Quantification of extrusion-induced excavation volume in shield tunnelling through squeezing cohesive-frictional ground.

When tunnelling through weak and highly deformable ground, relevant deformations occur ahead of the tunnel face and result in face displacements that are opposite to the advance direction ("extrusion"). In mechanised tunnelling, the cutterhead continuously removes the ground extruding at the tunnel face during advance. Therefore, the total excavation volume is higher than the volume that corresponds to the tunnel cross-section area. Previous investigations by the authors showed that the additional, extrusion-induced excavation volume may be considerable in the case of purely cohesive ground. This paper extends this work for the case of cohesive-frictional ground. Firstly, we quantify the extrusion-induced excavation volume for a wide parameter range by means of a numerical model that uses re-meshing to consider the continuous re-profiling taking place at the tunnel face during advance. The results of the performed parametric study are summarised in a single dimensionless design chart. Secondly, we derive a simple closed-form expression, which, despite the underlying simplifications, allows for an accurate and rapid quantification of the extrusion-induced excavation volume. The results show that face extrusion has an effect on excavation volume in the cohesive-frictional case, although less pronounced than when tunnelling through low-permeability clays with purely cohesive short-term strength. The study suggests that if the additional excavation volume due to face extrusion is not taken into account in the design of the mechanised advance, it may (a) pose logistical and disposal challenges and (b) lead to erroneous conclusions about the stability of the tunnel face, both of which affect the rate of tunnel advance.