Chapter Five - Environmental Effects of Blasting

Introduction

There are four environmental effects of blasting:

1. Flyrock
2. Ground vibrations
3. Airblast
4. Dust and gases

In addition to potential environmental impacts on non-mining neighbours, each of these has productivity and occupational health and safety implications. The most serious health and safety hazards are flyrock and gases.

Overview of Effects

• Flyrock: Potential cause of death, serious injury, and property damage
• Ground vibrations and airblast: Potential causes of property damage and human annoyance, but very unlikely to cause personal injury
• Dust and gases: Seldom caused in serious amounts by blasting

Flyrock, ground vibrations, and airblast all represent wasted explosive energy. Excessive amounts of these undesirable side effects are caused by improper blast design or lack of attention to geology. When excessive side effects occur, part of the explosive energy intended for rock fragmentation and displacement is lost to the surrounding rock and atmosphere.

Preblast Survey

When blasting in the vicinity of structures such as homes, hospitals, schools, and churches, a preblast survey documenting the condition of the structures is often beneficial.

Purposes of a Preblast Survey

1. Increases communications between the community and the mine operator

   • Good public relations is the operator's best means of reducing blasting complaints
   • Many companies have found preblast surveys to be an excellent investment

2. Provides a baseline record of the condition of a structure against which the effects of blasting can be assessed

   • When combined with a postblast survey, helps ensure equitable resolution of blast damage claims

Blasting Records

Good record keeping is essential to any successful blasting operation. A blasting record is useful in:

• Trouble-shooting the cause of undesirable blasting results (flyrock, airblast, ground vibrations, poor fragmentation)
• Providing excellent evidence in litigation on blast damage or nuisance

Blasting Record Contents

General Information:

• Date and time of blast
• Company name and location
• Blaster name and licence number
• Distance and direction to nearest dwelling, school, church, commercial, or institutional building

Weather Data:

• Temperature
• Wind direction and speed
• Cloud cover
• Inversions

Blast Parameters:

• Type of material blasted
• Number of holes, burden, spacing, depth, diameter
• Type of explosive used
• Maximum weight of explosive detonated within any 9-ms period
• Maximum number of holes detonated within any 9-ms period
• Total weight of explosives including primers
• Method of firing and type of circuit
• Type and length of stemming
• Whether mats or other protection used
• Type of delay detonators and periods used

Seismic Data:

• Vibration measurements (T, V, L, dB)
• Location of seismograph and distance from blast
• Name of person taking reading
• Name of person and firm analyzing the record

Additional Documentation:

• Sketch of blast pattern including delays
• Sketch of typical loaded hole
• Guard placement
• Reasons for any delays
• Precautions taken for flyrock, airblast, and ground vibrations
• Results of post-blast examination

Flyrock

Flyrock, primarily associated with surface blasting, is the most hazardous effect of blasting. It is a leading cause of onsite fatalities and equipment damage from blasting. Occasionally flyrock will cause serious injury and damage to persons and property beyond the mine limits.

Flyrock distances can range from zero (for a well-controlled coal strip-mine blast) to nearly a mile (for a poorly confined large, hardrock mine blast).

The term flyrock can be defined as an undesirable throw of material. Muckpile displacements of 100 feet are often desirable for certain types of loading equipment. Injuries occurring within the radius of expected muck displacement are not flyrock incidents but failures in blast area security.

Causes of Flyrock

Excessive flyrock is most often caused by an improperly designed or improperly loaded blast:

1. Burden dimension less than 25 times the charge diameter: Results in powder factor too high for the rock being blasted; excess explosive energy results in long flyrock distances

2. Excessively large burden: May cause violence in the collar zone, especially with:

   • Inadequate amount or ineffective type of stemming
   • Top priming (as opposed to center or toe priming)

3. Zones of weakness and voids: Often cause flyrock when explosive follows the line of least resistance and "blows out"

Alleviation of Flyrock

To prevent or correct flyrock problems:

1. Ensure proper burden and adequate collar distance

2. Use effective stemming: 1/4-inch size material makes better stemming than fines, particularly in wet boreholes

3. Consider deck charges: Lengthen stemming zone above main charge and use a small deck charge to reduce flyrock while still breaking caprock

4. Avoid top initiation: Particularly poor practice where flyrock is a problem

5. Use longer delays between rows: On the order of 10 milliseconds per foot of burden (take precautions against cutoffs)

6. Identify zones of weakness:

   • Consult with drill operator
   • Review past experience in the area
   • Check drill logs for abnormal lack of resistance to penetration
   • Place stemming (rather than explosive) in weak zones

7. Check column buildup during loading:

   • Slow buildup indicates voids - stem and continue above
   • Also ensures adequate room for stemming

