Post-blast examinations in mines
Learn about the legal requirements and best practices for post blast examinations.
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To assist technical experts in mining operations and the workplace parties with understanding the legal requirements in Regulation 854 – Mines and Mining Plants under the Occupational Health and Safety Act (OHSA) regarding post-blast examination procedures for mining workplaces.
This resource does not replace the Occupational Health and Safety Act (OHSA) and its regulations and should not be used as or considered legal advice. Health and safety inspectors apply the law based on the facts in the workplace.
This guideline is meant to:
- provide guidance and information to minimize and control the exposures of all workers to blast contaminants and other related hazards, especially those workers conducting post-blast gas examinations.
- increase awareness of adequate processes and controls so that workers, particularly those who conduct post blast examinations, are protected using safe work procedures.
Sections of Regulation 854 that apply
Sections 260, 254, 253, 152, 141.1, 141, 121, 74, 68, 64, 5, 5.1, 5.2, 5.3 of Regulation 854 – Mines and Mining Plants under the Occupational Health and Safety Act (OHSA) cover the important requirements.
On the surface
Sections 152,142,141.1, 141 5, 5.1, 5.2, 5.3 of Regulation 854 cover the important requirements.
Ontario workers have been killed and injured during post-blast clearing in underground mines. Workers risk exposure to post-blast gases and their associated hazards if suitable controls are not in place to dilute or clear contaminants. It is important that each mine comply with the applicable requirements of the OHSA and Regulation 854 and follow their written blasting procedures to protect the health and safety of workers during blasting activities. For all types of blasts and blasting situations, these procedures can include post-blast examination to protect workers from contaminants [section 260].
Serious injuries or fatalities can occur when post-blast examination teams are not properly trained or when workers are allowed to come into contact with post-blast gases due to inadequate controls, procedures, signage and barriers. These events can be avoided by complying with all applicable requirements and following best practices to ensure that work areas are free from blasting contaminants and have sufficient oxygen in the atmosphere before workers are permitted re-entry [clause 253(1)(a)].
Blasting hazards and potential controls
Airborne blasting contaminants
Blasting can generate large amounts of blasting contaminants, which are made up of gases such as carbon monoxide, nitrogen dioxide, oxides of nitrogen, sulphur dioxide, ammonia and hydrogen sulphide. Operations that use polyvinyl chloride (PVC) pipe to encase long holes, primarily due to fractured ground, may also experience levels of benzene gas. The airborne hazards produced by different blasting powders and agents can vary widely, depending on the manufacturer of the explosive and the type of wrapper used.
The volume of gases produced by explosives will vary depending on conditions such as:
- the hardness of the rock
- the water content of the holes
- the amount and type of explosives used
In high sulphide ore bodies, a secondary sulphide dust explosion may occur after the primary blast, generating large quantities of gases such as sulphur dioxide.
Workers underground risk exposure to post-blast gases if adequate controls are not in place to dilute or clear contaminants. Workers conducting post-blast examinations could be exposed to high concentrations of blasting contaminants.
The inhalation of airborne blasting contaminants can produce a variety of severe symptoms and biological effects, including irreversible tissue damage, which can interfere with normal body functions and may be fatal.
Mines must have measures in place to ensure that workers are not over-exposed to blasting contaminants after explosives are used.
Mines should establish procedures to conduct post-blast gas examinations safely, especially where high concentrations of airborne hazards are expected.
Dangerous ground conditions resulting from blasting
The use of explosives during blasting generates vibration, concussion and a seismic response, which can compromise ground support. This can create potential hazards such as:
- falls of ground
- loose ground
- damage to ground support such as screen, bolts, rebar and shotcrete
Workers conducting post-blast examinations could be exposed to the hazards of rock movements resulting from the blast and potential seismic events.
Following the use of explosives during blasting, workers could be injured or killed due to exposure to compromised ground conditions or ineffective ground support systems.
Where ground conditions or seismicity are likely to be expected as a result of the blast, mines should establish isolation and re-entry procedures to minimize the risk of exposure to workers who are conducting post-blast examinations.
Barricades used to prevent access to or isolate hazards in affected areas can be damaged or made ineffective as a result of blasting.
Following blasting, workers could access areas that had been previously isolated or barricaded from hazards including open holes, poor ground conditions or blast contaminants.
