Numerical simulation of rock fragmentation induced by a single TBM disc cutter close to a side free surface

A series of numerical simulations were performed to investigate the influence of side free surface on the rock fragmentation induced by single TBM disc cutter. The JH-2 model combined with a Rankine tensile cracking softening model was adopted to define the rock constitutive model to simulate the rock dynamic fracture process. The distance that allows for cracks just to extend to side free surface under a given penetration is defined as the critical distance, which distinguishes the rock fragmentation patterns between induced by disc cutter close to side free surface and conventional single cutter. The former rock fragmentation pattern is that rock chips in the form of wedge-shaped block spalling from rock specimen due to tensile cracks intersection. From the mean rolling force and the rock debris mass, the specific energy to reflect cutting efficiency was calculated for different distances to side free surface. The cutting efficiencies of former patterns are far higher that by conventional single cutter. The rock macro-fracture patterns and cutting efficiency were then compared with those obtained from linear cutting experiments. The critical distance for Baiyun granite is about 100 mm obtained from experiments, in good agreement with that deduced from numerical simulations. The height difference between the inner and outer cutterhead of the two-stage cutterhead should be more than 220 mm and the distance from first disc cutter on the inner cutterhead to free surface should be less than 100 mm for granite ground.