Grain-Based Modeling for Heterogeneous Rock Fragmentation Under Stressed Conditions and TBM Cutter Spacing Optimization

The initial stress is a crucial factor influencing the selection of cutter spacing and rock fragmentation efficiency in tunnel boring machine (TBM) operations. This study aims to investigate the cracking behavior at the grain scale and the optimal cutter spacing selection during rock fragmentation with a TBM under different initial stress conditions. A grain-based rock-breaking model (GB-RBM) that considers the actual immersion velocity is proposed to accurately represent the micromineral structure of rocks and realistically simulate rock breaking with a TBM. The microstructural parameters of the GB-RBM were obtained by calibrating the uniaxial compressive strength, tensile strength, elastic modulus, Poisson's ratio, and failure characteristics of granite. The accuracy of the GB-RBM in simulating heterogeneous rock breaking using TBM cutters was verified by comparison with multiple conventional simulation methods and experimental behaviors. A novel concept for the extraction and computation of fragments was proposed, accompanied by the establishment of an improved specific energy ( \({\text{SE}}_m\) ${\text{SE}}_m$ ) parameter that considers internal fragmentation within the rock. Comprehensive analyses were conducted on the fragmentation characteristics, cutting force, acoustic emission, intergranular and intragranular cracks, fragment distribution and area, and \({\text{SE}}_m\) ${\text{SE}}_m$ . The results indicated that the initial stress primarily inhibited the propagation of intergranular cracks during rock fragmentation. The presence of a free face significantly enhanced the rock-breaking efficiency of the TBM, resulting in noticeably better fragmentation than when the rock was constrained on both sides. The selection of the optimal cutter spacing was influenced by the initial stresses. Specifically, the optimal cutter spacing was 30–60 mm at 0 MPa and 70 mm at 10–40 MPa.