Watch the video on Excavation Safety.
Excavation is one of the most hazardous construction operations performed on residential jobsites. The weight of soil falling from the side of an open trench can crush workers at the bottom, seriously injuring or killing them. The greatest hazard in an excavation is a cave-in. Every year, workers continue to lose their lives in cave-ins. A single cubic yard of soil can weigh 2,700 pounds or more. The world weight lifting record for the press is 521.5 pounds, but a cubic yard of soil weighs five times that amount. Could you push back 500 pounds of soil falling into a trench? Could you breathe or even survive under the weight?
So why does a cave-in occur? Excavating puts stress on the soil. This stress is most highly concentrated at the base of the excavation wall. The excessive stress causes the soil to collapse, moving the area of stress higher up the excavation wall. The new area of stress collapses, moving the area of stress higher up the excavation wall again. Then, this area of stress collapses too. The collapses can occur in rapid succession before a worker has time to react.
A worker who is buried when the walls of an excavation collapse probably won't live more than four minutes. Even if the worker is buried only up to the waist, he could be severely injured or die from injuries caused by the weight of the collapsed soil. Most accidents occur in trenches. A trench is an excavation that's deeper than it is wide. Usually, there's no warning before a trench caves in. It's essential to take appropriate precautions to prevent cave-ins before workers are allowed into an excavation.
The good news is that almost all cave-in fatalities can be prevented by following safe excavation practices. The best practice for keeping workers safe when they're working in excavations is to first determine the soil type by consulting with a soils engineer. Once the soil type has been identified, there are four kinds of protective systems builders can use:
- Sloping. If the trench is fewer than 12 feet deep, sloping the sides of the trench is a good option. The angle of the slope will be determined by the soil type and can range from 34? in sandy soil to 90? in stable rock.
- Benching. Benching is a variation on sloping and consists of cutting the sides of the excavation to form one or more horizontal levels or steps with vertical or near-vertical surfaces between levels. The soil type determines how the excavation is benched.
- Shoring. When the trench is too deep to use sloping or benching, using supporting shores may be an option. Shoring supports the sides of the excavation with horizontal, vertical, and cross braces. The number of supports and positioning of the braces will depend upon the soil type and the depth and width of the trench.
- Shielding. A trench shield is a structure that can withstand a cave-in and is used specifically for trenches. The shield is a permanent or portable structure that can be moved along as work progresses. It can also be used in combination with sloping and benching.
Although the greatest danger to workers comes from cave-ins, other hazards exist as well. Excavated soil and building materials located near the excavation can roll into the excavation and onto the worker. Rising groundwater can undermine the excavation. Machinery operating near the edge of the excavation can cause it to cave in. Even vehicular traffic along a nearby road can undermine the excavation through vibrations.
OSHA requires that protective systems be installed in excavations so workers can go home healthy at the end of the day. A protective system safeguards workers from cave-ins and from materials falling into the excavation. A protective system is required unless the excavation is made entirely of stable rock or is less than 5 feet deep, and there's no indication of a potential cave-in. In hazardous soil conditions, even trenches fewer than 5 feet deep need a protective system.
Because many workers underestimate the weight of soil, the chance of cave-ins, injuries, and deaths due to poor safety practices continue around excavations. However, when builders take the time to determine the soil type of an excavation and use the right protective system, they can help keep their workers safe and avoid unnecessary injuries.
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