The influence of the round length on the stability of tunnel face and unsupported span

A number of research projects on the face stability of tunnels have been carried out in the past.
However, the influence of the round length has not been thoroughly investigated, although it is an essential factor for the stability and excavation costs. This paper presents typical failure mechanisms of the face including the unsupported span in relation to the round length in weak rock tunnels, whose behavior is not governed by discontinuities. A series of small-scale model tests and numerical analyses by PFC-2D (Particle Flow Code, Itasca) were carried out to define failure modes at the face and unsupported span. Five failure modes according to its shape and extent are suggested for excavation planning. The influence of the lining stiffness
and overburden on the overbreak and round length has been investigated in the model tests. Failure mechanisms, their initiation and propagation have been also investigated by PFC analyses and compared to the results of the model tests. Overbreak in the unsupported span does not affect the face stability as long as the face is initially stable and the round length is small enough not to cause excessive overbreak or daylight collapse. Therefore, the decision on the round length should be made in consideration of the interaction of overbreak and support rather than the face stability itself.