The main objectives of controlled blasting techniques are to minimize the overbreak and fracturing of rock beyond the designated boundary of excavation to achieve smooth post-blast surface and to control wild flyrock and ground vibration within permissible limits.
This objective is normally achieved by minimizing and making judicious use of explosives in the blast holes. Techniques such as line drilling, pre-splitting, smooth blasting, and cushion blasting are carried out to achieve controlled blasting.
Types of Controlled Blasting
1. Line Drilling
- Line drilling is carried out in a single row of closely spaced, unloaded, small diameter holes that are drilled along the excavation line.
- This provides a plane of weakness to which the primary blast can break and, to some extent, reflects the shock waves created by the explosion, reducing the shattering and stressing in the finished wall.
- Line drilling is best suited for homogenous formations where bedding planes, joints, and seams are at a minimum.
- The only place where it is applicable is in areas where even the light explosive loads associated with other controlled blasting techniques may cause damage beyond the excavation limit.
- The pre-splitting blasting involves a single row of holes drilled along the excavation line.
- Pre-split in the rock forms a discontinuous zone that minimizes or eliminates overbreak from the subsequent primary blast and produces a smooth-finish rock wall.
- Pre-splitting is also used to reduce ground vibration in some critical cases.
3. Smooth Blasting
- This method is widely accepted for controlling overbreak in canal, underground headings, and slopes.
- In smooth blasting, the holes are drilled along the excavation limits, lightly loaded with well-distributed charges, and fired at the end of the round.
- The holes for smooth blasting should be fired instantaneously or with minimum delay to achieve a shearing action and smooth walls with minimum overbreak.
- Smooth blasting and pre-splitting techniques differ mainly from the line drilling principle in that some or all of the holes are loaded with relatively light, well-distributed charges of explosives.
- The smooth blasting is also referred to as contour blasting or perimeter blasting.
4. Cushion Blasting
- Cushion blasting is like smooth wall blasting, a single row of holes is drilled along the excavation line, loaded with light, well-distributed charges, and fired after the main excavation is removed.
- In cushion blasting, the charged holes are further decoupled by reducing the diameter or using stemming material of crushed stone or sand to provide a crushing effect.
- This “cushions” the shock from the finished wall as the holes are detonated and minimizes the stresses and fractures in the finished wall.
- This technique is rarely used today because the reduction in decoupling could be achieved by the use of small diameter explosive cartridges, which serves the same purpose.
- The holes are blasted using the last delay number in the same blasting round, preferably with a jumping delay of 50 ms.
The suitable parameters for controlled blasting are established through trial blasts. Usually, it needs to establish the optimum hole spacing and the charge per hole.
When an explosive charge is detonated inside a blast hole, it is instantly converted into hot gases, and the expanding gases exert intense pressure on the blast hole walls. A high-intensity shock wave travels through the rock mass, which attenuates sharply with distance.
A major portion of the explosive energy passes beyond the fractured zone in the form of elastic ground vibrations. As seismic waves travel through the rock mass, they generate particle motions, which are termed as ground vibrations.
Even though the use of explosives has unwanted side effects in the form of vibration, explosives provide an inexpensive source of energy for rock excavation in mining and civil engineering projects.
Vibrations from blasting are transient, but the disturbance may result in permanent damage to property/structure. Thus it is crucial to control the adverse effects of blasting.
1. Factors Affecting Vibration
When a controlled blast is fired, the vibration level is controlled by two principal factors, distance and charge size. Obviously, it is safer to be far away from a blast than to be near it.
2. Safe Limits of Vibration
Various codes and standards have been prescribed for ground vibration limits in different countries. Some have so little that urban blasting is prohibited altogether, while others have more than the regularly allowed 50 mm/s maximum peak particle velocity at high excitation frequencies.
The recent trend is to refer to the frequency of the ground motion. Low-frequency waves cause more damage to structures, particularly in the case of multi-storied buildings. For low-frequency ground vibration, the safe level of vibration from this curve is 12.5 mm/s.
3. Vibration Control Procedures
The most common method of controlling ground vibration is by minimizing the charge weight per delay. Delay blasting permits to divide total charge into smaller charges, which are detonated in a predetermined sequence at specified intervals. Blasting without delay or sufficient delay numbers increases ground vibrations due to an increase in maximum charge per delay.
The vibration can be significantly reduced by optimizing blast design parameters. It is suggested to establish optimum burden, hole spacing, powder factor, and hookup to control vibration.
FAQs on Controlled Blasting
The main objectives of controlled blasting techniques are to minimize the over break and fracturing of rock beyond the designed boundary of excavation to achieve smooth post-blast surface and to control of fly rock and ground vibration within permissible limits.
The types in which controlled blasting is carried out are-
1. Line drilling
3. Smooth blasting
4. Cushion blasting
Line drilling is best suited to homogenous formations where bedding planes, joints, and seams are at a minimum.
Smooth blasting is widely used for controlling overbreak in canals, underground headings, and slopes.