The stability of the tunnel face is a top priority in a tunnel construction. The estimation of the earth pressure acting in front of the face is essential for the stability evaluation of the face, and many model experiments have been conducted to understand the characteristics of face stability. On the other hand, there are many cases in which the stability of the tunnel face is evaluated by limiting analysis assuming a constant slip region by applying the loosening earth pressure derived by Terzaghi as a boundary condition, but the actual slip region is not always constant and expanded progressively accompanying by the collapse of the face. There are few past studies that have considered this phenomenon. In this study, a estimation method of the tunnel face pressure are developed in consideration of the progress of face collapse. The proposed method is a kind of limit equilibrium method. Assuming a slip line with two ellipses with the same center and different radii, the first-order linear differential equation of earth pressure in the tunnel face direction is calculated from the equilibrium for infinitesimal elements in the circumferential direction.
The stability of the tunnel face is a top priority in a tunnel construction. In recent years, a method of construction proceeding with the face open has been widely implemented even in unconsolidated ground (e.g. urban NATM method). Since unconsolidated ground has relatively low strength, there is concern about face collapse. To predict face collapse, it is necessary to obtain the earth pressure acting on the face. Thus, accurate prediction of tunnel face pressure is extremely important.
Although many experimental studies have been conducted to investigate the ground behavior associated with tunnel excavation (e.g. [1]), the tunnel face behavior is quite complicated, and there is no established theory so far. One of the factors complicating the face behavior is the progressive failure, and the slip region is not always constant and expanded progressively accompanying by the collapse of the tunnel face [2]. On the other hand, in the past studies of face stability using the limit slip analysis [3, 4] and the rigid-plastic finite element method [5, 6], the stability of the final collapse at full face open can be evaluated, but the progressive failure cannot be usually considered.