Flyrock: Common Causes and Case Studies
Common Causes of Excessive Flyrock
Overloading of Holes
This practice is especially dangerous and can never be tolerated. The arbitrary increase in volume of explosives in your boreholes, for instance to get rid of old product, is an open invitation to the production of excessive flyrock.
Each shot should be analyzed and prepared with the amount of explosives being based on the particular rock formations, rock type, hole size and depth.
ALWAYS USE DRILLER'S LOGS
Representative powder factors should be maintained at all times.
Insufficient Stemming
Too little and/or poor quality stemming can lead to flyrock. Make certain that each hole is adequately filled, and with a fairly dense material - gravel and/or sand work the best.
Lack of Relief
Insufficient relief can create an excessive upward thrust of the material that is being blasted. Rather than the material collapsing upon itself, it follows the path of least resistance which will be straight up.
Lack of Burden
If the effective distance from a borehole to the free face is too close, severe multi-directional rock fragmentation may occur, creating excessive flyrock. Front row minimum burden distances should be carefully monitored.
Improper Delay Patterns
Irregular hole delaying can render any shot dangerous and can create uncontrollable variables. Proper delay sequences are essential for correct fragmentation and rock movement.
Improper delay patterns can cause rock to:
- Scatter wildly
- Be propelled great distances due to burden or relief distortions
Structural Abnormalities in Geological Formations
Caves, cracks, slips and mud seams are some of the hidden abnormalities that can be incurred by the blaster. These are dangerous because unchecked, they can:
- Alter the direction of the drill bit
- Cause an overloading of product into the holes due to the unseen cavity being filled with explosives, while the blaster thinks that only the standard hole diameter is being filled
Prevention Measures:
- It is extremely important that driller's logs are filled out
- There must be continuous communication between the blaster and the driller
- Measuring the product as it is being loaded is essential
- A weighted measuring tape is a common and effective tool for this purpose
- Loading poles marked with pertinent measurements are also good tools used to check holes
Case Studies: Flyrock Accidents
Case 1: Limestone Mine - Shelter Strike
A surface limestone miner was injured while blasting from a shelter. The shelter located about 60 feet in front and about 160 feet to the side was reportedly struck violently by flyrock.
The blaster was injured by striking the wall or being struck by steel equipment stored in the shelter as it rocked and pivoted.
Incident Result: Serious injury - fractured leg and fractured skull to one miner.
Case 2: Quartzite Quarry - Plant Area Strike
A plant operator's helper was injured by flyrock at a surface quartzite quarry. The blast was located 60 feet below and 650 feet horizontally from the plant.
The contract blaster indicated that flyrock had never before reached this distance. The plant personnel were involved with a repair problem and had forgotten about the scheduled blast.
Investigation Findings: Drilling accuracy problems contributed to the flyrock problem. The drill reportedly wandered while drilling some of the 3-1/2 inch diameter by 39 foot deep boreholes on a 9 x 9 foot pattern.
Incident Result: One person slightly injured.
Case 3: Limestone Quarry - Void Loading Error
A blasting contractor at a surface limestone quarry experienced difficulty in loading one hole. The specified weight of explosive was correct but the required stemming was 23 feet rather than the usual 10 to 12 feet.
Cause: The explosives materials had entered a void.
Flyrock from the shot reportedly struck two employees located about 1050 feet from the face and damaged five autos and two buildings.
Incident Result: Two injuries and extensive property damage.
Case 4: Rhyolite Quarry - Weak Zone
At a surface rhyolite quarry 12 boreholes, four inches in diameter were drilled on an 11 x 11 foot pattern. The shot was loaded with bulk water resistant blasting agents and ANFO and initiated using millisecond delay electric detonators with a rack bar blasting machine.
The 40 foot holes were primed near the top of the powder column and stemmed to the collar with 11 feet of drill cuttings.
Contributing Factor: A weak or weathered zone in the formation that was not compensated for in the borehole loading.
One person was injured and three autos were damaged on a U.S. highway near the mine beyond the property.
Incident Result: One injury and property damage.
Case 5: Granite Quarry - Insufficient Burden (Fatal)
Mine employees were hit by excessive flyrock at a surface granite quarry. The employees were in a surface maintenance shop located about 1700 feet from the blast site.
Blast Details:
- Boreholes: 6-1/2 inches in diameter by about 80 feet deep
- Pattern: 13 x 20 foot staggered pattern
- Bench height: 75 feet
- Subdrill: 6 feet
- Holes angled: 15 degrees
- Each hole was double primed using electric detonators and initiated in pairs using a sequential timer
Primary Cause: Insufficient burden at the base of the shot is suspected.
Incident Result: One fatality and one serious injury.
Case 6: Cast Blast - Video Taping Incident
A flyrock injury occurred at a surface mine. The victim was injured while video taping a cast blast from on top of an ANFO bulk truck that was located about 380 feet directly in front of the face.
Overburden from the 88 foot high bench was displaced over broken material in the bottom of the pit and out of the far side of the pit where the victim was located.
Blast Details:
- 61 holes containing about 53,400 pounds of explosives materials
- Cast an estimated 40 to 50 percent of the overburden
Incident Result: Broken leg and severe lacerations.
Case 7: Buffer Removal Error
A surface mine normally blasted to a buffer consisting of the broken material from a previous blast. Reportedly problems arose with drilling and the buffer was hauled away for production purposes.
Normal Practice:
- 6 inch diameter holes on the 45 foot high face
- Burden of about 15 feet
After Buffer Removal:
- Burden at the side was only 10 to 12 feet
Excessive flyrock up to 2500 feet resulted from the end of the blast. One miner was seriously injured in a building over 900 feet from the blast site.
Incident Result: Serious injury to one miner and extensive damage to vehicles and other mine property.
Case 8: Pit Service Area (Fatal)
After being notified by the blaster that a shot was going to be fired, the foreman drove his pickup truck to the pit service area. The service area was located 280 feet from the blasting area and was 70 feet lower in elevation.
When the shot was fired an undetermined sized ball of mud, clay, and small rocks was thrown into the pit service area where it struck the foreman's truck partially collapsing the truck's roof.
Incident Result: The foreman's head was crushed when the truck roof collapsed. FATAL.
Case 9: Highwall Excavation (Fatal)
A highwall was being excavated for future development of an underground mine opening. The drill operator assisted in preparing the round to be fired.
When the shot was fired, the drill operator was struck by flyrock and fatally injured.
Key Lessons Learned
This document is the property of Maritime Blasting Services Ltd., Moncton, NB