Drilling and blasting operation has been widely adopted for underground excavation of deep tunnels in hard rock. In this process, the rock mass is broken by the blasting-induced loads. The formation and evolution of blasting-induced damage zone around tunnel is, therefore, inevitable due to the initiation and propagation of cracks. There exist several factors affecting the outcomes of blasting. For instance, the effects of in-situ stresses on the results of tunnel blasting in deep hard rock have not been well investigated yet. In this study, the smooth blasting in the bench of TASQ tunnel at the Äspö Hard Rock Laboratory (HRL) is modelled by means of a powerful GPGPU parallelized hybrid finite-discrete element method, from which the rock fracture and fragmentation process, the stress redistribution induced by blast wave propagation and gas expansion, and the damage zone around the tunnel are analysed. The proposed method is characterised by the simulation of the transition from continuum to discontinuum in the surrounding rock mass, the decay pattern of blasting pressure exerted on the blasthole wall, and the gas expansion through the blasting-induced cracks. The numerical simulation results indicate that the in-situ stresses play an important role in the fracture propagation pattern around the tunnel. This study reveals the importance of understanding the mechanism of the blasting-induced fracture propagation under the influence of in-situ stresses in order to minimize the evolution of damage zone around a tunnel in practical drilling and blasting operations.

1. Introduction

Drilling and blasting is regarded as one of the most efficient approaches for breaking rock masses and for tunnelling in hard rocks at deep depth. However, in this process, the rock damage zone around tunnel due to the blast loading and in-situ stress redistribution is unavoidable, which draws attention in engineering fields. Considerable efforts have been dedicated to investigating these mechanisms of breakage in hard rocks. For instance, a number of field investigations in the Hard Rock Laboratory (HRL) in Sweden and the Underground Research Laboratory (URL) in Canada were conducted by some researchers (Ouchterlony, 1997; Martino and Chandler, 2004; Olsson et al., 2004; Kuzyk and Martino, 2008), from which the rock damage patterns around tunnel under various conditions were observed and assessed. Moreover, in the past few decades, many studies chose to implement numerical simulation approaches in modelling and analysing the blasting-induced rock damage behaviour around tunnels and underground excavations. The rock displacement, stress state redistribution, and damage expansion around a tunnel induced by blasting can be simulated using finite element method (FEM) and finite difference method (FDM) (Saiang and Nordlund, 2009; Wang et al., 2009; Yang et al., 2017). To further investigate the discontinuous behaviour of rock during the blasting process, distinct element method (DEM) and discontinuous deformation analysis (DDA) have been adopted by a few researchers (Cai, 2008; Jonsson et al., 2009; Saiang, 2010). However, the transition from continuum to discontinuum in rocks and the corresponding fracture and fragmentation process during blasting process are worth studying, considering their importance in revealing the internal mechanism of blasting-induced rock damage. In terms of numerical simulation, hybrid finite-discrete element method is capable for replicating such complex mechanisms of dynamic fracturing in rock masses.

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