by Sally Colby
Josh Flesher, assistant regional administrator for OSHA Region Six, had the unfortunate experience of being present during the recovery effort at a ditch collapse right before Christmas almost 15 years ago, and it’s something he’ll never forget.
“Two workers had been buried in a 28-foot trench that had collapsed,” said Flesher. “Fire and rescue was on site, attempting to do a rescue operation. I arrived on site at about 5 p.m. and was there until 9 the next morning. It took the local fire department that long to recover the bodies.”
Trench work is a dangerous but necessary activity on farms. Farmers often need to install drainage in fields, extend water lines to additional pastures or add to existing electrical lines.
Marko Kaar discussed excavation and trenching safety in a video produced under an OSHA Harwood grant. Kaar describes an excavation as ‘any man-made cut, cavity trench or depression formed by the removal of earth or soil. That means either mechanical or hand-removal.’
Kaar explained that the terms ‘excavation’ and ‘trench’ are often used interchangeably, but notes there is a difference. “A trench is an excavation that’s deeper than it is wide; usually not wider than 15 feet,” he said. “Some excavations are only dangerous near the slopes. The bottom line is that nearly all trenches are dangerous if not protected, due to the fact that they’re narrow and you could get trapped.”
One key to safe trench work is having a designated ‘competent person’ at the job site. That person’s job is to identify existing hazards and predict potential hazards that may occur either in the excavation or in the surrounding area. The competent person is present throughout the job, and continually assesses safety hazards and risks.
Cave-ins are the main cause of worker death in excavation work. “Collapses are18 times more likely to result in death than common construction accidents,” said Kaar. “Cave-ins aren’t the only dangers involved in excavation work. Other hazards include water accumulation, collapse of nearby structures and hazardous atmospheres produced by toxic gasses in the soil.” It’s important that everyone on the site, including the farm owner, is aware of potential hazards and knows to inform someone (such as the competent person or a job supervisor) of any impending danger such as the potential for a collapse or any other noticeable hazard.
Kaar explained what happens in a cave-in. When soil is removed, the surrounding soil becomes unstable and gravity forces it to collapse and fall. The resulting cave-in can entrap, bury or otherwise injure or immobilize people working in the excavation.
Most cave-ins occur in trenches that are five feet to 15 feet deep, and occur suddenly and with little or no warning. “On average, about 1,000 trench collapses occur each year in the United States,” said Kaar. “Over 95 percent of injuries occur in trenches without any protective system.” Kaar says it only takes about two to three feet of soil on a person’s chest to compress the lungs and prevent breathing, and as little as four to six minutes without oxygen to sustain considerable brain damage.
Factors such as rain and subsequent water accumulation can affect soil movement and increase the likelihood of a collapse. Temperature changes, vibration from heavy equipment working above the trench, and the shifting weight of soil piles surrounding the excavated trench can also influence trench stability. Soil that has been previously disturbed is more likely to cause problems because it often contains backfill or other material that rapidly break away from the native soil.
Kaar explained that knowing the soil type is an important safety factor in trench work, and that soil should be tested to determine the type. “Solid rock is the most stable and sand is the least stable,” he said. “Solid rock requires explosives for removal, and becomes less stable after blasting.”
Type A soil is stiff and cohesive without fractures or cracks, and is sometimes referred to as hardpan. Type C soil is granular, like sand or gravel, and type B soil is between A and C, and is usually subsoil or silty sand. Soil classification may have to be changed if conditions change, such as the addition of water to the site. Kaar said that soil classification can be difficult because of layering. For example, type B soil may be on the surface, with type C soil a few feet below. In any case, soil should be treated as type C unless proven otherwise.
Visual analysis of soil and the site should include inspection for surface water, sources of vibration, obvious cracks or fissures, previously disturbed soil or existing utility lines. Each factor can influence stability and the type of protection needed. It’s up to the project manager to document soil type and plan the appropriate trench protection system based on trench depth and environmental conditions surrounding the site. Kaar said that any trench protective system deeper than 20 feet must be designed by a registered professional engineer.
There are several ways to secure a trench, including sloping (angling) the edges or using support systems such as trench boxes. There are specs for sloping each soil type, but in general, the more granular the soil, the more gradual the incline. Shoring is another method of securing the sides of a trench, and must be designed according to soil type, depth and width of trench and the amount of space needed between the cross braces to perform the work in the trench. Shielding structures, or trench boxes, can also be used. These structures protect workers in the event of a cave-in, but the worker must be within the confines of the box for full protection.
No matter what kind of protective system is in place, the site should be continually monitored because any change in conditions such as weather or movement of heavy equipment can influence its stability. “Any surface encumbrance (such as a fallen tree) that can create a hazard must be removed or supported to safeguard workers,” said Kaar. “Before digging, utility companies should be contacted to locate sewer, telephone, fuel, electric or water lines.” Kaar cautions workers to watch overhead utilities, and to maintain a 10-foot distance between any lines and the maximum equipment height.
Be sure to allow a safe means of entry and exit for workers who will go into the trench, such as a ladder or a sturdy, non-slip ramp. The exit should provide a safe means for workers to get out quickly. Inspect ladders for structural defects prior to use and make sure the ladder extends at least three feet above the surface of the trench. Workers should not use loader buckets or backhoe shovels as a means to enter or exit trenches.
Workers on the ground should be sure to make eye contact with heavy equipment operators so that those operators are aware of others’ locations. If everyone on the ground wears reflective vests, equipment operators will have an easier time seeing them and add one more measure in avoiding serious or fatal accidents.