ABSTRACT

During boring tunnels at rocky environment with the classical New Austrian Tunneling Method, advancing with leaving the middle section unexcavated for tunnel face stability provides benefit without any risks, whereas moving ahead by leaving the core at soft (clayey) grounds, the distance between excavation grades increases and the time to put the tunnel cross-section into circular shape gets longer and undesired deformations occur. In order to avoid this situation, excavation should not be done at clay grounds by leaving the middle section unexcavated for the tunnel face stability.