Blasting

INTRODUCTION

As the country develops, the need increases for more roads, for improved roads, for commercial and residential buildings, and for more mining and quarrying, which, consequently, increases the need for more blasting. Blasting is essential to fracture large quantities of rock in order to more easily move the rock.  Breaking rock with hydraulic equipment, though much more accurate, is a slower and more labor-intensive process. As a consequence, this greater use of explosives increases the number of blasting claims reported to insurance companies. Even though advances in technology allow blasting companies to better control and predict the effects of a blast, the damage claims reported to insurance companies continue to grow. Extensive studies by the United States Bureau of Mines demonstrate that proper use of explosives rarely causes damage to nearby property. Generally, close investigations of insured properties show this is the case.

INVESTIGATION

Just as with a laboratory experiment in which the Scientific Method is used to determine proper and scientifically supportable conclusions, a similar method is needed to evaluate a claim of blasting damage. The first step in such an investigation is to inspect the property and record the damage. Photographs and sketches should be made of all damage.  Start by looking for the most fragile objects first, since they would be the ones most likely to exhibit damage from blasting.   Observe small loose items that could fall from a shelf, which would be the first items to be moved by vibrations.  If they have not moved, this is information that should be noted and photographed.

Next, record and photograph any cracks in the plaster or gypsum wallboard and look for cracks in the foundation. Plaster and gypsum wallboard are the most fragile building materials, and they exhibit cracking and other damages first.  Drywall and plaster will crack before masonry. These are known as threshold materials, in that they display damage and the lowest levels of vibration, or the low threshold of vibrations. These cracks should be closely examined with a magnifying glass in order to determine the age of the crack. Old paint, stains, cobwebs, mold, or debris found inside the cracks can often give an indication of the age of a crack, since any debris inside the crack postdates the forming of the crack, of course.  

In addition, observe the patterns of the cracks. When blasting vibrations encounter the foundation of a house, they move it rapidly up and down, back and forth, and sideways. At the same time, the upper part of the building attempts to remain at rest. These opposing actions cause diagonal stresses in both directions on the walls of the house. When the vibration is more intense than the flexural capacity of the wall material, the walls will crack. The resultant cracks will be opposing diagonal cracks typically at 45 degrees to the horizontal.  The stress in the walls can also cause like stresses and cracks in the ceilings. It is important to know, however, that cracks with similar patterns can result from other causes such as settlement, and it is sometimes difficult to determine if the damage was pre- existing. Excessive blasting will typically cause the extension of existing cracks before it creates new ones.

The next step in the data collection portion of the investigation is to contact the blasting company, who can be extremely helpful if they are willing to cooperate. A good blasting company will likely have a pre-blast survey of the building being investigated. The purpose of a pre-blast survey is to record the condition of the building prior to the blast, specifically focusing on any pre-existing damage. Typically, a pre-blast survey will consist of photographs with written comments. Recently it has become a common practice for the pre-blast inspector to videotape damage and verbally comment on the condition of the building.

The blasting company may also be able to provide blasting records and seismograph data. Seismographs record peak particle velocity (PPV), which is the speed in inches per seconds that earth particles move when the blast wave passes. Studies by the United States Bureau of Mines have determined that PPV is the best method to determine if the vibrations resulting from a blast have the potential to cause damage to a building. Blasting reports will also contain other helpful information such as the date and time of a shot, the material blasted, the type of explosive, the total amount of explosive, the number and depth of holes, the maximum weight of explosive per 8 ms delay, the location of the seismograph, and the seismograph reading. More information results in a better conclusion.

TYPES OF DAMAGE

There are basically three types of damage that result from blasting. They are damage from flyrock, damage from airblast, and damage from ground motion. Flyrock is flying rock and debris propelled into the air by the force of the blast. Flyrock can result from a poorly designed blast or from unexpected zones of weak rock.  On the rare occasions when excessive flyrock is produced, rock the size of a basketball can be hauled into the air. When flyrock damage occurs, it is easily recognizable. Airblast, which is often referred to as air overpressure, is the airborne shock wave produced by an explosion. There is general agreement among blasting experts, consultants, and government regulatory agencies that if any damage results from airblast, it will first be broken window glass. On the rare occasion when there is minor structural damage such as cracked plaster, it is always accompanied by broken windows. The data related to airblast generally is related to nuclear blast or catastrophic explosive accidents. Generally, flyrock and airblast result from a poorly designed blast or an accident. They are seldom the reason for blasting damage. When blasting damage is found, ground motion is usually the cause.

Ground motion is caused by the energy of an explosion that sends pressure waves traveling through the earth and along the surface of the earth, which cause ground vibrations or ground motions.  Vibrations along the surface of the earth and near the blast will travel as surface waves. The earth particles in the surface wave are moved or displaced from their normal position and rate at which this displacement occurs is called the particle velocity. As previously mentioned, studies by the United States Bureau of Mines show that particle velocity is the best indicator of ground motion and limiting particle velocity is the best way to control damage potential. Recommended safe blasting limits for low frequency ground vibrations, less than 40 Hz, are a maximum particle velocity of 0.50 in/sec for plaster and 0.75 in/sec for gypsum wallboard. For high frequency ground vibrations, greater than 40 Hz, the recommended maximum particle velocity is 2.0 in/sec for plaster and gypsum wallboard.

MISCONCEPTIONS

There are several common misconceptions about blasting that influence and mislead property owners. One is the belief that the damage is directly related to noise; the louder the blast the more damage that has likely occurred. The real effects of blasting are often psychological; the shaking of the building, the fright of injury, and the startling effect of an unanticipated blast are psychological factors that are not related to the actual damage to a structure. Property owners that experience these effects will often inspect their buildings more closely than usual and sometimes mistakenly identify pre-blast defects as blasting damage. The fact is, there is no relationship between loudness and damage.

Another misconception is that repetitive small blasts will cause property damage. This is untrue. Studies by the United States Bureau of Mines show that minor damage resulting from human activity and temperature changes in a house is equivalent to ground motion up to 1.2 in/sec. The threshold of human perception of a blast is recognized by experts to be a PPV of 0.03in/sec. Complaints seldom occur below a PPV of 0.08 in/sec. However, even at a PPV of 0.25 in/sec which is well within the safe blasting limits, complaints can be expected.

CONCLUSIONS

Typically, there is never a single blasting claim. It seems that either there are no damage claims or that everyone in the neighborhood reports for damage. It is important not to make the wrong decision when addressing a blasting claim or it can quickly become a difficult and unpleasant situation for everyone involved. Good judgment and a good investigation are necessary to make the right decision.