FDEM modeling of the fracture, fragmentation and collapse process of rock masses with pre-existing joints by smooth blasting during deep tunnelling

In this study, an in-house combined finite-discrete element method (FDEM) code is further developed and applied to investigate the fracture, fragmentation and collapse process of rock mass induced by smooth blasting during the excavation of a deep-buried tunnel with the presence of high in-situ stresses and complex pre-existing joints. The simulation approach in this study enables the modeling of the transition from continuous to discontinuous behavior of rock, the explosive-rock interaction via gas pressure variation and simplified gas flow model, and the incorporation of pre-existing joints with various lengths, dip angles, spacings and connectivity rates. The blasting-induced stress redistribution and the corresponding rock fracture, fragmentation and collapse process on site are simulated, which vividly replicate the formation of smooth tunnel sidewalls and the occurrence of rock collapse at the tunnel crown area incorporating pre-existing joints. The simulated rock collapse extent at the tunnel roof corresponds well with that from field observations. Furthermore, with the setup of various modeling cases, the effects of in-situ stresses, joint distributions and initiation time delay errors on the fracture propagation behavior in the excavation damaged zone (EDZ) are investigated in detail. The complex mechanisms regarding the critical roles of these three factors in the fracture propagation and EDZ development process are highlighted. The FDEM code in this study is expected to serve for the prevention or mitigation of the potential rock collapse induced by blasting excavation under the combined effects of in-situ stresses, blasting operation and geological structures.