Study on the optimum penetration depth by two TBM cutters under different cutter spacings

This paper investigates the optimum penetration depth and the synergistic effect of TBM (Tunnel Boring Machine) rock breaking under the different cutter spacings. Indentation tests, using two TBM cutters, were conducted on sandstone specimens based on an improved uniaxial testing. Afterwards, morphological measurement was performed to analyze the change of the groove volume, using a surface profilometer. Based on the evaluations of typical chips and cracks propagation, the fragmentation modes of the rock between two cutters under different penetration depths were proposed. In addition, the TBM rock breaking efficiency was studied. As the increase of the penetration depth, when the cutter spacing was constant, the results show that: (1) The rock fragmentation modes gradually changed from shear failure and shear-tensile failure to tensile failure. (2) Though penetration energy gradually increased, the groove volumes also increased, thus, the TBM rock breaking efficiency rises. (3) Specific energy decreased first and then gradually increased. It indicates that the optimum penetration depth was existent for different preset penetration depths. With the increase in cutter spacing, the optimum penetration depth also rised. According to the derivation and verification of the analytical formula, the internal relationship between the penetration depth and the fracture toughness was obtained, which was in line with the linear relationship between penetration depth and penetration energy. From a microscopic point of view, the sharp increase of penetration energy with increase of penetration depth can be explained.