Numerical prediction of crater blasting and bench blasting

1. IntroductionThe methods of crater blasting and bench blasting are widelyapplied in mining, quarrying and civil construction excavations.The efficiency of such operations depends on the knowledge ofcrater blasting and bench blasting. Crater blastings are distin-guished from bench blastings because in crater blasting the freeface is perpendicular to the borehole, whereas in bench blasting,at least two free faces are presented—one parallel to the boreholeand one perpendicular. Hence, cratering is a much more confinedblasting situation than benching.The processes of rock fracture and fragmentation around aborehole are strongly dependent on the parameters of theexplosive detonation and the dynamic response of rock, asdemonstrated in field experiments [1–4]. It is essential toimplement both experimental study and numerical study. Theexperimental study could generate an experimental database, andthe numerical study could simulate the processes of rockfragmentation through the use of numerical models. Currently,theoretical study on rock blasting is relatively difficult due to thecomplicated process of blasting and the response of rock. Mainlythe focus of theoretical study has been on the propagation of thepre-existing cracks under the gas pressure loading [5,6], andlesser attention has been paid to the initial cracks resulting fromstress wave loading.In rock blasting, it is generally agreed that two types ofloadings, i.e. stress wave (or shock wave) loading and explosiongas pressure loading, operate on the surrounding rock [7–9]. Thestress wave loading arises out of detonation of the explosivecolumn in a borehole. Generally, the detonation pressure exertedon the borehole wall at the moment of initiation can exceed10GPa. This high pressure on the borehole wall sets off a shockwave in the adjacent rock mass, but it soon decays to a highamplitude stress wave propagating at the velocity of longitudinalwave in the rock mass. It is immediately followed, albeit at a muchreduced velocity, by the longer duration gas pressure loading[10,11]. The stress wave initiates cracks around the borehole, andthe gas penetrates into these cracks and causes their furtherextension and propagation. This can be confirmed from aphotograph (a visual evidence) obtained from crater blasting tests[12], which clearly showed that cracks or fractures are formedbefore cratering occurs. Thus, both loadings playan important rolein the process of cratering and benching.This study will focus on the stress wave loading because thecracking process under stress wave loading is considered thecrucial stage as the nature of all subsequent crack extension,branching and coalescence would be largely governed by theinitial crack patterns generated by the stress wave loading. Thesehave very important bearings on control and predictions offragmentation of rock in actual blasting.Many researchers [13–25] have implemented numericalstudies on rock blasting, using various numerical codes andmodels to simulate the process of rock fracture and fragmentationin blasting. Zhu et al. [26,27] have developed dynamic numericalmodels through the use of AUTODYN code [28,29]. Three basicfracture zones, i.e. crushed zone, severely fractured zone andincipiently cracked zone around a borehole, as well as thecircumferential spalling cracks have been successfully simulatedin the blasting process. The fracturing mechanism under blastingloading has been analyzed, and the factors that influence rockfracturing have been discussed. This paper will continue theprevious study, with a focus on crater blasting and bench blasting.The objective of this study is to simulate the process of rockfracture and fragmentation in crater blasting and bench blasting,so as to obtain a better understandingof the dominant parametersthat control the results of crater blasting and bench blasting. Inorder to minimize the various variables and the associateduncertainties of the explosive and rock, an explosive, TNT, and arelatively homogeneous igneous rock, diorite, are employed. Usingthese models, the rock fracturing mechanism is analyzed, and