The assumption of excavation failure mode is based on the traditional wedge model which is not suited to usual clay soil and sandyclay layered soil. This paper presents a novel wedge model for the analysis of face stability, taking into consideration the real failure modes, the character difference of layered soil and the arching effect of soils. This model is an extension of the widely-used Terzaghi’s pressure equation. The results show that the critical supporting pressure, greatly affected by the overburden soil pressure, increases sharply with the increase of embded tunnel depth. The critical supporting pressure could be determined accurately by the suggested novel wedge model. The value of supporting pressure is closely related to the openangle α and dip angle  θ of the wedge itself and is unique when α=90°－θ. The rationality and feasibility of the novel wedge model and the determining method for critical supporting pressure were verified by a field case and the results could provide a reasonable reference for similar engineering conditions.