Studies on Seismic Energy of Ground Vibrations due to Blasting based on Signal Processing and Electrical Energy generation

Abstract

Blasting may be considered as the most crucial process in opencast mines. It is, therefore, important for mining engineers to understand the effect of blast design parameters on the results of blasting. Blasting operations in mines and quarries always result in ground vibrations, which are of major environmental concern. In general, a meager percentage of total explosive energy is utilized in rock fragmentation process, while the rest is wasted. Wasted explosive energy manifests in the form of various environmental effects such as ground vibrations, air over pressure and fly rock (Dowding, 1985). Ground vibrations caused by blasting cannot be totally eliminated, yet they can be minimized through a suitable blasting methodology. Substantial amount of research associated with identification of ground vibrations and assessing the blast performance in terms of intensity of ground vibrations has been carried out, so far. Nonetheless, very little research has gone into seismic energy and utilizing this energy in understanding the performance of blasts. Modern tools like high speed videography and seismic energy analysis reveal many aspects of fragmentation process, which otherwise are difficult to visualize and understand (Sastry, 2015). In the current research study, an attempt was made for the assessment and estimation of seismic energy dissipated into the ground due to blast induced ground vibrations at different distances from blast site. Studies were carried out in three mines having hard limestone formation, one soft limestone mine formation, one underground coal mine formation, two sandstone formations, and five quarries of hard granite rock formation. Initial studies were carried out by determining the geotechnical parameters influencing the propagation of ground vibrations in the laboratory, using the samples collected from mines and quarries of respective formations. Later, altogether 116 ground vibration events in hard limestone formation, 37 ground vibration events in soft limestone formation, 86 ground vibration events in an underground coal formation, 43 ground vibration events in sandstone formation, and 94 vibration events in granite formation were recorded resulting from various blast rounds using ground vibration monitors. Further, digital signal processing computation was done using Advanced Blastware and DADiSP software for all ground vibration waveforms. Mostof the blasts studied were recorded using High Speed Video Camera of 1000fps capacity for analyzing the blast dynamics. Multiple regression analysis was carried out for assessing the influence of Maximum Charge/Delay, Scaled Distance, Distance, and PPV on seismic energy. Also, ANOVA analysis was carried out for estimation of seismic energy with given blast design parameters using MATLAB. An attempt was made to tap electrical energy from blast induced ground vibrations using the Piezo-Generator (Piezo-Gen) circuit. Validation of Piezo-Gen circuit was done by comparing its output (generated voltage) with the vibration data obtained from geophones. It was evident from the results that the working of developed PiezoGen circuit is appropriate and analogous with vibration monitors. The developed Piezo-Gen circuits were placed adjacent to the seismographs at different short to long range distances to tap electrical energy from ground vibrations. In total, electrical energy was tapped from 66 blast induced ground vibrations in limestone formation, 36 in coal formation, 41 in sandstone formation and 94 in granite formation. Electrical voltage tapped from the blast induced ground vibrations during studies was used for running low powered VLSI systems as ambient power source. The tapped electrical energy was correlated with the PPV and seismic energy. Additionally, numerical modelling was carried out as a parametric study for predicting the seismic energy component resulting from a given blast. Altogether, 98 models were developed using SIMULIA Abaqus / CAE interface. Among them, 28 models are in limestone formation, 14 models are in coal formation, 15 models are in sandstone formation and 41 models are in granitic rock formation. Typical size of each developed model after running the job was upto 3.71GB in limestone formation, 461MB in underground coal formation, 6.02GB in sandstone formation and 5.47GB in granite formation. Each model job run took upto 8-27hrs for completion, in different rock formations. SIMULIA Abaqus based Finite Element Analysis (FEA), with both Python Scripting and Graphic User Interface (GUI) was used to estimate the magnitude of ground vibration intensity (PPV) resulting from a given blast. Additional parameter observed during a blast in the simulated models of four formations was stress components at integral points. Validation of results obtainedfrom developed models was done by comparing with the field results by carrying out three dimensional regression analysis. A proper correlation (>75%) between seismic energy and scaled distance was observed in all four rock formations. Also, from the regression analysis made, an excellent correlation (>90%) between seismic energy and electrical energy was observed in all formations. It indicated the possibility of assessing seismic energy dissipated by ground vibrations with the electrical energy generated by the developed Piezo-Gen circuit. From the numerical modelling analysis, higher stress values were observed at lower distances from blast location indicating dissipation of greater seismic energy. Also, PPV was found to increase in proportional to the distance in all four formations. From the three dimensional curve fitting analysis made among PPVs resulting from modelling study, PPVs obtained in field investigations, and electrical voltages obtained from Piezo-Gen circuit, a very good correlation between the modelling results and seismic data generated from vibration monitoring and electrical data generated from piezo electric generator was observed. Study indicated that the working of Piezo-Gen circuit in tapping ground vibrations is as accurate as traditional ground vibration monitors.