Blast evaluation using high speed video camera : a case study in Jharia coalfield

Typically a production blast lasts on an average 1 to 2 seconds. The event can be broken down into parts with duration ranging from a few hundred microseconds (the passage of explosively induced stress waves through the rock). through several tens of milliseconds (the detonation of individual bore holes through use of delay initiators), to a few seconds (the distribution of the resulting fragments into the muck pile). To assess the results of a production blast, it is necessary to obtain quantitative information on the size of fragments in the muck pile and the rate and velocity at which fragmented rock material is thrown away from the bench face. Thus, visualization of this small period is very important from the point of view of blast design which can be well taken care of by using highly sophisticated and fast monitoring instrument like high-speed video camera. In our experiment we used high speed video camera as the major tool The high speed video system comprising the video camera, color monitor, video cassette recorder, editor and color video printer was used to monitor and analyze a number of production blasts in BCCL. The camera used for the recording of blast events has a maximum shutter speed of 1/8000 seconds and the play back system provides a clear display of 50 frames per second i.e. a pause of 20 milliseconds between two consecutive frames. Encouraging results were obtained in respect of the relationship between the burden velocity and fragments size as well as the shape of muck pile profile. A burden velocity in the vicinity of 6 m/s was found to give best fragmentation under the given condition. The flying velocity and angle of flying for various rock fragments were also found out and found to vary from 13.16 m/s to 27.96 m/s and 30 to 135° respectively. The results, of course, need to be established more precisely by recording larger number more of blasts in future.