Geomechanical Characterisation of Hard Rocks for Disc Cutting in Deep Tunnels

Abstract

The disc cutting process for TBM excavation is dependent on the ability of the discs to initiate and propagate fractures into the tunnel face. At any depth, the geomechanical characteristics of the rock will determine how efficiently the fracture initiation and propagation processes occur. In deep tunnels the stresses induced at the tunnel boundary can lead to stress-induced failure mechanisms such as spalling and bursting. This paper examines the impact of geomechanical characteristics in combination with induced stresses at the tunnel face on the disc cutting process. TBM performance data and tunnel face maps were combined with mineralogy, grain size and fabric for deep tunnels in granitic and foliated massive rocks to determine how induced stresses enhance or hinder the fracture initiation and propagation processes. The impact of the induced stress varies with different geomechanical characteristics depending on the orientation and relative magnitudes of the stresses. In addition, stress rotation and relaxation ahead of the face can lead to stress-induced fracture creation at the face, which acts to precondition the rock prior to the cutters excavating. These results show that the sensitivity of different rock types to stress-related enhancement or hindrance of disc cutting must be taken into account for deep tunneling projects and are used to propose a geomechanical characterisation approach to identify potential for increased or reduced disc cutting efficiency in deep tunnels.