Abstract: The three-dimensional failure mechanism for the face stability of tunnel in pure clay under undrained condition is improved. As in the existing 3D translational multi-block collapse mechanism, the intersection of the circular truncated cylinder with the tunnel face is an ellipse surface that does not cover the entire circular face of the tunnel. The truncated elliptical cylinder is adapted to construct the 3D translational multi-block failure mechanism. In the present 3D mechanism, all the radial cross-sections of the rigid block are elliptical, and the intersection of the cylinder with the tunnel face is a circular face. The 3D collapse mechanism is further modified to intermix rotating multi-block and homogeneous shear zone, and the corresponding upper bound solution significantly improves the existing limit analysis. The validity of the combined upper bound solution for the tunnel stability in clays is demonstrated by comparing with the existing 3D finite element and centrifuge results.