Monitoring of large open cut rounds by VOD, PPV and gas pressure measurements

Abstract

A project with the goal to minimize the blast damage to the remaining pit walls has been carried out at the Aitik open pit mine in North Sweden. Factors like confinement during the blast, blast direction and size of blast holes in the contour were systematically changed. During the blasts VOD in the blast holes plus PPV and gas pressure in gauge holes behind the contour were monitored. The VOD measurements were used to check the explosive performance, to obtain the real initiation sequence and to identify the sources of the PPV pulses in the composite acceleration records. The PPV levels were used to establish a scaling law for the test area which later was used for blast damage assessments. The gas pressure measurements show that the ordinary rounds don't force any significant amounts of high pressure blast fumes into the walls, probably because the unstemmed blast holes allow venting of the fumes. In all gauge holes behind ordinary rounds, only subatmospheric pressures were recorded. This is probably explained by a shock wave initiated dynamic swelling movement that opens up fractures. Those fractures that are or become connected with the gauge hole increase its volume and reduce its pressure. The measured underpressures correlate relatively well with the measured residual swell values. The fracture dilation velocity is estimated from the maximum pressure drop rate in the pressure-time records. It lies in the range 0.05–2.0 m/s and it is considerably smaller than the swelling rate as given by the measured PPV-values. Further a shot pre-split line didn't transmit direct shock waves or blast fumes into the walls. The pre-split blast holes themselves though, despite being unstemmed, did however force pressurized blast fumes into the rock. Thus, they may be a source of blast damage.