In the U.S., more than 800 construction workers die every year while on the job. One of the most dangerous types of construction work is trenching. Trenching kills 40 construction workers every year. Workers can suffer death or serious injury within minutes of being caught in a trench cave-in. Walls can collapse suddenly and without warning, and workers do not have time to move out of the way. One cubic yard of dirt can weigh more than 3,000 pounds – which is about the weight of a car. This much weight falling onto an individual or group of individuals can fatally crush and suffocate workers.
What is the Difference Between a Trench and an Excavation?
A trench is defined by OSHA as a narrow underground excavation that is deeper than it is wide and is no wider than 15 feet. An excavation is defined as any man-made cut, cavity, trench, or depression that is made into the earth’s surface formed by earth removal.
What is a Competent Person?
A competent person is an individual, designated by the employer, who is capable of identifying existing and predictable hazards in the surroundings or working conditions which are unsanitary, hazardous or dangerous to workers, and who is authorized to take prompt corrective measures to eliminate them. Tasks of a competent person include: classifying soil, inspecting protective systems, designing structural ramps, monitoring water removal equipment, and conducting site inspections.
Soil classification is important to understand because the method of protection used in a trench is based off what type of soil is being excavated. There are four types of soil: Stable Rock, Type A, Type B, and Type C. Stability of the soil is the strongest as Stable Rock and the weakest as Type C soil. There can be more than one type of soil present in an excavation. Soil classification is determined by a competent person (company-designated) and is completed prior to the start of work in a trench. A simple thumb penetration test can be used in the field to help determine what type of soil is present in a trench. When performing this test, it must be done on undisturbed moist soil samples about the size of a baseball. This test should be performed as soon as possible after excavation to reduce the effects of drying or additional water. If a thumb indents the soil with difficulty, it is considered Type A soil. If the soil is indented to the cuticle with effort, the soil is considered Type B. If the soil is indented to the knuckle easily, it is considered Type C.
Stable Rock has the highest stability. It can be excavated with vertical sides and remain intact while exposed.
Type A soils have high stability. Examples of Type A soils include clay, silty clay, sandy clay, and clay loam. Soil cannot be classified as Type A if it: is fissured (cracks in the ground); is subject to vibration from heavy traffic, pile driving, or similar effects; or has been previously disturbed; or is part of a sloped, layered system where the layers dip into the excavation on a slope of 4:1 or greater; or is subject to other factors that would require it to be classified as a less-stable material.
Type B soils have medium stability. Examples of Type B soils include silt, sandy loam, medium clay and unstable rock. Soil can be classified as Type B if it is: previously disturbed soils, except those that would be classified as Type C; or soil that meets the requirements of Type A soil but is fissured or subject to vibration.
Type C soils have the least stability. Examples of Type C soils include gravel, sand, loamy sand, soft clay, submerged soil or dense unstable rock, or soil from which water is freely seeping.
Systems Available for Protection
The protective system used in a trench is chosen based off what type of soil is present. Protective systems must be in place if the trench is 5 feet deep or greater, unless the soil is Stable Rock. In some job sites, the contractor may require protective systems to be used in trenches that have depths that are less than 5 feet deep. There are four systems of protection that can be used in a trench: Benching, Sloping, Shielding, and Shoring. Each system has its own requirements that must be met to ensure it is the best system to be used in a trench.
Benching is excavating the sides of a trench to form one or a series of horizontal levels or steps, usually with vertical or near-vertical surfaces between levels. This form of protection cannot be used with Type C soils.
Sloping is cutting back the trench wall at an angle inclined away from the excavation. All type C soils must be sloped.
Shoring requires installing aluminum hydraulic or other types of supports to prevent soil movement and cave-ins.
Shielding uses trench boxes or other types of supports to prevent soil cave-ins. When using shielding as a form of protection, you must consider the soil classification, depth of cut, water content of soil, changes caused by the weather/climate, surcharge loads, and other operations being completed in the vicinity of the trench.
What Forms of Access and Egress Must be Available?
Ladders, steps, ramps, or other safe means of egress must be available for workers working in trench excavations 4 feet or deeper. The means of egress must be located in a position that requires workers to travel no more than 25 feet laterally within the trench. If a structural ramp is used solely for worker access or egress, it must be designed by a competent person. Structural ramps used for access or egress of equipment must also be designed by a competent person qualified in structural design.
- When working in or near trenches, it is important to be aware of your surroundings.
- Know where underground utilities are located prior to digging.
- Never enter a trench that has not been properly inspected prior to the start of each shift or after any occurrence that could have changed the conditions in the trench.
- Fissures, tension cracks, water seepage, or bulges at the bottom of a trench are all signs of a potential cave-in.
- A competent person must inspect the trench and fix any problems prior to work beginning.
- Check with the competent person before entering a trench that has standing or accumulating water or that has the potential to have an atmosphere lacking oxygen or that has hazardous fumes or vapors.
- If there is water present, the use of special support or shield systems, water removal to control the water level, or safety harnesses and lifelines must be in place when entering a trench.
- Be sure to test for atmospheric hazards such as low oxygen, hazardous fumes and toxic gases when working in trenches greater than 4 feet deep.
- Make sure there is a safe way to enter and exit the trench, such as a ladder, ramp, or steps.
- There must be a means of egress within 25 feet of all workers in a trench.
- Only enter a trench that has cave-in protection. Trenches should be protected by either benching, sloping, shoring, or shielding.
- Do not enter a trench if at least one of these protective systems is not in place, and do not work outside of any trench shields/shoring in unprotected trenches.
- Always wear a hard hat to prevent head injuries that may be caused by falling objects.
- Keep rocks, soil, materials, and equipment away from the edge of the trench.
- General rule of thumb is to keep them a minimum of 2 feet away.
- In some instances, it may not be possible to keep spoil piles at a distance. Be sure to evaluate all options prior to beginning work.
- If a trench collapse occurs, do not attempt to rescue your co-workers. Too frequently, untrained rescuers are killed after entering a collapsed trench.
REMEMBER: A trench does not have to be very deep to be dangerous. One cubic yard of dirt equals the weight of a car – it will crush you!
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