UPPER AND LOWER BOUND SOLUTIONS FOR THE FACE STABILITY OF SHALLOW CIRCULAR TUNNELS IN FRICTIONAL MATERIAL

With the recent increase in underground urban development as well as for transportation, tunnels need to be driven in increasingly difficult soil conditions. In most cases the ground itself is not stable and face stability is achieved by applying fluid pressure to the tunnel front. The question of determining the retaining pressure to be used has received special consideration in the past because of the concern for safety during construction, and also because of the damage that could be caused to surface structures by the failure of a shallow tunnel. The problem is three-dimensional and can be studied by using the limit state design method. Solutions are available for the case of a circular tunnel in purely cohesive ground, but very little is known of the face stability of a tunnel driven in sandy soils. This Paper approaches this latter problem from the point of view of limit analysis. Both safety against face collapse and blow-out are considered. Three upper bound solutions are derived from the consideration of mechanisms based on the motion of rigid conical blocks. The results are compared with lower bound solutions published in a previous article. A failure criterion is proposed for the tunnel face in the general case of a cohesive and frictional soil, and charts are provided to allow a bracketed estimate of the required retaining pressure. A comparison with centrifuge laboratory tests shows close agreement between the theoretical upper bound solutions and the face pressures at collapse measured experimentally.