Earthquake and Blasting Damage
Blasting is the technique used to break rock sufficiently so it can be excavated and removed. To accomplish this, drills are used to create a pattern of boreholes distributed evenly throughout the rock to be shattered. When explosives are detonated, they release huge amounts of energy in the form of heat, sound, and shockwaves (ground vibrations). The shockwaves and the force exerted on the rock produce vibrations in the rock and earth surrounding the blast site. The vibrations created can be compared to waves created when a rock is dropped into a calm lake: the waves spread out in all directions and gradually decrease as the distance from the source increases. Eventually, the vibrations completely die out – a decay phenomena known as attenuation.
Mine and construction blasting has been a heavily studied industry in regards to its adverse effects on people, buildings, and the overall environment surrounding the immediate area of the blasting site. Given this research, regulatory agencies have been able to generally agree upon and adopt several conventional criteria for restricting, regulating, and predicting the effects of mine and construction blasting.
In regards to blast-induced ground vibrations, most regulations limit the magnitude of the vibrations, which are measured as peak particle velocity (PPV). Scientific instruments such as seismographs, the same instruments used to measure the magnitude of an earthquake, are typically utilized to record blast vibration magnitudes. Commercial seismographs are capable of recording ground vibrations so small that they are unable to be perceived by humans or vibrations so large that they cause widespread destruction. However, these PPVs do not represent distances that the ground moves, but rather the speed at which the ground vibrates. Even during the strongest blast vibrations, the actual distance that the ground vibrates is only a few thousandths of an inch, less than the thickness of a sheet of paper, and not visible to the human eye. Given this important consideration, PPV is chosen as a unit for measuring blast-induced ground vibrations because it is more closely related to damage potential (for residences) than acceleration or actual ground displacement.
In regards to regulations, a magnitude of 2.0 inches per second (IPS) is the agreed-upon value of maximum allowable vibration magnitude at the habitable structure nearest the source of the blast. This does not suggest that damage occurs if the 2.0 IPS limit is exceeded, only that the threshold limit is conservative to avoid most damage. There is a great difference between these limits and the vibration necessary to cause structural damage. The use of the term “damage” is somewhat ambiguous as related to general ground vibrations. Damage to residential structures from values around the 2.0 IPS threshold is usually limited to cosmetic distresses to weak interior building surfaces and a general nuisance to persons sensing the vibrations. For comparison, damages to concrete structures such as competent foundations may first occur when PPV values approach around 10.0 IPS. However, on the opposite end of the spectrum, research has shown that houses with weak interior plaster surfaces may first sustain damage to these materials when PPV values exceed 0.75 IPS for frequencies below 40 Hz. .
Vibrations from blasting can be readily perceived by nearby residents. Research has shown that approximately 5 to 10 percent of people will judge peak particle velocities levels of 0.5 to 0.75 IPS as unacceptable. Additionally, vibration magnitudes below 0.5 IPS have been documented to cause hanging objects to swing, and be readily perceivable by people at rest.
| Median IPS |
Human Response |
Object’s Response |
Structure’s Response |
| 0.14 |
Not Felt |
|
|
| 0.26 |
Felt by Persons at Rest |
|
|
| 0.46 |
Duration Estimated |
Hanging Objects Swing. |
|
| 0.85 |
Jolt |
Objects Swing. Windows, Doors, Dishes Rattle. Glasses Clink. |
Walls and Frames May Crack |
| 1.56 |
Felt Outdoors. Sleepers Awakened. |
Liquids Disturbed. Objects Displaced or Upset. Doors Swing. Shutters, Pictures Move. Pendulum Clocks Affected. |
|
| 2.85 |
Felt by All. Many Frightened and Run Outside. |
Windows, Dishes Broken. Items Off Shelves. Pictures Off Walls, Furniture Moved, Overturned. |
Weak Plaster and Masonry May Fail. |
| 5.2 |
Difficult to Stand. |
Furniture Broken. |
Cracks in Masonry. Weak Chimneys Break at the Roofline. Fall of Plaster and Loose Bricks. |
| 9.5 |
|
|
Partial Collapse. Stucco, Non-Structural Walls, and Chimneys Fall. Houses Moved from Foundations. |
| 17 |
General Panic. |
|
Masonry Destroyed, Structural Wall Failure, Serious Damages, Structures Off of Foundations. |
| 32 |
|
|
General Destruction. Large Landslides. Soil Failures, Railroad Rails Slightly Bent. |
| 150 |
|
|
Pipelines Out of Service. |
| 300 |
|
|
Damage Nearly Total. Objects Thrown. |
True damage by vibration has a unique pattern. The first type of damage experienced by a structure is at the weakest material, such as the sheetrock or plaster. Large expanses of walls and ceilings act as a diaphragm, similar to the head of a drum: the centers of such diaphragms are the most elastic, and move to the greatest degree when vibrated. The edges of the diaphragms are tightly confined and tend to move less. Excessive vibration will initiate X-shaped cracks near the center of the wall or ceiling as the brittle material is moved.