Fractal analysis of limestone damage under successive impact by shield disc cutters

In tunnel constructions, when a shield machine crosses karst areas characterized by soft upper and hard lower layers, the presence of complex fractures and heterogeneities within the limestone significantly affects the tunneling efficiency. The debris produced during shield tunneling contains abundant information that can provide comprehensive feedback on ground conditions and shield machine operational status. Therefore, it is crucial to investigate the microscopic mechanical behavior of limestone under successive impact of shield disc cutters. To address this, a scaled disc cutter model was designed and placed in a dynamic impact mechanical test device for rock. Different successive impact pressures were applied to three groups of limestone to simulate the limestone-breaking process of shield disc cutters under realistic working conditions. Based on fractal mechanics and fracture mechanics theories, a fractal fitting function for crack propagation and failure during successive impact limestone-breaking by the disc cutter was established. The relationship between the fractal dimension of limestone fragmentation, microstructural parameters, and energy absorption was analyzed. The results demonstrated that the fractal dimension of limestone fragmentation predominantly ranged from 1.30 to 1.56, exhibiting clear fractal characteristics. When the fractal dimension approaches 1.5 or its increase reaches 20 %–27 %, it is crucial to promptly implement enhanced support measures to effectively prevent surrounding rock collapse. Moreover, there is a positive correlation between the fractal dimension of limestone fragmentation and the cumulative specific energy absorption. When the cumulative specific energy absorption reaches 60 J·cm⁻³ and the fractal dimension approaches 1.5, it is recommended to reduce the cutterhead pressure and advancement speed, and to conduct preventive maintenance on the cutting tools. These findings provide valuable insights for enhancing the efficiency of shield tunneling operations.