A large arch-wall-shaped cross-section with 28 m in span and 22 m in height formed in the Jishixia waterpower project when the spillway tunnel and the diversion tunnel intersect in bad geologic site. The surrounding rocks are distributed in layers and a weak intercalation goes through the top of a cross-section. To verify the schemes of excavation and supports
3D numerical simulation of excavating which tracks construction process is used to study the stresse and deformation distribution and the stability of the surrounding rocks. At the same time
the weak intercalation is taken into consideration. The results may have better reference values for similar projects in the future.