Workers could be exposed to adverse health effects, serious injury or death after inadvertently going into a hazardous area that had been previously barricaded.
Where required, barricades must be used isolate areas where there are known hazards such as loose ground, explosives, open holes, etc. Signs must also be used to warn workers of these areas. The location of barricades should always be marked or identified on plans. Procedures should be in place to ensure that all barricades remain effective after blasting.
Lack of visibility
Blasting operations can result in smoke and dust that interfere with visibility. The hazards associated with limited visibility may include exposure to loose ground, open holes, open brows, large concentrations of blast contaminants and overhead hazards, such as low hanging or broken pipes, that have resulted from the blast.
The general work force is not typically faced with the hazard of poor visibility after a blast. However, if the mine uses post-blast examination teams, these workers may experience a lack of visibility while conducting work to clear a mine of blast contaminants.
Workers on post-blast examination teams and others who have to work where there is limited visibility may be injured due to limited ability to see physical hazards or become ill because of blast contaminants.
Mines must comply with all applicable requirements related to visibility and should have procedures in place to ensure that workers do not encounter the hazards associated with low visibility.
Examples of blasting procedures
Many mines specify in their blasting procedures that effective barricades able to withstand the blast must be installed at the entrance to all overcuts and undercuts before a blast, as is required under sections 68 and 74 of Regulation 854. Other common procedures include:
- only allowing authorized workers who have the permission of the supervisor in charge of the workplace to proceed past a barricade for the purpose of making repairs or examining the workplace for hazards
- installing barricades or otherwise preventing worker access to open raises, holes and stopes following a blast
- ensuring that any barricades used are capable of withstanding the forces created by the blast
- coordinating blasting times where blasting at one mine may be a danger to workers in adjacent mines. If there is a disagreement, the owners or employers shall jointly determine blasting times as required by subsection 141(3) of Regulation 854
- ensuring that, before initiating any blasting, all other workers have left the workplace or the vicinity except those required to assist in blasting and guarding, and taking necessary precautions to ensure that all areas of the mine affected by the blasting are vacated, as required under clauses 141(1)(b)(ii) and (iii) of Regulation 854
- specifying which fans near the blast should be left on or turned off, which doors in the line of the concussion are to be left open and whether air-water sprays are to be installed and turned on in workplaces to be blasted.
- ensuring that, when explosives other than Fume Class 1 are used, no worker is exposed to fumes that endanger their health or safety as per section 121(b) of Regulation 854.
Many explosives and all primers currently in use exceed Fume Class 1. Refer to the latest “List of Authorized Explosives" as published by the Explosives Regulatory Division of Natural Resources Canada.
General post-blast procedures
To assist in complying with section 260, each underground mine should have procedures to ensure that contaminants have been removed or that the ventilation systems have rendered them harmless before workers are permitted to re-enter work areas. These procedures should be in writing and cover blasts and blasting situations.
Employers must take steps to limit the exposure of workers to hazardous substances and ensure that exposure to blasting contaminants does not exceed the legal requirements set out in Regulation 833 under the OHSA.
Threshold limit values (TLVs) should be adjusted in consideration of the workers’ shift length. This limit will be dependent on the type of gas, concentrations, exposure and the capability of the mine’s ventilation system to render contaminants harmless.
The procedures to protect workers may vary depending on the purpose of the blasting. For example, production blast procedures may be different from those for regular development or secondary blasting.
Depending on the mine and the location of the blast, the hoist operation procedures should account for the possibility of encountering contaminants while the conveyance is in operation.
The oncoming shift must be instructed by their supervisors to wait in designated areas (surface, refuge stations etc.), until the “all clear” has been given by the supervisor in charge of ensuring that the blast contaminants are cleared.
Some mines establish a “wait time” between shifts that is sufficient for the blasting contaminants to be cleared, diluted or removed naturally or by the mechanical ventilation system. This period of time is usually determined at the mine based on experience and calculation.
The affected areas must be checked by competent persons to ensure that contaminants have cleared, and the workplace is safe to re-enter.
Changes to the ventilation system may affect the length of the “wait time” following a blast, which should be re-evaluated and, if necessary, adjusted to ensure a safe workplace environment.
Procedures for specific types of blasts
These blasts use in excess of 1000 kg (2200 lbs.) of explosives in a single blast. Typical industry practice for production blasts includes the use of a procedure where a trained post-blast examination crew conduct an examination to ensure the area is clear of airborne contaminants and other hazards following the blast.
