Face Stability of Twin Tunnels Excavated in Opposite Directions Using Large-Diameter Slurry Shield Machines

The construction of twin tunnels excavated by simultaneous drive of two opposite tunnel boring machines presents a challenge for the tunneling industry, in particular the assurance of tunnel face stability due to a mutual effect when both tunnels are approaching each other. To ensure the stability of the tunnel face over the whole construction phase and along the complete alignment, it is important to correctly assess the response of the individual tunnel face with respect to the effect of the opposite tunneling process. For the assessment of such tunnel face stability, a three-dimensional (3D) numerical model that allows us to examine the 3D stress and pore water distribution at the tunnel face is used. This contribution presents a case study based on a real tunneling project in the city of Shenzhen, China, in which twin tunnels are excavated simultaneously in the opposite directions using two large-diameter slurry shield machines. The influence of slurry infiltration as well as the clear distance between the tunnel faces are investigated through comparison with simulations in which a standard face stability assessment technique is employed. It is shown that the additional seepage force induced by the pore pressure gradient between the two faces leads to the increase of the limit support stress ratio.