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 world weight lifting record for the press is 521.5 pounds. A single cubic yard of soil can weigh 2,700 pounds or more.
The greatest hazard in an excavation is a cave-in. A cave-in starts when excessive stress causes the soil at the base of the excavation to collapse, moving the area of stress higher up the excavation walls. The new area of stress collapses, moving the area of stress even higher. Collapses can occur in rapid succession before a worker has time to react. In addition to the excavation itself, external operations can put stress on the soil – from machinery operating near the excavation’s edge to rising groundwater and even vibrations from traffic along a nearby road.
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.
The good news is that almost all cave-in disasters can be prevented by following safe excavation practices. The Best Practice for keeping workers safe when they're working in an excavation 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 an excavation is fewer than 12' deep, sloping the sides of the excavation 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. It involves 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. If an excavation is too deep to use sloping or benching, shoring may be an option. Shoring supports the sides of the excavation with horizontal, vertical, and cross braces. The soil type and excavation dimensions determine how many supports are used and how the braces are positioned.
Shielding. A trench shield is a structure that can withstand a cave-in. It's used specifically for trenches. The shield can be an immovable structure, or it can be a portable one that's moved along as work progresses. Shielding can also be used in combination with sloping and benching.
OSHA requires that protective systems be installed in excavations, unless the excavation is made entirely of stable rock or is fewer than 5' deep with no risk of a cave-in. In hazardous soil conditions, even trenches fewer than 5' deep need a protective system. Because many workers underestimate the weight of soil, injuries and deaths continue to occur in 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 in excavations.
For more information about excavation safety, visit the Occupational Safety and Health Administration site, or sign up your team for Jobsite Safety II, an online training course from BuildIQ University. Other Best Practices® for preventing cosmetic and structural problems can be found in BuildIQ University’s House Placement and Excavation course.
