<p>Tunnel face stability is a critical concern in shield tunneling, since insufficient support pressure may cause face collapse, excessive ground deformation, and severe risks to construction safety. Existing three-dimensional failure mechanisms with the assumption of rigid truncated cones result in incomplete contact between the tunnel face and the failure region, thereby limiting their predictive accuracy in undrained soils. To overcome this limitation, this paper develops a modified three-dimensional failure mechanism. The instability zone ahead of the tunnel face is modeled as five rigid truncated elliptical cylinders with circular cross-sections, while the region above the face is represented by a vertical elliptical cylinder. Based on the upper bound theorem, an analytical solution for the limit support pressure is derived, and a practical fitting equation is established through systematic parametric analysis. Finally, the presented model is validated by two centrifuge tests as well as compared with classic models. A discussion is presented considering a linearly increasing undrained shear strength with depth. Results show that the model not only resolves the incomplete-contact limitation but also improves the accuracy of face stability assessment in undrained soils. Neglecting the increase of undrained shear strength with depth yields a conservative yet uneconomical estimate of the limit support pressure, with the effect being more pronounced in deep tunnels. Future improvements will focus on refining the failure mechanism representation to better match the actual upper failure region for deep tunnels and extending the mechanism to nonhomogeneous soils to enhance its applicability in practice.</p>