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Abstract
Blasting is the most common and economic way to excavate rockmass. In this process, explosive energy is utilised to produce vast quantities of energy instantaneously in the form of shock wave and gas pressure to decrease the strength of the rockmass. So that, the rockmass can be excavated easily. However, uncontrolled propagation of explosive energies result into unwanted damages to the surrounding rockmass beyond the desired excavation periphery. Such types of unwanted damages to the rockmass are called blast-induced-rockmass-damages (BIRD). These damages reduce the rockmass integrity and create problems in drilling for the next blast round holes or the stability of the highwall slopes. Thus, measurement and estimation of such damages are essential for a productive and safe mining operation. The techniques and instruments associated with measuring the damages are varied according to the physical ground conditions. In the surface blasting, the damage on the surface is called 'backbreak’, and it is measured by the physical offset surveying technique. However, the damages which are below the surface level and the minute cracked zone can be assessed using geophysical surveying methods. There are various prediction techniques available to estimate the blast-induced rock damage zones. Such techniques can be classified as vibration-based models, geophysical models, soft computing methods, empirical models, finite element methods etc. Among these, accessible, less computational, and easy to adopt method are vibration-based models. In the vibration-based models, the blast-induced ground vibration is estimated in the form of peak particle velocity (PPV), and then the damage zone is delineated by taking the threshold level of PPV. These models are of three types, namely- (i) extrapolation of farfield measurement, (ii) nearfield measurement and (iii) nearfield modelling.
In this study, two new 3D nearfield vibration-based models are developed to estimate the nearfield ground vibration (PPV). The first model is developed for the single charge column and the other one for the simultaneous blasting of multiple charge columns. The significance of these models is that these can estimate the PPV in any direction and in a 3D space. Three single hole test blast rounds and four multiple hole test blast rounds are designed to establish these
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proposed models. The blast-induced rock damage extents are observed for each blast. The threshold level of PPV for rock damage at the boundary of the damage zones are estimated from the developed nearfield vibration models. Considering the threshold level of PPV for rock damage, damage envelops are created. The accuracy of the delineated damage zones is determined. As these models are site- specific, for determining the damage zone through these models, the site-specific constants have to be determined. These constants are obtained from the regression analysis between the measured peak particle velocity, ݒ௧ (mm/sec) and the square root scale distance (SD) of that site.
The developed damage envelop is also compared with the simulated results obtained from the finite element analysis carried out in Ansys explicit dynamic platform for single hole test blast rounds. Similarly, the developed damage envelop for the multiple hole test blast rounds are compared with the damage zones estimated from multichannel analysis of the surface wave (MASW) method, one of the seismic survey methods, used to study the shear wave velocity pattern of the rockmass. The results obtained from these test blast rounds, clearly shows that the damage envelops obtained from the threshold level of PPV estimated using developed nearfield vibration models are providing the blast-induced damaged zone for single charge column as well as for the multiple hole blast rounds with sufficient accuracy (deviation less than 12 percent). Application of the 3D nearfield vibration-based damage prediction model is also applied to test its applicability in regular production blasting, where more number of holes are blasted with a complex delay sequence. Ten production blast rounds in limestone mine, and ten production blast rounds in overburden benches of the coal mine (Sandstone rockmass) are studied. It is found that the model is predicting the backbreak with a deviation of less than 30 percentage.
Keywords: Blast-induced damage, PPV, Ansys explicit dynamic, MASW, 3D nearfield vibration model, Threshold PPV