Protecting Against Injury and Damage

Despite careful planning and good blast design, flyrock may occasionally occur and must always be protected against:

• Measure and record abnormally long flyrock distances for future reference
• Size the guarded perimeter accounting for worst cases
• Post adequate number of guards at safe distances
• Ensure all persons within perimeter have safe cover and are adequately warned
• Remember that warning signs, prearranged blasting times, or warning sirens are seldom adequate for blast guarding
• Blaster should have commanding field of view so shot can be aborted if necessary

Ground Vibrations

All blasts create ground vibrations. When an explosive is detonated in a borehole it creates a shock wave that crushes the material around the borehole and creates many of the initial cracks needed for fragmentation. As this wave travels outward, it becomes a seismic (vibration) wave. As the wave passes a given piece of ground it causes that ground to vibrate (similar to circular ripples on water).

Measurement of Ground Vibrations

Ground vibrations are measured with seismographs in terms of:

• Amplitude (size of vibrations): Usually measured in velocity (inches per second)
• Frequency (number of times ground moves back and forth): Measured in hertz (cycles per second)

Effects of Ground Vibrations

• Excessively high levels can damage structures
• Even moderate to low levels can cause legal claims of damage and/or nuisance
• One of the best protections against claims is good public relations

Causes of Excessive Ground Vibrations

1. Too much explosive energy going into the ground
2. Improperly designed shot - energy not used in fragmentation goes into vibrations

The vibration level at a specific location is primarily determined by:

• Maximum weight of explosives used in any single delay period
• Distance from that location to the blast

Delays and Vibrations

Delays break the blast into a series of smaller, closely spaced individual blasts. These delayed blast segments do not add together (constructively interfere) if:

• Intervals are ≥ 9 milliseconds (for moderate blasts ~45.5 kg per delay)
• Greater separation may be required for large shots (~909 kg per delay), possibly 17 milliseconds

Note: For small, close-in blasts, a smaller delay may give adequate separation. For large blasts at long distances, longer delays are required for true separation.

Overconfinement Effects

A charge with a properly designed burden produces less vibration per pound of explosive than a charge with too much burden. Excessive subdrilling also causes higher vibration levels, particularly if the primer is placed in the subdrilling.

In multiple row blasts, there is a tendency for later rows to give higher vibration levels unless adequate relief is provided. Longer delays between rows (10 milliseconds per foot of burden) can help, but may increase chance of cutoffs.

Delay Timing and Direction

If delays proceed in sequence down a row, the vibrations in the direction that the sequence is proceeding will be highest (snowballing effect).

Vibration Damage Levels

There is no precise level at which damage begins to occur. The damage level depends on:

• Type, condition, and age of structure
• Type of ground on which structure is built
• Frequency of the vibration (hertz)

Recommended Maximum Levels (Bureau of Mines Research):

Complaints About Vibration

People tend to complain about vibrations far below the damage level. The tolerance level depends on:

• Health and fear of damage
• Attitude toward the mining operation
• Diplomacy of the mine operator
• How often and when blasts are fired
• Duration of the vibrations

Measuring Ground Vibrations

Types of Instruments:

1. Peak reading seismographs: Cheaper, easier to use, adequate for regulatory compliance
2. Time history recorders: More useful for understanding and troubleshooting problems

Instrument Configurations:

• Three-component (radial, transverse, vertical): Most common, specified by most regulations
• Vector sum: Measures 10-15% higher; satisfactory for regulatory compliance

Installation Options:

• Operator-attended: Cheaper but requires operator expense; can be moved for specific data
• Remotely installed: Records each blast without sending operator; install in protected locations

Installation Notes:

• When accelerations > 0.3g are expected, secure seismograph to ground (stakes, epoxy, or bolts)
• Where possible, bury the gage in the ground for high acceleration levels

Scaled Distance Equation

Where vibrations are not a serious problem, regulations often permit use of the scaled distance equation:

S.D. = D / √W

Where:

• S.D. = Scaled distance
• D = Distance from blast to structure (feet)
• W = Maximum charge weight per delay of ≥9 ms (pounds)

Example Values:

Techniques to Reduce Ground Vibrations

1. Reduce charge weight per delay:

   • Reduce number of blastholes per delay
   • Use sequential timer or combination of surface and in-hole delays
   • Use smaller diameter blastholes or lower bench height
   • Use delayed decks within each blasthole
   • Presplitting often requires delays

2. Avoid overly confined charges:

   • Don't use excessive burden or subdrilling
   • Don't place primer in subdrilling
   • Skip delay periods between rows if relief is inadequate

3. Increase delay length between charges (especially for large charges at large distances)

4. Arrange delay sequence: Place lowest delay in hole nearest structure of concern; propagate away from structure

5. Blast during high-activity periods (noon hour, after school dismissal); avoid quiet periods

Airblast

Airblast is a transient impulse that travels through the atmosphere. Much of the airblast produced by blasting has a frequency below 20 hertz and cannot be heard effectively by the human ear. However, all airblast (audible and inaudible) can cause a structure to vibrate in much the same way as ground vibrations.

