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In this study, we address the complex geological conditions encountered during tunnel boring machine (TBM) excavation, particularly the challenges posed by soft and hard composite strata. To overcome limitations of conventional numerical methods for analyzing rock fracture in such strata, we employ Peridynamics to simulate rock fragmentation induced by disc cutters. We propose a virtual disc cutter loading scheme that enhances computational efficiency by replacing traditional contact models. Through numerical examples, we demonstrate the efficacy of this loading scheme in simulating rock fragmentation within soft and hard composite strata. With this method, we conduct numerical studies on kerfs-assisted rock fragmentation in such geological conditions, finding that kerfs can help mitigate differences in cutter forces between soft and hard rocks, thus showing potential for improving TBM performance. Our findings offer insights into optimizing TBM operations in challenging geological environments. [ABSTRACT FROM AUTHOR]

• Reveal the formation mechanism of complex non-plane fractures near the wellbore. • The fracture initiation and propagation mechanisms of multiple hydraulic fractures are investigated. • Hydraulic fracture propagation is affected not only by stress shadow but also by inclined well orientation. • Stress shadowing effect in staged fracturing results in a large deflection angle of fractures when the horizontal principal stress difference is small. Multi-cluster fracturing in horizontal wells is an important stimulation technology for unconventional tight reservoirs. The interference among multiple simultaneously growing hydraulic fractures results in a non-uniform fluid distribution into fractures and non-planar fracture geometries. In this paper, the competition mechanism between perforations under the stress shadow, the formation mechanism of complex non-plane fractures near the wellbore, and the initiation and propagation mechanism of multiple hydraulic fractures are investigated by using the 3D lattice method. Also, the effects of fracture geometry, fracture spacing and rock mechanics parameters on the stress interaction among fractures are analyzed by the dislocation theory. The results showed the complexity of fracture initiation near the wellbore and the low efficiency of the perforation initiation in the case of spiral perforation, due to a strong interaction between perforations. However, these initial micro-cracks will eventually coalesce to be a macroscopical fracture which perpendicular to the direction of minimum principal stress. The study indicated that stress interaction and fracture propagation is significantly affected by inclined well orientation. The hydraulic fracture is more likely to deflect and propagate non-uniformly with the increasing width of hydraulic fracture, the decreasing spacing between perforation clusters and the increasing rock modulus. When the horizontal principal stress difference is small, it is obvious to cause a large deflection angle and sometimes furcation of fractures. This study can provide a theoretical basis and technical guidance for hydraulic fracturing design in tight reservoirs. [ABSTRACT FROM AUTHOR]

The free-surface-assisted rock fragmentation caused by TBM disc cutters is capable of overcoming the difficulties brought by abrasive or high strength hard rock stratum in tunneling practices. In this work, Peridynamics is proposed to study the phenomenon of free-surface-assisted rock fragmentation caused by disc cutters. Peridynamics can avoid the drawback of finite element methods in simulating complex rock fractures in continuum, and the inconvenience of discrete element methods whose parameters are often hard to calibrate. Through numerical examples, we demonstrate the capability of Peridynamics method in simulating rock fragmentation with only two calibration parameters. With the Peridynamics framework, we have captured the whole process of rock fragmentation caused by disc cutters, and we also study the size variation of 3D rock fragments with different free-surface height and distance from cutter to free surface, and the influences of spacing and loading styles of double cutters on fragmentation patterns. The paper provides a new and reliable numerical approach for modeling free-surface-assisted rock fragmentation, and can be used in optimizing the size of rock fragments, and designing cutter spacing of double disc cutters in the tunnel engineering. [ABSTRACT FROM AUTHOR]