Development blasts are used to advance mine workings. Development rounds use far less explosives than production blasts. Mines should ensure that, through a combination of procedures, measurement, records and past experience, the blasting gases are cleared to prevent workers from being inadvertently exposed to gas and contaminants when re-entering the workplace.
Mines often use small blasts, typically referred to as secondary blasts, for various purposes. Typically, these blasts are used to resize large muck usually associated with stopes, chutes, waste and ore passes. Procedures for carrying out these tasks should be developed at the mine. Workers must not be allowed in areas affected by blasting contaminants. In some mines, the material that is to be blasted may be moved directly to an exhaust airway, which will remove the blasting contaminants or render them harmless before a worker can be exposed.
There are many situations that may require a mine to use a different post-blast examination procedure. For example, if an auxiliary ventilation system used in a development heading has multiple fans set in series but is damaged by the blasting such that it cannot effectively clear the contaminants, an alternate procedure would need to be used.
Post-blast examination teams
A “Post-Blast Examination Team” (PBET) generally refers to those competent workers who check to ensure that the contaminants produced as a result of blasting have been removed or rendered harmless by the ventilation system.
In operations that use PBETs, there should be procedures dealing with the responsibilities of:
- the PBET itself
- persons debriefing the PBET
Employers that use PBETs should provide those workers with:
- copies of up-to-date ventilation plans for all the levels affected showing the information required in subsection 253(2) of Regulation 854
- if required or applicable, scaled prints showing the blast area(s), last row of holes to be loaded, possible location of open holes, open brows, location of barricades, fans, etc.
- pre and post-blast documents showing ventilation controls, including which fans were to be left on or turned off, which doors are to be left opened, etc. This procedure should also specify how the ventilation is supposed to be restored after the blast has taken place
- specific procedures detailing respiratory protection and information about occupational exposure limits if the PBET is required to intentionally travel or work in blasting contaminants
The supervisor or his/her designate must review the hazards likely to be encountered by the PBET, and review documents and plans with the team members. These may include:
- copies of up-to-date ventilation plans for all the levels affected, showing the information required in subsection 253(2) of Regulation 854
- if required or applicable, scaled prints showing the blast area(s), last row of holes to be loaded, possible location of open holes, open brows, location of barricades, fans, etc.
- pre- and post-blast documents showing ventilation controls, including which fans were to be left on or turned off, which doors are to be left opened, etc. These documents should also specify how the ventilation is supposed to be restored after the blast has taken place
- the locations to be sampled and contaminants to be checked to ensure the blasting gases have been removed
- any special procedures which may indicate what testing equipment or communication devices are to be used, what respiratory protection is to be worn and what vehicle, if any, is to be used, and when
- specific procedures detailing respiratory protection and contaminants limits if the PBET is required to intentionally travel or work in blasting contaminants
- when a PBET is required to go underground and clear the workings
- the maximum time limit the PBET is to be underground, giving consideration to shift schedules, the type of work to be performed and the expected time to complete the work
The supervisor should also review communication procedures with the PBET, which may include the following:
- the supervisor or designate should maintain direct communication with the PBET and should be the only person to communicate with them until such time as the clearance has been completed
- the supervisor or designate should keep a written record of communications and gas test/measurement results
- clear methods of communication should be established and maintained during gas checks, including the frequency of time for contact between the PBET and the supervisor
If required for post-blast examination, the supervisor is responsible for ensuring that the team is trained in the use of and has thorough knowledge of the following:
- respirators or self-containing breathing apparatus
- gas testing equipment
- communication methods and systems used, and location of phones and other communication devices used throughout the area being checked
- the normal function of the ventilation system, fans, doors, etc. in the area being checked
- the location of the area blasted
- post blast procedures for the mine and any specific procedures for the area blasted
- any vehicle that is used during blast clearance
Post blast examination team responsibilities
Prior to going underground, the PBET should pre-check their respirators, instrumentation, radios and any other equipment that is being brought along (barricades, signs, vehicles, etc.).
In many mines, the PBET is also expected to:
- perform testing in all areas affected by blasting contaminants
- monitor until these contaminants have been diluted to acceptable limits
- restore the ventilation system where possible and if trained and equipped to do so, by, for example, turning on fans, reconnecting vent tubing, closing doors, etc.