Measurement of Airblast

Airblast is measured with:

• Special gauges
• Pressure transducers
• Wide-response microphones

These instruments are often an integral part of blasting seismographs.

Measurement Parameters:

• Amplitude: Usually measured in decibels (sometimes pounds per square inch)
• Frequency: Measured in hertz

Maximum Recommended Airblast Levels

Note: Different instruments have different lower frequency limits. You need only meet one of these values, depending on the instrument used.

Causes of Airblast

1. Unconfined explosives: Uncovered detonating cord trunklines or mudcaps for secondary blasting

2. Inadequately confined borehole charges: Inadequate stemming, inadequate burden, mud seams

3. Movement of burden and ground surface: When the face moves out, it acts as a piston forming an air compression wave. Locations in front of the free face receive higher airblast levels than those behind.

Airblast and Complaints

Research shows that airblast from a typical blast has less potential than ground vibrations to cause damage to structures. However, it is frequently the cause of complaints.

When a person senses vibrations or experiences house rattling, it is usually impossible to tell whether ground vibrations or airblast is being sensed.

Measuring Airblast

Installation Guidelines:

• Gauge should be 3-5 feet above the ground
• At least 5 feet to one side of any structure (to prevent distortion from reflections)
• Use wind screens to cut background noise and protect microphone
• Protect remotely installed instruments from weather

Monitoring Options:

• Operator-attended: Cheaper but requires operator; more flexible for different locations
• Remotely installed: Records all blasts; disadvantage that loud noises near instrument can induce high readings
• Instruments recording entire airblast history recommended for remote monitoring (non-blasting events can be identified by non-characteristic wave trace)

Techniques to Reduce Airblast

1. Avoid unconfined explosives: Bury surface detonating cord (lighter core loads require less burial depth)

2. Ensure sufficient burden and stemming:

   • Coarse stemming material (1/4-inch) better than fines, especially with water in stemming zone
   • Use longer stemming where burden at crest has been robbed
   • Front row usually creates more airblast than subsequent rows

3. Compensate for geologic conditions:

   • Stem through mud seams, voids, or open bedding
   • Check column rise to avoid overloading solution cavities

4. Drill holes accurately to maintain designed burden (especially important with inclined holes)

5. Reorient or reduce height of high free faces facing built-up areas

6. Avoid collar priming (actually, collar priming is seldom desirable)

7. Avoid firing during:

   • Early morning, late afternoon, or night (temperature inversions most likely)
   • When significant wind is blowing toward built-up areas

8. Use longer delays between rows than between holes in a row (promotes forward rather than upward movement)

9. Avoid excessively long delays that may cause a hole to become unburdened before it fires

Public Reaction to Airblast

Public reaction can be reduced by:

• Blasting during high-activity periods (noon hour, after school dismissal)
• Avoiding blasting during quiet periods

Dust and Gases

Dust

Every blast generates some amount of dust, but the amounts do not present a serious problem. Other phases of the mining operation (loading, hauling, crushing, processing) produce considerably more dust than blasting.

• Well-controlled blasts create little or no dust
• Violent blasts may produce more than normal amounts
• Blasting is a relatively infrequent operation, so total daily dust production is insignificant

Mitigation:

• Thoroughly wet the muckpile before and during muckpile operations
• Allow appropriate time for dust to settle or be expelled by ventilation before miners enter the blast area

Gases

The most common toxic gases produced by blasting are:

Blast fumes are quickly diluted to below toxic levels by:

• Ventilation systems in underground mines
• Natural air movement in surface mines

Safety Precautions:

• Underground: Allow time for toxic gases to be expelled by ventilation before miners enter
• Surface: Wait for a short period before entering the immediate blast area, particularly if orange-brown fumes are present
• It is extremely rare for significant concentrations of toxic gases to leave the mining property

Causes of Excessive Nitrogen Oxides

If large amounts of orange-brown fumes are consistently present after blasts, the source should be determined and corrected:

1. Poor blasting agent mixtures
2. Degradation of blasting agents during storage
3. Use of non-water-resistant products in wet blastholes
4. Inefficient detonation due to loss of confinement

Warning: There have been instances, using highly aluminized explosives underground, of ignitable concentrations of flammable gases in the detonation products.