength

ABSTRACT In this paper, the required limiting pressure to be offered by the lining of circular tunnels formed in clay overlain by another clay layer under undrained condition has been computed. Both the layers are anisotropic in nature from the shear strength point of view. The upper layer is considered relatively either weaker or stronger than the lower layer i.e. undrained anisotropic shear strength of upper layer is relatively either smaller or greater than that of lower layer. Lower bound limit analysis coupled with finite element formulation and second order cone programming has been employed to perform the analysis. For different thickness of upper layer above crown of tunnel, the limiting lining pressure to maintain stability of tunnel has been obtained by varying thickness of lower clay layer, anisotropic parameters, normalised overburden pressure, undrained cohesion and unit weight of upper layer relative to lower layer. The solutions obtained from this analysis reveal that anisotropy in shear strength has significant effect on the required lining pressure to keep tunnel in stable condition.