Support Pressure for Stability of Horseshoe-Shaped Tunnels in Undrained Clay Using Lower-Bound Finite-Element Limit Analysis

In this paper, the peripheral stability of a horseshoe-shaped tunnel in undrained clay with linearly increasing and directionally dependent strength is studied using the lower-bound theorem of plastic analysis coupled with finite-element formulation and second-order cone programming. The present analysis computes required supporting pressure for a tunnel constructed in isotropic/anisotropic normally consolidated (NC)/lightly overconsolidated (LOC) clay with linearly varying undrained strength across the depth, and isotropic/anisotropic heavily overconsolidated (HOC) clay whose undrained strength remains constant throughout the depth. Dimensionless stability charts have been produced for expressing the undrained stability of horseshoe tunnels in terms of anisotropic and heterogeneous parameters, tunnel cover depth, and normalized overburden pressure. The combined influence of these parameters on the peripheral stability of tunnels and associated failure mechanisms have also been examined and discussed. Further, a few analyses were done to examine the advantage of a horseshoe-shaped cross section in comparison with a circular cross section.