Investigation on rock breaking and optimum spacing of TBM cutters under confining stress using a continuum-discontinuum method

In-situ stress is one of the important geological factors affecting Tunnel Boring Machine (TBM) performance. Meanwhile, cutter wear is also a nonnegligible issue in TBM tunneling. In this work, a continuum-discontinuum approach, the finite-discrete element method (FDEM) is employed to explore the impact of confining stress on rock breaking and optimum cutter spacing. Firstly, the evolution of system kinetic energy and rock surface displacement during confining stress equilibrium is revealed. Then, a series of adjacent cutter-indentation tests are performed to analyze the rock fracture pattern and its transformation law, cutter force and the vertical extent of the fracture zone, which is compared with the existing experimental and numerical results. Finally, a fragment extraction algorithm is adopted to count fragment shape and area, and the specific energy (SE) is quantitatively analyzed according to fragment area and cutter work. Besides, the cutter force and cutting performance with different existing wear are discussed. Results indicate that the maximum cutter force increases and the vertical crack propagation is more suppressed as confining stress increases. Overall, the rock fragment area increases first and then decreases with the increase of cutter spacing under all confining stresses, while the optimum cutter spacing increases as confining stress increases. Moreover, the rock breaking efficiency is the highest with a confining stress of 3 MPa and a cutter spacing of 114 mm. The existing wear increases the cutter work and reduces TBM performance. The findings are helpful in the design of TBM cutterhead in tunnel excavation under different in-situ stress conditions.