Stresses around Unsupported Tunnels in Rock

There is increasing interest around the world in using the underground space for civil engineering infrastructure. Analytical and numerical methods for the design of excavation and support of tunnels are slowly introduced into practice. When used, an assumption typically made is that the design of the tunnel can be done using a two dimensional approach. However, the 2-D approach is not adequate to predict the behavior near or ahead of the tunnel face. This paper provides a conceptual understanding of what are the stress changes induced in an elastic ground as a tunnel is excavated. This is done through a number of three-dimensional numerical simulations using a Finite Element Method. The results show that stresses are independent of the Young's modulus and Poisson's ratio of the rock. There is a volume of rock ahead of the tunnel face where the axial stresses are substantially reduced from those of the far-field. Such reduction becomes larger with an increase of the far-field axial stresses. With increasing the far-field vertical stress, the axial stress around the excavation becomes more asymmetric. What is also interesting is that the position of the point where yielding of the rock occurs first is located on the perimeter of the tunnel at some distance from the face. The point moves closer to the face as the far-field axial stress increases.