Stability assessment of unlined real horseshoe-shaped tunnels in anisotropic and heterogeneous undrained clay

In the urban underground transportation systems, especially, in case of mechanized tunneling, tunnel boring machine (TBM) of circular shape is mostly used for tunneling advancement. However, circular-shaped TBM particularly of large diameter have large waste of utilization area that is, small space utilization ratio. With the advancement in the machine manufacturing industries, non-circular-shaped TBM is now used, whose cross section allows to use the underground space efficiently to improve space utilization ratio. Despite the significance of this issue, there has not been much prior research reported in the literature for designing and analyzing underground-excavations/tunnels having horseshoe-shaped cross sections in anisotropic-heterogeneous clay. In the present study, peripheral stability analysis of real horseshoe-shaped unlined tunnels in saturated clay with direction reliant and linearly varying undrained shear strength has been performed using lower bound limit analysis coupled with finite elements and second-order cone programming. For a range of material properties and cover depth of tunnel, the stability charts for unlined tunnel have been generated in this study, which will serve as a useful tool for engineering practitioners. It has also been demonstrated that the underground excavations with horseshoe-shaped cross section is more stable as compared to that with equivalent circular-shaped cross section for the same value of utilization area.