Fumes from Blasting Operations
Fumes from Blasting Operations (From SLP-17)
Blasting operations produce toxic and nontoxic gases as a normal by-product regardless of the types of explosive materials used. Normally, prevailing winds or air currents readily dilute and dissipate to the atmosphere any gases generated in open pit blasting or outdoor construction blasting. However, there have been reports of incidents where carbon monoxide (CO) from outdoor blasting operations has migrated into underground enclosed spaces. Emergency responders reportedly detected CO at concentrations that approached or exceeded the threshold limit values (time-weighted-average of 25 ppm) established by the American Conference of Governmental and Industrial Hygienists (ACGIH).
In the incidents noted above, it appears that a unique set of circumstances combined to contribute to high concentrations of CO measured in the underground enclosed spaces following the detonation of the explosive materials. These circumstances included-
- The blasts were located very close to the underground enclosed spaces. Five of the blasts were within 20 to 50 feet of the underground enclosed spaces, three were 100 to 150 feet away, and one was nearly 500 feet away.
- The blastholes were stemmed to 50 percent or more of their depth. On firing the blasts, very little vertical displacement of the overburden and essentially little or no venting of the gas from the explosives occurred.
- In each case, excavation of the blasted material did not take place immediately after blasting, but remained in place.
- The underground enclosed spaces had some type of opening(s) through which the gas could readily enter. These entry points were typically drainage slots in floating slabs, pits for sump pumps, and floor drains. Unsealed entry points for new or existing utility lines also served as conduits and places of entry.
- In each case where persons were adversely affected by the CO, monitoring of the underground enclosed spaces for CO did not occur until after the toxic effects of the CO were felt.
Shots where blastholes are drilled though clay or dirt overburden and shot with the overburden in place to minimize vertical movement, control displacement, and prevent flyrock, may not allow venting of explosive gases from the blasted area and can cause accumulation of these gases in the broken rock below the surface of the ground. This condition may have little adverse effect when operations are remote and no dwellings are nearby the operation. Blasters should be aware that under certain blasting or geologic conditions, gases may migrate and collect in the basements of adjacent buildings or in nearby underground locations such as manholes, sumps, or tunnels.
In addition, in deep trench blasting or the blasting of access ramps where there is no vertical displacement of material and no venting of blastholes, gases from the blast may be trapped in the unexcavated rock. The gases may remain trapped until excavation of the shot rock. Blasters and excavators may encounter detonation gases in deep narrow trenches or while digging ramps-blasted-to-full-depth to grade. Employers should make operating employees aware that entrapped gases may be present in deep, narrow trenches or ramps where the underlying rock strata has been blasted with the overburden in place or the blast holes have been loaded to minimize swell or displacement of the blasted material. Where necessary, monitoring and/or venting practices to detect and eliminate entrapped gases should be employed.
To minimize any hazardous exposure from the gases produced by outdoor blasting, it is essential that the blaster -
- Be aware that lack of ground displacement may prevent venting of the blasted material and result in the entrapment of gases.
- Excavate blasted material as soon as possible after blasting. Excavation should start as close to the underground enclosed space as possible in order to provide for venting of any entrapped gases.
Additionally, it is recommended that the blaster -
- Be aware of and look for geologic pathways for CO such as old trenches, horizontal partings, faults, joints, hillseams, unconsolidated material, water, and voids that would allow movement of gas towards underground enclosed spaces.
- Be aware that when blasting very close to underground enclosed spaces, fractures caused by the detonation may create a pathway for the gases.
- Conduct a preblast survey to determine any possible problem areas when blasting near inhabited buildings or underground facilities (tunnels, manholes, etc.).
- Monitor possible problem areas to determine if any gases have migrated from the blasting operation.
- If gases are detected, use adequate and positive ventilation (open windows and exhaust fans) to limit the accumulation of gases at inhabited buildings or other facilities away from the blasting operation until the gas is removed from the ground.
- Keep accurate and complete records of all blasts. Blast records should include the names of the blaster-in-charge and crew, the exact blast site location, the weather conditions, site-specific loading and geologic data, vibration compliance data, a sketch of the blast site, and the blaster's signature.
Drilling monitoring holes between the blasting operation and the inhabited building or other area of concern can detect the movement of CO from the blast site. However, these monitoring holes, even on close spacing, may not intersect the geologic pathway and therefore may not allow detection of CO. These holes will not provide adequate passive venting of migrating gases. If located too close to the blast, these holes may actually create a hazard by allowing blast gases to rifle up them and create flyrock.
One technique has successfully and quickly removed migrated CO from the ground. This technique involved applying negative pressure to the earth and removing the CO from the ground surrounding the underground enclosed space. Even rudimentary systems involving placing a fan on top of a vertically buried large-diameter pipe with holes drilled in the side have worked.
Miners' concern over fumes when they used explosive materials for underground blasting goes back well over 100 years. Special efforts such as using low fume producing products and providing fresh air ventilation minimize miners' exposure to gases generated by underground blasting. For underground mining operations, explosive materials with IME Fume Class 1, 2, or 3 ratings can be ordered from an explosive supplier. Fume Class 1 explosives are recommended for use in poorly ventilated areas such as dead headings and blind-raises. Explosives complying with the requirements of Fume Class 2 and Fume Class 3 may be used if adequate ventilation has been provided. No explosives other than those rated in Fume Class 1, 2, or 3 should be used underground.
The Mine Safety and Health Administration (MSHA) regulates ventilation for underground metal and nonmetal mines in 30 Code of Federal Regulations (CFR), Part 57, Subpart G. MSHA regulates ventilation for underground coal mines in 30 CFR, Part 75, Subpart D. The Occupational Safety and Health Administration regulates ventilation for underground construction work in which blasting may be involved (including, shaft sinking, tunnels, caissons and cofferdams) in 29 CFR, Part 1926, Subpart S. These regulations cover monitoring and control techniques.
Originally approved by the IME Board of Governors December 2-3, 1993, amended August 23, 2001.
The above document is a good reference for dealing with NOx fumes from surface blasting operations.