- install barricades with warning signs to warn workers of any new hazards created by the blast, such as loose ground, unventilated areas, open holes, etc.
- report to the supervisor or designate the written record of communications and gas testing/measurement results
- inform the supervisor or his/her designate that the mine has been cleared of gases, or if required, that areas have been barricaded because of high gas concentrations or unsafe conditions
Debriefing of the post blast examination team
After the task has been completed, the supervisor should discuss with the PBET any concerns encountered during the clearing process.
If a potential or actual danger to the health or safety of a worker has not been remedied at the end of the shift, the supervisor must make a written record of the dangerous condition and any corrective actions taken as per the requirements in section 64. This record could include any ground condition problems that were noted and any blast damage. The supervisor must advise all affected workers about the dangerous condition (clauses 64(2)(a) to (c)) and should assign workers to correct any problems noted by the post-blast examination team.
If there is a danger or hazard to a worker, the area must be closed by barricades, fencing or other means and warning signs must be posted (as per section 68 of Regulation 854).
The need for respiratory protection must be reviewed as part of the post-blast examination procedures. Ventilation systems that have been disrupted or damaged by blasting have caused PBETs to be exposed to blasting contaminants. Mines should consider the options available to decide which form of protection would be appropriate for the PBET in any given situation. Workplaces should be aware of, and comply with, the requirements for respiratory protection set out in Regulation 833.
Some mines use oxygen-generating self-rescuers that are provided to the PBET. All self-rescuers are designed by the manufacturers for escape only. Team members must retreat when encountering levels of blasting gases above the limits set out in the mine’s procedures. This is normally any gas concentration in excess of a Short-Term Exposure Limit (STEL).
Some mines provide their team members Self Contained Breathing Apparatuses (SCBAs). Team members wearing SCBAs can travel through gases if conditions at the mine allow for safe travel to start fans, open/close vent doors and do other limited short-duration work that may be required.
Some mines set a maximum distance or time that the team may travel from the nearest source of fresh air based on the capacity of the SCBAs.
Filter-type self-rescuers do not cover all types of contaminants and should not be used for protection against blasting gases.
Workers requiring respiratory protection must be trained in its use, as per Regulation 833.
General re-entry procedures for all workers
Before re-entering the mine, trained and qualified personnel should confirm that the shaft is free of contaminants. With ramp entry mines, PBET workers can retreat if they encounter contaminants; however, in a shaft they could be lowered into contaminants with no means of retreat.
Workers should approach their workplaces by using a known fresh air route, noting such things as the functioning of auxiliary ventilation systems, the integrity of ventilation tubing and the possible presence of blasting contaminants or other hazards such as ground conditions resulting from the blast.
Should hazards, including contaminants, be encountered or suspected, the workers should be trained to retreat to a safe area until a thorough check is done by a competent person(s) to ensure the hazards have been removed.
Use of mine rescue teams
The post-blast procedures should provide clear guidance about when the PBET and the supervisor(s) in charge of the blast should use or obtain the assitance of mine rescue teams.
Where the blasting contaminants cannot be removed, and/or work must be carried out in elevated concentrations of blasting contaminants, a mine can conduct this reestablishment work, providing they properly equip, train, and take all the necessary precautions for the protection of workers.
If mine rescue equipment is used, a notice must be given immediately to the mine rescue officer and the inspector, as required by subsection 17(8) of Regulation 854.
Workers must be trained in re-entry procedures specific to the workplace. Where post-blast examinations are not conducted because they are not necessary in the circumstances to protect workers, the Joint Health and Safety Committee (JHSC), health and safety representatives and workers should nevertheless be trained in the recognition of blasting contaminants. Additionally, workers must be informed about where to seek solutions to concerns or answers to questions related to exposure.
Note: General knowledge of "toxic gases" and "oxygen deficiency" and the safety precautions to be observed is part of Module U0001 of the Common Core for Basic Underground Hard (or Soft) Rock Mining, required under section 11 of Regulation 854.
Joint health and safety committees
Joint health and safety committees, health and safety representatives and other appropriately experienced workers are useful resources to mines while developing safe and practical solutions for post-blast gas clearing. A key factor to workplace health and safety in Ontario is the Internal Responsibility System (IRS). The effective use of the IRS promotes a strong health and safety culture in workplaces where parties can come together to ensure a safe and healthy workplace.