TRENCH RESCUE HAZARDS:
The cardinal rule for anyone planning to enter a trench or confined space is, "Never trust your senses."
NOTE: Even though it may not be required by law, all guidelines and procedures use during a confined space emergency should be followed during a trench rescue.
The four most common conditions that constitute hazards are:
- Atmospheric
- Mechanical
- Physical
- Psychological
ATMOSPHERIC HAZARDS:
Of all of the hazards that can be found in a confined space, the atmospheric hazards are the most dangerous. Atmospheric hazards in confined spaces cannot be seen and are often fatal.
Oxygen Deficiency, and Oxygen enrichment:
Normal fresh air contains 20.8%oxygen. Any atmosphere containing less than 19.5% oxygen is considered to have an oxygen deficiency, Any atmosphere containing more than 23.5% is said to be enriched and adds to the explosion hazard.
Combustible Gas and Vapors:
Atmospheres may ignite or explode if a source of ignition is present or introduced into the environment. This category of atmospheric hazards includes naturally occurring gases and the vapors of a large group of liquids which are used as fuels and solvents. Some of these liquids evaporate easily. Both gases and vapors, when mixed with the required amount of air and a source of ignition, will burn and explode.
Many flammable gases or vapors are heavier than air. If they flow into a pit, a tank opening, or other confined space, they will present a serious fire or explosion hazard. Disposal of flammable liquids into sewers presents municipal workers with many potentially explosive situations. Gasoline, cleaning fluid vapors, and other natural gases are often found in sewers.
Toxic Gases or Vapors:
Atmospheres containing contaminants, even in low concentrations, can cause injury or death. This category includes asphyxiant and irritants.
One should always anticipate that any one or combination of the above atmospheric conditions might exist in trenches. In addition, hazards sometimes appear unexpectedly in normal situations due to inappropriate industrial waste disposal or leakage of toxic substances into earth strata and conduits.
Toxic substances are commonly found in industry as well as being generated by natural processes. Included are all gases and vapors which are known to provide disease, acute discomfort, bodily injury, or death.
Some gases, in very low concentrations, are mildly irritating to the respiratory and nervous systems. At higher levels they cause death.
Carbon monoxide is one of the common asphyxiant encountered in industry. It is formed by incomplete combustion whenever fuel containing carbon is burned. In addition to being a by-product of many industrial operations, it is produced in large amounts by internal combustion engines such as automobiles, diesel powered compressors, and forklifts. Substances which render the body incapable of utilizing an adequate oxygen supply are called chemical asphyxiant.
Hydrogen sulfide is even more toxic than carbon monoxide. Air containing as little as 100 PPM. or 0.01% hydrogen sulfide may cause death in a few hours. Hydrogen sulfide has a characteristic rotten egg odor. Since the gas renders the olfactory nerve (which controls the sense of smell) ineffective, a worker may be lulled into a false sense of security, not realizing that a toxic concentration of gas may be present.
Hydrogen sulfide is a hazard in many industries not only because of its use in manufacturing, but also because of its accidental occurrences. It is recognized as a major hazard in the oil refining industry, in sewage treatment, and wherever organic matter containing sulfur decomposes.
Atmospheric Monitoring:
Hazard Sector shall do atmospheric testing in the space to determine oxygen level, flammability, and toxicity. Based on readings, Hazard Sector should advise Command of the proper level of personal protective equipment. Any instruments used to monitor the confined space shall have:
1. An audio-alarm.
2. Be calibrated to 10% of the LEL of the calibrant gas.
3. Have the audio-alarm set at:
- O2 19.5% low, and O2 enriched 23.5%
- flammability 10% alarm set
- toxicity carbon monoxide 35 ppm
- hydrogen sulfide 10 ppm
4. Any O2 readings below 12%, Command should recognize that the LEL reading will not be accurate.
5. Hazard Sector shall give Command atmosphere readings at least every 5 minutes with an announcement of offensive or defensive mode (i.e., rescue or recovery).
Ventilation:
Ventilation Sector must consider the effects on the atmosphere that positive or negative pressure ventilation will have (i.e., increase or decrease flammability of atmosphere). It could require both positive and negative ventilation (pushing and pulling). This will be based on the vapor density or molecular weight of the product.
Ventilation Sector may consider negative pressure ventilation if there is only one entry point. Atmospheric monitoring will be required to ensure a non-explosive environment is present in the exhausted vapor area.
Ventilation Sector must also consider the effects the exhaust is having on the operation.
Ventilation Sector should consider the use of intrinsically safe fans when ever possible.
MECHANICAL HAZARDS:
Utilities, including electrical, gas and water should be secured and locked out. If it is not possible to lock/tagout/blankout, Safety Sector shall post a guard to assure the utilities are secured and not turned on during the operation.
Any product that is in or flowing (as in pipelines) in the trench/confined space must be secured and blanked off if possible. It may be determined that the space must be drained of any product prior to entry.
Any manufacturing or processing equipment must be shut down prior to entry. If possible, all equipment should be locked/tagged out and brought to a zero energy state.
Remember: Tags alone do not restrain! Padlocks and other physical restraints should always be used.
PHYSICAL HAZARDS:
Primary Collapse:
Not all trench rescues involve a collapse, but awareness of the potential of a primary collapse should be foremost on the mind of all rescue team members.
A collapse can occur at any time with minimal warning, some of the signs and factors of a potential collapse are:
- Vibration from traffic, and trench site equipment.
- Moisture
- Layered soil types.
- Soft pockets in the trench wall.
- Sloughing in the trench wall.
Secondary Collapse:
The probability of multiple repeat collapses after the initial collapse is extremely high!
Water:
Water can present engulfment and drowning hazards. Moisture can affect the cohesive properties of soil causing trench wall failure.
Temperature Extremes:
Extremely hot or cold temperatures can present problems for workers. For example, if the space has been steamed, it should be allowed to cool before any entry is made. If temperatures exceed 100 degrees, crew rotation on a regular basis is a must.
Noise:
Noise within a trench/confined space can be amplified because of design and acoustic properties of the space. Excessive noise can not only damage hearing, but can also affect communication, such as causing a shouted warning to go unheard. In addition, noise can cause undue vibrations in the trench.
Slick/Wet Surfaces:
Slips and falls can occur on a wet surface causing injury or death to workers. Also, a wet surface will increase the likelihood of an electric shock in areas where electric circuits, equipment, and tools are used.
Falling Objects:
Typically, excavation sites are littered with potential projectiles that can fall on rescuers working in a trench. Every attempt should be made to do preventive maintenance, clearing unnecessary objects from the lip of the trench.
NOTE: Do not move anything until the dig sight is identified, tools and other construction items may provide clues and help “zero” in on the location of the victim.
PSYCHOLOGICAL HAZARDS:
Claustrophobia:
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