The Influence of Velocity of Detonation on Blast-Induced Ground Vibration
The Influence of Velocity of Detonation on Blast-Induced Ground Vibration
Blasting operations are commonly used in mining, construction, and quarrying to fragment rock and facilitate excavation. However, one of the main issues associated with blasting is the generation of ground vibration, which can have detrimental effects on nearby structures, equipment, and the environment in the form of cracks, instability, and collapse.
Factors Affecting Ground Vibration:
- Charge length and VOD
- Delay time scattering and random fluctuations
- Rock properties and Geological conditions
In order to optimize blasting operations and minimize the effects of ground vibration we need to understand the influence of the Velocity of Detonation (VOD) on blast-induced ground vibration.
What is Velocity of Detonation (VOD) and How does it Affect Ground Vibration?
What is VOD? VOD delineates the speed at which a detonation wave moves through explosive material. Different characteristics of the explosive material, from chemical composition to density, yield diffferent VOD results, ranging from a few thousand meters to kilometers per second.
In order to understand the expected ground vibration from a blast, we must consider the Velocity of Detonation (VOD) or the rate at which the initiation propagates throughout the column of explosives, and the various factors that influence it.
One of those factors, the density of the explosive, has a massive impact on the VOD. When the explosive is ignited it creates a detonation wave, a very rapid wave of chemical reaction, that travels at a stable supersonic speed (the detonation velocity).
Typically, detonation velocities for pressed or cast high explosives range from 4000 - 7500 m/sec. As the detonation wave progresses through the condensed explosive, it converts the explosive within a fraction of a microsecond into very hot, dense, and high-pressure gas as shown in Fig. Pressures immediately behind the detonation front range from 2,700,000 - 4,900,000 psi. These pressures are called Chapman-Jouguet or CJ pressures. These pressures cause the explosives to initiate/detonate at a specific speed throughout the explosive column.
Higher detonation speeds (VOD) generate high explosive energy and increase the detonation pressure (DP) in the wall of the blast hole. High detonation pressure can cause smaller fragmentation sizes, better rock mass movement, and greater muckpile throw & spread, but also comes with increased backbreak, ground vibration, and the dominant frequency of the blasts.
Typically only 10-30% of the generated energy gets utilized breaking the rock and moving it whereas most of the energy gets converted into blast nuisances such as backbreak, flyrock, ground vibration, blast fumes, etc.
Thus, ground vibrations generated by blasting activities can be influenced by the VOD of an explosive. Explosives with higher VOD values typically produce higher-frequency vibrations, whereas lower VOD explosives tend to generate vibrations with a lower frequency content. So, high explosive energy generates high pressure which causes high ground vibration.
The Impact of Low Density Explosives on Ground Vibration
According to Sanchindrian et al. (2007), the use of explosive energy to fragment rock mass is the main objective of blasting, while the main challenge is to maximize the fragmentation energy and reduce the wastage of energy in other forms which can be controlled by optimizing blast design parameters. Moreover, the author Sujit et al. (2020) proposed that low-density explosives reduced blast-induced ground vibration by 40.99 to 42.04% because it generates less PPV due to lower VOD as shown in Fig.
Fig. Relation between distance vs time which shows the VOD of high and low-density explosives
The Impact of VOD and Geological Conditions on Ground Vibration
Blast results depend on matching blast design parameters and explosive with the physico-mechanical properties of the rock mass (called Impedance). Where it shows the density of rock mass * Longitudinal velocity of rock mass should be equal to the density of explosives * VOD of explosives. When the properties and characteristics of rock mass matches with the explosive properties with suitable design parameters, the utilization of energy would be high with less nuisance effect.
Higher rock mass strength needs a high explosive quantity with high density and VOD which directly impacts ground vibration. Conversely, for lower rock mass strength, the lower VOD and explosives density are required.
The Impact of VOD on Ground Vibration
1. VOD increases dominant frequency and PPV, however, its influence is limited to certain distances.
2. Low-density explosives having low VOD and high-density explosives having high VOD is directly proportional to the detonation pressure.
3. A high VOD increases fragmentation and blast movement but the direct correlation cannot be determined.
4. For soft and medium hard rock, use low-density explosives. For hard rock, use high-density explosives.
5. According to the research, an explosive with a higher VOD might cause vibration with a slightly higher frequency than an explosive with a lower VOD (Lonardi, 2021).
Designing Blasts to Minimize Ground Vibration
In order to limit the influence of high VOD multiple factors can be controlled for when designing a blast. These other factors include distance, explosive properties, coupling & decoupling of explosive, and electronic detonator delay timing.
While increasing the distance between a blast and the location sensitive to the ground vibration is one way to control the vibration at that location, it is rarely the feasible variable to manipulate. Instead, it tends to be one of the few conditions we cannot control, like the characteristics of the rock in which we are blasting. This can be designed around however, by effectively manipulating other blast design parameters.
When designing a blast for acceptable levels of ground vibration, parameters such as pre-splitting (smaller blasts around the boundary of the production holes before production blasting) can attenuate vibration at lower distance.
The division of explosives in multiple decks with suitable delay timing can be an effective way to control PPV. In this way the Max Instantaneous charge gets reduced. Reducing the max instantaneous charge is second only to distance in effectively reducing ground vibration and still obtaining good blast results.
Decoupling of explosive by using cartridge explosives, can also minimize PPV at certain ranges because the air gap between explosive and blast hole minimizes the explosive percentage.
Finally, using a precise delay time using an electronic detonator with less than 1% error is an easy step to minimize PPV. By this, the proportion of constructive interference between waveforms can be controlled.
Summary
Velocity Of Detonation is an important property of an explosive which defines the strength or energy of an explosive to fracture rock. But it can highly impact ground vibration if the percentage utilization is imbalanced.
Blasters should always use explosive property (VOD) according to the rock mass and other blast circumstances, not based solely on availability.
References
1. Kumar, S. and Mishra, A.K., (2020). Reduction of blast-induced ground vibration and utilization of explosive energy using low-density explosives for environmentally sensitive areas. Arabian Journal of Geosciences, 13, pp.1-10.
2. Sanchidrian JA, Segarra P, Lopez LM (2007). Energy components in rock blasting. Int J Rock Mech Min Sci 44:130–147. https://doi.org/10. 1016/j.ijrmms.2006.05.002.
3. Leidig, Mark, Jessie L. Bonner, Tim Rath, and Donald Murray (2010). Quantification of ground vibration differences from well-confined single-hole explosions with variable velocity of detonation. International Journal of Rock Mechanics and Mining Sciences 47, no. 1: 42-49.
4. Lonardi, Tommi, (2021). Effect of charge column’s velocity of detonation and location of initiation on blast-induced vibration’s frequency in rock mass, Master thesis.
5. Konya, C. J. and Walters Sr, E. J., (1991). Rock blasting and overbreak control, McLean, VA: U.S. Dept. of Transportation, Federal Highway Administration.
6. Lownds CM, Wallace BW (1986). The performance testing of permitted explosives for coal mines. Journal of the Southern African Institute of Mining and Metallurgy, 86(10):415–423.
7. Sayed-Ahmed, E.Y. and Naji, K.K., 2006. Residential Houses Cracking Due to Nearby Subsurface Construction Blasting: Critical Review of Current Safe Limits. In Civil Engineering Department, University of Qatar, Doha, Qatar, 1st International Structural Specialty Conference, Calgary, Alberta, Canada, Mar (pp. 23-26).
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