Blasting is one of the most popular methods of excavation in underground construction which is cost-effective and has the ability to demolish structures.
However, uncontrolled blasting can lead to adverse effects such as excessive fly-rock or over breakage, rock mass loosening, damaged rock support and tunnel failures.
Stress waves that are produced when explosives are detonated propagate through the medium as ground vibrations inducing blast damage to rocks and structures exceeding the limit of stress.
Types of Blasting Damages
The following types of physical damage which are typically seen in a blasting event are discussed below:
1. Fly-Rock Debris Impact
Fly-rock debris is soil and rock material that become airborne missiles that fly away at the time of detonation following the principle of projectile motion in its entire trajectory.
Fly-rock debris causes damage close to the blasting spot and generally, the impact comprises rocks on the side or top of the structure.
Penetration and embedding of the rock material can be seen when severe damage occurs and the structure contains wood or other soft material on the exterior.
Window glass can crack or break from the rock material and debris. Sometimes, it may happen that the detonation of explosives takes place at a surface which creates a crater by a dynamic charge detonated at the surface.
Knowing the distance above the surface, the actual charge can be placed and with an appropriate scaling factor, the amount of charge can then be determined by proportionality: the intensity of a blast decreases by the cube of the distance from the blasting spot.
2. Air Concussion Damage
Blasting at a construction site is done mainly to break a rock and efficient breaking requires that all the energy from the explosive be directed at the rock to be fractured.
However, as per the second law of thermodynamics, even in an efficient blast some amount of the total energy produced is used by the rapid expansion of gases and converted or expended to heat and air shock that propagate away from the blast site and decrease in intensity as the distance increases.
The intensity of air concussion can be measured by over pressure, also known as gage pressure, which is the amount of pressure in excess of the usual ambient pressure.
In a detonation blast, the rapid expansion of gases causes the over pressure or air concussion wavefront leading to a highly sharp pressure spike and this sharp over pressure front contains the primary destructive power of an air concussion from the blast.
As the over pressure front moves, it behaves similar to the spring-mass damped vibration system where a low negative pressure bounce appears pushing and throwing things down and taking debris along with the pressure front until the negative pressure subsides and everything comes to rest.
3. Air Shock Wave Damage
Air shock waves move over a significant distance from the epicentre and cause minor damage to structures typically to building windows.
Unlike the damages caused by air concussion, this type of damage is characterized by the fact that the damaging wave has skipped over a large span of the area causing no damage to that area.
The occurrence of air shock damages depends on two factors.
Firstly, a major explosion has to occur because the intensity of the blast must be large enough for the over pressure front to remain significant over a long distance.
Secondly, an atmospheric inversion layer at a considerable distance from the center where cold air is trapped over warm air.
4. Ground Vibrations
A huge amount of energy is released in a blast to fracture rocks and displace earth which a great fraction transmits through the earth in the form of vibrational waves that radiated away from the center.
Blasts generate majorly two types of ground vibrations:
- Longitudinal or P waves which cause sinusoidal-like displacements in the direction of a travelling wave radially away at the speed of sound from the blast center and consists of alternating elastic compression and tension of the matter through which the wavefront passes
- Shear or S waves which cause sinusoidal-like displacements transverse to the direction of travelling wave through the ground at a velocity of little more than half the speed of P waves. P waves are least important with respect to the damage caused to the surface structures while S waves are responsible for most of the damage to the structures. Collectively S and P waves are called body waves.
Ground vibrations originating from a blast site and travelling to nearby structures are essentially transmitted by shear waves or “S” waves.
Hence for a structure to be affected by shear waves, the surface between the structure and the blast site must be continuous with no significant breaks in the surface or changes in composition.
“S” waves cannot jump through the air from one side of a discontinuity to the other.
Drastic changes in surface composition can also change the way “S” waves behave. Fluid or similar materials rapidly dampen the transmission of “S” waves.
Practically, lakes, rivers, wet sand, marshes etc. can be considered to be non-transmitting media for “S” waves which quickly dampen them.
Damages typical of blasting ground vibrations usually follow a “top-down” pattern of severity, where the most severe damages occur at the top of the structure and the least severe damages occur at the bottom.
If the claimed damages do not follow this “top-down” pattern of similarity, it's probably they were not caused by nearby blasting work.
During the blasting demolition of high-rise buildings in metropolitan regions, the influence of vibrations on surrounding structures is considered to be one of the most important factors.
The ground motion caused by blasting demolition is usually a combination of blasting vibration, backlash vibration and touchdown vibration and it could damage nearby structures if its amplitude is sufficiently large.
Case Study
Jackhammer work was done across the street from a neighborhood bar and grill in a Midwestern city.
New pavement was laid after the old one was broken up. The bar and restaurant were located in an earlier structure with stucco over bricks.
The stucco on the front outside wall of the bar and grill split from the brickwork and fell off the building onto the pavement in front of it just as the street work was being done.
There was no other harm. The jackhammer vibrations loosened the stucco, causing it to slide off the structure, according to the building owner.
Even though the street work was mostly finished, it was decided to break up a tiny section of the new pavement with the same jackhammers.
The work was to be completed in the same manner as before. A seismograph was set up close to the site on the building where the stucco had separated while this operation was taking place.
As the simulated work progressed, seismograph measurements were taken. The vibrations observed were considerably below those that could have caused any damage to the structure.
The stucco wall was examined more closely due to the low vibrational results. The wall had suffered extensive water seepage from the roof, with the seepage being particularly severe in the area where the stucco had separated from the brick.
The brickwork had no stucco attaching to it, and the stucco had peeled off in one piece, according to observations. The meteorological records on the night before the accident indicated that it had rained.
As a result, it was determined that the stucco had detached not as a result of extreme vibration caused by building equipment, but rather as a result of rainwater eroding the bond between the stucco and the brickwork.
March 20, 2022 - BY SIFS India