Date Published: March 22, 2019
Publisher: Public Library of Science
Author(s): Xiaoming Guan, Caixia Guo, Ben Mou, Leilei Shi, Wajid Mumtaz.
The reasonable delay time of millisecond-delay blasting using digital electronic detonators can significantly reduce the vibration effects induced by tunnel blasting. This study proposes a method for calculating the delay time for cut holes, easer holes and periphery holes, considering the rocks breaking effect as well as wave superposition theory. And then according to the actual layout diagram of the tunnel holes, the delay time calculation formulas of different holes are put forward. Then the delay times were calculated according to the formulas and applied in the field tests. The velocities, rocks breaking and wave superposition cancellation of the vibration using different delay times are analyzed with digital electronic detonators. Then the optimum delay times of different holes were obtained and applied to New Hongyan tunnel project. The velocity and frequency of the vibration with digital electronic detonators are analyzed, compared with non-electronic detonators. The effects of charge and delay time on the velocity and principal frequency of a blasting seismic wave are discussed. The results indicate that the delay time for the holes must be prioritized to achieve breaking effects in the rock with the simultaneous formation of a new free surface, next considering the wave superposition cancellation. When the delay time of cut holes was 5 ms, the rocks breaking effect and wave superposition cancellation effect both worked well. The velocity of the vibration induced by the cut holes blasting was about only 0.46–0.51 cm/s. When the delay time was 6 ms or much longer, the rocks breaking effect would fail. With regard to the easer holes and periphery holes, the optimum delay time of them were all 5ms. The vertical peak particle velocity was reduced from 2.974 cm/s to 0.901 cm/s with digital electronic detonators. Therefore, the velocity had decreased by 69.70% than non-electronic detonators, which was caused by reducing the single simultaneous explosive charge and setting optimum delay time. The proposed delay time calculation method is demonstrated to be sufficiently accurate and can thus be used as a guideline to reduce tunnel blasting vibrations.
Complex environmental tunnels that pass through dense buildings are constructed using the drilling and blasting method are becoming increasingly popular [1,2,3]. Tunnel blasting vibration often causes damage to surrounding buildings and affects the daily work and lives of nearby inhabitants [4,5,6,7]. Therefore, reasonable measures must be taken to control the blasting vibrations. The main concept of the millisecond blasting technique is to control the detonation time and ignition sequence of detonators to achieve the expected vibration suppression effectiveness [8,9,10]. At present, non-electric millisecond detonators are widely used in tunnel blasting vibration control; however, such detonators have many limitations. Because non-electric millisecond detonators are detonated at the same time with several holes, the blasting vibration can be reduced by reducing the cyclical footage, using small charges and dividing the blasting. However, this approach reduces the construction efficiency and extends the construction period. When using the millisecond blasting technique, the delay time accuracy of non-electric detonators is rather low, with a delay error of approximately ± (10–150) milliseconds, because non-electric detonators achieve millisecond delay by controlling the chemical burning rate . Therefore, blasting with non-electric detonators causes uncertainties and instabilities in blasting vibration control that prevent waveform interference from achieving the desired vibration reduction.
There are many theories of millisecond delay time calculation for tunnel blasting, such as the method of short delay blasts in quarries considering the breaking effect of rocks , the U. Langefors’ theory to reduce the blasting vibration considering the wave superposition cancellation , and so on. In the previous researches, the breaking effect of rocks and wave superposition cancellation effect were often considered separately, and above both effects were rarely considered at the same time. Besides the calculation method of delay time of different types holes are not the same. The delay time is supposed to determine according to the types of holes. Therefore, the holes are firstly classified into two types: 1) cut holes, 2) easer holes and periphery holes. Considering the rocks breaking effect as well as wave superposition theory, the theoretic calculation methods for above two kinds holes are put forward. And then according to the actual holes layout diagram and construction parameters, the delay time formulas of different holes are studied. Then the delay times were calculated according to the formulas. The several delay times were checked in the field tests. After analyzing the velocity, rocks breaking effect and wave superposition cancellation of the vibration induced by different delay time, the optimum time could be obtained. The block diagram of the method of delay time for tunnel blasting is given in Fig 1.
This study developed a method for calculating the delay time for tunnel millisecond delay blasting using digital electronic detonators and investigated the influence of charge and delay time on the peak velocity and dominant frequency of the blasting seismic wave through a case study of a tunnel blasting project. The following conclusions are drawn from this research: