Investigation of the effect of in-situ stresses and loading rate on blasting induced fracture propagation

Rock blasting process is very complicated and is affected by many factors such as internal and external factors. Internal factors are related to controllable parameters such as blasthole and bench dimensions and explosive parameters. External factors are uncontrollable and in blast design must be determined precisely. These factors such as magnitude and direction of in situ stresses, rock properties, ground water and rock jointing dominate the amount of fractures induced by an explosive. In underground blasting, the effect of in situ stresses is more sensible and extent and direction of fractures depend on theses stresses. On the other hand, the blast loading rate (i.e. applied pressure waveform) which depends on explosive properties and also blasthole dimensions are controllable and affect the pattern of different fracture zones around blasthole. Therefore, to control overbreak and to aim desired breakage of rock mass, all parameters specially two mentioned parameters should be considered and measured carefully. For this purpose, two dimensional distinct element code (UDEC) was used to study fracture initiation and propagation process in a rock domain. Mohr-Columb material model was employed to allow rock failure. The analysis consists of two different parts. At part one, the in situ stress effect on fracture pattern is investigated. The results of conducted analysis demonstrate that magnitude and direction of these stresses have significant effect on the amount and direction of blasting induced fractures. The main fractures are intended to more propagation in the direction parallel to main stress and in directions which are perpendicular to major stress, there are a few long fractures. In the second part, the effect of stress waveform on fracture initiation and propagation was examined. The main features in this part were the dimensions of fracture zones generated around blasthole and the length of major fractures. The results indicate that higher stress-loading rate increases the number of radial cracks, thereby releasing intense magnitude of stress around the running cracks. At lower stress-loading rates, the number of cracks and crack arrest caused by stress released at adjacent cracks were reduced. This led to longer crack extension. These analyses showed that when the preferential cracks developed earlier, crack extension was longer.