June 1, 2012 by Andrew Snook
In April of 2012, a 53-year-old construction worker lost his life when he was buried alive in a tragic trenching accident in Lachine, Que., despite the best efforts of co-workers and emergency rescue staff. Working in trenches is a dangerous job. From potential cave-ins to falling objects, workers have to constantly watch for a variety of deadly hazards.
Rick Preszcator, regional safety director for EllisDon in Toronto, Ont., has more than 27 years of experience in the construction industry and knows what needs to be done on the jobsite to ensure everyone gets home safe.
“Pre-planning is the biggest thing,” he says. “If you are using the proper tools and the proper hazard assessments you should not have accidents.”
Constantly assessing worksites is vital for keeping people safe in the trenches. Preszcator says daily pre-job safety assessments (PSAs) are a tool that helps workers stop, think and put controls in place. “If you do something like a PSA daily, it’s a good way to prevent complacency.”
The safety culture within the construction sector has changed dramatically over the past two decades. It has a higher presence now than it did 20 years ago, and most companies have full-time safety employees in place to keep workers out of harm’s way. Educating the workforce has become a top priority.
“We do a lot of in-house training. You have to make everyone aware of potential hazards, the tools you’ll use and the procedures for doing it. Everyone knows what everyone else is doing. It’s careful planning and careful preparation,” explains Preszcator.
He says the number of incidents and accidents 15 years ago—even 10 years ago—have gone way down, a comment echoed by Mike Chappell, provincial coordinator for the Construction Health and Safety Program (CHSP) at the Ontario Ministry of Labour.
Trench collapses were a major problem from about 1982 to the early 1990s and awareness has played a significant factor in reducing the number of trenching accidents.
“Understanding the potential [hazard] is part of the problem. In the past, people may have recognized that it would collapse, but they wouldn’t think it was going to collapse at that moment,” Chappell explains. “They all thought ‘Well if it did collapse we would all go down with a shovel and dig them out and carry on,’ so they took those risks, and most of the time they got away with them.”
The combination of increased enforcement by the Ministry of Labour and its focus on trenching has caused more of the workers and employers in the trenching business to provide shoring protection.
Tip #1 – Use the proper cave-in protection for the job
There are three ways to provide proper trenching protection: sloping, shoring and trench boxes. Remember to check the Ministry’s safety guidelines and requirements before you dig. Do your research and find out what type of protection is best suited for the soil conditions on your jobsite.
The most common type of protection used is sloping the walls of the trench—often used with shoring or trench boxes.
The second is a shoring system—timber or hydraulic, for example—which is used to support the trench walls to prevent the movement of soil, underground utilities, roadways and foundations.
The third type of protection is a trench box for protecting workers from cave-ins. Each trench box should be able to withstand the maximum lateral load expected at a given depth in a particular soil condition. There are also slide rail systems, a kind of hybrid of trench box and shoring protection. It’s a component shoring system, comprised of steel panels and posts that are similar to trench shields.
Tip #2 – Inspect the system regularly
No matter what protection system you choose, it needs to be inspected regularly. Hydraulic shoring should be checked for leaks in hoses and cylinders, bent bases, broken or cracked nipples, and any other possible damages or defects. Timber shoring should always be inspected pre-installation—to remove damaged or defective lumber—and afterwards for signs of crushing. Trench boxes should be checked for structural damage, cracks in welds and other defects.
Tip #3 – Keep the trench accessible
Whether sloping, shoring or trench boxes are used for protection, workers must be provided with ladders so they can enter and exit trenches safely. The ladders must be placed inside the protected area, securely tied off at the top and extend above the shoring or box by a minimum of one metre.
Tip #4 – Watch the soil
One of the most significant factors that endangers the lives of workers within trenches is the type of soil being excavated.
There are four types of soil set out in construction regulations that range from Type 1, which is a hard soil to dig that tends to remain vertical and not release water when excavated, to Type 4 soils that can be excavated easily using a hydraulic backhoe but require constant support and containment (See Soil Type sidebar for details).
Soil types can change quickly depending on a variety of factors. It’s vital to reassess the soil at a jobsite regularly, especially when weather has changed.
Tip #5 – Be wary of vibrations, even from afar
Vibrations caused by machines—even 500 feet away—can make a difference to the structural integrity of a trench. Ongoing operations like earth moving or compaction and vehicular traffic may seriously affect the integrity of trench walls. Type 4 soils are extremely sensitive to vibration and other disturbances due to a high moisture count.
Tip #6 – Never enter a trench alone
If a trench is unsafe workers should not enter it, and never enter a trench, even a protected one, when alone.
“The chance of being rescued is slim to begin with, but the chance of being rescued when alone is zero,” says Chappell. “The problem with soils, especially soils in the city, is that we don’t know when [a collapse] will occur. It could occur within seconds of being excavated, or it could occur several hours or several days later.”
Tip #7 – Watch where you step
Workers inside the trenches aren’t the only ones that experience trenching-related injuries. One of the most common ways people get hurt on-site is by falling over the edge of a trench or sliding down the edge of a bank.
Tip #8 – Keep an eye out for falling debris
In addition to cave-ins and falls, working in trenches comes with hazards such as piping, stones or debris rolling down an excavated bank and striking workers.
Tip #9 – Remember to look up
While a great deal of focus on the ground being dug up is necessary to ensure worker safety—locating underground utilities and setting up the necessary safety measures, for example—one of the biggest hazards that are sometimes overlooked is overhead powerlines.
According to Infrastructure Health & Safety Association’s (IHSA) publication, Trenching Safety: Introduction to Trenching Hazards, constructors must have written procedures in place to prevent equipment from intruding on minimum safe distances.
For powerline voltage ratings of 750 to 150,000 volts, equipment must maintain a minimum distance of 3 metres (10 feet). For 150,000 to 250,000 volts, equipment must have a minimum safe distance of 4.5 metres (15 feet); and for powerlines with voltage ratings that are more than 250,000 volts, a minimum safe distance of 6 metres (20 feet) must be maintained.
Tip #10 – Use extra protection whenever possible
In addition to standard protection, such as hardhats, eye protection and Grade 1 safety boots, another pro
tection measure workers can take is wearing a safety harness and attaching themselves to a lifeline. Depending on the soil condition within the trench, workers may also be required to wear rubber safety boots.
Tip #11 – Have emergency plans in place
Every company should have an emergency plan in place to deal with potential trenching accidents. “In the case of a cave-in you don’t have a lot of time,” says Preszcator.
Rescuing a worker who has been trapped in a cave-in is a raceagainst time. The weight of the soil in a typical collapse is several thousand pounds, and it only takes 40 pounds of pressure on the average human chest on a sustained level to prevent them from breathing. “Essentially, if you’re buried in a trench collapse, the weight of the soil pushes the air out of your lungs and then it’s a ticking clock, and a very short ticking clock, before the worker is killed… my experience with trenching is that it’s seldom that you see a positive outcome when a worker is buried,” says Chappell.
Tip #12 – Make sure everyone receives training
There is a wide variety of trench safety training programs offered through public and private companies, and many unions do their own training as well. The Ministry of Labour offers videos and literature on trench safety on its website free of charge for the public.
Preszcator says if you plan your work and work your plan, then all of your safety measures should be in place.
“The bottom line is you have to make it a safe work environment. Everybody goes home at the end of the day.”