Insights into the Effect of Water Content on Mudstone Fragmentation and Cutter Force during TBM Cutter Indentation via the Combined Finite-Discrete Element Method

In the Tunnel Boring Machines (TBMs) construction of soft rock tunnels, adverse geological conditions, such as water-rich and wet strata are often encountered. Water in strata can change the stress state and weaken the mechanical performance of rock mass, further affecting the tunneling process and rock breaking efficiency. To study the effect of water content on mudstone fragmentation characteristics and cutter force during TBM cutter indentation, the water softening effect is considered in the moisture diffusion-fracture coupling model based on the finite-discrete element method (FDEM). Then, the micro parameters (fracture energy release rate, swelling coefficient and moisture diffusion coefficient) of mudstone are calibrated by uniaxial, triaxial compression and swelling tests. Subsequently, the water content and swelling deformation evolution of mudstone samples during water migration are revealed. Finally, the coupling model is utilized to investigate the fragmentation characteristics and cutter force of mudstone samples with different water contents under various confining pressures during TBM cutter indentation. The fragmentation zone, cutter force, acoustic emission (AE), and crack number during cutter indentation are quantitatively analyzed. Results indicate that the vertical extent of fragmentation range (Ev) decreases with the increase of water content, while the horizontal extent of fragmentation zone (Eh) does not decrease obviously until the water content reaches 3%. The increase of confining pressure reduces the effect of water content on Ev, and the increase of water content also reduces the effect of confining pressure on Ev, indicating that confining pressure and water content have a mutual inhibition effect on Ev. Besides, both the peak value of normal force and the fluctuation range of tangential force for the TBM cutter decrease with the increase of the water content. The AE counts show that the failure pattern of mudstone is mainly shear failure during TBM cutter indentation. Specifically, many shear cracks are generated in the compression zone under the cutter, while few tensile cracks propagate in mudstone on both sides and deeper below the cutter. For mudstone under different confining pressures, the proportion of shear cracks increases with the increase of water content. Moreover, the effect of cutter penetration velocity on mudstone fragmentation characteristics and cutter force is discussed. The results in this paper have theoretical guidance for TBM construction of soft rock tunnels in water-rich and wet strata. A FDEM moisture diffusion-fracture coupling model is first applied to study the effect of water content on rock fragmentation by TBM cutter.Shear failure is the main failure pattern of mudstone during TBM cutter indentation.The fragmentation zone and cutter force are greatly affected by water content during TBM cutter indentation.Water content and confining pressure have a mutual inhibition effect on the vertical extent of fragmentation zone.