Three-dimensional analysis of excavation face stability of shallow tunnels

•Face stability analysis of shallow tunnels by analytical and numerical methods.•Tunnel structure becomes self-supporting for very high soil cohesion values.•A 15-cm thick shotcrete was required to determine limit support pressure values.•At higher tunnel depth, more pressure needed to support excavation face. Several studies have been focused on minimizing construction risks of underground works. However, additional studies regarding excavation face stability, which has great influence on global performance of shallow tunnels, will offer more security criteria to engineers. The aim of this work is to compare the results from an existing analytical and a numerical method in assessing the behavior of the excavation face subjected to different overburden depth and ground conditions. Typical parameters of shallow soils were considered for all simulations. The analytical method was applied to assess the excavation face pressure for different tunnel covers. Numerical methods were applied using ABAQUS 3D® finite elements software, which considered a continuous, homogeneous medium in a 6-meter-wide tunnel excavated at different depths, and assessed the need for support and stabilization pressure on the face. Furthermore, the influence of a well on the excavation line was also considered for such assessment. As result, excavation face was defined as stable by the analytical method; nevertheless, it was obtained a very low safety factor. On the other hand, results from the numerical method showed the need to apply pressure on the excavation face. Those limit support pressure values at tunnel face showed a direct correlation with the increase of cover-to-tunnel diameter ratio. Finally, the numerical method showed a more realistic result when compared with the analytical method.