Geological controls on the breakthrough of tunnel boring machines in hard rock crystalline terrains

Geological factors including the orientation, condition and frequency of discontinuities in rock mass, and also intact rock properties such as strength and brittleness are crucial parameters for performance analysis of hard rock TBMs. These data along with machine specifications such as thrust and power allow the appraisal and prediction of machine penetration rates. Recently completed projects include the Queens freshwater, Manapouri Second tailrace hydropower and the Milyang hydropower tunnels are assessed to investigate the effect of geological and rock mass conditions on the penetrability of utilized full face tunneling machines. Compilation of the experiences and datasets obtained from these projects indicates that even though intact rock properties including mineralogy, texture, metamorphic grade, hardness, strength and brittleness have an effect on the breakthrough of the machines, the most significant and controlling geological parameters are the orientation, condition and frequency of discontinuities in rock mass encountered along the tunnel. Thus, the geological conditions of the site should be investigated in early stage of the constructed tunnel and continuously updated until projects are completed.