Numerical analysis for failure and deformation assessment of the waterway tunnel, Wabe Hydropower Project, Central Ethiopia

In designing suitable support systems and ensuring safe excavation of a tunnel, deformation and block failure assessment around the opening is a crucial aspect of tunneling. In this study, a distinct element modeling approach was employed to evaluate the distribution of failed blocks, failure modes, and displacements of the tunnels to gain insight into support recommendations for the Wabe Hydropower Project in central Ethiopia. For this purpose, three representative numerical models were developed considering different rock mass along the tunnel alignment. Subsequently, the influence region classification technique was introduced, and the models were systematically classified into three distinct regions. This technique enabled the consideration of blocky rock mass as discontinuum through the direct inclusion of field-measured joints with average spacings of 0.2, 0.56, and 1.2 m into a region surrounding the tunnel opening. The simulation results indicated that tunnels in closely jointed rock mass behave anisotropic, with failed blocks following the joint inclinations of N253/72 and N035/79 and exhibiting a tensile failure mode. Tunneling in the fault zone induced a shear failure mode, with a significant distribution of failed blocks aligned in the maximum principal stress direction. However, under low horizontal in situ stress, both shear and tensile failure could exist, with tensile failure affecting the roof and floor. Furthermore, tunnels in closely jointed rock mass are primarily influenced by horizontal displacement, whereas tunneling in fault zones led to both greater horizontal and vertical convergences, with horizontal displacement being more significant. Finally, the obtained results were used to propose support recommendations.