Western Illinois University: Macomb Campus
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Trenching & Excavation Safety
Trenching and shoring operations performed deeper than four feet pose several hazards to workers. Employees need to know and follow all safety requirements in order to get in and out of trenches safely. Follow safe work practices for working in trenches. Place secured ladders that extend a few feet above the trench every 25 feet for safe evacuation. Wear all PPE required. Enter a trench only if you have a specific task to complete inside. Back fill the trench as soon as possible after work inn trench is complete.
Naturally occurring issues - subsidence is created when the weight of soil, which averages 2700 lbs. per cubic yard, is pushed against the walls of a trench causing them to sag and bulge. Subsidence can cause or worsen cracking. Top soil (or tension) cracks are also caused by subsidence. Subsidence can cause soil layers to slide over one another and spill into the trench; shifting between bedrock and soil layers cause cracks that can break off in the trench.
Water with a weight of 62 lbs. per cubic foot, can increase pressure against an excavation wall creating hydrostatic pressure. Water can turn the trench floor into unstable mud; it can also be a hazard if it freezes and expands cracks apart causing soil to slide into the excavation. Pumps, dikes and special shoring techniques can be used to remove water.
Manmade hazards - the weight of the spoils, or the soil removed from the trench, can cause spillage or cave-in unless it is placed at least two feet away from the trench edge. Jack hammers and large equipment can vibrate the soil and enlarge cracks if operated near the trench. Since soil around lines and pipes may be unstable, supports man need to be rigged to keep them from breaking.
Hazard prevention - sloping involves grading the angle of the trench back to prevent the walls from collapsing. The angle is known as the angle of repose. The type of soil and water presence determines the angle’s grade while how far bag you dig is determined by how far down you must dig. The average angle of repose is 45 degrees, resulting in a one foot dig back for every foot down. For unstable soils, the ratio must be larger for digging back.
Shielding is a box constructed of metal or wood, known as a trench shield, is used to protect cave-ins in narrow work areas. The trench shield should be placed as close to the bottom of the trench as possible and extend above the upper edge; you’re only protected when you are inside the shield.
Shoring is used when access is narrow and susceptible to vehicular traffic vibration. The uprights of the shore’s frame are placed vertically and tightly as possible against the trench walls. Stringers are placed horizontally against the uprights. Braces are hydraulic jacks or adjustable poles that provide support against the uprights and stringers. Install shoring as straight as possible from the top down for maximum support. Inspect shoring daily or when conditions have changed that may affect the soil.
Sheeting - install sheeting made of metal or wood for deep or large trenching operations. Install sheeting below the point at which water may enter the trench to keep out water; extend it above the trench walls to keep out sliding spoils.
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